Magnetic Resonance Imaging (
MRI), or
nuclear magnetic resonance imaging (
NMRI2D-FT Nuclear magnetic resonance imaging , or Two-dimensional Fourier transform nuclear magnetic resonance imaging , is primarily a non-invasive imaging technique most commonly used in biomedical research and medical radiology/nuclear medicine/MRI to visualize structures and functions of the living...
), is primarily a
medical imagingMedical imaging refers to the techniques and processes used to create images of the human body for clinical purposes or medical science .As a discipline and in its widest sense, it is part of biological imaging and incorporates...
technique most commonly used in
radiologyRadiology is the branch or specialty of medicine that deals with the study and application of imaging technology like x-ray and radiation to diagnosing and treating disease....
to visualize the internal structure and function of the body. MRI provides much greater
contrastContrast 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 than
computed tomographyComputed tomography is a medical imaging method employing tomography created by computer processing. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.CT...
(CT) does, making it especially useful in
neurologicalNeurology 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,...
(brain), musculoskeletal,
cardiovascularCardiovascular magnetic resonance imaging , sometimes known as cardiac MRI, is a medical imaging technology for the non-invasive assessment of the function and structure of the cardiovascular system. It is derived from and based on the same basic principles as Magnetic Resonance Imaging MRI but...
, and
oncologicalOncology is the branch of medicine dealing with tumors . A medical professional who practices oncology is an oncologist. The term originates from the Greek "Ογκολογία" derived from onkos , meaning bulk, mass, or tumor, and the suffix -logy , meaning "study of" or "to talk about".Oncology is...
(cancer) imaging. Unlike CT, it uses no
ionizing radiationIonizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
, but uses a powerful
magneticIn physics, the term magnetism is used to describe how materials respond on the microscopic level to an applied magnetic field; to categorize the magnetic phase of a material. For example, the most well known form of magnetism is ferromagnetism such that some ferromagnetic materials produce their...
field to align the
nuclear magnetizationThe 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 (usually)
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
atomThe atom is a basic unit of matter consisting 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 water in the body.
Radio frequencyRadio frequency is a frequency, or rate of oscillation, of electromagnetic radiation within the range of about 3 Hz to 300 GHz. This range corresponds to the frequency of alternating current electrical signals used to produce and detect radio waves...
(RF) fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. This signal can be manipulated by additional magnetic fields to build up enough information to construct an image of the body.
Magnetic Resonance Imaging is a relatively new technology. The first MR image was published in 1973 and the first cross-sectional image of a living mouse was published in January 1974. The first studies performed on humans were published in 1977. By comparison, the first human
X-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 10 to 0.01 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...
image was taken in 1895.
Magnetic Resonance Imaging was developed from knowledge gained in the study of
nuclear magnetic resonanceNuclear magnetic resonance is a property that magnetic nuclei have in a magnetic field and applied electromagnetic pulse, which cause the nuclei to absorb energy from the EM pulse and radiate this energy back out...
. In its early years the technique was referred to as nuclear magnetic resonance imaging (NMRI). However, as the word
nuclear was associated in the public mind with
ionizing radiationIonizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
exposure it is generally now referred to simply as MRI. Scientists still use the term NMRI when discussing non-medical devices operating on the same principles. The term Magnetic Resonance Tomography (MRT) is also sometimes used.
How MRI works
The body is
largely composed of water moleculesIn medicine, body water is all of the water content of the human body. A significant fraction of the human body is water. Lean muscle tissue contains about 99% water by weight. Blood contains 2% water, body fat contains 1/2% water and bone has 97% water. Skin also contains much water...
which each contain two
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
nucleiThe nucleus is the very dense region consisting of nucleons at the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus, with a very small contribution from the orbiting electrons....
or
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
s. When a person goes inside the powerful
magnetic fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
of the scanner, the magnetic moments of these protons align with the direction of the field.
A radio frequency electromagnetic field is then briefly turned on, causing the protons to alter their alignment relative to the field. When this field is turned off the protons return to the original magnetization alignment. These alignment changes create a signal which can be detected by the scanner. The frequency the protons resonate at depends on the strength of the magnetic field. The position of protons in the body can be determined by applying additional magnetic fields during the scan which allows an image of the body to be built up. These are created by turning gradients coils on and off which creates the knocking sounds heard during an MR scan.
Diseased tissue, such as tumors, can be detected because the protons in different tissues return to their equilibrium state at different rates. By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue.
Contrast agents may be injected
intravenouslyIntravenous therapy or IV therapy is the giving of liquid substances directly into a vein. It can be intermittent or continuous; continuous administration is called an intravenous drip. The word intravenous simply means "within a vein", but is most commonly used to refer to IV therapy...
to enhance the appearance of
blood vesselThe blood vessels are the part of the circulatory system that transport 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
inflammationInflammation is 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 as well as initiate the healing process for the tissue. Inflammation is not a...
. Contrast agents may also be directly injected into a joint in the case of arthrograms, MR images of joints. Unlike
CTComputed tomography is a medical imaging method employing tomography created by computer processing. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.CT...
, MRI uses no
ionizing radiationIonizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
and is generally a very safe procedure. Patients with some metal implants,
cochlear implantA 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. The cochlear implant is often referred to as a bionic ear...
s, and cardiac pacemakers are prevented from having an MRI scan due to effects of the strong magnetic field and powerful radio frequency pulses.
MRI is used to image every part of the body, and is particularly useful for neurological conditions, for disorders of the muscles and joints, for evaluating tumors, and for showing abnormalities in the heart and blood vessels.
Nuclear magnetism
Subatomic particles have the
quantum mechanicalQuantum mechanics is a set of principles describing the physical reality at the atomic level of matter and the subatomic . These descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation...
property of
spinIn particle physics and quantum mechanics, spin is a fundamental characteristic property of elementary particles including the force carriers , composite particles , and atomic nuclei....
. Certain nuclei such as
1H (protons),
2H,
3He,
23Na or
31P, have a non–zero spin and therefore a magnetic moment. In the case of the so-called spin-
{{Redirect|MRI}}
{{mergefrom|2D-FT NMRI and Spectroscopy|discuss=Talk:Magnetic_resonance_imaging#Merge_2D-FT_NMRI_and_Spectroscopy|date=July 2009}}
Magnetic Resonance Imaging (
MRI), or
nuclear magnetic resonance imaging (
NMRI2D-FT Nuclear magnetic resonance imaging , or Two-dimensional Fourier transform nuclear magnetic resonance imaging , is primarily a non-invasive imaging technique most commonly used in biomedical research and medical radiology/nuclear medicine/MRI to visualize structures and functions of the living...
), is primarily a
medical imagingMedical imaging refers to the techniques and processes used to create images of the human body for clinical purposes or medical science .As a discipline and in its widest sense, it is part of biological imaging and incorporates...
technique most commonly used in
radiologyRadiology is the branch or specialty of medicine that deals with the study and application of imaging technology like x-ray and radiation to diagnosing and treating disease....
to visualize the internal structure and function of the body. MRI provides much greater
contrastContrast 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 than
computed tomographyComputed tomography is a medical imaging method employing tomography created by computer processing. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.CT...
(CT) does, making it especially useful in
neurologicalNeurology 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,...
(brain), musculoskeletal,
cardiovascularCardiovascular magnetic resonance imaging , sometimes known as cardiac MRI, is a medical imaging technology for the non-invasive assessment of the function and structure of the cardiovascular system. It is derived from and based on the same basic principles as Magnetic Resonance Imaging MRI but...
, and
oncologicalOncology is the branch of medicine dealing with tumors . A medical professional who practices oncology is an oncologist. The term originates from the Greek "Ογκολογία" derived from onkos , meaning bulk, mass, or tumor, and the suffix -logy , meaning "study of" or "to talk about".Oncology is...
(cancer) imaging. Unlike CT, it uses no
ionizing radiationIonizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
, but uses a powerful
magneticIn physics, the term magnetism is used to describe how materials respond on the microscopic level to an applied magnetic field; to categorize the magnetic phase of a material. For example, the most well known form of magnetism is ferromagnetism such that some ferromagnetic materials produce their...
field to align the
nuclear magnetizationThe 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 (usually)
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
atomThe atom is a basic unit of matter consisting 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 water in the body.
Radio frequencyRadio frequency is a frequency, or rate of oscillation, of electromagnetic radiation within the range of about 3 Hz to 300 GHz. This range corresponds to the frequency of alternating current electrical signals used to produce and detect radio waves...
(RF) fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. This signal can be manipulated by additional magnetic fields to build up enough information to construct an image of the body.{{rp|36}}
Magnetic Resonance Imaging is a relatively new technology. The first MR image was published in 1973 and the first cross-sectional image of a living mouse was published in January 1974. The first studies performed on humans were published in 1977. By comparison, the first human
X-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 10 to 0.01 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...
image was taken in 1895.
Magnetic Resonance Imaging was developed from knowledge gained in the study of
nuclear magnetic resonanceNuclear magnetic resonance is a property that magnetic nuclei have in a magnetic field and applied electromagnetic pulse, which cause the nuclei to absorb energy from the EM pulse and radiate this energy back out...
