Magnetic resonance imaging

Magnetic resonance imaging

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Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging
Medical imaging
Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...

 technique used in radiology
Radiology
Radiology is a medical specialty that employs the use of imaging to both diagnose and treat disease visualized within the human body. Radiologists use an array of imaging technologies to diagnose or treat diseases...

 to visualize detailed internal structures. MRI makes use of the property of nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...

 (NMR) to image nuclei of atoms inside the body.

An MRI machine uses a powerful magnetic
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...

 field to align the magnetization
Nuclear magnetic moment
The nuclear magnetic moment is the magnetic moment of an atomic nucleus and arises from the spin of the protons and neutrons. It is mainly a magnetic dipole moment; the quadrupole moment does cause some small shifts in the hyperfine structure as well....

 of some atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s in the body, and radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

 fields to systematically alter the alignment of this magnetization. This causes the nuclei to produce a rotating magnetic field detectable by the scanner—and this information is recorded to construct an image of the scanned area of the body.
Strong magnetic field gradients cause nuclei at different locations to rotate at different speeds. 3-D spatial information can be obtained by providing gradients in each direction.

MRI provides good contrast
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...

 between the different soft tissues of the body, which makes it especially useful in imaging the brain
Neurology
Neurology is a medical specialty dealing with disorders of the nervous system. Specifically, it deals with the diagnosis and treatment of all categories of disease involving the central, peripheral, and autonomic nervous systems, including their coverings, blood vessels, and all effector tissue,...

, muscles, the heart, and cancers
Oncology
Oncology is a branch of medicine that deals with cancer...

 compared with other medical imaging
Medical imaging
Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...

 techniques such as computed tomography
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...

 (CT) or X-ray
Medical radiography
Radiography is the use of ionizing electromagnetic radiation such as X-rays to view objects. Although not technically radiographic techniques, imaging modalities such as PET and MRI are sometimes grouped in radiography because the radiology department of hospitals handle all forms of imaging...

s. Unlike CT scans or traditional X-rays, MRI uses no ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

.

How MRI works


The body is largely composed of water molecules
Body water
In medicine, body water is the water content of the human body. A significant fraction of the human body is water. Arthur Guyton 's Textbook of Medical Physiology states that "the total amount of water in a man of average weight is approximately 40 litres, averaging 57 percent of his total body...

. Each water molecule has two hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

 or proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s. When a person is inside the powerful magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 of the scanner, the magnetic moments of some of these molecules become aligned with the direction of the field. A radio frequency transmitter is briefly turned on, producing a further varying electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...

. The photons of this field have just the right energy, known as the resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

 frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

, to be absorbed and flip the spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...

 of the aligned protons in the body. The frequency at which the protons resonate depends on the strength of the applied magnetic field. After the field is turned off, those protons which absorbed energy revert to the original lower-energy spin-down state.
A hydrogen dipole has two spins, 1 high spin and 1 low. In low spin both dipole and field are in parallel direction and in high spin case it is antiparallel. They release the difference in energy as a photon, and the released photons are detected by the scanner as an electromagnetic signal, similar to radio waves.

As a result of conservation of energy
Conservation of energy
The nineteenth century law of conservation of energy is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time...

, the resonant frequency also dictates the frequency of the released photons. The photons released when the field is removed have an energy — and therefore a frequency — which depends on the energy absorbed while the field was active. It is this relationship between field-strength and frequency that allows the use of nuclear magnetic resonance for imaging. An image can be constructed because the protons in different tissues return to their equilibrium state at different rates, which is a difference that can be detected. Five different tissue variables — spin density, T1 and T2 relaxation times and flow and spectral shifts can be used to construct images. By changing the settings on the scanner, this effect is used to create contrast between different types of body tissue or between other properties, as in fMRI and diffusion MRI
Diffusion MRI
Diffusion MRI is a magnetic resonance imaging method that produces in vivo images of biological tissues weighted with the local microstructural characteristics of water diffusion, which is capable of showing connections between brain regions...

.

The 3D
Three-dimensional space
Three-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...

 position from which photons were released is learned by applying additional fields during the scan. This is done by passing electric currents through specially-wound solenoid
Solenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...

s, known as gradient coils. These fields make the magnetic field strength vary depending on the position within the patient, which in turn makes the frequency of released photons dependent on their original position in a predictable manner, and the original locations can be mathematically recovered from the resulting signal by the use of inverse Fourier transform
Fourier transform
In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...

.

Contrast agents may be injected intravenously
Intravenous therapy
Intravenous therapy or IV therapy is the infusion of liquid substances directly into a vein. The word intravenous simply means "within a vein". Therapies administered intravenously are often called specialty pharmaceuticals...

 to enhance the appearance of blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

s, tumors or inflammation
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

. Contrast agents may also be directly injected into a joint in the case of arthrogram
Arthrogram
An arthrogram is a series of images, often X-rays, of a joint after injection of a contrast medium. The injection is normally done under a local anesthetic.The radiologist performs the study utilizing fluoroscopy or ultrasound to guide the placement of the needle into the joint and then injects an...

s, MRI images of joints. Unlike CT
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...

, MRI uses no ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

 and is generally a very safe procedure. Nonetheless the strong magnetic fields and radio pulses can affect metal implants, including cochlear implant
Cochlear implant
A cochlear implant is a surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf or severely hard of hearing...

s and cardiac pacemaker
Cardiac pacemaker
right|thumb|350px|Image showing the cardiac pacemaker which is the SA nodeThe contraction of heart muscle in all animals with hearts is initiated by chemical impulses. The rate at which these impulses fire controls the heart rate...

s. In the case of cochlear implants, the US FDA has approved some implants for MRI compatibility. In the case of cardiac pacemakers, the results can sometimes be lethal, so patients with such implants are generally not eligible for MRI.

Since the gradient coils are within the bore of the scanner, there are large forces between them and the main field coils, producing most of the noise that is heard during operation. Without efforts to damp this noise, it can approach 130 decibels (dB) with strong fields (see also the subsection on acoustic noise).

MRI is used to image every part of the body, and is particularly useful for tissues with many hydrogen nuclei and little density contrast, such as the brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

, muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

, connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 and most tumor
Tumor
A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...

s.

History


In the 1950s, Herman Carr
Herman Carr
Herman Y. Carr was an American physicist and pioneer of magnetic resonance imaging.Carr was born in Alliance, Ohio. He received his BS, MS and PHD from Harvard University, where he studied under Edward Purcell. He later moved to Rutgers University, where he was professor. He retired in 1987.In...

 reported on the creation of a one-dimensional MR image. Paul Lauterbur
Paul Lauterbur
Paul Christian Lauterbur was an American chemist who shared the Nobel Prize in Physiology or Medicine in 2003 with Peter Mansfield for his work which made the development of magnetic resonance imaging possible.Dr...

 expanded on Carr's technique and developed a way to generate the first MRI images, in 2D and 3D, using gradients. In 1973, Lauterbur published the first nuclear magnetic resonance image. and the first cross-sectional image of a living mouse was published in January 1974. Nuclear magnetic resonance imaging is a relatively new technology first developed at the University of Nottingham
University of Nottingham
The University of Nottingham is a public research university based in Nottingham, United Kingdom, with further campuses in Ningbo, China and Kuala Lumpur, Malaysia...

, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

. Peter Mansfield
Peter Mansfield
Sir Peter Mansfield, FRS, , is a British physicist who was awarded the 2003 Nobel Prize in Physiology or Medicine for his discoveries concerning magnetic resonance imaging . The Nobel Prize was shared with Paul Lauterbur, who also contributed to the development of MRI...

, a physicist and professor at the university, then developed a mathematical technique that would allow scans to take seconds rather than hours and produce clearer images than Lauterbur had.
In a 1971 paper in the journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

, Dr. Raymond Damadian, an Armenian-American
Armenian-American
Armenian Americans are citizens of the United States whose ancestry originates wholly or partly in Armenia. During the United States 2000 Census, 385,488 respondents indicated either full or partial Armenian ancestry...

 physician, scientist, and professor at the Downstate Medical Center State University of New York
State University of New York
The State University of New York, abbreviated SUNY , is a system of public institutions of higher education in New York, United States. It is the largest comprehensive system of universities, colleges, and community colleges in the United States, with a total enrollment of 465,000 students, plus...

 (SUNY), reported that tumor
Tumor
A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...

s and normal tissue can be distinguished in vivo by nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...

 ("NMR"). He suggested that these differences could be used to diagnose cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

, though later research would find that these differences, while real, are too variable for diagnostic purposes. Damadian's initial methods were flawed for practical use, relying on a point-by-point scan of the entire body and using relaxation rates, which turned out to not be an effective indicator of cancerous tissue.

While researching the analytical properties of magnetic resonance, Damadian created the world's first magnetic resonance imaging machine in 1972. He filed the first patent for an MRI machine, U.S. patent #3,789,832 on March 17, 1972, which was later issued to him on February 5, 1974. As the National Science Foundation
National Science Foundation
The National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...

 notes, "The patent included the idea of using NMR to 'scan' the human body to locate cancerous tissue." However, it did not describe a method for generating pictures from such a scan or precisely how such a scan might be done. Damadian along with Larry Minkoff and Michael Goldsmith, subsequently went on to perform the first MRI body scan of a human being on July 3, 1977. These studies performed on humans were published in 1977.

In recording the history of MRI, Mattson and Simon (1996) credit Damadian with describing the concept of whole-body NMR scanning, as well as discovering the NMR tissue relaxation differences that made this feasible.

2003 Nobel Prize


Reflecting the fundamental importance and applicability of MRI in medicine, Paul Lauterbur
Paul Lauterbur
Paul Christian Lauterbur was an American chemist who shared the Nobel Prize in Physiology or Medicine in 2003 with Peter Mansfield for his work which made the development of magnetic resonance imaging possible.Dr...

 of the University of Illinois at Urbana-Champaign
University of Illinois at Urbana-Champaign
The University of Illinois at Urbana–Champaign is a large public research-intensive university in the state of Illinois, United States. It is the flagship campus of the University of Illinois system...

 and Sir Peter Mansfield
Peter Mansfield
Sir Peter Mansfield, FRS, , is a British physicist who was awarded the 2003 Nobel Prize in Physiology or Medicine for his discoveries concerning magnetic resonance imaging . The Nobel Prize was shared with Paul Lauterbur, who also contributed to the development of MRI...

 of the University of Nottingham
University of Nottingham
The University of Nottingham is a public research university based in Nottingham, United Kingdom, with further campuses in Ningbo, China and Kuala Lumpur, Malaysia...

 were awarded the 2003 Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine administered by the Nobel Foundation, is awarded once a year for outstanding discoveries in the field of life science and medicine. It is one of five Nobel Prizes established in 1895 by Swedish chemist Alfred Nobel, the inventor of dynamite, in his will...

 for their "discoveries concerning magnetic resonance imaging". The Nobel citation acknowledged Lauterbur's insight of using magnetic field gradients to determine spatial localization, a discovery that allowed rapid acquisition of 2D images. Mansfield was credited with introducing the mathematical formalism and developing techniques for efficient gradient utilization and fast imaging. The actual research that won the prize was done almost 30 years before, while Paul Lauterbur was at Stony Brook University in New York.

The award was vigorously protested by Raymond Vahan Damadian, founder of FONAR Corporation, who claimed that he invented the MRI, and that Lauterbur and Mansfield had merely refined the technology. An ad hoc
Ad hoc
Ad hoc is a Latin phrase meaning "for this". It generally signifies a solution designed for a specific problem or task, non-generalizable, and not intended to be able to be adapted to other purposes. Compare A priori....

 group, called "The Friends of Raymond Damadian", took out full-page advertisements in the New York Times and The Washington Post
The Washington Post
The Washington Post is Washington, D.C.'s largest newspaper and its oldest still-existing paper, founded in 1877. Located in the capital of the United States, The Post has a particular emphasis on national politics. D.C., Maryland, and Virginia editions are printed for daily circulation...

 entitled "The Shameful Wrong That Must Be Righted", demanding that he be awarded at least a share of the Nobel Prize. Also, even earlier, in the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

, Vladislav Ivanov
Vladislav Ivanov (physicist)
Vladislav Ivanov is a Soviet physicist who in 1960 filed a document with the USSR State Committee for Inventions and Discovery at Leningrad for a Magnetic Resonance Imaging device, although this was not approved until the 1970s....

 filed (in 1960) a document with the USSR State Committee for Inventions and Discovery at Leningrad for a Magnetic Resonance Imaging device, although this was not approved until the 1970s. In a letter to Physics Today
Physics Today
Physics Today, created in 1948, is the membership journal of the American Institute of Physics. It is provided to 130,000 members of twelve physics societies, including the American Physical Society...

, Herman Carr
Herman Carr
Herman Y. Carr was an American physicist and pioneer of magnetic resonance imaging.Carr was born in Alliance, Ohio. He received his BS, MS and PHD from Harvard University, where he studied under Edward Purcell. He later moved to Rutgers University, where he was professor. He retired in 1987.In...

 pointed out his own even earlier use of field gradients for one-dimensional MR imaging.

Applications



In clinical practice, MRI is used to distinguish pathologic tissue (such as a brain tumor
Brain tumor
A brain tumor is an intracranial solid neoplasm, a tumor within the brain or the central spinal canal.Brain tumors include all tumors inside the cranium or in the central spinal canal...

) from normal tissue. One advantage of an MRI scan is that it is harmless to the patient. It uses strong magnetic fields and non-ionizing radiation in the radio frequency range, unlike CT scans and traditional X-rays
Radiography
Radiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....

, which both use ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

.

While CT provides good spatial resolution (the ability to distinguish two separate structures an arbitrarily small distance from each other), MRI provides comparable resolution with far better contrast resolution
Contrast resolution
Contrast resolution is the ability to distinguish between differences in intensity in an image. The measure is used in medical imaging to quantify the quality of acquired images. It is a difficult quantity to define, because it depends on the human observer as much as the quality of the actual image...

 (the ability to distinguish the differences between two arbitrarily similar but not identical tissues). The basis of this ability is the complex library of pulse sequences that the modern medical MRI scanner includes, each of which is optimized to provide image contrast based on the chemical sensitivity of MRI.
For example, with particular values of the echo time (TE) and the repetition time (TR), which are basic parameters of image acquisition, a sequence takes on the property of T2-weighting. On a T2-weighted scan, water- and fluid-containing tissues are bright (most modern T2 sequences are actually fast T2 sequences) and fat-containing tissues are dark. The reverse is true for T1-weighted images. Damaged tissue tends to develop edema
Edema
Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...

, which makes a T2-weighted sequence sensitive for pathology, and generally able to distinguish pathologic tissue from normal tissue. With the addition of an additional radio frequency pulse and additional manipulation of the magnetic gradients, a T2-weighted sequence can be converted to a FLAIR sequence, in which free water is now dark, but edematous tissues remain bright. This sequence in particular is currently the most sensitive way to evaluate the brain for demyelinating
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...

 diseases, such as multiple sclerosis
Multiple sclerosis
Multiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...

.

The typical MRI examination consists of 5–20 sequences, each of which are chosen to provide a particular type of information about the subject tissues. This information is then synthesized by the interpreting physician
Physician
A physician is a health care provider who practices the profession of medicine, which is concerned with promoting, maintaining or restoring human health through the study, diagnosis, and treatment of disease, injury and other physical and mental impairments...

.

T1-weighted MRI


T1-weighted scans are a standard basic scan, in particular differentiating fat from water - with water darker and fat brighter use a gradient echo (GRE) sequence, with short TE and short TR. This is one of the basic types of MR contrast and is a commonly run clinical scan. The T1 weighting can be increased (improving contrast) with the use of an inversion pulse as in an MP-RAGE sequence. Due to the short repetition time (TR) this scan can be run very fast allowing the collection of high resolution 3D datasets. A T1 reducing gadolinium contrast agent is also commonly used, with a T1 scan being collected before and after administration of contrast agent to compare the difference. In the brain T1-weighted scans provide good gray matter/white matter contrast; in other words, T1-weighted images highlight fat deposition.

T2-weighted MRI



T2-weighted scans are another basic type. Like the T1-weighted scan, fat is differentiated from water - but in this case fat shows darker, and water lighter. For example, in the case of cerebral and spinal study, the CSF (cerebrospinal fluid) will be lighter in T2-weighted images. These scans are therefore particularly well suited to imaging edema, with long TE and long TR. Because the spin echo
Spin echo
In magnetic resonance, a spin echo is the refocusing of precessing spin magnetisation by a pulse of resonant radiation. Modern nuclear magnetic resonance and magnetic resonance imaging rely heavily on this effect....

 sequence is less susceptible to inhomogeneities in the magnetic field, these images have long been a clinical workhorse.

T-weighted MRI


T weighted scans use a gradient echo (GRE) sequence, with long TE and long TR. The gradient echo sequence used does not have the extra refocusing pulse used in spin echo
Spin echo
In magnetic resonance, a spin echo is the refocusing of precessing spin magnetisation by a pulse of resonant radiation. Modern nuclear magnetic resonance and magnetic resonance imaging rely heavily on this effect....

 so it is subject to additional losses above the normal T2 decay (referred to as T2′), these taken together are called T. This also makes it more prone to susceptibility losses at air/tissue boundaries, but can increase contrast for certain types of tissue, such as venous blood.

