Neuroimaging includes the use of various techniques to either directly or indirectly
imageImaging is the representation or reproduction of an object's outward form; especially a visual representation .-Imaging methodologies and technologies:*Chemical imaging, the simultaneous measurement of spectra and pictures...
the
structureNeuroanatomy is the study of the anatomy of nervous tissue and neural structures of the nervous system. In vertebrate animals, the routes that the myriad nerves take from the brain to the rest of the body , and the internal structure of the brain in particular, are both extremely elaborate...
, function/
pharmacologyPharmacology is the study of drug action. More specifically, it is the study of the interactions that occur between a living organism and exogenous chemicals that alter normal biochemical function. If substances have medicinal properties, they are considered pharmaceuticals...
of 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...
. It is a relatively new discipline within
medicineMedicine is the art and science of healing. It encompasses a range of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
and
neuroscienceNeuroscience is the scientific study of the nervous system. Such studies span the structure, function, evolutionary history, development, genetics, biochemistry, physiology, pharmacology, informatics, computational neuroscience and pathology of the nervous system.The International Brain Research...
/
psychologyPsychology is an academic and applied discipline involving the systematic, and sometimes scientific, study of human or animal mental functions and behavior...
.
Overview
Neuroimaging falls into two broad categories:
- Structural imaging, which deals with the structure of the brain and the diagnosis of gross (large scale) intracranial disease (such as tumor), and injury, and
- functional imaging
Functional neuroimaging is the use of neuroimaging technology to measure an aspect of brain function, often with a view to understanding the relationship between activity in certain brain areas and specific mental functions...
, which is used to diagnose metabolic diseases and lesions on a finer scale (such as Alzheimer's disease) and also for neurological and cognitive psychologyCognitive psychology is a discipline within psychology that investigates the internal mental processes of thought such as visual processing, memory, problem solving, and language....
research and building brain-computer interfaceA brain–computer interface , sometimes called a direct neural interface or a brain–machine interface, is a direct communication pathway between a brain and an external device...
s.
Functional imaging enables, for example, the processing of information by centers in the brain to be visualized directly. Such processing causes the involved area of the brain to increase metabolism and "light up" on the scan.
History
In 1918 the American neurosurgeon
Walter DandyWalter Edward Dandy was an American neurosurgeon and scientist. He is considered one of the founding fathers of neurosurgery, and is credited with numerous discoveries and innovations, including the description of the circulation of cerebrospinal fluid in the brain, surgical treatment of...
introduced the technique of ventriculography.
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...
images of the
ventricular systemThe ventricular system is a set of structures containing cerebrospinal fluid in the brain. It is continuous with the central canal of the spinal cord.-Components:The system comprises four ventricles:* right and left lateral ventricles* third ventricle...
within the brain were obtained by injection of filtered air directly into one or both lateral ventricles of the brain. Dandy also observed that air introduced into the subarachnoid space via lumbar spinal puncture could enter the cerebral ventricles and also demonstrate the cerebrospinal fluid compartments around the base of the brain and over its surface. This technique was called
pneumoencephalographyPneumoencephalography is a medical procedure in which cerebrospinal fluid is drained to a small amount from around the brain and replaced with air, oxygen, or helium to allow the structure of the brain to show up more clearly on an X-ray picture...
.
In 1927
Egas MonizAntónio Caetano de Abreu Freire Egas Moniz known as Egas Moniz ; was a Portuguese neurologist and researcher....
, professor of neurology in
LisbonLisbon is the capital and largest city of Portugal. It is also the seat of the district of Lisbon and the main city of the Lisbon region...
and recipient of the Nobel Prize for Physiology or Medicine in 1949, introduced cerebral angiography, whereby both normal and abnormal blood vessels in and around the brain could be visualized with great accuracy.
In the early 1970s,
Allan McLeod CormackAllan MacLeod Cormack was a South African-born American physicist who won the 1979 Nobel Prize in Physiology or Medicine for his work on x-ray computed tomography ....
and Godfrey Newbold Hounsfield introduced computerized axial tomography (CAT or CT scanning), and ever more detailed anatomic images of the brain became available for diagnostic and research purposes. Cormack and Hounsfield won the 1979 Nobel Prize for Physiology or Medicine for their work. Soon after the introduction of CAT in the early 1980s, the development of
radioligandA radioligand is a radioactive biochemical substance that is used for diagnosis or for research-oriented study of the receptor systems of the body....
s allowed
single photon emission computed tomographySingle photon emission computed tomography is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera. However, it is able to provide true 3D information...