. In its early years the technique was referred to as nuclear magnetic resonance imaging (NMRI). However, as the word
nuclear was associated in the public mind with
ionizing radiationIonizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
exposure it is generally now referred to simply as MRI. Scientists still use the term NMRI when discussing non-medical devices operating on the same principles. The term Magnetic Resonance Tomography (MRT) is also sometimes used.
How MRI works
{{Unreferenced section|date=September 2009}}
The body is
largely composed of water moleculesIn medicine, body water is all of the water content of the human body. A significant fraction of the human body is water. Lean muscle tissue contains about 99% water by weight. Blood contains 2% water, body fat contains 1/2% water and bone has 97% water. Skin also contains much water...
which each contain two
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
nucleiThe nucleus is the very dense region consisting of nucleons at the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus, with a very small contribution from the orbiting electrons....
or
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
s. When a person goes inside the powerful
magnetic fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
of the scanner, the magnetic moments of these protons align with the direction of the field.
A radio frequency electromagnetic field is then briefly turned on, causing the protons to alter their alignment relative to the field. When this field is turned off the protons return to the original magnetization alignment. These alignment changes create a signal which can be detected by the scanner. The frequency the protons resonate at depends on the strength of the magnetic field. The position of protons in the body can be determined by applying additional magnetic fields during the scan which allows an image of the body to be built up. These are created by turning gradients coils on and off which creates the knocking sounds heard during an MR scan.
Diseased tissue, such as tumors, can be detected because the protons in different tissues return to their equilibrium state at different rates. By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue.
Contrast agents may be injected
intravenouslyIntravenous therapy or IV therapy is the giving of liquid substances directly into a vein. It can be intermittent or continuous; continuous administration is called an intravenous drip. The word intravenous simply means "within a vein", but is most commonly used to refer to IV therapy...
to enhance the appearance of
blood vesselThe blood vessels are the part of the circulatory system that transport 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
inflammationInflammation is 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 as well as initiate the healing process for the tissue. Inflammation is not a...
. Contrast agents may also be directly injected into a joint in the case of arthrograms, MR images of joints. Unlike
CTComputed tomography is a medical imaging method employing tomography created by computer processing. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.CT...
, MRI uses no
ionizing radiationIonizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
and is generally a very safe procedure. Patients with some metal implants,
cochlear implantA 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. The cochlear implant is often referred to as a bionic ear...
s, and cardiac pacemakers are prevented from having an MRI scan due to effects of the strong magnetic field and powerful radio frequency pulses.
MRI is used to image every part of the body, and is particularly useful for neurological conditions, for disorders of the muscles and joints, for evaluating tumors, and for showing abnormalities in the heart and blood vessels.
Nuclear magnetism
{{main article|Nuclear magnetic resonance}}
Subatomic particles have the
quantum mechanicalQuantum mechanics is a set of principles describing the physical reality at the atomic level of matter and the subatomic . These descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation...
property of
spinIn particle physics and quantum mechanics, spin is a fundamental characteristic property of elementary particles including the force carriers , composite particles , and atomic nuclei....
. Certain nuclei such as
1H (protons),
2H,
3He,
23Na or
31P, have a non–zero spin and therefore a magnetic moment. In the case of the so-called spin-
{{Redirect|MRI}}
{{mergefrom|2D-FT NMRI and Spectroscopy|discuss=Talk:Magnetic_resonance_imaging#Merge_2D-FT_NMRI_and_Spectroscopy|date=July 2009}}
Magnetic Resonance Imaging (
MRI), or
nuclear magnetic resonance imaging (
NMRI2D-FT Nuclear magnetic resonance imaging , or Two-dimensional Fourier transform nuclear magnetic resonance imaging , is primarily a non-invasive imaging technique most commonly used in biomedical research and medical radiology/nuclear medicine/MRI to visualize structures and functions of the living...
), is primarily a
medical imagingMedical imaging refers to the techniques and processes used to create images of the human body for clinical purposes or medical science .As a discipline and in its widest sense, it is part of biological imaging and incorporates...
technique most commonly used in
radiologyRadiology is the branch or specialty of medicine that deals with the study and application of imaging technology like x-ray and radiation to diagnosing and treating disease....
to visualize the internal structure and function of the body. MRI provides much greater
contrastContrast 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 than
computed tomographyComputed tomography is a medical imaging method employing tomography created by computer processing. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.CT...
(CT) does, making it especially useful in
neurologicalNeurology 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,...
(brain), musculoskeletal,
cardiovascularCardiovascular magnetic resonance imaging , sometimes known as cardiac MRI, is a medical imaging technology for the non-invasive assessment of the function and structure of the cardiovascular system. It is derived from and based on the same basic principles as Magnetic Resonance Imaging MRI but...
, and
oncologicalOncology is the branch of medicine dealing with tumors . A medical professional who practices oncology is an oncologist. The term originates from the Greek "Ογκολογία" derived from onkos , meaning bulk, mass, or tumor, and the suffix -logy , meaning "study of" or "to talk about".Oncology is...
(cancer) imaging. Unlike CT, it uses no
ionizing radiationIonizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
, but uses a powerful
magneticIn physics, the term magnetism is used to describe how materials respond on the microscopic level to an applied magnetic field; to categorize the magnetic phase of a material. For example, the most well known form of magnetism is ferromagnetism such that some ferromagnetic materials produce their...
field to align the
nuclear magnetizationThe 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 (usually)
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
atomThe atom is a basic unit of matter consisting 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 water in the body.
Radio frequencyRadio frequency is a frequency, or rate of oscillation, of electromagnetic radiation within the range of about 3 Hz to 300 GHz. This range corresponds to the frequency of alternating current electrical signals used to produce and detect radio waves...
(RF) fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. This signal can be manipulated by additional magnetic fields to build up enough information to construct an image of the body.{{rp|36}}
Magnetic Resonance Imaging is a relatively new technology. The first MR image was published in 1973 and the first cross-sectional image of a living mouse was published in January 1974. The first studies performed on humans were published in 1977. By comparison, the first human
X-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 10 to 0.01 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...
image was taken in 1895.
Magnetic Resonance Imaging was developed from knowledge gained in the study of
nuclear magnetic resonanceNuclear magnetic resonance is a property that magnetic nuclei have in a magnetic field and applied electromagnetic pulse, which cause the nuclei to absorb energy from the EM pulse and radiate this energy back out...
. In its early years the technique was referred to as nuclear magnetic resonance imaging (NMRI). However, as the word
nuclear was associated in the public mind with
ionizing radiationIonizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
exposure it is generally now referred to simply as MRI. Scientists still use the term NMRI when discussing non-medical devices operating on the same principles. The term Magnetic Resonance Tomography (MRT) is also sometimes used.
How MRI works
{{Unreferenced section|date=September 2009}}
The body is
largely composed of water moleculesIn medicine, body water is all of the water content of the human body. A significant fraction of the human body is water. Lean muscle tissue contains about 99% water by weight. Blood contains 2% water, body fat contains 1/2% water and bone has 97% water. Skin also contains much water...
which each contain two
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
nucleiThe nucleus is the very dense region consisting of nucleons at the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus, with a very small contribution from the orbiting electrons....
or
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
s. When a person goes inside the powerful
magnetic fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
of the scanner, the magnetic moments of these protons align with the direction of the field.
A radio frequency electromagnetic field is then briefly turned on, causing the protons to alter their alignment relative to the field. When this field is turned off the protons return to the original magnetization alignment. These alignment changes create a signal which can be detected by the scanner. The frequency the protons resonate at depends on the strength of the magnetic field. The position of protons in the body can be determined by applying additional magnetic fields during the scan which allows an image of the body to be built up. These are created by turning gradients coils on and off which creates the knocking sounds heard during an MR scan.
Diseased tissue, such as tumors, can be detected because the protons in different tissues return to their equilibrium state at different rates. By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue.
Contrast agents may be injected
intravenouslyIntravenous therapy or IV therapy is the giving of liquid substances directly into a vein. It can be intermittent or continuous; continuous administration is called an intravenous drip. The word intravenous simply means "within a vein", but is most commonly used to refer to IV therapy...
to enhance the appearance of
blood vesselThe blood vessels are the part of the circulatory system that transport 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
inflammationInflammation is 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 as well as initiate the healing process for the tissue. Inflammation is not a...
. Contrast agents may also be directly injected into a joint in the case of arthrograms, MR images of joints. Unlike
CTComputed tomography is a medical imaging method employing tomography created by computer processing. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.CT...
, MRI uses no
ionizing radiationIonizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
and is generally a very safe procedure. Patients with some metal implants,
cochlear implantA 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. The cochlear implant is often referred to as a bionic ear...
s, and cardiac pacemakers are prevented from having an MRI scan due to effects of the strong magnetic field and powerful radio frequency pulses.
MRI is used to image every part of the body, and is particularly useful for neurological conditions, for disorders of the muscles and joints, for evaluating tumors, and for showing abnormalities in the heart and blood vessels.
Nuclear magnetism
{{main article|Nuclear magnetic resonance}}
Subatomic particles have the
quantum mechanicalQuantum mechanics is a set of principles describing the physical reality at the atomic level of matter and the subatomic . These descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation...
property of
spinIn particle physics and quantum mechanics, spin is a fundamental characteristic property of elementary particles including the force carriers , composite particles , and atomic nuclei....