Spin density weighted MRI


Spin density, also called proton density, weighted scans try to have no contrast from either T2 or T1 decay, the only signal change coming from differences in the amount of available spins (hydrogen nuclei in water). It uses a spin echo
Spin echo
In magnetic resonance, a spin echo is the refocusing of precessing spin magnetisation by a pulse of resonant radiation. Modern nuclear magnetic resonance and magnetic resonance imaging rely heavily on this effect....

 or sometimes a gradient echo sequence, with short TE and long TR.

Diffusion MRI



Diffusion MRI
Diffusion MRI
Diffusion MRI is a magnetic resonance imaging method that produces in vivo images of biological tissues weighted with the local microstructural characteristics of water diffusion, which is capable of showing connections between brain regions...

 measures the diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 of water molecules in biological tissues. In an isotropic medium (inside a glass of water for example), water molecules naturally move randomly according to turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...

 and Brownian motion
Brownian motion
Brownian motion or pedesis is the presumably random drifting of particles suspended in a fluid or the mathematical model used to describe such random movements, which is often called a particle theory.The mathematical model of Brownian motion has several real-world applications...

. In biological tissues however, where the Reynolds number is low enough for flows to be laminar
Laminar flow
Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents...

, the diffusion may be anisotropic. For example, a molecule inside the axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

 of a neuron has a low probability of crossing the myelin
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...

 membrane. Therefore the molecule moves principally along the axis of the neural fiber. If it is known that molecules in a particular voxel
Voxel
A voxel is a volume element, representing a value on a regular grid in three dimensional space. This is analogous to a pixel, which represents 2D image data in a bitmap...

 diffuse principally in one direction, the assumption can be made that the majority of the fibers in this area are going parallel to that direction.

The recent development of diffusion tensor imaging (DTI) enables diffusion to be measured in multiple directions and the fractional anisotropy in each direction to be calculated for each voxel. This enables researchers to make brain maps of fiber directions to examine the connectivity of different regions in the brain (using tractography
Tractography
In neuroscience, tractography is a procedure to demonstrate the neural tracts.It uses special techniques of magnetic resonance imaging , and computer-based image analysis.The results are presented in two- and three-dimensional images....

) or to examine areas of neural degeneration and demyelination in diseases like Multiple Sclerosis
Multiple sclerosis
Multiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...

.

Another application of diffusion MRI is diffusion-weighted imaging (DWI). Following an ischemic stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

, DWI is highly sensitive to the changes occurring in the lesion. It is speculated that increases in restriction (barriers) to water diffusion, as a result of cytotoxic edema (cellular swelling), is responsible for the increase in signal on a DWI scan. The DWI enhancement appears within 5–10 minutes of the onset of stroke symptoms (as compared with computed tomography
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...

, which often does not detect changes of acute infarct for up to 4–6 hours) and remains for up to two weeks. Coupled with imaging of cerebral perfusion, researchers can highlight regions of "perfusion/diffusion mismatch" that may indicate regions capable of salvage by reperfusion therapy.

Like many other specialized applications, this technique is usually coupled with a fast image acquisition sequence, such as echo planar imaging sequence.

Magnetization Transfer MRI


Magnetization transfer (MT) refers to the transfer of longitudinal magnetization from free water protons to hydration water protons in NMR and MRI.

In magnetic resonance imaging of molecular solutions, such as protein solutions, two types of water molecules, free (bulk) and hydration (bound), are found. Free water protons have faster average rotational frequency and hence less fixed water molecules that may cause local field inhomogeneity. Because of this uniformity, most free water protons have resonance frequency lying narrowly around the normal proton resonance frequency of 63 MHz (at 1.5 teslas). This also results in slower transverse magnetization dephasing and hence longer T2. Conversely, hydration water molecules are slowed down by interaction with solute molecules and hence create field inhomogeneities that lead to wider resonance frequency spectrum.

In free liquids, protons, which may be viewed classically as small magnetic dipoles, exhibit translational and rotational motions. These moving dipoles disturb the surrounding magnetic field however on long enough time-scales (which may be nanoseconds) the average field caused by the motion of protons is zero. This is known as “motional averaging” or narrowing and is characteristic of protons moving freely in liquid. On the other hand, protons bound to macromolecules, such as proteins, tend to have a fixed orientation and so the average magnetic field in close proximity to such structures does not average to zero. The result is a spatial pattern in the magnetic field that gives rise to a residual dipolar coupling (range of precession frequencies) for the protons experiencing the magnetic field. The wide frequency distribution appears as a broad spectrum that may be several kHz wide. The net signal from these protons disappears very quickly, in inverse proportion to the width, due to the loss of coherence of the spins, i.e. T2 relaxation. Due to exchange mechanisms, such as spin transfer or proton chemical exchange, the (incoherent) spins bound to the macromolecules continually switch places with (coherent) spins in the bulk media and establish a dynamic equilibrium.

Magnetization transfer: Although there is no measurable signal from the bound spins, or the bound spins that exchange into the bulk media, their longitudinal magnetization is preserved and may recover only via the relatively slow process of T1 relaxation. If the longitudinal magnetization of just the bound spins can be altered, then the effect can be measured in the spins of the bulk media due to the exchange processes. The magnetization transfer sequence applies RF saturation at a frequency that is far off resonance for the narrow line of bulk water but still on resonance for the bound protons with a spectral linewidth of kHz. This causes saturation of the bound spins which exchange into the bulk water, resulting in a loss of longitudinal magnetization and hence signal decrease in the bulk water. This provides an indirect measure of macromolecular content in tissue. Implementation of magnetization transfer involves choosing suitable frequency offsets and pulse shapes to saturate the bound spins sufficiently strongly, within the safety limits of specific absorption rate for RF irradiation.

T1rho MRI



T1ρ (T1rho): Molecules have a kinetic energy that is a function of the temperature and is expressed as translational and rotational motions, and by collisions between molecules. The moving dipoles disturb the magnetic field but are often extremely rapid so that the average effect over a long time-scale may be zero. However, depending on the time-scale, the interactions between the dipoles do not always average away. At the slowest extreme the interaction time is effectively infinite and occurs where there are large, stationary field disturbances (e.g. a metallic implant). In this case the loss of coherence is described as a "static dephasing". T2* is a measure of the loss of coherence in an ensemble of spins that include all interactions (including static dephasing). T2 is a measure of the loss of coherence that excludes static dephasing, using an RF pulse to reverse the slowest types of dipolar interaction. There is in fact a continuum of interaction time-scales in a given biological sample and the properties of the refocusing RF pulse can be tuned to refocus more than just static dephasing. In general, the rate of decay of an ensemble of spins is a function of the interaction times and also the power of the RF pulse. This type of decay, occurring under the influence of RF, is known as T1ρ. It is similar to T2 decay but with some slower dipolar interactions refocused as well as the static interactions, hence T1ρ≥T2.

Fluid attenuated inversion recovery (FLAIR)



Fluid Attenuated Inversion Recovery (FLAIR) is an inversion-recovery pulse sequence used
to null signal from fluids. For example, it can be used in brain imaging to suppress cerebrospinal fluid (CSF) so as to bring out the periventricular hyperintense lesions, such as multiple sclerosis (MS) plaques. By carefully choosing the inversion time TI (the time between the inversion and excitation pulses), the signal from any particular tissue can be suppressed.

Magnetic resonance angiography




Magnetic resonance angiography (MRA) generates pictures of the arteries to evaluate them for stenosis
Stenosis
A stenosis is an abnormal narrowing in a blood vessel or other tubular organ or structure.It is also sometimes called a stricture ....

 (abnormal narrowing) or aneurysm
Aneurysm
An aneurysm or aneurism is a localized, blood-filled balloon-like bulge in the wall of a blood vessel. Aneurysms can commonly occur in arteries at the base of the brain and an aortic aneurysm occurs in the main artery carrying blood from the left ventricle of the heart...

s (vessel wall dilatations, at risk of rupture). MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs (called a "run-off"). A variety of techniques can be used to generate the pictures, such as administration of a paramagnetic contrast agent (gadolinium
Gadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...

) or using a technique known as "flow-related enhancement" (e.g. 2D and 3D time-of-flight sequences), where most of the signal on an image is due to blood that recently moved into that plane, see also FLASH MRI
FLASH MRI
FLASH MRI is a basic measuring principle for rapid MRI invented in 1985 by Jens Frahm, Axel Haase, W Hänicke, KD Merboldt, and D Matthaei at the in Göttingen, Germany...

. Techniques involving phase accumulation (known as phase contrast angiography) can also be used to generate flow velocity maps easily and accurately. Magnetic resonance venography (MRV) is a similar procedure that is used to image veins. In this method, the tissue is now excited inferiorly, while signal is gathered in the plane immediately superior to the excitation plane—thus imaging the venous blood that recently moved from the excited plane.

Magnetic resonance gated intracranial CSF dynamics (MR-GILD)


Magnetic resonance gated intracranial cerebrospinal fluid (CSF) or liquor dynamics (MR-GILD) technique is an MR sequence based on bipolar gradient pulse used to demonstrate CSF pulsatile flow in ventricles, cisterns, aqueduct of Sylvius and entire intracranial CSF pathway. It is a method for analyzing CSF circulatory system dynamics in patients with CSF obstructive lesions such as normal pressure hydrocephalus. It also allows visualization of both arterial and venous pulsatile blood flow in vessels without use of contrast agents.
Diastolic time data acquisition (DTDA). Systolic time data acquisition (STDA).

Magnetic resonance spectroscopy


Magnetic resonance spectroscopy
In vivo magnetic resonance spectroscopy
In vivo magnetic resonance spectroscopy is a specialised technique associated to magnetic resonance imaging ....

 (MRS) is used to measure the levels of different metabolites in body tissues. The MR signal produces a spectrum of resonances that correspond to different molecular arrangements of the isotope being "excited". This signature is used to diagnose certain metabolic disorders, especially those affecting the brain, and to provide information on tumor metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

.

Magnetic resonance spectroscopic imaging (MRSI) combines both spectroscopic and imaging methods to produce spatially localized spectra from within the sample or patient. The spatial resolution is much lower (limited by the available SNR
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

), but the spectra in each voxel contains information about many metabolites. Because the available signal is used to encode spatial and spectral information, MRSI requires high SNR achievable only at higher field strengths (3 T and above).

Functional MRI




Functional MRI (fMRI) measures signal changes in the brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

 that are due to changing neural
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

 activity. The brain is scanned at low resolution but at a rapid rate (typically once every 2–3 seconds). Increases in neural activity cause changes in the MR signal via T changes; this mechanism is referred to as the BOLD (blood-oxygen-level dependent) effect. Increased neural activity causes an increased demand for oxygen, and the vascular
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

 system actually overcompensates for this, increasing the amount of oxygenated hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

 relative to deoxygenated hemoglobin. Because deoxygenated hemoglobin attenuates the MR signal, the vascular response leads to a signal increase that is related to the neural activity. The precise nature of the relationship between neural activity and the BOLD signal is a subject of current research. The BOLD effect also allows for the generation of high resolution 3D maps of the venous vasculature within neural tissue.

While BOLD signal is the most common method employed for neuroscience studies in human subjects, the flexible nature of MR imaging provides means to sensitize the signal to other aspects of the blood supply. Alternative techniques employ arterial spin labeling (ASL) or weight the MRI signal by cerebral blood flow (CBF) and cerebral blood volume (CBV). The CBV method requires injection of a class of MRI contrast agents that are now in human clinical trials. Because this method has been shown to be far more sensitive than the BOLD technique in preclinical studies, it may potentially expand the role of fMRI in clinical applications. The CBF method provides more quantitative information than the BOLD signal, albeit at a significant loss of detection sensitivity.

Real-time MRI


Real-time MRI
Real-time MRI
Real-time magnetic resonance imaging refers to the continuous monitoring of moving objects in real time. Because MRIis based on time-consuming scanning of k-space, real-time MRI was possible only with low image quality or low temporal resolution...

 refers to the continuous monitoring (“filming”) of moving objects in real time. While many different strategies have been developed over the past two decades, a recent development reported a real-time MRI technique based on radial FLASH
FLASH MRI
FLASH MRI is a basic measuring principle for rapid MRI invented in 1985 by Jens Frahm, Axel Haase, W Hänicke, KD Merboldt, and D Matthaei at the in Göttingen, Germany...

 and iterative reconstruction
Iterative reconstruction
Iterative reconstruction refers to iterative algorithms used to reconstruct 2D and 3D images in certain imaging techniques.For example, in computed tomography an image must be reconstructed from projections of an object...

 that yields a temporal resolution of 20 to 30 milliseconds for images with an in-plane resolution of 1.5 to 2.0 mm. The new method promises to add important information about diseases of the joints and the heart. In many cases MRI examinations may become easier and more comfortable for patients.

Interventional MRI


The lack of harmful effects on the patient and the operator make MRI well-suited for "interventional radiology
Interventional radiology
Interventional radiology is a specialty of radiology, in which image-guided procedures are used to diagnose and treat a multitude of diseases across all body systems...

", where the images produced by a MRI scanner are used to guide minimally invasive procedures. Of course, such procedures must be done without any ferromagnetic instruments.

A specialized growing subset of interventional MRI is that of intraoperative MRI in which the MRI is used in the surgical process. Some specialized MRI systems have been developed that allow imaging concurrent with the surgical procedure. More typical, however, is that the surgical procedure is temporarily interrupted so that MR images can be acquired to verify the success of the procedure or guide subsequent surgical work.

Radiation therapy simulation


Because of MRI's superior imaging of soft tissues, it is now being utilized to specifically locate tumors within the body in preparation for radiation therapy treatments. For therapy simulation, a patient is placed in specific, reproducible, body position and scanned. The MRI system then computes the precise location, shape and orientation of the tumor mass, correcting for any spatial distortion inherent in the system. The patient is then marked or tattooed with points that, when combined with the specific body position, permits precise triangulation for radiation therapy.

Current density imaging


Current density imaging
Current density imaging
Current density imaging is an extension of magnetic resonance imaging , developed at the University of Toronto.It employs two techniques for spatially mapping electrical current pathways through tissue:...

 (CDI) endeavors to use the phase information from images to reconstruct current densities within a subject. Current density imaging works because electrical currents generate magnetic fields, which in turn affect the phase of the magnetic dipoles during an imaging sequence.

Magnetic resonance guided focused ultrasound


In MRgFUS therapy, ultrasound beams are focused on a tissue—guided and controlled using MR thermal imaging—and due to the significant energy deposition at the focus, temperature within the tissue rises to more than 65 °C (150 °F), completely destroying it. This technology can achieve precise ablation
Ablation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...

 of diseased tissue. MR imaging provides a three-dimensional view of the target tissue, allowing for precise focusing of ultrasound energy. The MR imaging provides quantitative, real-time, thermal images of the treated area. This allows the physician to ensure that the temperature generated during each cycle of ultrasound energy is sufficient to cause thermal ablation within the desired tissue and if not, to adapt the parameters to ensure effective treatment.

Multinuclear imaging


Hydrogen is the most frequently imaged nucleus in MRI because it is present in biological tissues in great abundance, and because its high gyromagnetic ratio gives a strong signal. However, any nucleus with a net nuclear spin could potentially be imaged with MRI. Such nuclei include helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

-3, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

-13, fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...

-19, oxygen-17
Oxygen-17
Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...

, sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

-23, phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

-31 and xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...

-129. 23Na and 31P are naturally abundant in the body, so can be imaged directly. Gaseous isotopes such as 3He or 129Xe must be hyperpolarized
Hyperpolarization (physics)
Hyperpolarization is the nuclear spin polarization of a material far beyond thermal equilibrium conditions. It is commonly applied to gases such as 129Xe and 3He which are then used, for instance, in hyperpolarized magnetic resonance imaging of the lungs....

 and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions. 17O
Oxygen-17
Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...

 and 19F can be administered in sufficient quantities in liquid form (e.g. 17O
Oxygen-17
Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...

-water) that hyperpolarization is not a necessity.

Multinuclear imaging is primarily a research technique at present. However, potential applications include functional imaging and imaging of organs poorly seen on 1H MRI (e.g. lungs and bones) or as alternative contrast agents. Inhaled hyperpolarized 3He can be used to image the distribution of air spaces within the lungs. Injectable solutions containing 13C or stabilized bubbles of hyperpolarized 129Xe have been studied as contrast agents for angiography and perfusion imaging. 31P can potentially provide information on bone density and structure, as well as functional imaging of the brain.