(SPECT) and
positron emission tomographyPositron emission tomography is a nuclear medicine imaging technique which produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
(PET) of the brain.
More or less concurrently,
magnetic resonance imagingMagnetic Resonance Imaging , or nuclear magnetic resonance imaging , is primarily a medical imaging technique most commonly used in radiology to visualize the internal structure and function of the body...
(MRI or MR scanning) was developed by researchers including
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...
and
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...
, who were awarded the Nobel Prize for Physiology or Medicine in 2003. In the early 1980s MRI was introduced clinically, and during the 1980s a veritable explosion of technical refinements and diagnostic MR applications took place. Scientists soon learned that the large blood flow changes measured by PET could also be imaged by the correct type of MRI.
Functional magnetic resonance imagingFunctional MRI or functional Magnetic Resonance Imaging is a type of specialized MRI scan. It measures the haemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging...
(fMRI) was born, and
since the 1990s, fMRI has come to dominate the brain mapping field due to its low invasiveness, lack of radiation exposure, and relatively wide availability. As noted above fMRI is also beginning to dominate the field of stroke treatment.
In early 2000s the field of neuroimaging reached the stage where limited practical applications of functional brain imaging have become feasible. The main application area is crude forms of brain-computer interface.
Computed axial tomography
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) or
Computed Axial Tomography (CAT) scanning uses a series of x-rays of the head taken from many different directions. Typically used for quickly viewing
brain injuriesAn acquired brain injury is damage to the brain acquired after birth. It usually affects cognitive, physical, emotional, social or independent functioning and can result from traumatic brain injury and nontraumatic brain injury...
, CT scanning uses a computer program that performs a numerical integral calculation (the inverse
Radon transformthumb|right|Radon transform of the [[indicator function]] of two squares shown in the image below. Lighter regions indicate larger function values. Black indicates zero.thumb|right|Original function is equal to one on the white region and zero on the dark region....
) on the measured x-ray series to estimate how much of an x-ray beam is absorbed in a small volume of the brain. Typically the information is presented as cross sections of the brain.
In approximation, the denser a material is, the whiter a volume of it will appear on the scan (just as in the more familiar "flat" X-rays). CT scans are primarily used for evaluating swelling from tissue damage in the brain and in assessment of ventricle size. Modern CT scanning can provide reasonably good images in a matter of minutes.
Diffuse optical imaging
Diffuse optical imaging (DOI) or diffuse optical tomography (DOT) is 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...
modality which uses near
infraredInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
light to generate images of the body. The technique measures the optical absorption of haemoglobin, and relies on the
absorption spectrumA material's absorption spectrum shows the fraction of incident electromagnetic radiation absorbed by the material over a range of frequencies. Atoms, for example, have absorption lines at wavelengths corresponding to the differences between the energy levels of its atomic orbitals. Each chemical...
of haemoglobin varying with its oxygenation status.
Event-related optical signal
Event-related optical signalEvent-related optical signal is a brain-scanning technique which uses infrared light through optical fibers to measure changes in optical properties of active areas of the cerebral cortex...
(EROS) is a brain-scanning technique which uses infrared light through optical fibers to measure changes in optical properties of active areas of the cerebral cortex. Whereas techniques such as diffuse optical imaging (DOT) and near infrared spectroscopy (NIRS) measure optical absorption of haemoglobin, and thus are based on blood flow, EROS takes advantage of the scattering properties of the neurons themselves, and thus provides a much more direct measure of cellular activity. EROS can pinpoint activity in the brain within millimeters (spatially) and within milliseconds (temporally). Its biggest downside is the inability to detect activity more than a few centimeters deep. EROS is a new, relatively inexpensive technique that is non-invasive to the test subject. It was developed at the University of Illinois at Urbana-Champaign where it is now used in the Cognitive Neuroimaging Laboratory of Dr. Gabriele Gratton and Dr. Monica Fabiani.
Magnetic resonance imaging
Magnetic resonance imagingMagnetic Resonance Imaging , or nuclear magnetic resonance imaging , is primarily a medical imaging technique most commonly used in radiology to visualize the internal structure and function of the body...