. Certain nuclei such as
1H (protons),
2H,
3He,
23Na or
31P, have a non–zero spin and therefore a magnetic moment. In the case of the so-called spin-{{frac nuclei, such as
1H, there are two spin states, sometimes referred to as "up" and "down". Nuclei such as
12C have no unpaired neutrons or protons, and no net spin; however, the isotope
13C does.
When these spins are placed in a strong external magnetic field they
precessPrecession refers to a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
around an axis along the direction of the field. Protons align in two energy "
eigenstatesIn mathematics, eigenvalue, eigenvector, and eigenspace are related concepts in the field of linear algebra. Linear algebra studies linear transformations, which are represented by matrices acting on vectors. Eigenvalues, eigenvectors and eigenspaces are properties of a matrix...
" (the "
Zeeman effectThe Zeeman effect is the splitting of a spectral line into several components in the presence of a static magnetic field. It is analogous to the Stark effect, the splitting of a spectral line into several components in the presence of an electric field...
"): one low-energy and one high-energy, which are separated by a very small splitting energy.
Resonance and relaxation
{{main|Relaxation (NMR)}}
{{Unreferenced section|date=September 2009}}
In the static magnetic fields commonly used in MRI, the energy difference between the nuclear spin states corresponds to a radio frequency
photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
. Resonant absorption of energy by the protons due to an external oscillating magnetic field will occur at the
Larmor frequencyIn physics, Larmor precession is the precession of the magnetic moments of electrons, atomic nuclei, and atoms about an external magnetic field...
for the particular nucleus.
The net magnetization vector has two states. The longitudinal magnetization is due to a tiny excess of protons in the lower energy state. This gives a net polarization parallel to the external field. Application of an RF pulse can tip sideways (with i.e. a so-called 90° pulse) or even reverse (with a so-called 180° pulse) this net polarization vector. The transverse magnetization is due to quantum mechanical behavior not so far described in this article.
The recovery of longitudinal magnetization is called longitudinal or
T1 relaxationIn 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...
and occurs exponentially with a time constant
T1 The loss of phase coherence in the transverse plane is called transverse or
T2 relaxation.
T1 is thus associated with the
enthalpyIn thermodynamics and molecular chemistry, the enthalpy is a thermodynamic property of a thermodynamic system. It can be used to calculate the heat transfer during a quasistatic process taking place in a closed thermodynamic system under constant pressure...
of the spin system (the number of nuclei with parallel versus anti-parallel spin) while
T2 is associated with its
entropyEntropy is a concept of information maintaining great importance in physics, chemistry, and information theory...
(the number of nuclei in phase).
When the radio frequency pulse is turned off, the transverse vector component produces an oscillating magnetic field which induces a small current in the receiver coil. This signal is called the
free induction decayIn Fourier Transform NMR, a free induction decay is the observable NMR signal generated by non-equilibrium nuclear spin magnetisation precessing about the magnetic field ....
(FID). In an idealized
nuclear magnetic resonanceNuclear magnetic resonance is a property that magnetic nuclei have in a magnetic field and applied electromagnetic pulse, which cause the nuclei to absorb energy from the EM pulse and radiate this energy back out...
experiment, the FID decays approximately exponentially with a time constant
T2, but in practical MRI small differences in the static magnetic field at different spatial locations ("inhomogeneities") cause the Larmor frequency to vary across the body creating destructive interference which shortens the FID. The time constant for the observed decay of the FID is called the
T{{su|p=*|b=2}} relaxation time, and is always shorter than
T2. Also, when the radio frequency pulse is turned off, the longitudinal magnetization starts to recover exponentially with a time constant
T1.
In MRI, the static magnetic field is caused to vary across the body (a field gradient), so that different spatial locations become associated with different precession frequencies. Usually these field gradients are pulsed, and it is the almost infinite variety of RF and gradient pulse sequences that gives MRI its versatility. Application of field gradient destroys the FID signal, but this can be recovered and measured by a refocusing gradient (to create a so-called "gradient echo"), or by a radio frequency pulse (to create a so-called "spin-echo"). The whole process can be repeated when some
T1-relaxation has occurred and the thermal equilibrium of the spins has been more or less restored.
Typically in soft tissues
T1 is around one second while
T2 and
T{{su|p=*|b=2}} are a few tens of milliseconds, but these values vary widely between different tissues (and different external magnetic fields), giving MRI its tremendous soft tissue contrast.
Contrast agents work by altering (shortening) the relaxation parameters, especially
T1.
Imaging
A number of schemes have been devised for combining field gradients and radio frequency excitation to create an image:
- 2D or 3D reconstruction from projections, much as in Computed Tomography
Computed tomography is a medical imaging method employing tomography created by computer processing. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.CT...
.
- Building the image point-by-point or line-by-line.
- Gradients in the RF field rather than the static field.
Although each of these schemes is occasionally used in specialist applications, the majority of MR Images today are created either by the
Two-Dimensional Fourier Transform (2DFT2D-FT Nuclear magnetic resonance imaging , or Two-dimensional Fourier transform nuclear magnetic resonance imaging , is primarily a non-invasive imaging technique most commonly used in biomedical research and medical radiology/nuclear medicine/MRI to visualize structures and functions of the living...
) technique with slice selection, or by the Three-Dimensional Fourier Transform (3DFT) technique. Another name for 2DFT is spin-warp. What follows here is a description of the 2DFT technique with slice selection.
The 3DFT technique is rather similar except that there is no slice selection and phase-encoding is performed in two separate directions.
Another scheme which is sometimes used, especially in brain scanning or where images are needed very rapidly, is called echo-planar imaging (EPI): In this case, each RF excitation is followed by a train of gradient echoes with different spatial encoding.
Image contrast and contrast enhancement
Image
contrastContrast 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...
is created by differences in the strength of the NMR signal recovered from different locations within the sample. This depends upon the relative density of excited nuclei (usually water protons), on differences in relaxation times (
T1,
T2, and
T{{su|p=*|b=2}}) of those nuclei after the pulse sequence, and often on other parameters discussed under specialized MR scans. Contrast in most MR images is actually a mixture of all these effects, but careful design of the imaging pulse sequence allows one contrast mechanism to be emphasized while the others are minimized. The ability to choose different contrast mechanisms gives MRI tremendous flexibility. In the brain,
T1-weighting causes the nerve connections of
white matterWhite matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...
to appear white, and the congregations of neurons of
gray matter"Gray Matter" is a short story by Stephen King, first published in the October 1973 issue of Cavalier magazine, and later collected in King's collection Night Shift...
to appear gray, while
cerebrospinal fluid (CSF)Cerebrospinal fluid , Liquor cerebrospinalis, is a clear bodily fluid that occupies the subarachnoid space and the ventricular system around and inside the brain. In essence, the brain "floats" in it....
appears dark. The contrast of white matter, gray matter and cerebrospinal fluid is reversed using
T2 or
T{{su|p=*|b=2}} imaging, whereas proton-density-weighted imaging provides little contrast in healthy subjects. Additionally, functional parameters such as
cerebral blood flow (CBF)Cerebral blood flow, or CBF, is the blood supply to the brain in a given time. In an adult, CBF is typically 750 millitres per minute or 15% of the cardiac output. This equates to 50 to 54 millilitres of blood per 100 grams of brain tissue per minute. CBF is tightly regulated to meet the brain's...
, cerebral blood volume (CBV) or blood oxygenation can affect
T1,
T2 and
T{{su|p=*|b=2}} and so can be encoded with suitable pulse sequences.
In some situations it is not possible to generate enough image contrast to adequately show the
anatomyAnatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy and plant anatomy...
or
pathologyPathology is the study and diagnosis of disease through examination of organs, tissues, bodily fluids, and whole bodies...
of interest by adjusting the imaging parameters alone, in which case a
contrast agentA medical contrast medium is a substance used to enhance the contrast of structures or fluids within the body in medical imaging...
may be administered. This can be as simple as
waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
, taken orally, for imaging the stomach and small bowel. However, most
contrast agents used in MRIMRI 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...
are selected for their specific magnetic properties. Most commonly, a paramagnetic contrast agent (usually a
gadoliniumGadolinium is a chemical element that has the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. Gadolinium has exceptionally high absorption of neutrons and therefore is used for shielding in neutron radiography and in nuclear reactors...
compound) is given. Gadolinium-enhanced tissues and fluids appear extremely bright on
T1-weighted images. This provides high sensitivity for detection of vascular tissues (
e.g., tumors) and permits assessment of brain perfusion (
e.g., in stroke). There have been concerns raised recently regarding the toxicity of gadolinium-based contrast agents and their impact on persons with impaired kidney function. The
American College of RadiologyThe American College of Radiology , founded in 1923, is a non-profit professional medical organization composed of diagnostic radiologists, radiation oncologists, interventional radiologists, nuclear medicine physicians, and medical physicists. It is based in Reston, Virginia, with offices in...
released
screening criteria for patients intended to be given gadolinium-based contrast agents to identify potential risk factors for negative reactions. Special actions may be taken, such as
hemodialysisIn medicine, hemodialysis is a method for removing waste products such as potassium and urea, as well as free water from the blood when the kidneys are in renal failure. Hemodialysis is one of three renal replacement therapies .Hemodialysis can be an outpatient or inpatient therapy...
following a contrast MRI scan for
renally-impairedRenal failure or kidney failure is a situation in which the kidneys fail to function adequately. It is divided into acute and chronic forms; either form may be due to a large number of other medical problems....
patients.