Susceptibility weighted imaging (SWI)



Susceptibility weighted imaging (SWI), is a new type of contrast in MRI different from spin density, T1, or T2 imaging. This method exploits the susceptibility differences between tissues and uses a fully velocity compensated, three dimensional, RF spoiled, high-resolution, 3D gradient echo scan. This special data acquisition and image processing produces an enhanced contrast magnitude image very sensitive to venous blood, hemorrhage and iron storage. It is used to enhance the detection and diagnosis of tumors, vascular and neurovascular diseases (stroke and hemorrhage, multiple sclerosis, Alzheimer's), and also detects traumatic brain injuries that may not be diagnosed using other methods

Other specialized MRI techniques


New methods and variants of existing methods are often published when they are able to produce better results in specific fields. Examples of these recent improvements are T
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Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a
medical imaging
Medical imaging
Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...

 technique used in radiology
Radiology
Radiology is a medical specialty that employs the use of imaging to both diagnose and treat disease visualized within the human body. Radiologists use an array of imaging technologies to diagnose or treat diseases...

 to visualize detailed internal structures. MRI makes use of the property of nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...

 (NMR) to image nuclei of atoms inside the body.

An MRI machine uses a powerful magnetic
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...

 field to align the magnetization
Nuclear magnetic moment
The nuclear magnetic moment is the magnetic moment of an atomic nucleus and arises from the spin of the protons and neutrons. It is mainly a magnetic dipole moment; the quadrupole moment does cause some small shifts in the hyperfine structure as well....

 of some atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s in the body, and radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

 fields to systematically alter the alignment of this magnetization. This causes the nuclei to produce a rotating magnetic field detectable by the scanner—and this information is recorded to construct an image of the scanned area of the body.{{Rp|36}}
Strong magnetic field gradients cause nuclei at different locations to rotate at different speeds. 3-D spatial information can be obtained by providing gradients in each direction.

MRI provides good contrast
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...

 between the different soft tissues of the body, which makes it especially useful in imaging the brain
Neurology
Neurology is a medical specialty dealing with disorders of the nervous system. Specifically, it deals with the diagnosis and treatment of all categories of disease involving the central, peripheral, and autonomic nervous systems, including their coverings, blood vessels, and all effector tissue,...

, muscles, the heart, and cancers
Oncology
Oncology is a branch of medicine that deals with cancer...

 compared with other medical imaging
Medical imaging
Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...

 techniques such as computed tomography
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...

 (CT) or X-ray
Medical radiography
Radiography is the use of ionizing electromagnetic radiation such as X-rays to view objects. Although not technically radiographic techniques, imaging modalities such as PET and MRI are sometimes grouped in radiography because the radiology department of hospitals handle all forms of imaging...

s. Unlike CT scans or traditional X-rays, MRI uses no ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

.

How MRI works


{{refimprove section|date=October 2011}}
{{Main|Physics of Magnetic Resonance Imaging}}
The body is largely composed of water molecules
Body water
In medicine, body water is the water content of the human body. A significant fraction of the human body is water. Arthur Guyton 's Textbook of Medical Physiology states that "the total amount of water in a man of average weight is approximately 40 litres, averaging 57 percent of his total body...

. Each water molecule has two hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

 or proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s. When a person is inside the powerful magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 of the scanner, the magnetic moments of some of these molecules become aligned with the direction of the field. A radio frequency transmitter is briefly turned on, producing a further varying electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...

. The photons of this field have just the right energy, known as the resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

 frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

, to be absorbed and flip the spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...

 of the aligned protons in the body. The frequency at which the protons resonate depends on the strength of the applied magnetic field. After the field is turned off, those protons which absorbed energy revert to the original lower-energy spin-down state.
A hydrogen dipole has two spins, 1 high spin and 1 low. In low spin both dipole and field are in parallel direction and in high spin case it is antiparallel. They release the difference in energy as a photon, and the released photons are detected by the scanner as an electromagnetic signal, similar to radio waves.

As a result of conservation of energy
Conservation of energy
The nineteenth century law of conservation of energy is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time...

, the resonant frequency also dictates the frequency of the released photons. The photons released when the field is removed have an energy — and therefore a frequency — which depends on the energy absorbed while the field was active. It is this relationship between field-strength and frequency that allows the use of nuclear magnetic resonance for imaging. An image can be constructed because the protons in different tissues return to their equilibrium state at different rates, which is a difference that can be detected. Five different tissue variables — spin density, T1 and T2 relaxation times and flow and spectral shifts can be used to construct images. By changing the settings on the scanner, this effect is used to create contrast between different types of body tissue or between other properties, as in fMRI and diffusion MRI
Diffusion MRI
Diffusion MRI is a magnetic resonance imaging method that produces in vivo images of biological tissues weighted with the local microstructural characteristics of water diffusion, which is capable of showing connections between brain regions...

.

The 3D
Three-dimensional space
Three-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...

 position from which photons were released is learned by applying additional fields during the scan. This is done by passing electric currents through specially-wound solenoid
Solenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...

s, known as gradient coils. These fields make the magnetic field strength vary depending on the position within the patient, which in turn makes the frequency of released photons dependent on their original position in a predictable manner, and the original locations can be mathematically recovered from the resulting signal by the use of inverse Fourier transform
Fourier transform
In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...

.

Contrast agents may be injected intravenously
Intravenous therapy
Intravenous therapy or IV therapy is the infusion of liquid substances directly into a vein. The word intravenous simply means "within a vein". Therapies administered intravenously are often called specialty pharmaceuticals...

 to enhance the appearance of blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

s, tumors or inflammation
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

. Contrast agents may also be directly injected into a joint in the case of arthrogram
Arthrogram
An arthrogram is a series of images, often X-rays, of a joint after injection of a contrast medium. The injection is normally done under a local anesthetic.The radiologist performs the study utilizing fluoroscopy or ultrasound to guide the placement of the needle into the joint and then injects an...

s, MRI images of joints. Unlike CT
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...

, MRI uses no ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

 and is generally a very safe procedure. Nonetheless the strong magnetic fields and radio pulses can affect metal implants, including cochlear implant
Cochlear implant
A cochlear implant is a surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf or severely hard of hearing...

s and cardiac pacemaker
Cardiac pacemaker
right|thumb|350px|Image showing the cardiac pacemaker which is the SA nodeThe contraction of heart muscle in all animals with hearts is initiated by chemical impulses. The rate at which these impulses fire controls the heart rate...

s. In the case of cochlear implants, the US FDA has approved some implants for MRI compatibility. In the case of cardiac pacemakers, the results can sometimes be lethal, so patients with such implants are generally not eligible for MRI.

Since the gradient coils are within the bore of the scanner, there are large forces between them and the main field coils, producing most of the noise that is heard during operation. Without efforts to damp this noise, it can approach 130 decibels (dB) with strong fields (see also the subsection on acoustic noise).

MRI is used to image every part of the body, and is particularly useful for tissues with many hydrogen nuclei and little density contrast, such as the brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

, muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

, connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 and most tumor
Tumor
A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...

s.

History


In the 1950s, Herman Carr
Herman Carr
Herman Y. Carr was an American physicist and pioneer of magnetic resonance imaging.Carr was born in Alliance, Ohio. He received his BS, MS and PHD from Harvard University, where he studied under Edward Purcell. He later moved to Rutgers University, where he was professor. He retired in 1987.In...

 reported on the creation of a one-dimensional MR image. Paul Lauterbur
Paul Lauterbur
Paul Christian Lauterbur was an American chemist who shared the Nobel Prize in Physiology or Medicine in 2003 with Peter Mansfield for his work which made the development of magnetic resonance imaging possible.Dr...

 expanded on Carr's technique and developed a way to generate the first MRI images, in 2D and 3D, using gradients. In 1973, Lauterbur published the first nuclear magnetic resonance image. and the first cross-sectional image of a living mouse was published in January 1974. Nuclear magnetic resonance imaging is a relatively new technology first developed at the University of Nottingham
University of Nottingham
The University of Nottingham is a public research university based in Nottingham, United Kingdom, with further campuses in Ningbo, China and Kuala Lumpur, Malaysia...

, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

. Peter Mansfield
Peter Mansfield
Sir Peter Mansfield, FRS, , is a British physicist who was awarded the 2003 Nobel Prize in Physiology or Medicine for his discoveries concerning magnetic resonance imaging . The Nobel Prize was shared with Paul Lauterbur, who also contributed to the development of MRI...

, a physicist and professor at the university, then developed a mathematical technique that would allow scans to take seconds rather than hours and produce clearer images than Lauterbur had.
In a 1971 paper in the journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

, Dr. Raymond Damadian, an Armenian-American
Armenian-American
Armenian Americans are citizens of the United States whose ancestry originates wholly or partly in Armenia. During the United States 2000 Census, 385,488 respondents indicated either full or partial Armenian ancestry...

 physician, scientist, and professor at the Downstate Medical Center State University of New York
State University of New York
The State University of New York, abbreviated SUNY , is a system of public institutions of higher education in New York, United States. It is the largest comprehensive system of universities, colleges, and community colleges in the United States, with a total enrollment of 465,000 students, plus...

 (SUNY), reported that tumor
Tumor
A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...

s and normal tissue can be distinguished in vivo by nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...

 ("NMR"). He suggested that these differences could be used to diagnose cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

, though later research would find that these differences, while real, are too variable for diagnostic purposes. Damadian's initial methods were flawed for practical use, relying on a point-by-point scan of the entire body and using relaxation rates, which turned out to not be an effective indicator of cancerous tissue.

While researching the analytical properties of magnetic resonance, Damadian created the world's first magnetic resonance imaging machine in 1972. He filed the first patent for an MRI machine, U.S. patent #3,789,832 on March 17, 1972, which was later issued to him on February 5, 1974. As the National Science Foundation
National Science Foundation
The National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...

 notes, "The patent included the idea of using NMR to 'scan' the human body to locate cancerous tissue." However, it did not describe a method for generating pictures from such a scan or precisely how such a scan might be done. Damadian along with Larry Minkoff and Michael Goldsmith, subsequently went on to perform the first MRI body scan of a human being on July 3, 1977. These studies performed on humans were published in 1977.

In recording the history of MRI, Mattson and Simon (1996) credit Damadian with describing the concept of whole-body NMR scanning, as well as discovering the NMR tissue relaxation differences that made this feasible.

2003 Nobel Prize


Reflecting the fundamental importance and applicability of MRI in medicine, Paul Lauterbur
Paul Lauterbur
Paul Christian Lauterbur was an American chemist who shared the Nobel Prize in Physiology or Medicine in 2003 with Peter Mansfield for his work which made the development of magnetic resonance imaging possible.Dr...

 of the University of Illinois at Urbana-Champaign
University of Illinois at Urbana-Champaign
The University of Illinois at Urbana–Champaign is a large public research-intensive university in the state of Illinois, United States. It is the flagship campus of the University of Illinois system...

 and Sir Peter Mansfield
Peter Mansfield
Sir Peter Mansfield, FRS, , is a British physicist who was awarded the 2003 Nobel Prize in Physiology or Medicine for his discoveries concerning magnetic resonance imaging . The Nobel Prize was shared with Paul Lauterbur, who also contributed to the development of MRI...

 of the University of Nottingham
University of Nottingham
The University of Nottingham is a public research university based in Nottingham, United Kingdom, with further campuses in Ningbo, China and Kuala Lumpur, Malaysia...

 were awarded the 2003 Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine administered by the Nobel Foundation, is awarded once a year for outstanding discoveries in the field of life science and medicine. It is one of five Nobel Prizes established in 1895 by Swedish chemist Alfred Nobel, the inventor of dynamite, in his will...

 for their "discoveries concerning magnetic resonance imaging". The Nobel citation acknowledged Lauterbur's insight of using magnetic field gradients to determine spatial localization, a discovery that allowed rapid acquisition of 2D images. Mansfield was credited with introducing the mathematical formalism and developing techniques for efficient gradient utilization and fast imaging. The actual research that won the prize was done almost 30 years before, while Paul Lauterbur was at Stony Brook University in New York.

The award was vigorously protested by Raymond Vahan Damadian, founder of FONAR Corporation, who claimed that he invented the MRI, and that Lauterbur and Mansfield had merely refined the technology. An ad hoc
Ad hoc
Ad hoc is a Latin phrase meaning "for this". It generally signifies a solution designed for a specific problem or task, non-generalizable, and not intended to be able to be adapted to other purposes. Compare A priori....

 group, called "The Friends of Raymond Damadian", took out full-page advertisements in the New York Times and The Washington Post
The Washington Post
The Washington Post is Washington, D.C.'s largest newspaper and its oldest still-existing paper, founded in 1877. Located in the capital of the United States, The Post has a particular emphasis on national politics. D.C., Maryland, and Virginia editions are printed for daily circulation...

 entitled "The Shameful Wrong That Must Be Righted", demanding that he be awarded at least a share of the Nobel Prize. Also, even earlier, in the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

, Vladislav Ivanov
Vladislav Ivanov (physicist)
Vladislav Ivanov is a Soviet physicist who in 1960 filed a document with the USSR State Committee for Inventions and Discovery at Leningrad for a Magnetic Resonance Imaging device, although this was not approved until the 1970s....

 filed (in 1960) a document with the USSR State Committee for Inventions and Discovery at Leningrad for a Magnetic Resonance Imaging device, although this was not approved until the 1970s. In a letter to Physics Today
Physics Today
Physics Today, created in 1948, is the membership journal of the American Institute of Physics. It is provided to 130,000 members of twelve physics societies, including the American Physical Society...

, Herman Carr
Herman Carr
Herman Y. Carr was an American physicist and pioneer of magnetic resonance imaging.Carr was born in Alliance, Ohio. He received his BS, MS and PHD from Harvard University, where he studied under Edward Purcell. He later moved to Rutgers University, where he was professor. He retired in 1987.In...

 pointed out his own even earlier use of field gradients for one-dimensional MR imaging.

Applications


{{Unreferenced section|date=September 2009}}
In clinical practice, MRI is used to distinguish pathologic tissue (such as a brain tumor
Brain tumor
A brain tumor is an intracranial solid neoplasm, a tumor within the brain or the central spinal canal.Brain tumors include all tumors inside the cranium or in the central spinal canal...

) from normal tissue. One advantage of an MRI scan is that it is harmless to the patient. It uses strong magnetic fields and non-ionizing radiation in the radio frequency range, unlike CT scans and traditional X-rays
Radiography
Radiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....

, which both use ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

.

While CT provides good spatial resolution (the ability to distinguish two separate structures an arbitrarily small distance from each other), MRI provides comparable resolution with far better contrast resolution
Contrast resolution
Contrast resolution is the ability to distinguish between differences in intensity in an image. The measure is used in medical imaging to quantify the quality of acquired images. It is a difficult quantity to define, because it depends on the human observer as much as the quality of the actual image...

 (the ability to distinguish the differences between two arbitrarily similar but not identical tissues). The basis of this ability is the complex library of pulse sequences that the modern medical MRI scanner includes, each of which is optimized to provide image contrast based on the chemical sensitivity of MRI.
For example, with particular values of the echo time (TE) and the repetition time (TR), which are basic parameters of image acquisition, a sequence takes on the property of T2-weighting. On a T2-weighted scan, water- and fluid-containing tissues are bright (most modern T2 sequences are actually fast T2 sequences) and fat-containing tissues are dark. The reverse is true for T1-weighted images. Damaged tissue tends to develop edema
Edema
Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...

, which makes a T2-weighted sequence sensitive for pathology, and generally able to distinguish pathologic tissue from normal tissue. With the addition of an additional radio frequency pulse and additional manipulation of the magnetic gradients, a T2-weighted sequence can be converted to a FLAIR sequence, in which free water is now dark, but edematous tissues remain bright. This sequence in particular is currently the most sensitive way to evaluate the brain for demyelinating
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...

 diseases, such as multiple sclerosis
Multiple sclerosis
Multiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...

.

The typical MRI examination consists of 5–20 sequences, each of which are chosen to provide a particular type of information about the subject tissues. This information is then synthesized by the interpreting physician
Physician
A physician is a health care provider who practices the profession of medicine, which is concerned with promoting, maintaining or restoring human health through the study, diagnosis, and treatment of disease, injury and other physical and mental impairments...

.

T1-weighted MRI


{{Main|Spin-lattice relaxation time}}

T1-weighted scans are a standard basic scan, in particular differentiating fat from water - with water darker and fat brighter use a gradient echo (GRE) sequence, with short TE and short TR. This is one of the basic types of MR contrast and is a commonly run clinical scan. The T1 weighting can be increased (improving contrast) with the use of an inversion pulse as in an MP-RAGE sequence. Due to the short repetition time (TR) this scan can be run very fast allowing the collection of high resolution 3D datasets. A T1 reducing gadolinium contrast agent is also commonly used, with a T1 scan being collected before and after administration of contrast agent to compare the difference. In the brain T1-weighted scans provide good gray matter/white matter contrast; in other words, T1-weighted images highlight fat deposition.

T2-weighted MRI


{{Main|Spin-spin relaxation time}}
T2-weighted scans are another basic type. Like the T1-weighted scan, fat is differentiated from water - but in this case fat shows darker, and water lighter. For example, in the case of cerebral and spinal study, the CSF (cerebrospinal fluid) will be lighter in T2-weighted images. These scans are therefore particularly well suited to imaging edema, with long TE and long TR. Because the spin echo
Spin echo
In magnetic resonance, a spin echo is the refocusing of precessing spin magnetisation by a pulse of resonant radiation. Modern nuclear magnetic resonance and magnetic resonance imaging rely heavily on this effect....

 sequence is less susceptible to inhomogeneities in the magnetic field, these images have long been a clinical workhorse.