(MRI) uses magnetic fields and radio waves to produce high quality two- or three-dimensional images of brain structures without use of ionizing radiation (X-rays) or radioactive tracers. During an MRI, a large cylindrical
magnetA magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials and attracts or repels other magnets.A permanent magnet is an object made from a...
creates a
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...
around the head of the patient through which radio waves are sent. When the magnetic field is imposed, each point in space has a unique
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...
at which the signal is received and transmitted (Preuss). Sensors read the frequencies and a computer uses the information to construct an image. The detection mechanisms are so precise that changes in structures over time can be detected.
Using MRI, scientists can create images of both surface and subsurface structures with a high degree of
anatomicalAnatomy 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...
detail. MRI scans can produce cross sectional images in any direction from top to bottom, side to side, or front to back. The problem with original MRI technology was that while it provides a detailed assessment of the physical appearance, water content, and many kinds of subtle derangements of structure of the brain (such as inflammation or bleeding), it fails to provide information about the metabolism of the brain (i.e. how actively it is functioning) at the time of imaging. A distinction is therefore made between "MRI imaging" and "functional MRI imaging" (fMRI), where MRI provides only structural information on the brain while fMRI yields both structural and functional data.
Functional magnetic resonance imaging
Functional magnetic resonance imagingFunctional MRI or functional Magnetic Resonance Imaging is a type of specialized MRI scan. It measures the haemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging...
(fMRI) relies on the paramagnetic properties of oxygenated and deoxygenated
hemoglobinHemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates, and the tissues of some invertebrates....
to see images of changing blood flow in the brain associated with neural activity. This allows images to be generated that reflect which brain structures are activated (and how) during performance of different tasks.
Most fMRI scanners allow subjects to be presented with different visual images, sounds and touch stimuli, and to make different actions such as pressing a button or moving a joystick. Consequently, fMRI can be used to reveal brain structures and processes associated with perception, thought and action. The resolution of fMRI is about 2-3 millimeters at present, limited by the spatial spread of the hemodynamic response to neural activity. It has largely superseded PET for the study of brain activation patterns. PET, however, retains the significant advantage of being able to identify specific brain
receptorsIn biochemistry, a receptor is a protein molecule, embedded in either the plasma membrane or cytoplasm of a cell, to which a mobile signaling molecule may attach...
(or
transportersMonoamine transporters are structures in nerve-cell membranes that function as neurotransmitter transporters transferring monoamine neurotransmitters in or out of cells.-Types:There are several different monoamine transporters:*The dopamine transporter, DAT....
) associated with particular neurotransmitters through its ability to image radiolabelled receptor "ligands" (receptor ligands are any chemicals that stick to receptors).
As well as research on healthy subjects, fMRI is increasingly used for the medical diagnosis of disease. Because fMRI is exquisitely sensitive to blood flow, it is extremely sensitive to early changes in the brain resulting from ischemia (abnormally low blood flow), such as the changes which follow
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...
. Early diagnosis of certain types of stroke is increasingly important in neurology, since substances which dissolve blood clots may be used in the first few hours after certain types of stroke occur, but are dangerous to use afterwards. Brain changes seen on fMRI may help to make the decision to treat with these agents.
With between 72% and 90% accuracy where chance would achieve 0.8%, fMRI techniques can decide which of a set of known images the subject is viewing.
Electroencephalography
ElectroencephalographyElectroencephalography is the recording of electrical activity along the scalp produced by the firing of neurons within the brain . In clinical contexts, EEG refers to the recording of the brain's spontaneous electrical activity over a short period of time, usually 20–40 minutes, as recorded from...
(EEG) is an imaging technique used to measure the electric fields in the brain via electrodes placed on the scalp of a human. EEG offers a very direct measurement of neural electrical activity with very high temporal resolution but relatively low spatial resolution.
MagnetoEncephaloGraphy
MagnetoencephalographyMagnetoencephalography is an imaging technique used to measure the magnetic fields produced by electrical activity in the brain via extremely sensitive devices such as superconducting quantum interference devices . These measurements are commonly used in both research and clinical settings...
(MEG) is an imaging technique used to measure the magnetic fields produced by electrical activity in the brain via extremely sensitive devices such as
superconducting quantum interference devicesSuperconducting Quantum Interference Devices are very sensitive magnetometers used to measure extremely small magnetic fields, based on superconducting loops containing Josephson junctions....