More recently,
superparamagneticSuperparamagnetism is a form of magnetism, which appear in small ferromagnetic or ferrimagnetic nanoparticles. In small enough nanoparticles, magnetization can randomly flip direction under the influence of temperature. The typical time between two flips is called the Néel relaxation time...
contrast agents,
e.g.,
iron oxideIron oxides are chemical compounds composed of iron and oxygen. Altogether, there are sixteen known iron oxides and oxyhydroxides.-Oxides:* FeO, iron oxide, * Fe3O4, iron oxide,...
nanoparticleIn nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties. It is further classified according to size: in terms of diameter, fine particles cover a range between 100 and 2500 nanometers, while ultrafine particles, on the other...
s, have become available. These agents appear very dark on
T{{su|p;=*|b=2}}-weighted images and may be used for liver imaging, as normal
liverThe liver is a vital organ present in vertebrates and some other animals; it has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
tissue retains the agent, but abnormal areas (
e.g., scars, tumors) do not. They can also be taken orally, to improve visualization of the
gastrointestinal tractThe human gastrointestinal tract , digestive tract, guts or gut is the system of organs within humans that takes in food, digests it to extract energy and nutrients, and expels the remaining matter...
, and to prevent water in the gastrointestinal tract from obscuring other organs (
e.g., the
pancreasThe pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to...
).
DiamagneticDiamagnetism is the property of an object which causes it to create a magnetic field in opposition of an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the...
agents such as
barium sulfateBarium sulfate is a white crystalline solid with the chemical formula BaSO4. It is insoluble in water and other traditional solvents but is soluble in concentrated sulfuric acid. The mineral barite is composed largely of barium sulfate and is a common ore of barium...
have also been studied for potential use in the
gastrointestinal tractThe human gastrointestinal tract , digestive tract, guts or gut is the system of organs within humans that takes in food, digests it to extract energy and nutrients, and expels the remaining matter...
, but are less frequently used.
K-space
{{main article|K-space (MRI)}}
In 1983 Ljunggren and Tweig independently introduced the k-space formalism, a technique that proved invaluable in unifying different MR imaging techniques. They showed that the demodulated MR signal
S(
t) generated by freely precessing nuclear spins in the presence of a linear magnetic field gradient
G equals the Fourier transform of the effective spin density. Mathematically:
where:
In other words, as time progresses the signal traces out a trajectory in k-space with the velocity vector of the trajectory proportional to the vector of the applied magnetic field gradient.
By the term
effective spin density we mean the true spin density corrected for the effects of
T1 preparation,
T2 decay, dephasing due to field inhomogeneity, flow, diffusion,
etc. and any other phenomena that affect that amount of transverse magnetization available to induce signal in the RF probe.
From the basic k-space formula, it follows immediately that we reconstruct an image simply by taking the inverse Fourier transform of the sampled data,
viz.
Using the k-space formalism, a number of seemingly complex ideas became simple. For example, it becomes very easy to understand the role of phase encoding (the so-called spin-warp method). In a standard spin echo or gradient echo scan, where the readout (or view) gradient is constant (
e.g. G), a single line of k-space is scanned per RF excitation. When the phase encoding gradient is zero, the line scanned is the
kx axis. When a non-zero phase-encoding pulse is added in between the RF excitation and the commencement of the readout gradient, this line moves up or down in k-space,
i.e., we scan the line
ky = constant.
The k-space formalism also makes it very easy to compare different scanning techniques. In single-shot EPI, all of k-space is scanned in a single shot, following either a sinusoidal or zig-zag trajectory. Since alternating lines of k-space are scanned in opposite directions, this must be taken into account in the reconstruction. Multi-shot EPI and fast spin echo techniques acquire only part of k-space per excitation. In each shot, a different interleaved segment is acquired, and the shots are repeated until k-space is sufficiently well-covered. Since the data at the center of k-space represent lower spatial frequencies than the data at the edges of k-space, the
TE value for the center of k-space determines the image's
T2 contrast.
The importance of the center of k-space in determining image contrast can be exploited in more advanced imaging techniques. One such technique is spiral acquisition - a rotating magnetic field gradient is applied, causing the trajectory in k-space to spiral out from the center to the edge. Due to
T2 and
T{{su|p=*|b=2}} decay the signal is greatest at the start of the acquisition, hence acquiring the center of k-space first improves
contrast to noise ratio (CNR) when compared to conventional zig-zag acquisitions, especially in the presence of rapid movement.
Since and are conjugate variables (with respect to the Fourier transform) we can use the Nyquist theorem to show that the step in k-space determines the field of view of the image (maximum frequency that is correctly sampled) and the maximum value of k sampled determines the resolution,
i.e.
(These relationships apply to each axis independently.)
Example of a pulse sequence
{{Unreferenced section|date=September 2009}}
In the timing diagram, the horizontal axis represents time. The vertical axis represents: (top row) amplitude of radio frequency pulses; (middle rows) amplitudes of the three orthogonal magnetic field gradient pulses; and (bottom row) receiver analog-to-digital converter (ADC). Radio frequencies are transmitted at the Larmor frequency of the nuclide to be imaged. For example, for
1H in a magnetic field of 1 T, a frequency of 42.5781 MHz would be employed. The three field gradients are labeled
GX (typically corresponding to a patient's left-to-right direction and colored red in diagram),
GY (typically corresponding to a patient's front-to-back direction and colored green in diagram), and
GZ (typically corresponding to a patient's head-to-toe direction and colored blue in diagram). Where negative-going gradient pulses are shown, they represent reversal of the gradient direction,
i.e., right-to-left, back-to-front or toe-to-head. For human scanning, gradient strengths of 1–100 mT/m are employed: Higher gradient strengths permit better resolution and faster imaging. The pulse sequence shown here would produce a transverse (axial) image.
The first part of the pulse sequence, SS, achieves 'slice selection'. A shaped pulse (shown here with a sinc modulation) causes a 90° nutation of longitudinal nuclear magnetization within a slab, or slice, creating transverse magnetization. The second part of the pulse sequence, PE, imparts a phase shift upon the slice-selected nuclear magnetization, varying with its location in the Y direction. The third part of the pulse sequence, another slice selection (of the same slice) uses another shaped pulse to cause a 180° rotation of transverse nuclear magnetization within the slice. This transverse magnetisation refocuses to form a spin echo at a time TE. During the spin echo, a frequency-encoding (FE) or readout gradient is applied, making the resonant frequency of the nuclear magnetization vary with its location in the X direction. The signal is sampled
nFE times by the ADC during this period, as represented by the vertical lines. Typically
nFE of between 128 and 512 samples are taken.
The longitudinal magnetisation is then allowed to recover somewhat and after a time TR the whole sequence is repeated
nPE times, but with the phase-encoding gradient incremented (indicated by the horizontal hatching in the green gradient block). Typically
nPE of between 128 and 512 repetitions are made.
The negative-going lobes in
GX and
GZ are imposed to ensure that, at time TE (the spin echo maximum), phase only encodes spatial location in the Y direction.
Typically TE is between 5 ms and 100 ms, while TR is between 100 ms and 2000 ms.
After the two-dimensional matrix (typical dimension between 128 × 128 and 512 × 512) has been acquired, producing the so-called K-space data, a two-dimensional Fourier transform is performed to provide the familiar MR image. Either the magnitude or phase of the Fourier transform can be taken, the former being far more common.
Scanner construction and operation
The major components of an MRI scanner are: the main magnet, which polarizes the sample, the shim coils for correcting inhomogeneities in the main magnetic field, the gradient system which is used to localize the MR signal and the RF system, which excites the sample and detects the resulting NMR signal. The whole system is controlled by one or more computers.
Magnet
The magnet is the largest and most expensive component of the scanner, and the remainder of the scanner is built around it. The strength of the magnet is measured in
tesla (T)The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honor of the Serbian-American inventor, physicist, and electrical engineer Nikola Tesla...
. Clinical magnets generally have a field strength in the range 0.1—3.0 T, with research systems available up to 9.4 T for human use and 21 T for animal systems.
Just as important as the strength of the main magnet is its precision. The straightness of the magnetic lines within the center (or, as it is technically known, the iso-center) of the magnet needs to be near-perfect. This is known as homogeneity. Fluctuations (inhomogeneities in the field strength) within the scan region should be less than three parts per million (3 ppm). Three types of magnets have been used:
- Permanent magnet: Conventional magnets made from ferromagnetic materials (e.g., steel alloys containing rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a collection of seventeen chemical elements in the periodic table, namely scandium, yttrium, and the fifteen lanthanoids...
s such as neodymiumNeodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal which tarnishes in air. Neodymium was discovered in 1885. Its various compounds are present in trace amounts in minerals monazite and bastnäsite. Pure neodymium is not found naturally and it is...