T{{su|p=*|b=2}}-weighted MRI


T{{su|p=*|b=2}} weighted scans use a gradient echo (GRE) sequence, with long TE and long TR. The gradient echo sequence used does not have the extra refocusing pulse used in spin echo
Spin echo
In magnetic resonance, a spin echo is the refocusing of precessing spin magnetisation by a pulse of resonant radiation. Modern nuclear magnetic resonance and magnetic resonance imaging rely heavily on this effect....

 so it is subject to additional losses above the normal T2 decay (referred to as T2′), these taken together are called T{{su|p=*|b=2}}. This also makes it more prone to susceptibility losses at air/tissue boundaries, but can increase contrast for certain types of tissue, such as venous blood.

Spin density weighted MRI


Spin density, also called proton density, weighted scans try to have no contrast from either T2 or T1 decay, the only signal change coming from differences in the amount of available spins (hydrogen nuclei in water). It uses a spin echo
Spin echo
In magnetic resonance, a spin echo is the refocusing of precessing spin magnetisation by a pulse of resonant radiation. Modern nuclear magnetic resonance and magnetic resonance imaging rely heavily on this effect....

 or sometimes a gradient echo sequence, with short TE and long TR.

Diffusion MRI


{{Main|Diffusion MRI}}
Diffusion MRI
Diffusion MRI
Diffusion MRI is a magnetic resonance imaging method that produces in vivo images of biological tissues weighted with the local microstructural characteristics of water diffusion, which is capable of showing connections between brain regions...

 measures the diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 of water molecules in biological tissues. In an isotropic medium (inside a glass of water for example), water molecules naturally move randomly according to turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...

 and Brownian motion
Brownian motion
Brownian motion or pedesis is the presumably random drifting of particles suspended in a fluid or the mathematical model used to describe such random movements, which is often called a particle theory.The mathematical model of Brownian motion has several real-world applications...

. In biological tissues however, where the Reynolds number is low enough for flows to be laminar
Laminar flow
Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents...

, the diffusion may be anisotropic. For example, a molecule inside the axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

 of a neuron has a low probability of crossing the myelin
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...

 membrane. Therefore the molecule moves principally along the axis of the neural fiber. If it is known that molecules in a particular voxel
Voxel
A voxel is a volume element, representing a value on a regular grid in three dimensional space. This is analogous to a pixel, which represents 2D image data in a bitmap...

 diffuse principally in one direction, the assumption can be made that the majority of the fibers in this area are going parallel to that direction.

The recent development of diffusion tensor imaging (DTI) enables diffusion to be measured in multiple directions and the fractional anisotropy in each direction to be calculated for each voxel. This enables researchers to make brain maps of fiber directions to examine the connectivity of different regions in the brain (using tractography
Tractography
In neuroscience, tractography is a procedure to demonstrate the neural tracts.It uses special techniques of magnetic resonance imaging , and computer-based image analysis.The results are presented in two- and three-dimensional images....

) or to examine areas of neural degeneration and demyelination in diseases like Multiple Sclerosis
Multiple sclerosis
Multiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...

.

Another application of diffusion MRI is diffusion-weighted imaging (DWI). Following an ischemic stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

, DWI is highly sensitive to the changes occurring in the lesion. It is speculated that increases in restriction (barriers) to water diffusion, as a result of cytotoxic edema (cellular swelling), is responsible for the increase in signal on a DWI scan. The DWI enhancement appears within 5–10 minutes of the onset of stroke symptoms (as compared with computed tomography
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...

, which often does not detect changes of acute infarct for up to 4–6 hours) and remains for up to two weeks. Coupled with imaging of cerebral perfusion, researchers can highlight regions of "perfusion/diffusion mismatch" that may indicate regions capable of salvage by reperfusion therapy.

Like many other specialized applications, this technique is usually coupled with a fast image acquisition sequence, such as echo planar imaging sequence.

Magnetization Transfer MRI


{{Main|Magnetization transfer}}

Magnetization transfer (MT) refers to the transfer of longitudinal magnetization from free water protons to hydration water protons in NMR and MRI.

In magnetic resonance imaging of molecular solutions, such as protein solutions, two types of water molecules, free (bulk) and hydration (bound), are found. Free water protons have faster average rotational frequency and hence less fixed water molecules that may cause local field inhomogeneity. Because of this uniformity, most free water protons have resonance frequency lying narrowly around the normal proton resonance frequency of 63 MHz (at 1.5 teslas). This also results in slower transverse magnetization dephasing and hence longer T2. Conversely, hydration water molecules are slowed down by interaction with solute molecules and hence create field inhomogeneities that lead to wider resonance frequency spectrum.

In free liquids, protons, which may be viewed classically as small magnetic dipoles, exhibit translational and rotational motions. These moving dipoles disturb the surrounding magnetic field however on long enough time-scales (which may be nanoseconds) the average field caused by the motion of protons is zero. This is known as “motional averaging” or narrowing and is characteristic of protons moving freely in liquid. On the other hand, protons bound to macromolecules, such as proteins, tend to have a fixed orientation and so the average magnetic field in close proximity to such structures does not average to zero. The result is a spatial pattern in the magnetic field that gives rise to a residual dipolar coupling (range of precession frequencies) for the protons experiencing the magnetic field. The wide frequency distribution appears as a broad spectrum that may be several kHz wide. The net signal from these protons disappears very quickly, in inverse proportion to the width, due to the loss of coherence of the spins, i.e. T2 relaxation. Due to exchange mechanisms, such as spin transfer or proton chemical exchange, the (incoherent) spins bound to the macromolecules continually switch places with (coherent) spins in the bulk media and establish a dynamic equilibrium.

Magnetization transfer: Although there is no measurable signal from the bound spins, or the bound spins that exchange into the bulk media, their longitudinal magnetization is preserved and may recover only via the relatively slow process of T1 relaxation. If the longitudinal magnetization of just the bound spins can be altered, then the effect can be measured in the spins of the bulk media due to the exchange processes. The magnetization transfer sequence applies RF saturation at a frequency that is far off resonance for the narrow line of bulk water but still on resonance for the bound protons with a spectral linewidth of kHz. This causes saturation of the bound spins which exchange into the bulk water, resulting in a loss of longitudinal magnetization and hence signal decrease in the bulk water. This provides an indirect measure of macromolecular content in tissue. Implementation of magnetization transfer involves choosing suitable frequency offsets and pulse shapes to saturate the bound spins sufficiently strongly, within the safety limits of specific absorption rate for RF irradiation.

T1rho MRI


{{Main|Spin-lattice relaxation time in the rotating frame}}
T1ρ (T1rho): Molecules have a kinetic energy that is a function of the temperature and is expressed as translational and rotational motions, and by collisions between molecules. The moving dipoles disturb the magnetic field but are often extremely rapid so that the average effect over a long time-scale may be zero. However, depending on the time-scale, the interactions between the dipoles do not always average away. At the slowest extreme the interaction time is effectively infinite and occurs where there are large, stationary field disturbances (e.g. a metallic implant). In this case the loss of coherence is described as a "static dephasing". T2* is a measure of the loss of coherence in an ensemble of spins that include all interactions (including static dephasing). T2 is a measure of the loss of coherence that excludes static dephasing, using an RF pulse to reverse the slowest types of dipolar interaction. There is in fact a continuum of interaction time-scales in a given biological sample and the properties of the refocusing RF pulse can be tuned to refocus more than just static dephasing. In general, the rate of decay of an ensemble of spins is a function of the interaction times and also the power of the RF pulse. This type of decay, occurring under the influence of RF, is known as T1ρ. It is similar to T2 decay but with some slower dipolar interactions refocused as well as the static interactions, hence T1ρ≥T2.

Fluid attenuated inversion recovery (FLAIR)


{{Main|Fluid attenuated inversion recovery}}
Fluid Attenuated Inversion Recovery (FLAIR) is an inversion-recovery pulse sequence used
to null signal from fluids. For example, it can be used in brain imaging to suppress cerebrospinal fluid (CSF) so as to bring out the periventricular hyperintense lesions, such as multiple sclerosis (MS) plaques. By carefully choosing the inversion time TI (the time between the inversion and excitation pulses), the signal from any particular tissue can be suppressed.

Magnetic resonance angiography



{{Main|Magnetic resonance angiography}}
Magnetic resonance angiography (MRA) generates pictures of the arteries to evaluate them for stenosis
Stenosis
A stenosis is an abnormal narrowing in a blood vessel or other tubular organ or structure.It is also sometimes called a stricture ....

 (abnormal narrowing) or aneurysm
Aneurysm
An aneurysm or aneurism is a localized, blood-filled balloon-like bulge in the wall of a blood vessel. Aneurysms can commonly occur in arteries at the base of the brain and an aortic aneurysm occurs in the main artery carrying blood from the left ventricle of the heart...

s (vessel wall dilatations, at risk of rupture). MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs (called a "run-off"). A variety of techniques can be used to generate the pictures, such as administration of a paramagnetic contrast agent (gadolinium
Gadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...

) or using a technique known as "flow-related enhancement" (e.g. 2D and 3D time-of-flight sequences), where most of the signal on an image is due to blood that recently moved into that plane, see also FLASH MRI
FLASH MRI
FLASH MRI is a basic measuring principle for rapid MRI invented in 1985 by Jens Frahm, Axel Haase, W Hänicke, KD Merboldt, and D Matthaei at the in Göttingen, Germany...

. Techniques involving phase accumulation (known as phase contrast angiography) can also be used to generate flow velocity maps easily and accurately. Magnetic resonance venography (MRV) is a similar procedure that is used to image veins. In this method, the tissue is now excited inferiorly, while signal is gathered in the plane immediately superior to the excitation plane—thus imaging the venous blood that recently moved from the excited plane.

Magnetic resonance gated intracranial CSF dynamics (MR-GILD)


Magnetic resonance gated intracranial cerebrospinal fluid (CSF) or liquor dynamics (MR-GILD) technique is an MR sequence based on bipolar gradient pulse used to demonstrate CSF pulsatile flow in ventricles, cisterns, aqueduct of Sylvius and entire intracranial CSF pathway. It is a method for analyzing CSF circulatory system dynamics in patients with CSF obstructive lesions such as normal pressure hydrocephalus. It also allows visualization of both arterial and venous pulsatile blood flow in vessels without use of contrast agents.
Diastolic time data acquisition (DTDA). Systolic time data acquisition (STDA).

Magnetic resonance spectroscopy


{{Main|In vivo magnetic resonance spectroscopy}}
{{Main|Nuclear magnetic resonance spectroscopy}}

Magnetic resonance spectroscopy
In vivo magnetic resonance spectroscopy
In vivo magnetic resonance spectroscopy is a specialised technique associated to magnetic resonance imaging ....

 (MRS) is used to measure the levels of different metabolites in body tissues. The MR signal produces a spectrum of resonances that correspond to different molecular arrangements of the isotope being "excited". This signature is used to diagnose certain metabolic disorders, especially those affecting the brain, and to provide information on tumor metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

.

Magnetic resonance spectroscopic imaging (MRSI) combines both spectroscopic and imaging methods to produce spatially localized spectra from within the sample or patient. The spatial resolution is much lower (limited by the available SNR
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

), but the spectra in each voxel contains information about many metabolites. Because the available signal is used to encode spatial and spectral information, MRSI requires high SNR achievable only at higher field strengths (3 T and above).

Functional MRI


{{Main|Functional magnetic resonance imaging}}

Functional MRI (fMRI) measures signal changes in the brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

 that are due to changing neural
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

 activity. The brain is scanned at low resolution but at a rapid rate (typically once every 2–3 seconds). Increases in neural activity cause changes in the MR signal via T{{su|p=*|b=2}} changes; this mechanism is referred to as the BOLD (blood-oxygen-level dependent) effect. Increased neural activity causes an increased demand for oxygen, and the vascular
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

 system actually overcompensates for this, increasing the amount of oxygenated hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

 relative to deoxygenated hemoglobin. Because deoxygenated hemoglobin attenuates the MR signal, the vascular response leads to a signal increase that is related to the neural activity. The precise nature of the relationship between neural activity and the BOLD signal is a subject of current research. The BOLD effect also allows for the generation of high resolution 3D maps of the venous vasculature within neural tissue.

While BOLD signal is the most common method employed for neuroscience studies in human subjects, the flexible nature of MR imaging provides means to sensitize the signal to other aspects of the blood supply. Alternative techniques employ arterial spin labeling (ASL) or weight the MRI signal by cerebral blood flow (CBF) and cerebral blood volume (CBV). The CBV method requires injection of a class of MRI contrast agents that are now in human clinical trials. Because this method has been shown to be far more sensitive than the BOLD technique in preclinical studies, it may potentially expand the role of fMRI in clinical applications. The CBF method provides more quantitative information than the BOLD signal, albeit at a significant loss of detection sensitivity.

Real-time MRI



{{Main|Real-time MRI}}
Real-time MRI
Real-time MRI
Real-time magnetic resonance imaging refers to the continuous monitoring of moving objects in real time. Because MRIis based on time-consuming scanning of k-space, real-time MRI was possible only with low image quality or low temporal resolution...

 refers to the continuous monitoring (“filming”) of moving objects in real time. While many different strategies have been developed over the past two decades, a recent development reported a real-time MRI technique based on radial FLASH
FLASH MRI
FLASH MRI is a basic measuring principle for rapid MRI invented in 1985 by Jens Frahm, Axel Haase, W Hänicke, KD Merboldt, and D Matthaei at the in Göttingen, Germany...

 and iterative reconstruction
Iterative reconstruction
Iterative reconstruction refers to iterative algorithms used to reconstruct 2D and 3D images in certain imaging techniques.For example, in computed tomography an image must be reconstructed from projections of an object...

 that yields a temporal resolution of 20 to 30 milliseconds for images with an in-plane resolution of 1.5 to 2.0 mm. The new method promises to add important information about diseases of the joints and the heart. In many cases MRI examinations may become easier and more comfortable for patients.

Interventional MRI


{{Main|Interventional MRI}}

The lack of harmful effects on the patient and the operator make MRI well-suited for "interventional radiology
Interventional radiology
Interventional radiology is a specialty of radiology, in which image-guided procedures are used to diagnose and treat a multitude of diseases across all body systems...

", where the images produced by a MRI scanner are used to guide minimally invasive procedures. Of course, such procedures must be done without any ferromagnetic instruments.

A specialized growing subset of interventional MRI is that of intraoperative MRI in which the MRI is used in the surgical process. Some specialized MRI systems have been developed that allow imaging concurrent with the surgical procedure. More typical, however, is that the surgical procedure is temporarily interrupted so that MR images can be acquired to verify the success of the procedure or guide subsequent surgical work.

Radiation therapy simulation


Because of MRI's superior imaging of soft tissues, it is now being utilized to specifically locate tumors within the body in preparation for radiation therapy treatments. For therapy simulation, a patient is placed in specific, reproducible, body position and scanned. The MRI system then computes the precise location, shape and orientation of the tumor mass, correcting for any spatial distortion inherent in the system. The patient is then marked or tattooed with points that, when combined with the specific body position, permits precise triangulation for radiation therapy.{{Citation needed|date=December 2010}}

Current density imaging


Current density imaging
Current density imaging
Current density imaging is an extension of magnetic resonance imaging , developed at the University of Toronto.It employs two techniques for spatially mapping electrical current pathways through tissue:...

 (CDI) endeavors to use the phase information from images to reconstruct current densities within a subject. Current density imaging works because electrical currents generate magnetic fields, which in turn affect the phase of the magnetic dipoles during an imaging sequence.{{Citation needed|date=December 2010}}

Magnetic resonance guided focused ultrasound


In MRgFUS therapy, ultrasound beams are focused on a tissue—guided and controlled using MR thermal imaging—and due to the significant energy deposition at the focus, temperature within the tissue rises to more than 65 °C (150 °F), completely destroying it. This technology can achieve precise ablation
Ablation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...

 of diseased tissue. MR imaging provides a three-dimensional view of the target tissue, allowing for precise focusing of ultrasound energy. The MR imaging provides quantitative, real-time, thermal images of the treated area. This allows the physician to ensure that the temperature generated during each cycle of ultrasound energy is sufficient to cause thermal ablation within the desired tissue and if not, to adapt the parameters to ensure effective treatment.

Multinuclear imaging


Hydrogen is the most frequently imaged nucleus in MRI because it is present in biological tissues in great abundance, and because its high gyromagnetic ratio gives a strong signal. However, any nucleus with a net nuclear spin could potentially be imaged with MRI. Such nuclei include helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

-3, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

-13, fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...

-19, oxygen-17
Oxygen-17
Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...

, sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

-23, phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

-31 and xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...

-129. 23Na and 31P are naturally abundant in the body, so can be imaged directly. Gaseous isotopes such as 3He or 129Xe must be hyperpolarized
Hyperpolarization (physics)
Hyperpolarization is the nuclear spin polarization of a material far beyond thermal equilibrium conditions. It is commonly applied to gases such as 129Xe and 3He which are then used, for instance, in hyperpolarized magnetic resonance imaging of the lungs....

 and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions. 17O
Oxygen-17
Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...

 and 19F can be administered in sufficient quantities in liquid form (e.g. 17O
Oxygen-17
Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...