) can be used to provide the static magnetic field. A permanent magnet that is powerful enough to be used in an MRI will be extremely large and bulky; they can weigh over 100 tonnes. Permanent magnet MRIs are very inexpensive to maintain; this cannot be said of the other types of MRI magnets, but there are significant drawbacks to using permanent magnets. They are only capable of achieving weak field strengths compared to other MRI magnets (usually less than 0.4 T) and they are of limited precision and stability. Permanent magnets also present special safety issues; since their magnetic fields cannot be "turned off," ferromagnetic objects are virtually impossible to remove from them once they come into direct contact. Permanent magnets also require special care when they are being brought to their site of installation.
- Resistive electromagnet: A solenoid
A solenoid is a three-dimensional coil. In physics, the term solenoid refers to a loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it...
wound from copper wire is an alternative to a permanent magnet. An advantage is low initial cost, but field strength and stability are limited. The electromagnet requires considerable electrical energy during operation which can make it expensive to operate. This design is essentially obsolete.
- Superconducting electromagnet
A superconducting magnet is an electromagnet that is built using coils of superconducting wire. They must be cooled to cryogenic temperatures during operation...
: When a niobium-titaniumNiobium-titanium is an alloy of niobium and titanium, used industrially as a type II superconductor wire for superconducting magnets...
or niobium-tinNiobium-tin or triniobium-tin is a metallic chemical compound of niobium and tin , used industrially as a type II superconductor...
alloy is cooled by liquid heliumHelium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atm is approximately 0.125 g/mL Helium-4 was first liquefied on 10...
to 4 K (−269 °C, −452 °F) it becomes a superconductor, losing resistance to flow of electrical current. An electromagnet constructed with superconductors can have extremely high field strengths, with very high stability. The construction of such magnets is extremely costly, and the cryogenic helium is expensive and difficult to handle. However, despite their cost, helium cooled superconducting magnets are the most common type found in MRI scanners today.
Most superconducting magnets have their coils of superconductive wire immersed in liquid helium, inside a vessel called a
cryostatA Cryostat is a vessel, similar in construction to a vacuum flask, or Dewar used to maintain cold cryogenic temperatures.-Magnetic Resonance Imaging and Research magnet types:...
. Despite thermal insulation, ambient heat causes the helium to slowly boil off. Such magnets, therefore, require regular topping-up with liquid helium. Generally a
cryocoolerCryocoolers are the devices used to reach cryogenic temperatures by cycling certain gases.A cryostat is likely to be used to reach and/or maintain similar conditions or keep some environment in cryogenic stasis.-See also:* Cryogenic processor...
, also known as a coldhead, is used to recondense some helium vapor back into the liquid helium bath. Several manufacturers now offer 'cryogenless' scanners, where instead of being immersed in liquid helium the magnet wire is cooled directly by a cryocooler.
Magnets are available in a variety of shapes. However, permanent magnets are most frequently 'C' shaped, and superconducting magnets most frequently cylindrical. However, C-shaped superconducting magnets and box-shaped permanent magnets have also been used.
Magnetic field strength is an important factor in determining image quality. Higher magnetic fields increase
signal-to-noise ratioSignal-to-noise ratio is an electrical engineering measurement, also used in other fields , defined as the ratio of a signal power to the noise power corrupting the signal...
, permitting higher resolution or faster scanning. However, higher field strengths require more costly magnets with higher maintenance costs, and have increased safety concerns. A field strength of 1.0–1.5 T is a good compromise between cost and performance for general medical use. However, for certain specialist uses (
e.g., brain imaging) higher field strengths are desirable, with some hospitals now using 3.0 T scanners.
Shims
When MR scanner is placed in the hospital its main magnetic field is far from being homogeneous enough to be used for scanning. That is why before doing fine turning of field using sample magnetic field of the magnet must be measured and shimmed.
Afterward a sample is placed into the scanner, the main magnetic field is distorted by
susceptibilityIn electromagnetism the magnetic susceptibility is the degree of magnetization of a material in response to an applied magnetic field....
boundaries within that sample, causing signal dropout (regions showing no signal) and spatial distortions in acquired images. For humans or animals the effect is particularly pronounced at air-tissue boundaries such as the sinuses (due to paramagnetic oxygen in air) making, for example, the frontal lobes of the brain difficult to image. To restore field homogeneity a set of shim coils is included in the scanner. These are resistive coils, usually at room temperature, capable of producing field corrections distributed as several orders of
spherical harmonicsIn mathematics, the spherical harmonics are the angular portion of a set of solutions to Laplace's equation. Represented in a system of spherical coordinates, Laplace's spherical harmonics are a specific set of spherical harmonics that forms an orthogonal system, first introduced by Pierre Simon...
.
After placing the sample in the scanner, the
B0 field is 'shimmed' by adjusting currents in the shim coils. Field homogeneity is measured by examining an
FIDIn Fourier Transform NMR, a free induction decay is the observable NMR signal generated by non-equilibrium nuclear spin magnetisation precessing about the magnetic field ....
signal in the absence of field gradients. The FID from a poorly shimmed sample will show a complex decay envelope, often with many humps. Shim currents are then adjusted to produce a large amplitude exponentially decaying FID, indicating a homogeneous
B0 field. The process is usually automated.
Gradients
Gradient coils are used to spatially encode the positions of protons by varying the magnetic field linearly across the imaging volume. The Larmor frequency will then vary as a function of position in the x, y and z-axes.
Gradient coils are usually resistive electromagnets powered by sophisticated amplifiers which permit rapid and precise adjustments to their field strength and direction. Typical gradient systems are capable of producing gradients from 20 mT/m to 100 mT/m (
i.e., in a 1.5 T magnet, when a maximal z-axis gradient is applied, the field strength may be 1.45 T at one end of a 1 m long bore and 1.55 T at the other). It is the magnetic gradients that determine the plane of imaging - because the orthogonal gradients can be combined freely, any plane can be selected for imaging.
Scan speed is dependent on performance of the gradient system. Stronger gradients allow for faster imaging, or for higher resolution; similarly, gradients systems capable of faster switching can also permit faster scanning. However, gradient performance is limited by safety concerns over nerve stimulation.
Some important characteristics of gradient amplifiers and gradient coils are slew rate and gradient strength. As mentioned earlier, a gradient coil will create an additional, linearly varying magnetic field that adds or subtracts from the main magnetic field. This additional magnetic field will have components in all 3 directions,
viz. X, Y and Z; however, only the component along the magnetic field (usually called the Z-axis, hence denoted
Gz) is useful for imaging. Along any given axis, the gradient will add to the magnetic field on one side of the zero position and subtract from it on the other side. Since the additional field is a gradient, it has units of
gaussThe gauss, abbreviated as G, is the cgs unit of measurement of a magnetic field B , named after the German mathematician and physicist Carl Friedrich Gauss. One gauss is defined as one maxwell per square centimeter.-Unit name and convention:This unit is named after Carl Friedrich Gauss...
per cm or millitesla (mT) per meter. High performance gradient coils used in MRI are typically capable of producing a gradient magnetic field of approximate 30 mT per meter or higher for a 1.5 T MRI. The slew rate of a gradient system is a measure of how quickly the gradients can be ramped on or off. Typical higher performance gradients have a slew rate of up to 100–200 T·m
−1·s
−1. The slew rate depends both on the gradient coil (it takes more time to ramp up or down a large coil than a small coil) and on the performance of the gradient amplifier (it takes a lot of voltage to overcome the inductance of the coil) and has adequate influence on image quality.
Radio frequency system
The radio frequency (RF) transmission system consists of an RF synthesizer, power amplifier and transmitting coil. This is usually built into the body of the scanner. The power of the transmitter is variable, but high-end whole-body scanners may have a peak output power of up to 35 kW, and be capable of sustaining average power of 1 kW. The receiver consists of the coil, pre-amplifier and signal processing system. While it is possible to scan using the integrated coil for RF transmission and MR signal reception, if a small region is being imaged, then better image quality (i.e. higher signal-to-noise ratio) is obtained by using a close-fitting smaller coil. A variety of coils are available which fit closely around parts of the body such as the head, knee, wrist, breast, or internally, e.g. the rectum.
A recent development in MRI technology has been the development of sophisticated multi-element phased array coils which are capable of acquiring multiple channels of data in parallel. This 'parallel imaging' technique uses unique acquisition schemes that allow for accelerated imaging, by replacing some of the spatial coding originating from the magnetic gradients with the spatial sensitivity of the different coil elements. However, the increased acceleration also reduces the signal-to-noise ratio and can create residual artifacts in the image reconstruction. Two frequently used parallel acquisition and reconstruction schemes are known as SENSE and GRAPPA. A detailed review of parallel imaging techniques can be found here:
Applications
{{Unreferenced section|date=September 2009}}
In clinical practice, MRI is used to distinguish pathologic tissue (such as a
brain tumorA brain tumor is an abnormal growth of cells within the brain or inside the skull, which can be cancerous or non-cancerous .It is defined as any intracranial tumor created by abnormal and uncontrolled cell division, normally either in the brain itself , in the cranial nerves...
) from normal tissue. One advantage of an MRI scan is that it is believed to be harmless to the patient. It uses strong magnetic fields and non-ionizing radiation in the radio frequency range. Compare this to CT scans and
traditional X-raysRadiography is the use of the property of X-rays to cross materials to view inside objects. The impact on society of this technique has also been immense : application fields are medical, non-destructive testing, food inspection, security, archeology, ......
which involve doses of
ionizing radiationIonizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
and may increase the risk of malignancy, especially in a fetus.
While CT provides good spatial resolution (the ability to distinguish two structures an arbitrarily small distance from each other as separate), MRI provides comparable resolution with far better
contrast resolutionContrast or is referred to as the ability of an imaging modality such as magnetic resonance imaging or fluoroscopy to distinguish between various structures of an acquired image by the perception of differences in their apparent signal intensities....
(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 will take on the property of -weighting. On a -weighted scan, water- and fluid-containing tissues are bright (most modern sequences are actually
fast sequences) and fat-containing tissues are dark. The reverse is true for -weighted images. Damaged tissue tends to develop
edemaEdema or oedema , formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin, or in one or more cavities of the body...
, which makes a -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 -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
demyelinatingMyelin 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...
diseases, such as
multiple sclerosisMultiple sclerosis is an idiopathic disease of suspected autoimmune cause, in which the body's immune response attacks a person's central nervous system , leading to demyelination. Disease onset usually occurs in young adults, and it is more common in females...
.
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
physicianA physician — also known as medical practitioner, doctor of medicine, medical doctor, or simply doctor — practices the ancient profession of medicine, which is concerned with maintaining or restoring human health through the study, diagnosis, and treatment of disease or injury...
.
-weighted MRI
-weighted scans 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 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 reducing gadolinium contrast agent is also commonly used, with a scan being collected before and after administration of contrast agent to compare the difference. In the brain -weighted scans provide good gray matter/white matter contrast.
-weighted MRI
-weighted scans use a
spin echoIn nuclear magnetic resonance, spin echo refers to the refocusing of precessing nuclear spin magnetisation by a 180° pulse of resonant radiofrequency....
(SE) sequence, with long TE and long TR. They have long been the clinical workhorse as the spin echo sequence is less susceptible to inhomogeneities in the magnetic field. They are particularly well suited to edema as they are sensitive to water content (edema is characterized by increased water content).
*-weighted MRI
- (pronounced "T 2 star") 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 so it is subject to additional losses above the normal decay (referred to as '), these taken together are called *. 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 or decay, the only signal change coming from differences in the amount of available spins. It uses a spin echo or sometimes a gradient echo sequence, with short TE and long TR.
Diffusion MRI
{{Main|Diffusion MRI}}
Diffusion MRIDiffusion MRI is a magnetic resonance imaging method that produces in vivo images of biological tissues weighted with the local microstructural characteristics of water diffusion...
measures the
diffusionMolecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material...
of water molecules in biological tissues. In an isotropic medium (inside a glass of water for example) water molecules naturally move randomly according to
Brownian motionBrownian motion is the seemingly random movement of particles suspended in a fluid or the mathematical model used to describe such random movements, often called a particle theory....
. In biological tissues however, the diffusion may be anisotropic. For example a molecule inside the
axonAn axon or nerve fiber is a long, slender projectionof a nerve cell, or neuron, that conducts electrical impulsesaway from the neuron's cell body or soma....
of a neuron has a low probability of crossing the
myelinMyelin 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...
membrane. Therefore the molecule will move principally along the axis of the neural fiber. If we know that molecules in a particular
voxelA 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 we can make the assumption 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
tractographyIn 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 SclerosisMultiple sclerosis is an idiopathic disease of suspected autoimmune cause, in which the body's immune response attacks a person's central nervous system , leading to demyelination. Disease onset usually occurs in young adults, and it is more common in females...
.
Another application of diffusion MRI is diffusion-weighted imaging (DWI). Following an ischemic
strokeA stroke 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 thrombosis or embolism or due to 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 tomographyComputed tomography is a medical imaging method employing tomography created by computer processing. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.CT...
, 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}}
{{Unreferenced section|date=September 2009}}
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, 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 tesla). This also results in slower transverse magnetization dephasing and hence longer . Conversely, hydration water molecules are slowed down by interaction with solute molecules and hence create field inhomogeneities that lead to wider resonance frequency spectrum.
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), signal from any particular tissue can be suppressed.
Magnetic resonance angiography
{{main|Magnetic resonance angiography}}
{{Unreferenced section|date=September 2009}}
Magnetic resonance angiography (MRA) is used to generate pictures of the arteries in order to evaluate them for
stenosisA 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
aneurysmAn aneurysm or aneurism , is a localized, blood-filled dilation of a blood vessel caused by disease or weakening of the vessel wall....
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 (
gadoliniumGadolinium is a chemical element that has the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. Gadolinium has exceptionally high absorption of neutrons and therefore is used for shielding in neutron radiography and in nuclear reactors...
) 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 which has recently moved into that plane, see also
FLASH MRIFLASH MRI is a basic measuring principle for rapid MRI invented in 1985 by Jens Frahm, Axel Haase 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, and thus imaging the venous blood which has 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}}
{{main|2D-FT NMRI and Spectroscopy}}
Magnetic resonance spectroscopyIn vivo magnetic resonance spectroscopy is a specialised technique associated with magnetic resonance imaging .-Uses:...
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, as well as to provide information on tumor
metabolismMetabolism is the set of chemical reactions that occur in living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter,...
.
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
SNRSignal-to-noise ratio is an electrical engineering measurement, also used in other fields , defined as the ratio of a signal power to the noise power corrupting the signal...
), 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 (1.5T and above).
Functional MRI
{{main|Functional magnetic resonance imaging}}
Functional MRI (fMRI) measures signal changes in the
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
that are due to changing
neuralA neuron is an excitable cell in the nervous system that processes and transmits information by electrochemical signaling. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves...
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 * 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
vascularThe blood vessels are the part of the circulatory system that transport 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
hemoglobinHemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates, and 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.
Interventional MRI
{{main|Interventional MRI}}
{{Unreferenced section|date=September 2009}}
The lack of harmful effects on the patient and the operator make MRI well-suited for "interventional radiology", 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
{{Unreferenced section|date=September 2009}}
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 which, when combined with the specific body position, will permit precise triangulation for radiation therapy.
Current density imaging
{{Unreferenced section|date=September 2009}}
Current density imagingCurrent 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. To date no successful CDI has been performed using biological currents, but several studies have been published which involve applied currents through a pair of electrodes.
Magnetic resonance guided focused ultrasound
{{Unreferenced section|date=September 2009}}
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, completely destroying it. This technology can achieve precise "
ablationAblation means removal of material from the surface of an object by vaporization, chipping, or other erosive processes. The term occurs in space physics associated with atmospheric reentry, in glaciology, medicine, and passive fire protection.-Space physics:...
" 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
{{Unreferenced section|date=September 2009}}
Hydrogen is the most frequently imaged nucleus in MRI because it is present in biological tissues in great abundance. However, any nucleus which has a net nuclear spin could potentially be imaged with MRI. Such nuclei include
heliumHelium is the chemical element with atomic number 2, and 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,
carbonCarbon 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,
fluorineFluorine is the chemical element with atomic number 9, represented by the symbol F. Fluorine forms a single bond with itself in elemental form, resulting in the diatomic F
2 molecule. F
2 is a supremely reactive, poisonous, pale, yellowish brown gas. Elemental fluorine is the...
-19,
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
-17,
sodiumSodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
-23,
phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate rocks. Elemental phosphorus exists in two major forms - white phosphorus and red phosphorus...
-31 and
xenonXenon is a chemical element represented by the symbol Xe. Its atomic number is 54. 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
hyperpolarizedHyperpolarization is the nuclear spin polarization of a material far beyond thermal equilibrium conditions. It is commonly applied to gases such as 129Xe, 3He which are then used, for instance, in hyperpolarized magnetic resonance imaging....
and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions.
17O,
13C and
19F can be administered in sufficient quantities in liquid form (e.g.
17O-water,
13C-
glucoseGlucose , a monosaccharide also known as - grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology. The living cell uses it as a source of energy and metabolic intermediate...
solutions or perfluorocarbons) 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)
{{main|Susceptibility weighted imaging}}
Susceptibility weighted imaging (SWI), is a new type of contrast in MRI different from spin density, , or 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
MRI is a new and active field of research and new methods and variants are often published when they are able to get better results in specific fields. Examples of these recent improvements are
*-weightedIn 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 ( 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 NMRNuclear magnetic resonance in the geomagnetic field is conventionally referred to as Earth's field NMR . Note that the same acronym is used for electric 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, requiring long scan times to capture spectroscopic data or build up MRI images.
Research with
atomic magnetometerA magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument. Magnetism varies from place to place and differences in Earth's magnetic field can be caused by the differing nature of rocks and the interaction...
s have discussed the possibility for cheap and portable MRI instruments without the large magnet.
MRI versus CT
A
computed tomographyComputed tomography is a medical imaging method employing tomography created by computer processing. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.CT...
(CT) scanner uses
X-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 10 to 0.01 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...
s, a type of
ionizing radiationIonizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
, to acquire its 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 calcifications (calcium based) within the body (carbon based flesh), or of structures (vessels, bowel). MRI, on the other hand, uses non-ionizing
radio frequencyRadio frequency is a frequency, or rate of oscillation, of electromagnetic radiation within the range of about 3 Hz to 300 GHz. This range corresponds to the frequency of alternating current electrical signals used to produce and detect radio waves...