-water) that hyperpolarization is not a necessity.{{Citation needed|date=December 2010}}

Multinuclear imaging is primarily a research technique at present. However, potential applications include functional imaging and imaging of organs poorly seen on 1H MRI (e.g. lungs and bones) or as alternative contrast agents. Inhaled hyperpolarized 3He can be used to image the distribution of air spaces within the lungs. Injectable solutions containing 13C or stabilized bubbles of hyperpolarized 129Xe have been studied as contrast agents for angiography and perfusion imaging. 31P can potentially provide information on bone density and structure, as well as functional imaging of the brain. {{Citation needed|date=December 2010}}

Susceptibility weighted imaging (SWI)


{{Main|Susceptibility weighted imaging}}
Susceptibility weighted imaging (SWI), is a new type of contrast in MRI different from spin density, T1, or T2 imaging. This method exploits the susceptibility differences between tissues and uses a fully velocity compensated, three dimensional, RF spoiled, high-resolution, 3D gradient echo scan. This special data acquisition and image processing produces an enhanced contrast magnitude image very sensitive to venous blood, hemorrhage and iron storage. It is used to enhance the detection and diagnosis of tumors, vascular and neurovascular diseases (stroke and hemorrhage, multiple sclerosis, Alzheimer's), and also detects traumatic brain injuries that may not be diagnosed using other methods

Other specialized MRI techniques


New methods and variants of existing methods are often published when they are able to produce better results in specific fields. Examples of these recent improvements are T
{{Redirect|MRI|other meanings of MRI or Mri|MRI (disambiguation)}}
{{globalize|article|USA|2name=the United States|date=August 2010}}
{{Technical|date=January 2011}}
{{Interventions infobox
| Name = Magnetic resonance imaging
| Image = MR Knee.jpg
| Caption = Sagittal MR image of the knee
| ICD10 = B?3?ZZZ
| ICD9unlinked = {{ICD9proc|88.91}}-{{ICD9proc|88.97}}
| MeshID = D008279
| OPS301 = {{OPS301|3-80...3-84}}
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}}
Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a
medical imaging
Medical imaging
Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...

 technique used in radiology
Radiology
Radiology is a medical specialty that employs the use of imaging to both diagnose and treat disease visualized within the human body. Radiologists use an array of imaging technologies to diagnose or treat diseases...

 to visualize detailed internal structures. MRI makes use of the property of nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...

 (NMR) to image nuclei of atoms inside the body.

An MRI machine uses a powerful magnetic
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...

 field to align the magnetization
Nuclear magnetic moment
The nuclear magnetic moment is the magnetic moment of an atomic nucleus and arises from the spin of the protons and neutrons. It is mainly a magnetic dipole moment; the quadrupole moment does cause some small shifts in the hyperfine structure as well....

 of some atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s in the body, and radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

 fields to systematically alter the alignment of this magnetization. This causes the nuclei to produce a rotating magnetic field detectable by the scanner—and this information is recorded to construct an image of the scanned area of the body.{{Rp|36}}
Strong magnetic field gradients cause nuclei at different locations to rotate at different speeds. 3-D spatial information can be obtained by providing gradients in each direction.

MRI provides good contrast
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...

 between the different soft tissues of the body, which makes it especially useful in imaging the brain
Neurology
Neurology is a medical specialty dealing with disorders of the nervous system. Specifically, it deals with the diagnosis and treatment of all categories of disease involving the central, peripheral, and autonomic nervous systems, including their coverings, blood vessels, and all effector tissue,...

, muscles, the heart, and cancers
Oncology
Oncology is a branch of medicine that deals with cancer...

 compared with other medical imaging
Medical imaging
Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...

 techniques such as computed tomography
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...

 (CT) or X-ray
Medical radiography
Radiography is the use of ionizing electromagnetic radiation such as X-rays to view objects. Although not technically radiographic techniques, imaging modalities such as PET and MRI are sometimes grouped in radiography because the radiology department of hospitals handle all forms of imaging...

s. Unlike CT scans or traditional X-rays, MRI uses no ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

.

How MRI works


{{refimprove section|date=October 2011}}
{{Main|Physics of Magnetic Resonance Imaging}}
The body is largely composed of water molecules
Body water
In medicine, body water is the water content of the human body. A significant fraction of the human body is water. Arthur Guyton 's Textbook of Medical Physiology states that "the total amount of water in a man of average weight is approximately 40 litres, averaging 57 percent of his total body...

. Each water molecule has two hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

 or proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s. When a person is inside the powerful magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 of the scanner, the magnetic moments of some of these molecules become aligned with the direction of the field. A radio frequency transmitter is briefly turned on, producing a further varying electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...

. The photons of this field have just the right energy, known as the resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

 frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

, to be absorbed and flip the spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...

 of the aligned protons in the body. The frequency at which the protons resonate depends on the strength of the applied magnetic field. After the field is turned off, those protons which absorbed energy revert to the original lower-energy spin-down state.
A hydrogen dipole has two spins, 1 high spin and 1 low. In low spin both dipole and field are in parallel direction and in high spin case it is antiparallel. They release the difference in energy as a photon, and the released photons are detected by the scanner as an electromagnetic signal, similar to radio waves.

As a result of conservation of energy
Conservation of energy
The nineteenth century law of conservation of energy is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time...

, the resonant frequency also dictates the frequency of the released photons. The photons released when the field is removed have an energy — and therefore a frequency — which depends on the energy absorbed while the field was active. It is this relationship between field-strength and frequency that allows the use of nuclear magnetic resonance for imaging. An image can be constructed because the protons in different tissues return to their equilibrium state at different rates, which is a difference that can be detected. Five different tissue variables — spin density, T1 and T2 relaxation times and flow and spectral shifts can be used to construct images. By changing the settings on the scanner, this effect is used to create contrast between different types of body tissue or between other properties, as in fMRI and diffusion MRI
Diffusion MRI
Diffusion MRI is a magnetic resonance imaging method that produces in vivo images of biological tissues weighted with the local microstructural characteristics of water diffusion, which is capable of showing connections between brain regions...

.

The 3D
Three-dimensional space
Three-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...

 position from which photons were released is learned by applying additional fields during the scan. This is done by passing electric currents through specially-wound solenoid
Solenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...

s, known as gradient coils. These fields make the magnetic field strength vary depending on the position within the patient, which in turn makes the frequency of released photons dependent on their original position in a predictable manner, and the original locations can be mathematically recovered from the resulting signal by the use of inverse Fourier transform
Fourier transform
In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...

.

Contrast agents may be injected intravenously
Intravenous therapy
Intravenous therapy or IV therapy is the infusion of liquid substances directly into a vein. The word intravenous simply means "within a vein". Therapies administered intravenously are often called specialty pharmaceuticals...

 to enhance the appearance of blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

s, tumors or inflammation
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

. Contrast agents may also be directly injected into a joint in the case of arthrogram
Arthrogram
An arthrogram is a series of images, often X-rays, of a joint after injection of a contrast medium. The injection is normally done under a local anesthetic.The radiologist performs the study utilizing fluoroscopy or ultrasound to guide the placement of the needle into the joint and then injects an...

s, MRI images of joints. Unlike CT
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...

, MRI uses no ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

 and is generally a very safe procedure. Nonetheless the strong magnetic fields and radio pulses can affect metal implants, including cochlear implant
Cochlear implant
A cochlear implant is a surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf or severely hard of hearing...

s and cardiac pacemaker
Cardiac pacemaker
right|thumb|350px|Image showing the cardiac pacemaker which is the SA nodeThe contraction of heart muscle in all animals with hearts is initiated by chemical impulses. The rate at which these impulses fire controls the heart rate...

s. In the case of cochlear implants, the US FDA has approved some implants for MRI compatibility. In the case of cardiac pacemakers, the results can sometimes be lethal, so patients with such implants are generally not eligible for MRI.

Since the gradient coils are within the bore of the scanner, there are large forces between them and the main field coils, producing most of the noise that is heard during operation. Without efforts to damp this noise, it can approach 130 decibels (dB) with strong fields (see also the subsection on acoustic noise).

MRI is used to image every part of the body, and is particularly useful for tissues with many hydrogen nuclei and little density contrast, such as the brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

, muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

, connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 and most tumor
Tumor
A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...

s.

History


In the 1950s, Herman Carr
Herman Carr
Herman Y. Carr was an American physicist and pioneer of magnetic resonance imaging.Carr was born in Alliance, Ohio. He received his BS, MS and PHD from Harvard University, where he studied under Edward Purcell. He later moved to Rutgers University, where he was professor. He retired in 1987.In...

 reported on the creation of a one-dimensional MR image. Paul Lauterbur
Paul Lauterbur
Paul Christian Lauterbur was an American chemist who shared the Nobel Prize in Physiology or Medicine in 2003 with Peter Mansfield for his work which made the development of magnetic resonance imaging possible.Dr...

 expanded on Carr's technique and developed a way to generate the first MRI images, in 2D and 3D, using gradients. In 1973, Lauterbur published the first nuclear magnetic resonance image. and the first cross-sectional image of a living mouse was published in January 1974. Nuclear magnetic resonance imaging is a relatively new technology first developed at the University of Nottingham
University of Nottingham
The University of Nottingham is a public research university based in Nottingham, United Kingdom, with further campuses in Ningbo, China and Kuala Lumpur, Malaysia...

, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

. Peter Mansfield
Peter Mansfield
Sir Peter Mansfield, FRS, , is a British physicist who was awarded the 2003 Nobel Prize in Physiology or Medicine for his discoveries concerning magnetic resonance imaging . The Nobel Prize was shared with Paul Lauterbur, who also contributed to the development of MRI...

, a physicist and professor at the university, then developed a mathematical technique that would allow scans to take seconds rather than hours and produce clearer images than Lauterbur had.
In a 1971 paper in the journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

, Dr. Raymond Damadian, an Armenian-American
Armenian-American
Armenian Americans are citizens of the United States whose ancestry originates wholly or partly in Armenia. During the United States 2000 Census, 385,488 respondents indicated either full or partial Armenian ancestry...

 physician, scientist, and professor at the Downstate Medical Center State University of New York
State University of New York
The State University of New York, abbreviated SUNY , is a system of public institutions of higher education in New York, United States. It is the largest comprehensive system of universities, colleges, and community colleges in the United States, with a total enrollment of 465,000 students, plus...

 (SUNY), reported that tumor
Tumor
A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...

s and normal tissue can be distinguished in vivo by nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...

 ("NMR"). He suggested that these differences could be used to diagnose cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

, though later research would find that these differences, while real, are too variable for diagnostic purposes. Damadian's initial methods were flawed for practical use, relying on a point-by-point scan of the entire body and using relaxation rates, which turned out to not be an effective indicator of cancerous tissue.

While researching the analytical properties of magnetic resonance, Damadian created the world's first magnetic resonance imaging machine in 1972. He filed the first patent for an MRI machine, U.S. patent #3,789,832 on March 17, 1972, which was later issued to him on February 5, 1974. As the National Science Foundation
National Science Foundation
The National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...

 notes, "The patent included the idea of using NMR to 'scan' the human body to locate cancerous tissue." However, it did not describe a method for generating pictures from such a scan or precisely how such a scan might be done. Damadian along with Larry Minkoff and Michael Goldsmith, subsequently went on to perform the first MRI body scan of a human being on July 3, 1977. These studies performed on humans were published in 1977.

In recording the history of MRI, Mattson and Simon (1996) credit Damadian with describing the concept of whole-body NMR scanning, as well as discovering the NMR tissue relaxation differences that made this feasible.

2003 Nobel Prize


Reflecting the fundamental importance and applicability of MRI in medicine, Paul Lauterbur
Paul Lauterbur
Paul Christian Lauterbur was an American chemist who shared the Nobel Prize in Physiology or Medicine in 2003 with Peter Mansfield for his work which made the development of magnetic resonance imaging possible.Dr...

 of the University of Illinois at Urbana-Champaign
University of Illinois at Urbana-Champaign
The University of Illinois at Urbana–Champaign is a large public research-intensive university in the state of Illinois, United States. It is the flagship campus of the University of Illinois system...

 and Sir Peter Mansfield
Peter Mansfield
Sir Peter Mansfield, FRS, , is a British physicist who was awarded the 2003 Nobel Prize in Physiology or Medicine for his discoveries concerning magnetic resonance imaging . The Nobel Prize was shared with Paul Lauterbur, who also contributed to the development of MRI...

 of the University of Nottingham
University of Nottingham
The University of Nottingham is a public research university based in Nottingham, United Kingdom, with further campuses in Ningbo, China and Kuala Lumpur, Malaysia...

 were awarded the 2003 Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine administered by the Nobel Foundation, is awarded once a year for outstanding discoveries in the field of life science and medicine. It is one of five Nobel Prizes established in 1895 by Swedish chemist Alfred Nobel, the inventor of dynamite, in his will...

 for their "discoveries concerning magnetic resonance imaging". The Nobel citation acknowledged Lauterbur's insight of using magnetic field gradients to determine spatial localization, a discovery that allowed rapid acquisition of 2D images. Mansfield was credited with introducing the mathematical formalism and developing techniques for efficient gradient utilization and fast imaging. The actual research that won the prize was done almost 30 years before, while Paul Lauterbur was at Stony Brook University in New York.

The award was vigorously protested by Raymond Vahan Damadian, founder of FONAR Corporation, who claimed that he invented the MRI, and that Lauterbur and Mansfield had merely refined the technology. An ad hoc
Ad hoc
Ad hoc is a Latin phrase meaning "for this". It generally signifies a solution designed for a specific problem or task, non-generalizable, and not intended to be able to be adapted to other purposes. Compare A priori....

 group, called "The Friends of Raymond Damadian", took out full-page advertisements in the New York Times and The Washington Post
The Washington Post
The Washington Post is Washington, D.C.'s largest newspaper and its oldest still-existing paper, founded in 1877. Located in the capital of the United States, The Post has a particular emphasis on national politics. D.C., Maryland, and Virginia editions are printed for daily circulation...

 entitled "The Shameful Wrong That Must Be Righted", demanding that he be awarded at least a share of the Nobel Prize. Also, even earlier, in the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

, Vladislav Ivanov
Vladislav Ivanov (physicist)
Vladislav Ivanov is a Soviet physicist who in 1960 filed a document with the USSR State Committee for Inventions and Discovery at Leningrad for a Magnetic Resonance Imaging device, although this was not approved until the 1970s....

 filed (in 1960) a document with the USSR State Committee for Inventions and Discovery at Leningrad for a Magnetic Resonance Imaging device, although this was not approved until the 1970s. In a letter to Physics Today
Physics Today
Physics Today, created in 1948, is the membership journal of the American Institute of Physics. It is provided to 130,000 members of twelve physics societies, including the American Physical Society...

, Herman Carr
Herman Carr
Herman Y. Carr was an American physicist and pioneer of magnetic resonance imaging.Carr was born in Alliance, Ohio. He received his BS, MS and PHD from Harvard University, where he studied under Edward Purcell. He later moved to Rutgers University, where he was professor. He retired in 1987.In...

 pointed out his own even earlier use of field gradients for one-dimensional MR imaging.

Applications


{{Unreferenced section|date=September 2009}}
In clinical practice, MRI is used to distinguish pathologic tissue (such as a brain tumor
Brain tumor
A brain tumor is an intracranial solid neoplasm, a tumor within the brain or the central spinal canal.Brain tumors include all tumors inside the cranium or in the central spinal canal...

) from normal tissue. One advantage of an MRI scan is that it is harmless to the patient. It uses strong magnetic fields and non-ionizing radiation in the radio frequency range, unlike CT scans and traditional X-rays
Radiography
Radiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....

, which both use ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

.

While CT provides good spatial resolution (the ability to distinguish two separate structures an arbitrarily small distance from each other), MRI provides comparable resolution with far better contrast resolution
Contrast resolution
Contrast resolution is the ability to distinguish between differences in intensity in an image. The measure is used in medical imaging to quantify the quality of acquired images. It is a difficult quantity to define, because it depends on the human observer as much as the quality of the actual image...

 (the ability to distinguish the differences between two arbitrarily similar but not identical tissues). The basis of this ability is the complex library of pulse sequences that the modern medical MRI scanner includes, each of which is optimized to provide image contrast based on the chemical sensitivity of MRI.
For example, with particular values of the echo time (TE) and the repetition time (TR), which are basic parameters of image acquisition, a sequence takes on the property of T2-weighting. On a T2-weighted scan, water- and fluid-containing tissues are bright (most modern T2 sequences are actually fast T2 sequences) and fat-containing tissues are dark. The reverse is true for T1-weighted images. Damaged tissue tends to develop edema
Edema
Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...

, which makes a T2-weighted sequence sensitive for pathology, and generally able to distinguish pathologic tissue from normal tissue. With the addition of an additional radio frequency pulse and additional manipulation of the magnetic gradients, a T2-weighted sequence can be converted to a FLAIR sequence, in which free water is now dark, but edematous tissues remain bright. This sequence in particular is currently the most sensitive way to evaluate the brain for demyelinating
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...

 diseases, such as multiple sclerosis
Multiple sclerosis
Multiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...