(RF) signals to acquire its images and is best suited for non-calcified tissue, though MR images can also be acquired from bones and teeth as well as fossils.
CT may be enhanced by use of
contrast agentsA medical contrast medium is a substance used to enhance the contrast of structures or fluids within the body in medical imaging...
containing elements of a higher atomic number than the surrounding flesh such as
iodineIodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
or
bariumBarium is a chemical element. It has the symbol Ba, atomic number 56, and is the fifth element in Group 2. Barium is a soft silvery metallic alkaline earth metal. It 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...
. Contrast agents for MRI are those which have
paramagneticParamagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field. Paramagnetic materials are attracted to magnetic fields, hence have a relative magnetic permeability of ≥1 .The magnetic moment induced by the applied field is linear in the field...
properties, e.g.
gadoliniumGadolinium is a chemical element that has the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. Gadolinium has exceptionally high absorption of neutrons and therefore is used for shielding in neutron radiography and in nuclear reactors...
and
manganeseManganese 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...
.
Both CT and MRI scanners can generate multiple two-dimensional cross-sections (slices) of tissue and three-dimensional reconstructions. Unlike CT, which uses only X-ray attenuation to generate image contrast, MRI has a long list of properties that may be used to generate image contrast. By variation of scanning parameters, tissue contrast can be altered and enhanced in various ways to detect different features. (See Applications above.)
MRI can generate cross-sectional images in any
planeIn mathematics, a plane is a flat surface. Planes can arise as subspaces of some higher dimensional space, as with the walls of a room, or they may enjoy an independent existence in their own right, as in the setting of Euclidean geometry....
(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-
isotropicIsotropy is uniformity in all directions. Precise definitions depend on the subject area. The word is made up from Greek iso and tropos . 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 due to less motion artifact. Furthermore, CT usually is more widely available, faster, less expensive, and may be less likely to require the person to be sedated or anesthetized.
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
{{Globalize/USA}}
{{Unreferenced section|date=September 2009}}
MRI equipment is expensive. 1.5 tesla scanners often cost between $1 million and $1.5 million USD. 3.0 tesla scanners often cost between $2 million and $2.3 million USD. Construction of MRI suites can cost up to $500,000 USD, 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. Some insurance companies require preapproval of an MRI procedure as a condition for coverage.
In the US, the 2007
Deficit Reduction ActDeficit Reduction Act may refer to various US legislation, including:# Gramm-Rudman-Hollings Deficit Reduction Act# Omnibus Budget Reconciliation Act of 1993# Deficit Reduction Act of 2005...
(DRA) 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}}
Safety
Death and injuries have occurred from projectiles created by the magnetic field, although few compared to the millions of examinations administered.
MRI makes use of powerful magnetic fields which, though they have not been demonstrated to cause direct biological damage, can interfere with metallic and electromechanical devices. Additional (small) risks are presented by the radio frequency systems, components or elements of the MRI system's operation, elements of the scanning procedure and medications that may be administered to facilitate MRI imaging.
Of great concern is the dramatic increase in the number of reported MRI accidents to the U.S. Food and Drug Administration (FDA). Since 2004, the last year in which a decline in the number of MRI accidents was reported, the full spectrum of MRI accidents has increased significantly in the following years. The 2008
FDA accident report data culminates in a 277% increase over the 2004 rate.
There are many steps that the MRI patient and referring physician can take to help reduce the remaining risks, including providing a full, accurate and thorough medical history to the MRI provider.
Several of the specific MRI safety considerations are identified below:
Implants and foreign bodies
{{Unreferenced section|date=September 2009}}
PacemakersA pacemaker is a medical device which uses electrical impulses, delivered by electrodes contacting the heart muscles, to regulate the beating of the heart...
are generally considered an absolute
contraindicationIn medicine, a contraindication is a condition or factor that increases the risks involved in using a particular drug, carrying out a medical procedure, or engaging in a particular activity....
towards MRI scanning, though highly specialized protocols have been developed to permit scanning of select pacing devices. Several cases of arrhythmia or death have been reported in patients with pacemakers who have undergone MRI scanning without appropriate precautions. Other electronic implants have varying contraindications, depending upon scanner technology, and implant properties, scanning protocols and anatomy being imaged.
Many other forms of medical or biostimulation implants may be contraindicated for MRI scans. These may include
vagus nerve stimulatorsVagus nerve stimulation is an adjunctive treatment for certain types of intractable epilepsy and major depression. VNS uses an implanted stimulator that sends electric impulses to the left vagus nerve in the neck via a lead wire implanted under the skin.- Cyberonics VNS Device :VNS implantation...
,
implantable cardioverter-defibrillatorAn 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. The device is programmed to detect cardiac arrhythmia and correct it by delivering a jolt of...
s, loop recorders, insulin pumps,
cochlear implantA 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. The cochlear implant is often referred to as a bionic ear...
s, deep brain stimulators, and many others. Medical device patients should always present complete information (manufacturer, model, serial number and date of implantation) about all implants to both the referring physician and to the radiologist or technologist before entering the room for the MRI scan.
While these implants pose a current problem, scientists and manufacturers are working on improved designs which will further minimize the risks that MRI scans pose to medical device operations. One such development in the works is a nano-coating for implants intended to screen them from the radio frequency waves, helping to make MRI exams available to patients currently prohibited from receiving them. The current
article for this is from New Scientist.
Ferromagnetic foreign bodies (e.g.
shellA shell is a payload-carrying projectile, which, as opposed to shot, contains an explosive or other filling, though modern usage includes large solid projectiles previously termed shot...
fragments), or metallic implants (e.g. surgical prostheses,
aneurysmAn aneurysm or aneurism , is a localized, blood-filled dilation of a blood vessel caused by disease or weakening of the vessel wall....
clips) are also potential risks, and safety aspects need to be considered on an individual basis. 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, thermal injury from radio-frequency
induction heatingInduction 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. An induction heater consists of an electromagnet, through which a high-frequency...
of the object, or failure of an implanted device. These issues are especially problematic when dealing with the eye. Most MRI centers require an
orbital x-rayOrbital x-ray or orbital radiography is a specialized form of radiography used to study the orbit of the eye. It is useful for detecting problems in the eye arising from injury or disease...
to be performed on anyone suspected of having metal fragments in their eyes, something not uncommon in
metalworkingMetalworking is the process of working with metals to create individual parts, assemblies, or large scale structures. The term covers a wide range of work from large ships and bridges to precise engine parts and delicate jewellery. It therefore includes a correspondingly wide range of skills,...
.
Because of its non-ferromagnetic nature and poor electrical conductivity,
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the “space age metal”, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color.Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other...
and its alloys are useful for long term implants and surgical instruments intended for use in
image-guided surgeryImage-guided surgery is the general term used for any surgical procedure where the surgeon uses indirect visualization to operate, i.e., by employing imaging instruments in real time, such as fiber optic guides, internal video cameras, flexible or rigid endoscopes, ultrasonography, etc...
. In particular, not only is titanium safe from movement from the magnetic field, but artifacts around the implant are less frequent and less severe than with more ferromagnetic materials e.g. stainless steel. Artifacts from metal frequently appear as regions of empty space around the implant - frequently called 'black-hole artifact' e.g. a 3mm titanium alloy coronary stent may appear as a 5mm diameter region of empty space on MRI, whereas around a stainless steel stent, the artifact may extend for 10–20 mm or more.
In 2006, a new 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.
Though the current classification system was originally developed for regulatory-approved medical devices, it is being applied to all manner of items, appliances and equipment intended for use in the MR environment.
In the case of pacemakers, the risk is thought to be primarily RF induction in the pacing electrodes/wires causing inappropriate pacing of the heart, rather than the magnetic field affecting the pacemaker itself. Much research and development is being undertaken, and many tools are being developed in order to predict the effects of the RF fields inside the body.
Patients who have been prescribed MRI exams who are concerned about safety may be interested in the
10 Questions To Ask Your MRI Provider.
MRI providers who wish to measure the degree to which they have effectively addressed the safety issues for patients and staff may be interested in the
MRI Suite Safety Calculator provided through a radiology website.
Projectile or missile effect
As a result of the very high strength of the magnetic field needed to produce scans (frequently up to 60,000 times the earth's own magnetic field effects), there are several incidental safety issues addressed in MRI facilities. Missile-effect accidents, where ferromagnetic objects are attracted to the center of the magnet, have resulted in injury and death. A
video simulation of a fatal projectile effect accident illustrates the extreme power that contemporary MRI equipment can exert on ferromagnetic objects.
In order to help reduce the risks of projectile accidents, ferromagnetic objects and devices are typically prohibited in proximity to the MRI scanner, with non-ferromagnetic versions of many tools and devices typically retained by the scanning facility. Patients undergoing MRI examinations are required to remove all metallic objects, often by changing into a gown or
scrubsScrubs are the shirts and trousers or gowns worn by nurses, surgeons, and other operating room personnel when "scrubbing in" for surgery. They are designed to be simple with minimal places for dirt to hide, easy to launder, and cheap to replace if damaged or stained irreparably. The wearing of...