.

The typical MRI examination consists of 5–20 sequences, each of which are chosen to provide a particular type of information about the subject tissues. This information is then synthesized by the interpreting physician
Physician
A physician is a health care provider who practices the profession of medicine, which is concerned with promoting, maintaining or restoring human health through the study, diagnosis, and treatment of disease, injury and other physical and mental impairments...

.

T1-weighted MRI


{{Main|Spin-lattice relaxation time}}

T1-weighted scans are a standard basic scan, in particular differentiating fat from water - with water darker and fat brighter use a gradient echo (GRE) sequence, with short TE and short TR. This is one of the basic types of MR contrast and is a commonly run clinical scan. The T1 weighting can be increased (improving contrast) with the use of an inversion pulse as in an MP-RAGE sequence. Due to the short repetition time (TR) this scan can be run very fast allowing the collection of high resolution 3D datasets. A T1 reducing gadolinium contrast agent is also commonly used, with a T1 scan being collected before and after administration of contrast agent to compare the difference. In the brain T1-weighted scans provide good gray matter/white matter contrast; in other words, T1-weighted images highlight fat deposition.

T2-weighted MRI


{{Main|Spin-spin relaxation time}}
T2-weighted scans are another basic type. Like the T1-weighted scan, fat is differentiated from water - but in this case fat shows darker, and water lighter. For example, in the case of cerebral and spinal study, the CSF (cerebrospinal fluid) will be lighter in T2-weighted images. These scans are therefore particularly well suited to imaging edema, with long TE and long TR. Because the spin echo
Spin echo
In magnetic resonance, a spin echo is the refocusing of precessing spin magnetisation by a pulse of resonant radiation. Modern nuclear magnetic resonance and magnetic resonance imaging rely heavily on this effect....

 sequence is less susceptible to inhomogeneities in the magnetic field, these images have long been a clinical workhorse.

T{{su|p=*|b=2}}-weighted MRI


T{{su|p=*|b=2}} weighted scans use a gradient echo (GRE) sequence, with long TE and long TR. The gradient echo sequence used does not have the extra refocusing pulse used in spin echo
Spin echo
In magnetic resonance, a spin echo is the refocusing of precessing spin magnetisation by a pulse of resonant radiation. Modern nuclear magnetic resonance and magnetic resonance imaging rely heavily on this effect....

 so it is subject to additional losses above the normal T2 decay (referred to as T2′), these taken together are called T{{su|p=*|b=2}}. This also makes it more prone to susceptibility losses at air/tissue boundaries, but can increase contrast for certain types of tissue, such as venous blood.

Spin density weighted MRI


Spin density, also called proton density, weighted scans try to have no contrast from either T2 or T1 decay, the only signal change coming from differences in the amount of available spins (hydrogen nuclei in water). It uses a spin echo
Spin echo
In magnetic resonance, a spin echo is the refocusing of precessing spin magnetisation by a pulse of resonant radiation. Modern nuclear magnetic resonance and magnetic resonance imaging rely heavily on this effect....

 or sometimes a gradient echo sequence, with short TE and long TR.

Diffusion MRI


{{Main|Diffusion MRI}}
Diffusion MRI
Diffusion MRI
Diffusion MRI is a magnetic resonance imaging method that produces in vivo images of biological tissues weighted with the local microstructural characteristics of water diffusion, which is capable of showing connections between brain regions...

 measures the diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 of water molecules in biological tissues. In an isotropic medium (inside a glass of water for example), water molecules naturally move randomly according to turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...

 and Brownian motion
Brownian motion
Brownian motion or pedesis is the presumably random drifting of particles suspended in a fluid or the mathematical model used to describe such random movements, which is often called a particle theory.The mathematical model of Brownian motion has several real-world applications...

. In biological tissues however, where the Reynolds number is low enough for flows to be laminar
Laminar flow
Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents...

, the diffusion may be anisotropic. For example, a molecule inside the axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

 of a neuron has a low probability of crossing the myelin
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...

 membrane. Therefore the molecule moves principally along the axis of the neural fiber. If it is known that molecules in a particular voxel
Voxel
A voxel is a volume element, representing a value on a regular grid in three dimensional space. This is analogous to a pixel, which represents 2D image data in a bitmap...

 diffuse principally in one direction, the assumption can be made that the majority of the fibers in this area are going parallel to that direction.

The recent development of diffusion tensor imaging (DTI) enables diffusion to be measured in multiple directions and the fractional anisotropy in each direction to be calculated for each voxel. This enables researchers to make brain maps of fiber directions to examine the connectivity of different regions in the brain (using tractography
Tractography
In neuroscience, tractography is a procedure to demonstrate the neural tracts.It uses special techniques of magnetic resonance imaging , and computer-based image analysis.The results are presented in two- and three-dimensional images....

) or to examine areas of neural degeneration and demyelination in diseases like Multiple Sclerosis
Multiple sclerosis
Multiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...

.

Another application of diffusion MRI is diffusion-weighted imaging (DWI). Following an ischemic stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

, DWI is highly sensitive to the changes occurring in the lesion. It is speculated that increases in restriction (barriers) to water diffusion, as a result of cytotoxic edema (cellular swelling), is responsible for the increase in signal on a DWI scan. The DWI enhancement appears within 5–10 minutes of the onset of stroke symptoms (as compared with computed tomography
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...

, which often does not detect changes of acute infarct for up to 4–6 hours) and remains for up to two weeks. Coupled with imaging of cerebral perfusion, researchers can highlight regions of "perfusion/diffusion mismatch" that may indicate regions capable of salvage by reperfusion therapy.

Like many other specialized applications, this technique is usually coupled with a fast image acquisition sequence, such as echo planar imaging sequence.

Magnetization Transfer MRI


{{Main|Magnetization transfer}}

Magnetization transfer (MT) refers to the transfer of longitudinal magnetization from free water protons to hydration water protons in NMR and MRI.

In magnetic resonance imaging of molecular solutions, such as protein solutions, two types of water molecules, free (bulk) and hydration (bound), are found. Free water protons have faster average rotational frequency and hence less fixed water molecules that may cause local field inhomogeneity. Because of this uniformity, most free water protons have resonance frequency lying narrowly around the normal proton resonance frequency of 63 MHz (at 1.5 teslas). This also results in slower transverse magnetization dephasing and hence longer T2. Conversely, hydration water molecules are slowed down by interaction with solute molecules and hence create field inhomogeneities that lead to wider resonance frequency spectrum.

In free liquids, protons, which may be viewed classically as small magnetic dipoles, exhibit translational and rotational motions. These moving dipoles disturb the surrounding magnetic field however on long enough time-scales (which may be nanoseconds) the average field caused by the motion of protons is zero. This is known as “motional averaging” or narrowing and is characteristic of protons moving freely in liquid. On the other hand, protons bound to macromolecules, such as proteins, tend to have a fixed orientation and so the average magnetic field in close proximity to such structures does not average to zero. The result is a spatial pattern in the magnetic field that gives rise to a residual dipolar coupling (range of precession frequencies) for the protons experiencing the magnetic field. The wide frequency distribution appears as a broad spectrum that may be several kHz wide. The net signal from these protons disappears very quickly, in inverse proportion to the width, due to the loss of coherence of the spins, i.e. T2 relaxation. Due to exchange mechanisms, such as spin transfer or proton chemical exchange, the (incoherent) spins bound to the macromolecules continually switch places with (coherent) spins in the bulk media and establish a dynamic equilibrium.

Magnetization transfer: Although there is no measurable signal from the bound spins, or the bound spins that exchange into the bulk media, their longitudinal magnetization is preserved and may recover only via the relatively slow process of T1 relaxation. If the longitudinal magnetization of just the bound spins can be altered, then the effect can be measured in the spins of the bulk media due to the exchange processes. The magnetization transfer sequence applies RF saturation at a frequency that is far off resonance for the narrow line of bulk water but still on resonance for the bound protons with a spectral linewidth of kHz. This causes saturation of the bound spins which exchange into the bulk water, resulting in a loss of longitudinal magnetization and hence signal decrease in the bulk water. This provides an indirect measure of macromolecular content in tissue. Implementation of magnetization transfer involves choosing suitable frequency offsets and pulse shapes to saturate the bound spins sufficiently strongly, within the safety limits of specific absorption rate for RF irradiation.

T1rho MRI


{{Main|Spin-lattice relaxation time in the rotating frame}}
T1ρ (T1rho): Molecules have a kinetic energy that is a function of the temperature and is expressed as translational and rotational motions, and by collisions between molecules. The moving dipoles disturb the magnetic field but are often extremely rapid so that the average effect over a long time-scale may be zero. However, depending on the time-scale, the interactions between the dipoles do not always average away. At the slowest extreme the interaction time is effectively infinite and occurs where there are large, stationary field disturbances (e.g. a metallic implant). In this case the loss of coherence is described as a "static dephasing". T2* is a measure of the loss of coherence in an ensemble of spins that include all interactions (including static dephasing). T2 is a measure of the loss of coherence that excludes static dephasing, using an RF pulse to reverse the slowest types of dipolar interaction. There is in fact a continuum of interaction time-scales in a given biological sample and the properties of the refocusing RF pulse can be tuned to refocus more than just static dephasing. In general, the rate of decay of an ensemble of spins is a function of the interaction times and also the power of the RF pulse. This type of decay, occurring under the influence of RF, is known as T1ρ. It is similar to T2 decay but with some slower dipolar interactions refocused as well as the static interactions, hence T1ρ≥T2.

Fluid attenuated inversion recovery (FLAIR)


{{Main|Fluid attenuated inversion recovery}}
Fluid Attenuated Inversion Recovery (FLAIR) is an inversion-recovery pulse sequence used
to null signal from fluids. For example, it can be used in brain imaging to suppress cerebrospinal fluid (CSF) so as to bring out the periventricular hyperintense lesions, such as multiple sclerosis (MS) plaques. By carefully choosing the inversion time TI (the time between the inversion and excitation pulses), the signal from any particular tissue can be suppressed.

Magnetic resonance angiography



{{Main|Magnetic resonance angiography}}
Magnetic resonance angiography (MRA) generates pictures of the arteries to evaluate them for stenosis
Stenosis
A stenosis is an abnormal narrowing in a blood vessel or other tubular organ or structure.It is also sometimes called a stricture ....

 (abnormal narrowing) or aneurysm
Aneurysm
An aneurysm or aneurism is a localized, blood-filled balloon-like bulge in the wall of a blood vessel. Aneurysms can commonly occur in arteries at the base of the brain and an aortic aneurysm occurs in the main artery carrying blood from the left ventricle of the heart...

s (vessel wall dilatations, at risk of rupture). MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs (called a "run-off"). A variety of techniques can be used to generate the pictures, such as administration of a paramagnetic contrast agent (gadolinium
Gadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...

) or using a technique known as "flow-related enhancement" (e.g. 2D and 3D time-of-flight sequences), where most of the signal on an image is due to blood that recently moved into that plane, see also FLASH MRI
FLASH MRI
FLASH MRI is a basic measuring principle for rapid MRI invented in 1985 by Jens Frahm, Axel Haase, W Hänicke, KD Merboldt, and D Matthaei at the in Göttingen, Germany...

. Techniques involving phase accumulation (known as phase contrast angiography) can also be used to generate flow velocity maps easily and accurately. Magnetic resonance venography (MRV) is a similar procedure that is used to image veins. In this method, the tissue is now excited inferiorly, while signal is gathered in the plane immediately superior to the excitation plane—thus imaging the venous blood that recently moved from the excited plane.

Magnetic resonance gated intracranial CSF dynamics (MR-GILD)


Magnetic resonance gated intracranial cerebrospinal fluid (CSF) or liquor dynamics (MR-GILD) technique is an MR sequence based on bipolar gradient pulse used to demonstrate CSF pulsatile flow in ventricles, cisterns, aqueduct of Sylvius and entire intracranial CSF pathway. It is a method for analyzing CSF circulatory system dynamics in patients with CSF obstructive lesions such as normal pressure hydrocephalus. It also allows visualization of both arterial and venous pulsatile blood flow in vessels without use of contrast agents.
Diastolic time data acquisition (DTDA). Systolic time data acquisition (STDA).

Magnetic resonance spectroscopy


{{Main|In vivo magnetic resonance spectroscopy}}
{{Main|Nuclear magnetic resonance spectroscopy}}

Magnetic resonance spectroscopy
In vivo magnetic resonance spectroscopy
In vivo magnetic resonance spectroscopy is a specialised technique associated to magnetic resonance imaging ....

 (MRS) is used to measure the levels of different metabolites in body tissues. The MR signal produces a spectrum of resonances that correspond to different molecular arrangements of the isotope being "excited". This signature is used to diagnose certain metabolic disorders, especially those affecting the brain, and to provide information on tumor metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

.

Magnetic resonance spectroscopic imaging (MRSI) combines both spectroscopic and imaging methods to produce spatially localized spectra from within the sample or patient. The spatial resolution is much lower (limited by the available SNR
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

), but the spectra in each voxel contains information about many metabolites. Because the available signal is used to encode spatial and spectral information, MRSI requires high SNR achievable only at higher field strengths (3 T and above).

Functional MRI


{{Main|Functional magnetic resonance imaging}}

Functional MRI (fMRI) measures signal changes in the brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

 that are due to changing neural
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

 activity. The brain is scanned at low resolution but at a rapid rate (typically once every 2–3 seconds). Increases in neural activity cause changes in the MR signal via T{{su|p=*|b=2}} changes; this mechanism is referred to as the BOLD (blood-oxygen-level dependent) effect. Increased neural activity causes an increased demand for oxygen, and the vascular
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

 system actually overcompensates for this, increasing the amount of oxygenated hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

 relative to deoxygenated hemoglobin. Because deoxygenated hemoglobin attenuates the MR signal, the vascular response leads to a signal increase that is related to the neural activity. The precise nature of the relationship between neural activity and the BOLD signal is a subject of current research. The BOLD effect also allows for the generation of high resolution 3D maps of the venous vasculature within neural tissue.

While BOLD signal is the most common method employed for neuroscience studies in human subjects, the flexible nature of MR imaging provides means to sensitize the signal to other aspects of the blood supply. Alternative techniques employ arterial spin labeling (ASL) or weight the MRI signal by cerebral blood flow (CBF) and cerebral blood volume (CBV). The CBV method requires injection of a class of MRI contrast agents that are now in human clinical trials. Because this method has been shown to be far more sensitive than the BOLD technique in preclinical studies, it may potentially expand the role of fMRI in clinical applications. The CBF method provides more quantitative information than the BOLD signal, albeit at a significant loss of detection sensitivity.

Real-time MRI



{{Main|Real-time MRI}}
Real-time MRI
Real-time MRI
Real-time magnetic resonance imaging refers to the continuous monitoring of moving objects in real time. Because MRIis based on time-consuming scanning of k-space, real-time MRI was possible only with low image quality or low temporal resolution...

 refers to the continuous monitoring (“filming”) of moving objects in real time. While many different strategies have been developed over the past two decades, a recent development reported a real-time MRI technique based on radial FLASH
FLASH MRI
FLASH MRI is a basic measuring principle for rapid MRI invented in 1985 by Jens Frahm, Axel Haase, W Hänicke, KD Merboldt, and D Matthaei at the in Göttingen, Germany...

 and iterative reconstruction
Iterative reconstruction
Iterative reconstruction refers to iterative algorithms used to reconstruct 2D and 3D images in certain imaging techniques.For example, in computed tomography an image must be reconstructed from projections of an object...

 that yields a temporal resolution of 20 to 30 milliseconds for images with an in-plane resolution of 1.5 to 2.0 mm. The new method promises to add important information about diseases of the joints and the heart. In many cases MRI examinations may become easier and more comfortable for patients.

Interventional MRI


{{Main|Interventional MRI}}

The lack of harmful effects on the patient and the operator make MRI well-suited for "interventional radiology
Interventional radiology
Interventional radiology is a specialty of radiology, in which image-guided procedures are used to diagnose and treat a multitude of diseases across all body systems...

", where the images produced by a MRI scanner are used to guide minimally invasive procedures. Of course, such procedures must be done without any ferromagnetic instruments.

A specialized growing subset of interventional MRI is that of intraoperative MRI in which the MRI is used in the surgical process. Some specialized MRI systems have been developed that allow imaging concurrent with the surgical procedure. More typical, however, is that the surgical procedure is temporarily interrupted so that MR images can be acquired to verify the success of the procedure or guide subsequent surgical work.

Radiation therapy simulation


Because of MRI's superior imaging of soft tissues, it is now being utilized to specifically locate tumors within the body in preparation for radiation therapy treatments. For therapy simulation, a patient is placed in specific, reproducible, body position and scanned. The MRI system then computes the precise location, shape and orientation of the tumor mass, correcting for any spatial distortion inherent in the system. The patient is then marked or tattooed with points that, when combined with the specific body position, permits precise triangulation for radiation therapy.{{Citation needed|date=December 2010}}

Current density imaging


Current density imaging
Current density imaging
Current density imaging is an extension of magnetic resonance imaging , developed at the University of Toronto.It employs two techniques for spatially mapping electrical current pathways through tissue:...

 (CDI) endeavors to use the phase information from images to reconstruct current densities within a subject. Current density imaging works because electrical currents generate magnetic fields, which in turn affect the phase of the magnetic dipoles during an imaging sequence.{{Citation needed|date=December 2010}}

Magnetic resonance guided focused ultrasound


In MRgFUS therapy, ultrasound beams are focused on a tissue—guided and controlled using MR thermal imaging—and due to the significant energy deposition at the focus, temperature within the tissue rises to more than 65 °C (150 °F), completely destroying it. This technology can achieve precise ablation
Ablation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...

 of diseased tissue. MR imaging provides a three-dimensional view of the target tissue, allowing for precise focusing of ultrasound energy. The MR imaging provides quantitative, real-time, thermal images of the treated area. This allows the physician to ensure that the temperature generated during each cycle of ultrasound energy is sufficient to cause thermal ablation within the desired tissue and if not, to adapt the parameters to ensure effective treatment.

Multinuclear imaging


Hydrogen is the most frequently imaged nucleus in MRI because it is present in biological tissues in great abundance, and because its high gyromagnetic ratio gives a strong signal. However, any nucleus with a net nuclear spin could potentially be imaged with MRI. Such nuclei include helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

-3, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

-13, fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...

-19, oxygen-17
Oxygen-17
Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...

, sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

-23, phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

-31 and xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...

-129. 23Na and 31P are naturally abundant in the body, so can be imaged directly. Gaseous isotopes such as 3He or 129Xe must be hyperpolarized
Hyperpolarization (physics)
Hyperpolarization is the nuclear spin polarization of a material far beyond thermal equilibrium conditions. It is commonly applied to gases such as 129Xe and 3He which are then used, for instance, in hyperpolarized magnetic resonance imaging of the lungs....

 and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions. 17O
Oxygen-17
Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...

 and 19F can be administered in sufficient quantities in liquid form (e.g. 17O
Oxygen-17
Oxygen-17 is a low abundant isotope of oxygen . Being the only stable isotope of oxygen possessing a nuclear spin and the unique characteristic of field-independent relaxation it enables NMR studies of metabolic pathways of compounds incorporating oxygen at high magnetic fields Oxygen-17 is a low...

-water) that hyperpolarization is not a necessity.{{Citation needed|date=December 2010}}

Multinuclear imaging is primarily a research technique at present. However, potential applications include functional imaging and imaging of organs poorly seen on 1H MRI (e.g. lungs and bones) or as alternative contrast agents. Inhaled hyperpolarized 3He can be used to image the distribution of air spaces within the lungs. Injectable solutions containing 13C or stabilized bubbles of hyperpolarized 129Xe have been studied as contrast agents for angiography and perfusion imaging. 31P can potentially provide information on bone density and structure, as well as functional imaging of the brain. {{Citation needed|date=December 2010}}

Susceptibility weighted imaging (SWI)


{{Main|Susceptibility weighted imaging}}
Susceptibility weighted imaging (SWI), is a new type of contrast in MRI different from spin density, T1, or T2 imaging. This method exploits the susceptibility differences between tissues and uses a fully velocity compensated, three dimensional, RF spoiled, high-resolution, 3D gradient echo scan. This special data acquisition and image processing produces an enhanced contrast magnitude image very sensitive to venous blood, hemorrhage and iron storage. It is used to enhance the detection and diagnosis of tumors, vascular and neurovascular diseases (stroke and hemorrhage, multiple sclerosis, Alzheimer's), and also detects traumatic brain injuries that may not be diagnosed using other methods

Other specialized MRI techniques


New methods and variants of existing methods are often published when they are able to produce better results in specific fields. Examples of these recent improvements are T{{su
Relaxation (NMR)
In nuclear magnetic resonance spectroscopy and magnetic resonance imaging the term relaxation describes several processes by which nuclear magnetization prepared in a non-equilibrium state return to the equilibrium distribution. In other words, relaxation describes how fast spins "forget" the...

 turbo spin-echo (T2 TSE MRI), double inversion recovery MRI (DIR-MRI) or phase-sensitive inversion recovery MRI (PSIR-MRI), all of them able to improve imaging of the brain lesions. Another example is MP-RAGE (magnetization-prepared rapid acquisition with gradient echo), which improves images of multiple sclerosis cortical lesions.

Portable instruments


Portable magnetic resonance instruments are available for use in education and field research. Using the principles of Earth's field NMR
Earth's field NMR
Nuclear magnetic resonance in the geomagnetic field is conventionally referred to as Earth's field NMR . EFNMR is a special case of low field NMR....

, they have no powerful polarizing magnet, so that such instruments can be small and inexpensive. Some can be used for both EFNMR spectroscopy and MRI imaging. The low strength of the Earth's field results in poor signal to noise ratios (SNR), requiring long scan times to capture spectroscopic data or build up MRI images. However, the extremely low noise floor of SQUID-based MRI detectors, and the low density of thermal noise in the low-frequency operating range (tens of kiloHertz) may result in usable SNR approaching that of mid-field conventional instruments. Further, the ultra-low field technologies enable electron spin resonance detection, and potentially imaging, at safe operating frequencies (NASA Technical Brief).

Research with atomic magnetometer
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...

s have discussed the possibility for cheap and portable MRI instruments without the large magnet.

MRI versus CT


A computed tomography
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...

 (CT) scanner uses X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s, a type of ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

, to acquire images, making it a good tool for examining tissue composed of elements of a higher atomic number than the tissue surrounding them, such as bone and calcification
Calcification
Calcification is the process in which calcium salts build up in soft tissue, causing it to harden. Calcifications may be classified on whether there is mineral balance or not, and the location of the calcification.-Causes:...

s (calcium based) within the body (carbon based flesh), or of structures (vessels, bowel). MRI, on the other hand, uses non-ionizing radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

 (RF) signals to acquire its images and is best suited for soft tissue (although MRI can also be used to acquire images of bones, teeth and even fossils).

In contrast, CT images are generated purely by X-ray attenuation, while a variety of properties may be used to generate contrast in MR images. By variation of scanning parameters, tissue contrast can be altered to enhance different features in an image (see Applications for more details). Both CT and MR images may be enhanced by the use of contrast agents
Contrast medium
A medical contrast medium is a substance used to enhance the contrast of structures or fluids within the body in medical imaging...

. Contrast agents for CT contain elements of a high atomic number, relative to tissue, such as iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

 or barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...

, while contrast agents for MRI have paramagnetic
Paramagnetism
Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...

 properties, such as gadolinium
Gadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...

 and manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

, used to alter tissue relaxation
Relaxation (NMR)
In nuclear magnetic resonance spectroscopy and magnetic resonance imaging the term relaxation describes several processes by which nuclear magnetization prepared in a non-equilibrium state return to the equilibrium distribution. In other words, relaxation describes how fast spins "forget" the...

 times.

CT and MRI scanners are able to generate multiple two-dimensional cross-sections (tomographs, or "slices") of tissue and three-dimensional reconstructions.
MRI can generate cross-sectional images in any plane
Plane (mathematics)
In mathematics, a plane is a flat, two-dimensional surface. A plane is the two dimensional analogue of a point , a line and a space...

 (including oblique planes). In the past, CT was limited to acquiring images in the axial (or near axial) plane. The scans used to be called Computed Axial Tomography scans (CAT scans). However, the development of multi-detector CT scanners with near-isotropic
Isotropy
Isotropy is uniformity in all orientations; it is derived from the Greek iso and tropos . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary...

 resolution, allows the CT scanner to produce data that can be retrospectively reconstructed in any plane with minimal loss of image quality. For purposes of tumor detection and identification in the brain, MRI is generally superior. However, in the case of solid tumors of the abdomen and chest, CT is often preferred as it suffers less from motion artifacts. Furthermore, CT usually is more widely available, faster, and less expensive. However, CT has the disadvantage of exposing the patient to harmful ionizing radiation.

MRI is also best suited for cases when a patient is to undergo the exam several times successively in the short term, because, unlike CT, it does not expose the patient to the hazards of ionizing radiation.

Economics of MRI


MRI equipment is expensive. 1.5 tesla scanners often cost between {{USD|1 million}} and {{USD|1.5 million}}. 3.0 tesla scanners often cost between {{USD|2 million}} and {{USD|2.3 million}}. Construction of MRI suites can cost up to {{USD|500,000}}, or more, depending on project scope.

MRI scanners have been significant sources of revenue for healthcare providers in the US. This is because of favorable reimbursement rates from insurers and federal government programs. Insurance reimbursement is provided in two components, an equipment charge for the actual performance of the MRI scan and professional charge for the radiologist's review of the images and/or data. In the US Northeast, an equipment charge might be $3,500 and a professional charge might be $350 although the actual fees received by the equipment owner and interpreting physician are often significantly less and depend on the rates negotiated with insurance companies or determined by governmental action as in the Medicare Fee Schedule. For example, an orthopedic surgery group in Illinois billed a charge of $1,116 for a knee MRI in 2007 but the Medicare reimbursement in 2007 was only $470.91. Many insurance companies require preapproval of an MRI procedure as a condition for coverage.

In the US, the Deficit Reduction Act of 2005
Deficit Reduction Act of 2005
The Deficit Reduction Act of 2005 is a United States Act of Congress concerning the budget, that became law in 2006.-Legislative history:The Senate's version passed after a tie-breaking vote was cast by Vice President Dick Cheney. The bill passed the chamber with no Democrats and five Republicans...

 significantly reduced reimbursement rates paid by federal insurance programs for the equipment component of many scans, shifting the economic landscape. Many private insurers have followed suit.{{Citation needed|date=November 2008}}
In France, the cost of an MRI exam is approximately 150 Euros. This covers three basic scans including one with an intravenous contrast agent, as well as a consultation with the technician and a written report to the patient's physician. {{Citation needed|date=November 2011}}

Safety


A number of features of MRI scanning can give rise to risks.

These include:
  • Powerful magnetic fields
  • Cryogenic liquids
  • Noise
  • Claustrophobia


In addition, in cases where MRI contrast agent
MRI contrast agent
MRI contrast agents are a group of contrast media used to improve the visibility of internal body structures in magnetic resonance imaging . The most commonly used compounds for contrast enhancement are gadolinium-based. MRI contrast agents alter the relaxation times of tissues and body cavities...

s are used, these also typically have associated risks.

Magnetic field


Most forms of medical or biostimulation implants are generally considered contraindication
Contraindication
In medicine, a contraindication is a condition or factor that serves as a reason to withhold a certain medical treatment.Some contraindications are absolute, meaning that there are no reasonable circumstances for undertaking a course of action...

s for MRI scanning. These include pacemakers
Artificial pacemaker
A pacemaker is a medical device that uses electrical impulses, delivered by electrodes contacting the heart muscles, to regulate the beating of the heart...

, vagus nerve stimulators
Vagus nerve stimulation
Vagus nerve stimulation is an adjunctive treatment for certain types of intractable epilepsy and treatment-resistant depression.- Mechanism of action :...

, implantable cardioverter-defibrillator
Implantable cardioverter-defibrillator
An implantable cardioverter-defibrillator is a small battery-powered electrical impulse generator which is implanted in patients who are at risk of sudden cardiac death due to ventricular fibrillation and ventricular tachycardia. The device is programmed to detect cardiac arrhythmia and correct it...

s, loop recorders, insulin pumps, cochlear implant
Cochlear implant
A cochlear implant is a surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf or severely hard of hearing...

s, deep brain stimulators and capsules retained from capsule endoscopy
Capsule endoscopy
Capsule endoscopy is a way to record images of the digestive tract for use in medicine. The capsule is the size and shape of a pill and contains a tiny camera. After a patient swallows the capsule, it takes pictures of the inside of the gastrointestinal tract...

. Patients are therefore always asked for complete information about all implants before entering the room for an MRI scan. Several deaths have been reported in patients with pacemakers who have undergone MRI scanning without appropriate precautions.{{Citation needed|date=October 2010}} To reduce such risks, implants are increasingly being developed to make them able to be safely scanned, and specialized protocols have been developed to permit the safe scanning of selected implants and pacing devices. Cardiovascular stents are considered safe, however.

Ferromagnetic foreign bodies such as shell
Shell (projectile)
A shell is a payload-carrying projectile, which, as opposed to shot, contains an explosive or other filling, though modern usage sometimes includes large solid projectiles properly termed shot . Solid shot may contain a pyrotechnic compound if a tracer or spotting charge is used...

 fragments, or metallic implants such as surgical prostheses and aneurysm
Aneurysm
An aneurysm or aneurism is a localized, blood-filled balloon-like bulge in the wall of a blood vessel. Aneurysms can commonly occur in arteries at the base of the brain and an aortic aneurysm occurs in the main artery carrying blood from the left ventricle of the heart...

 clips are also potential risks. Interaction of the magnetic and radio frequency fields with such objects can lead to trauma due to movement of the object in the magnetic field or thermal injury from radio-frequency induction heating
Induction heating
Induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal...

 of the object.{{Citation needed|date=October 2010}}

Titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 and its alloys are safe from movement from the magnetic field.

In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 a classification system for implants and ancillary clinical devices has been developed by ASTM International and is now the standard supported by the US Food and Drug Administration:
  • MR-Safe — The device or implant is completely non-magnetic, non-electrically conductive, and non-RF reactive, eliminating all of the primary potential threats during an MRI procedure.
  • MR-Conditional — A device or implant that may contain magnetic, electrically conductive or RF-reactive components that is safe for operations in proximity to the MRI, provided the conditions for safe operation are defined and observed (such as 'tested safe to 1.5 teslas' or 'safe in magnetic fields below 500 gauss in strength').
  • MR-Unsafe — Nearly self-explanatory, this category is reserved for objects that are significantly ferromagnetic and pose a clear and direct threat to persons and equipment within the magnet room.


The very high strength of the magnetic field can also cause "missile-effect" accidents, where ferromagnetic objects are attracted to the center of the magnet, and there have been incidences of injury and death. To reduce the risks of projectile accidents, ferromagnetic objects and devices are typically prohibited in proximity to the MRI scanner and patients undergoing MRI examinations are required to remove all metallic objects, often by changing into a gown or scrubs
Scrubs (clothing)
Scrubs are the shirts and trousers or gowns worn by nurses , surgeons, and other operating room personnel when "scrubbing in" for surgery. In the United Kingdom, they are sometimes known as Theatre Blues. They are designed to be simple with minimal places for dirt to hide, easy to launder, and...

 and ferromagnetic detection devices are used by some sites.

There is no evidence for biological harm from even very powerful static magnetic fields.

Radio frequency energy


A powerful radio transmitter is needed for excitation of proton spins. This can heat the body to the point of risk of hyperthermia
Hyperthermia
Hyperthermia is an elevated body temperature due to failed thermoregulation. Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate...

 in patients, particularly in obese patients or those with thermoregulation disorders{{Citation needed|reason=lack of evidence supporting this claim|date=July 2010}}. Several countries have issued restrictions on the maximum specific absorption rate
Specific absorption rate
Specific absorption rate is a measure of the rate at which energy is absorbed by the body when exposed to a radio frequency electromagnetic field; although, it can also refer to absorption of other forms of energy by tissue, including ultrasound . It is defined as the power absorbed per mass of...

 that a scanner may produce.

Peripheral nerve stimulation (PNS)


The rapid switching on and off of the magnetic field gradients is capable of causing nerve stimulation. Volunteers report a twitching sensation when exposed to rapidly switched fields, particularly in their extremities. The reason the peripheral nerves are stimulated is that the changing field increases with distance from the center of the gradient coils (which more or less coincides with the center of the magnet). Note however that when imaging the head, the heart is far off-center and induction of even a tiny current into the heart must be avoided at all costs.{{Citation needed|date=January 2010}} Although PNS was not a problem for the slow, weak gradients used in the early days of MRI, the strong, rapidly switched gradients used in techniques such as EPI, fMRI, diffusion MRI, etc. are indeed capable of inducing PNS. American and European regulatory agencies insist that manufacturers stay below specified dB/dt limits (dB/dt is the change in field per unit time) or else prove that no PNS is induced for any imaging sequence. As a result of dB/dt limitation, commercial MRI systems cannot use the full rated power of their gradient amplifiers.

Acoustic noise


Switching of field gradients causes a change in the Lorentz force
Lorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...

 experienced by the gradient coils, producing minute expansions and contractions of the coil itself. As the switching is typically in the audible frequency range, the resulting vibration produces loud noises (clicking or beeping). This is most marked with high-field machines and rapid-imaging techniques in which sound intensity can reach 120 dB(A) (equivalent to a jet engine at take-off), and therefore appropriate ear protection is essential for anyone inside the MRI scanner room during the examination.

Cryogens


As described in Physics of Magnetic Resonance Imaging, many MRI scanners rely on cryogenic liquids to enable superconducting capabilities of the electromagnetic coils within. Though the cryogenic liquids used are non-toxic, their physical properties present specific hazards.

An unintentional shut-down of a superconducting electromagnet
Superconducting magnet
A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire can conduct much larger electric currents than ordinary wire, creating intense magnetic fields...