.
New ferromagnetic-only detection devices are proving highly effective in supplementing conventional screening techniques in many leading hospitals and imaging centers and are now recommended by the American College of Radiology's
Guidance Document for Safe MR Practices: 2007, the United States' Veterans Administration's
Design Guide and the Joint Commission's
Sentinel Event Alert #38.
The magnetic field and the associated risk of missile-effect accidents remains a permanent hazard — as superconductive MRI magnets retain their magnetic field, even in the event of a power outage.
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
hyperthermiaHyperthermia 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. Several countries have issued restrictions on the maximum
specific absorption rateSpecific absorption rate is a measure of the rate at which energy is absorbed by the body when exposed to a radio frequency electromagnetic field. It is defined as the power absorbed per mass of tissue and has units of watts per kilogram. SAR is usually averaged either over the whole body, or...
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. 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 forceIn 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) .
Appropriate use of ear protection is essential for anyone inside the MRI scanner room during the examination.
Cryogens
As described above in 'Scanner Construction And Operation', many MRI scanners rely on cryogenic liquids to enable superconducting capabilities of the electromagnetic coils within. Though the cryogenic liquids most frequently used are non-toxic, their physical properties present specific hazards.
An emergency shut-down of a
superconducting electromagnetA superconducting magnet is an electromagnet that is built using coils of superconducting wire. They must be cooled to cryogenic temperatures during operation...
, an operation known as "quenching", involves the rapid boiling of liquid
heliumHelium is the chemical element with atomic number 2, and 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.
Liquid
heliumHelium is the chemical element with atomic number 2, and 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. 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.
Since a quench results in rapid loss of all cryogens in the magnet, recommissioning the magnet is extremely expensive and time-consuming. Spontaneous quenches are uncommon, but may also be triggered by equipment malfunction, improper cryogen fill technique, contaminates 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
gadoliniumGadolinium is a chemical element that has the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. Gadolinium has exceptionally high absorption of neutrons and therefore is used for shielding in neutron radiography and in nuclear reactors...
. In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT. Anaphylactoid reactions 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 CTRadiocontrast 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 fibrosisNephrogenic 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, but it seems to be associated with exposure to gadolinium in patients with severe kidney failure...
, 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 StatesThe 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
dialysisIn medicine, dialysis is primarily used to provide an artificial replacement for lost kidney function due to renal failure...
should be performed as soon as possible after the scan is complete, in order 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.
{{Citations missing|section called "Pregnancy"|date=September 2008}}
Pregnancy
No effects of MRI on the fetus have been demonstrated.
In particular, MRI avoids the use of
ionizing radiationIonizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
, 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
organogenesisIn 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;
gadoliniumGadolinium is a chemical element that has the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. Gadolinium has exceptionally high absorption of neutrons and therefore is used for shielding in neutron radiography and in nuclear reactors...
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
ultrasoundUltrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Although this limit varies from person to person, it is approximately 20 kilohertz in healthy, young adults and thus, 20 kHz serves as a useful lower limit in describing ultrasound...
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
teratomaA teratoma is a kind of tumor . Definitive diagnosis of a teratoma is based on its histology. A teratoma is a 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 surgeryFetal surgery covers a broad range of surgical techniques used in treatment of birth defects where the fetus is operated on while still in the pregnant uterus. Open fetal surgery involves completely opening up the uterus in order to operate on the fetus...
, other
fetal interventionFetal 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 procedureThe 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
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Due to the construction of some MRI scanners, they can be potentially unpleasant to lie in. Older models of closed bore MRI systems feature a fairly long tube or tunnel. The part of the body being imaged needs to 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
claustrophobiaClaustrophobia is the fear of having no escape, and being closed in. It is typically classified as an anxiety disorder and often results in panic attack...
are sometimes unable to tolerate an MRI scan without management. Modern scanners may have larger bores (up to 70 cm) and scan times are shorter. This means that claustrophobia is less of an issue, and many patients now find MRI an innocuous and easily tolerated 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
- 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 with a 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
- Scan Rooms with lighting, sound and images on the wall. Some rooms come with images on the walls or ceiling.
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.
Acoustic noise associated with the operation of an MRI scanner can also exacerbate the discomfort associated with the procedure.
Nephrogenic systemic fibrosis (NSF) 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, but it seems to be associated with exposure to gadolinium (which is frequently used as a contrast substance for MRIs) in patients with severe kidney failure.
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 RadiologyThe American College of Radiology , founded in 1923, is a non-profit professional medical organization 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 for any period of time. 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 upon which it is based, 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
isotropicIsotropy is uniformity in all directions. Precise definitions depend on the subject area. The word is made up from Greek iso and tropos . 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 detectionEdge detection is a terminology in image processing and computer vision, particularly in the areas of feature detection and feature extraction, to refer to algorithms which aim at identifying points in a digital image at which the image brightness changes sharply or more formally has...
image processing algorithms. From this, a 3-dimensional model can be constructed and displayed on screen. Multiple models can be constructed from various different 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 renderingVolume rendering is a technique 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 will only display surfaces which meet a threshold density, and will only display the surface that is 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
segmentationIn 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.
2003 Nobel Prize
Reflecting the fundamental importance and applicability of MRI in the medical field,
Paul LauterburPaul 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-ChampaignThe University of Illinois at Urbana-Champaign is a public research university in the state of Illinois, United States. It is the oldest and largest campus in the University of Illinois system....
and
Sir Peter MansfieldSir 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 NottinghamThe University of Nottingham is a public research university in the city of Nottingham, England, with further campuses in Ningbo, China and Kuala Lumpur, Malaysia...
were awarded the 2003
Nobel Prize in Physiology or MedicineThe Nobel Prize in Physiology or Medicine is awarded once a year by the Swedish Karolinska Institute. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in Physics, Chemistry, Literature, Peace, and Physiology or Medicine...
for their "discoveries concerning magnetic resonance imaging". The Nobel Prize committee acknowledged Lauterbur's insight of using magnetic field gradients to introduce 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 for which the prize was awarded was done almost 30 years ago, 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 was the inventor of MRI, and that Lauterbur and Mansfield had merely refined the technology. An
ad hocAd hoc is a Latin phrase which means "for this [purpose]". It generally signifies a solution designed for a specific problem or task, non-generalizable, and which cannot be adapted to other purposes....
group, called "The Friends of Raymond Damadian", took out full-page advertisements in the
New York Times and
The Washington PostThe Washington Post is the newspaper with the largest circulation in Washington, D.C. and is the city's oldest paper, founded in 1877. Being located in the nation's capital, it has a particular emphasis on national politics and international affairs...
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 UnionThe Union of Soviet Socialist Republics was a constitutionally socialist state that existed in Eurasia from 1922 to 1991. The name is a translation of the , tr. Soyuz Sovetskikh Sotsialisticheskikh Respublik, abbreviated СССР, SSSR. The common short name is Soviet Union, from , Sovetskiy Soyuz...
,
Vladislav IvanovVladislav 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 1970's. In a letter to
Physics TodayPhysics Today magazine, 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 CarrHerman 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.
See also
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- Earth's field NMR
Nuclear magnetic resonance in the geomagnetic field is conventionally referred to as Earth's field NMR . Note that the same acronym is used for electric 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 refers to the techniques and processes used to create images of the human body for clinical purposes or medical science .As a discipline and in its widest sense, it is part of biological imaging and incorporates...
- 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 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 is a multidisciplinary journal published by Elsevier, encompassing biology, physics, and clinical science as they relate to the development and use of MRI technology. The publication of Magnetic Resonance Imaging began in 1982, and is currently managed by editor-in-chief...
- 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 100µm³ .-Nomenclature:...
- 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 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...
- Neuroimaging software
- Nephrogenic fibrosing dermopathy
- Nobel Prize controversies
The Nobel Prize controversies are contentious disputes regarding the Nobel Prize.-The Prize:The Nobel Prizes are a series of awards which were posthumously instituted by bequest of Alfred Nobel . They are currently awarded to persons and organizations that have served humanity in the fields of...
{{Col-3-of-3}}
{{commons}}
- Nuclear magnetic resonance
Nuclear magnetic resonance is a property that magnetic nuclei have in a magnetic field and applied electromagnetic pulse, which cause the nuclei to absorb energy from the EM pulse and radiate this energy back out...
(NMR)
- 2D-FT NMRI and Spectroscopy
2D-FT Nuclear magnetic resonance imaging , or Two-dimensional Fourier transform nuclear magnetic resonance imaging , is primarily a non-invasive imaging technique most commonly used in biomedical research and medical radiology/nuclear medicine/MRI to visualize structures and functions of the living...
- Relaxation
Relaxation may refer to:*Relaxation, the usual term in Hypnotism, Meditation, and Physiology*Relaxation , processes by which nuclear magnetization returns to the equilibrium distribution...
- Robinson oscillator
The Robinson oscillator is an electronic circuit used in the field of Nuclear Magnetic Resonance . The oscillator forms the underlying basis of Magnetic Resonance Imaging systems used in many hospitals. It was invented by the British physicist Neville Robinson.- References :* Deschamps, P.,...
- 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...
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External links
{{Medical imaging}}
{{DEFAULTSORT:Magnetic Resonance Imaging}}