, an event known as "quench", involves the rapid boiling of liquid helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 from the device. If the rapidly expanding helium cannot be dissipated through an external vent, sometimes referred to as 'quench pipe', it may be released into the scanner room where it may cause displacement of the oxygen and present a risk of asphyxiation.

Oxygen deficiency monitors are usually used as a safety precaution. Liquid helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

, the most commonly used cryogen in MRI, undergoes near explosive expansion as it changes from liquid to a gaseous state. The use of an oxygen monitor is important to ensure that oxygen levels safe for patient/physicians. Rooms built in support of superconducting MRI equipment should be equipped with pressure relief mechanisms and an exhaust fan, in addition to the required quench pipe.

Because a quench results in rapid loss of cryogens from the magnet, recommissioning the magnet is expensive and time-consuming. Spontaneous quenches are uncommon, but a quench may also be triggered by equipment malfunction, improper cryogen fill technique, contaminants inside the cryostat, or extreme magnetic or vibrational disturbances.

Contrast agents


{{Main|MRI contrast agent}}
The most commonly used intravenous contrast agents are based on chelates of gadolinium
Gadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...

. In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT. Anaphylactoid reactions
Anaphylaxis
Anaphylaxis is defined as "a serious allergic reaction that is rapid in onset and may cause death". It typically results in a number of symptoms including throat swelling, an itchy rash, and low blood pressure...

 are rare, occurring in approx. 0.03–0.1%. Of particular interest is the lower incidence of nephrotoxicity, compared with iodinated agents, when given at usual doses—this has made contrast-enhanced MRI scanning an option for patients with renal impairment, who would otherwise not be able to undergo contrast-enhanced CT
Radiocontrast
Radiocontrast agents are a type of medical contrast medium used to improve the visibility of internal bodily structures in an X-ray based imaging techniques such as computed tomography or radiography...

.

Although gadolinium agents have proved useful for patients with renal impairment, in patients with severe renal failure requiring dialysis there is a risk of a rare but serious illness, nephrogenic systemic fibrosis
Nephrogenic systemic fibrosis
Nephrogenic systemic fibrosis or nephrogenic fibrosing dermopathy is a rare and serious syndrome that involves fibrosis of skin, joints, eyes, and internal organs. Its cause is not fully understood...

, that may be linked to the use of certain gadolinium-containing agents. The most frequently linked is gadodiamide, but other agents have been linked too. Although a causal link has not been definitively established, current guidelines in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 are that dialysis patients should only receive gadolinium agents where essential, and that dialysis
Dialysis
In medicine, dialysis is a process for removing waste and excess water from the blood, and is primarily used to provide an artificial replacement for lost kidney function in people with renal failure...

 should be performed as soon as possible after the scan to remove the agent from the body promptly. In Europe, where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released. Recently a new contrast agent named gadoxetate
Gadoxetic acid
Gadoxetic acid is a gadolinium-based MRI contrast agent. Its salt, gadoxetate disodium, is marketed as Primovist in Europe and Eovist in the United states by Bayer HealthCare Pharmaceuticals....

, brand name Eovist (US) or Primovist (EU), was approved for diagnostic use: this has the theoretical benefit of a dual excretion path.

Pregnancy


No effects of MRI on the fetus have been demonstrated.
In particular, MRI avoids the use of ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

, to which the fetus is particularly sensitive. However, as a precaution, current guidelines recommend that pregnant women undergo MRI only when essential. This is particularly the case during the first trimester of pregnancy, as organogenesis
Organogenesis
In animal development, organogenesis is the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism. Internal organs initiate development in humans within the 3rd to 8th weeks in utero...

 takes place during this period. The concerns in pregnancy are the same as for MRI in general, but the fetus may be more sensitive to the effects—particularly to heating and to noise. However, one additional concern is the use of contrast agents; gadolinium
Gadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...

 compounds are known to cross the placenta and enter the fetal bloodstream, and it is recommended that their use be avoided.

Despite these concerns, MRI is rapidly growing in importance as a way of diagnosing and monitoring congenital defects of the fetus because it can provide more diagnostic information than ultrasound
Ultrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

 and it lacks the ionizing radiation of CT. MRI without contrast agents is the imaging mode of choice for pre-surgical, in-utero diagnosis and evaluation of fetal tumors, primarily teratoma
Teratoma
A teratoma is an encapsulated tumor with tissue or organ components resembling normal derivatives of all three germ layers. There are rare occasions when not all three germ layers are identifiable...

s, facilitating open fetal surgery
Fetal surgery
Fetal surgery is any of a broad range of surgical techniques that are used to treat birth defects in fetuses who are still in the pregnant uterus.* Open fetal surgery involves completely opening the uterus to operate on the fetus....

, other fetal intervention
Fetal intervention
Fetal intervention involves in utero surgical treatment of a fetus. Procedures include open fetal surgery, the most invasive, and the less invasive fetendo and fetal image-guided surgery.Fetal intervention is relatively new...

s, and planning for procedures (such as the EXIT procedure
EXIT procedure
The EXIT procedure, or ex utero intrapartum treatment procedure, is a specialized surgical delivery procedure used to deliver babies who have airway compression...

) to safely deliver and treat babies whose defects would otherwise be fatal.

Claustrophobia and discomfort


MRI scans can be unpleasant. Older closed bore MRI systems have a fairly long tube or tunnel. The part of the body being imaged must lie at the center of the magnet, which is at the absolute center of the tunnel. Because scan times on these older scanners may be long (occasionally up to 40 minutes for the entire procedure), people with even mild claustrophobia
Claustrophobia
Claustrophobia is the fear of having no escape and being closed in small spaces or rooms...

 are sometimes unable to tolerate an MRI scan without management. Some modern scanners have larger bores (up to 70 cm) and scan times are shorter. This means that claustrophobia could be less of an issue, and additional patients may now find MRI to be a tolerable procedure.

Nervous patients may still find the following strategies helpful:
  • Advance preparation
    • visiting the scanner to see the room and practice lying on the table
    • visualization techniques
    • chemical sedation
    • general anesthesia
  • Coping while inside the scanner
    • having a loved one in the room to hold hand, reassure them, etc.
    • holding a "panic button"
    • closing eyes as well as covering them (e.g. washcloth, eye mask)
    • listening to music on headphones or watching a movie, using mirror-glasses and a projection screen or via a Head-mounted display
      Head-mounted display
      A head-mounted display or helmet mounted display, both abbreviated HMD, is a display device, worn on the head or as part of a helmet, that has a small display optic in front of one or each eye .- Overview :...

      , while in the machine.


Many newer MRI systems place a diagonal mirror above the eyes to allow the patient to look down the tunnel rather than at the bore wall immediately above their faces.

Alternative scanner designs, such as open or upright systems, can also be helpful where these are available. Though open scanners have increased in popularity, they produce inferior scan quality because they operate at lower magnetic fields than closed scanners. However, commercial 1.5 tesla open systems have recently become available, providing much better image quality than previous lower field strength open models.

For babies and young children chemical sedation or general anesthesia are the norm, as these subjects cannot be instructed to hold still during the scanning session. Obese patients and pregnant women may find the MRI machine to be a tight fit. Pregnant women may also have difficulty lying on their backs for an hour or more without moving.

Guidance


Safety issues, including the potential for biostimulation device interference, movement of ferromagnetic bodies, and incidental localized heating, have been addressed in the American College of Radiology
American College of Radiology
The American College of Radiology , founded in 1923, is a non-profit professional medical association composed of diagnostic radiologists, radiation oncologists, interventional radiologists, nuclear medicine physicians, and medical physicists. It is based in Reston, Virginia, with offices in...

's White Paper on MR Safety, which was originally published in 2002 and expanded in 2004. The ACR White Paper on MR Safety has been rewritten and was released early in 2007 under the new title ACR Guidance Document for Safe MR Practices.

In December 2007, the Medicines in Healthcare product Regulation Agency (MHRA), a UK healthcare regulatory body, issued their Safety Guidelines for Magnetic Resonance Imaging Equipment in Clinical Use.

In February 2008, the Joint Commission, a US healthcare accrediting organization, issued a Sentinel Event Alert #38, their highest patient safety advisory, on MRI safety issues.

In July 2008, the United States Veterans Administration, a federal governmental agency serving the healthcare needs of former military personnel, issued a substantial revision to their MRI Design Guide, which includes physical or facility safety considerations.

The European Physical Agents Directive


The European Physical Agents (Electromagnetic Fields) Directive is legislation adopted in European legislature. Originally scheduled to be required by the end of 2008, each individual state within the European Union must include this directive in its own law by the end of 2012. Some member nations passed complying legislation and are now attempting to repeal their state laws in expectation that the final version of the EU Physical Agents Directive will be substantially revised prior to the revised adoption date.

The directive applies to occupational exposure to electromagnetic fields (not medical exposure) and was intended to limit workers’ acute exposure to strong electromagnetic fields, as may be found near electricity substations, radio or television transmitters or industrial equipment. However, the regulations impact significantly on MRI, with separate sections of the regulations limiting exposure to static magnetic fields, changing magnetic fields and radio frequency energy. Field strength limits are given, which may not be exceeded. An employer may commit a criminal offense by allowing a worker to exceed an exposure limit, if that is how the Directive is implemented in a particular member state.

The Directive is based on the international consensus of established effects of exposure to electromagnetic fields, and in particular the advice of the European Commissions's advisor, the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The aims of the Directive, and the ICNIRP guidelines it is based on, are to prevent exposure to potentially harmful fields. The actual limits in the Directive are very similar to the limits advised by the Institute of Electrical and Electronics Engineers, with the exception of the frequencies produced by the gradient coils, where the IEEE limits are significantly higher.

Many Member States of the EU already have either specific EMF regulations or (as in the UK) a general requirement under workplace health and safety legislation to protect workers against electromagnetic fields. In almost all cases the existing regulations are aligned with the ICNIRP limits so that the Directive should, in theory, have little impact on any employer already meeting their legal responsibilities.

The introduction of the Directive has brought to light an existing potential issue with occupational exposures to MRI fields. There are at present very few data on the number or types of MRI practice that might lead to exposures in excess of the levels of the Directive. There is a justifiable concern amongst MRI practitioners that if the Directive were to be enforced more vigorously than existing legislation, the use of MRI might be restricted, or working practices of MRI personnel might have to change.

In the initial draft a limit of static field strength to 2 T was given. This has since been removed from the regulations, and whilst it is unlikely to be restored as it was without a strong justification, some restriction on static fields may be reintroduced after the matter has been considered more fully by ICNIRP. The effect of such a limit might be to restrict the installation, operation and maintenance of MRI scanners with magnets of 2 T and stronger. As the increase in field strength has been instrumental in developing higher resolution and higher performance scanners, this would be a significant step back. This is why it is unlikely to happen without strong justification.

Individual government agencies and the European Commission have now formed a working group to examine the implications on MRI and to try to address the issue of occupational exposures to electromagnetic fields from MRI.

The principle


Because contemporary MRI scanners offer isotropic
Isotropy
Isotropy is uniformity in all orientations; it is derived from the Greek iso and tropos . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary...

, or near isotropic, resolution, display of images does not need to be restricted to the conventional axial images. Instead, it is possible for a software program to build a volume by 'stacking' the individual slices one on top of the other. The program may then display the volume in an alternative manner.

3D rendering techniques


Surface rendering: A threshold value of greyscale density is chosen by the operator (e.g. a level that corresponds to fat). A threshold level is set, using edge detection
Edge detection
Edge detection is a fundamental tool in image processing and computer vision, particularly in the areas of feature detection and feature extraction, which aim at identifying points in a digital image at which the image brightness changes sharply or, more formally, has discontinuities...

 image processing algorithms. From this, a 3-dimensional model can be constructed and displayed on screen. Multiple models can be constructed from various thresholds, allowing different colors to represent each anatomical component such as bone, muscle, and cartilage. However, the interior structure of each element is not visible in this mode of operation.

Volume rendering
Volume rendering
In scientific visualization and computer graphics, volume rendering is a set of techniques used to display a 2D projection of a 3D discretely sampled data set.A typical 3D data set is a group of 2D slice images acquired by aCT, MRI, or MicroCT scanner....

: Surface rendering is limited in that it only displays surfaces that meet a threshold density, and only displays the surface closest to the imaginary viewer. In volume rendering, transparency and colors are used to allow a better representation of the volume to be shown in a single image - e.g. the bones of the pelvis could be displayed as semi-transparent, so that even at an oblique angle, one part of the image does not conceal another.

Image segmentation


Where different structures have similar threshold density, it can become impossible to separate them simply by adjusting volume rendering parameters. The solution is called segmentation
Segmentation (image processing)
In computer vision, segmentation refers to the process of partitioning a digital image into multiple segments . The goal of segmentation is to simplify and/or change the representation of an image into something that is more meaningful and easier to analyze...

, a manual or automatic procedure that can remove the unwanted structures from the image.

See also


{{Col-begin}}
{{Col-1-of-3}}
  • Earth's field NMR
    Earth's field NMR
    Nuclear magnetic resonance in the geomagnetic field is conventionally referred to as Earth's field NMR . EFNMR is a special case of low field NMR....

     (EFNMR)
  • Electron-spin resonance (spin physics)
  • History of brain imaging
  • Medical imaging
    Medical imaging
    Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...

  • Magnetic immunoassay
    Magnetic immunoassay
    Magnetic immunoassay is a novel type of diagnostic immunoassay using magnetic beads as labels in lieu of conventional enzymes , radioisotopes or fluorescent moieties . This assay involves the specific binding of an antibody to its antigen, where a magnetic label is conjugated to one element of...

  • Jemris
    Jemris
    Jemris is an open source MRI sequence design and simulation framework written in C++.It was designed to most generally and numerically integrate the Bloch equation in a single-core or parallel fashion for protons over a time course of a sequence on almost arbitrary samples with arbitrary excitation...

     (open source MRI simulator)

{{Col-2-of-3}}
  • Magnetic Resonance Imaging (journal)
    Magnetic Resonance Imaging (journal)
    Magnetic Resonance Imaging is a peer-reviewed scientific journal published by Elsevier, encompassing biology, physics, and clinical science as they relate to the development and use of magnetic resonance imaging technology. Magnetic Resonance Imaging was established in 1982 and the current...

  • Magnetic resonance microscopy
    Magnetic resonance microscopy
    Magnetic Resonance Microscopy is Magnetic Resonance Imaging at a microscopic level. A strict definition is MRI having voxel resolutions of better than 100 µm³ .-Nomenclature:...

  • Magnetic particle imaging
    Magnetic Particle Imaging
    Magnetic particle imaging is a tomographic imaging technique that measures the magnetic fields generated by magnetic particles in a tracer. Researchers at Philips Research have used the technique to achieve resolutions finer than one millimeter. Magnetic Particle Imaging has potential applications...

     (MPI)
  • Magnetic resonance elastography
    Magnetic resonance elastography
    Magnetic resonance elastography is a medical imaging technique that images propagating mechanical waves using MRI. It non-invasively measures the stiffness of biological tissues. Pathological tissues are often harder than the surrounding normal tissue...

  • Molecular breast imaging
    Molecular breast imaging
    Molecular breast imaging can refer to:* Scintimammography* Positron emission mammography...

  • Neuroimaging software
  • Nephrogenic fibrosing dermopathy
  • Nobel Prize controversies
    Nobel Prize controversies
    Subsequent to his death in 1896, the will of Swedish industrialist Alfred Nobel established the Nobel Prizes. Annual prizes were to be awarded for service to humanity in the fields of physics, chemistry, physiology or medicine, literature, and peace. Similarly, the Bank of Sweden Prize in Economic...


{{Col-3-of-3}}
{{Commons}}
  • Nuclear magnetic resonance
    Nuclear magnetic resonance
    Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...

     (NMR)
  • 2D-FT NMRI and Spectroscopy
  • Relaxation
    Relaxation (NMR)
    In nuclear magnetic resonance spectroscopy and magnetic resonance imaging the term relaxation describes several processes by which nuclear magnetization prepared in a non-equilibrium state return to the equilibrium distribution. In other words, relaxation describes how fast spins "forget" the...

  • Robinson oscillator
    Robinson oscillator
    The Robinson oscillator is an electronic circuit originally devised for use in the field of continuous wave Nuclear Magnetic Resonance . It was a development of the Marginal oscillator. Strictly one should distinguish between the marginal oscillator and the Robinson oscillator, although sometimes...

  • Rabi cycle
    Rabi cycle
    In physics, the Rabi cycle is the cyclic behaviour of a two-state quantum system in the presence of an oscillatory driving field. A two-state system has two possible states, and if they are not degenerate energy levels the system can become "excited" when it absorbs a quantum of energy.The effect...

  • Virtopsy
    Virtopsy
    Virtopsy is a portmanteau of virtual and autopsy. It is a way of performing a non- or minimally-invasive autopsy by scanning a corpse. Virtopsy is a registered name of the research team in Bern, Switzerland.- Virtopsies in popular culture :...


{{col-end}}

External links


{{Commons category}}


{{Medical imaging}}
{{DEFAULTSORT:Magnetic Resonance Imaging}}