(SQUIDs). MEG offers a very direct measurement neural electrical activity (compared to fMRI for example) with very high temporal resolution but relatively low spatial resolution. The advantage of measuring the magnetic fields produced by neural activity is that they are not distorted by surrounding tissue, unlike the electric fields measured by EEG (particularly the skull and scalp).
There are many uses for the MEG, including assisting surgeons in localizing a pathology, assisting researchers in determining the function of various parts of the brain, neurofeedback, and others.
Positron emission tomography
Positron emission tomographyPositron emission tomography is a nuclear medicine imaging technique which produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
(PET) measures emissions from radioactively labeled metabolically active chemicals that have been injected into the bloodstream. The emission data are computer-processed to produce 2- or 3-dimensional images of the distribution of the chemicals throughout the brain. The
positronThe positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1, a spin of , and the same mass as an electron. When a low-energy positron collides with a low-energy electron, annihilation occurs, resulting in the production...
emitting radioisotopes used are produced by a
cyclotronA cyclotron is a type of particle accelerator. Cyclotrons accelerate charged particles using a high-frequency, alternating voltage...
, and chemicals are labeled with these radioactive atoms. The labeled compound, called a
radiotracer, is injected into the bloodstream and eventually makes its way to the brain. Sensors in the PET scanner detect the radioactivity as the compound accumulates in various regions of the brain. A computer uses the data gathered by the sensors to create multicolored 2- or 3-dimensional images that show where the compound acts in the brain. Especially useful are a wide array of ligands used to map different aspects of neurotransmitter activity, with by far the most commonly used PET tracer being a labeled form of glucose (see FDG).
The greatest benefit of PET scanning is that different compounds can show blood flow and oxygen and
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...
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,...
in the tissues of the working brain. These measurements reflect the amount of brain activity in the various regions of the brain and allow to learn more about how the brain works. PET scans were superior to all other metabolic imaging methods in terms of resolution and speed of completion (as little as 30 seconds), when they first became available. The improved resolution permitted better study to be made as to the area of the brain activated by a particular task. The biggest drawback of PET scanning is that because the radioactivity decays rapidly, it is limited to monitoring short tasks. Before fMRI technology came online, PET scanning was the preferred method of functional (as opposed to structural) brain imaging, and it still continues to make large contributions to
neuroscienceNeuroscience is the scientific study of the nervous system. Such studies span the structure, function, evolutionary history, development, genetics, biochemistry, physiology, pharmacology, informatics, computational neuroscience and pathology of the nervous system.The International Brain Research...
.
PET scanning is also used for diagnosis of brain disease, most notably because brain tumors, strokes, and neuron-damaging diseases which cause dementia (such as Alzheimer's disease) all cause great changes in brain metabolism, which in turn causes easily detectable changes in PET scans. PET is probably most useful in early cases of certain dementias (with classic examples being
Alzheimer's diseaseAlzheimer's disease , also called Alzheimer disease, Senile Dementia of the Alzheimer Type or simply Alzheimer's, is the most common form of dementia. This incurable, degenerative, and terminal disease was first described by German psychiatrist and neuropathologist Alois Alzheimer in 1906 and was...
and
Pick's diseasePick's disease, also known as Pick disease and PiD, is a rare neurodegenerative disease. While the term Pick's disease was once used to represent a specific group of clinical syndromes with symptoms attributable to frontal and temporal lobe dysfunction, it is now used to mean a specific pathology...
) where the early damage is too diffuse and makes too little difference in brain volume and gross structure to change CT and standard MRI images enough to be able to reliably differentiate it from the "normal" range of cortical atrophy which occurs with aging (in many but not all) persons, and which does
not cause clinical dementia.
Single photon emission computed tomography
Single photon emission computed tomographySingle photon emission computed tomography is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera. However, it is able to provide true 3D information...
(SPECT) is similar to PET and uses
gamma rayGamma rays are electromagnetic radiation of high frequency . They are produced by sub-atomic particle interactions, such as electron-positron annihilation, neutral pion decay, radioactive decay, fusion, fission or inverse Compton scattering in astrophysical processes...
emitting radioisotopes and a
gamma cameraA gamma camera is a device used to image gamma radiation emitting radioisotopes, a technique known as scintigraphy. The applications of scintigraphy include early drug development and nuclear medical imaging to view and analyse images of the human body or the distribution of medically injected,...
to record data that a computer uses to construct two- or three-dimensional images of active brain regions SPECT relies on an injection of radioactive tracer, which is rapidly taken up by the brain but does not redistribute. Uptake of SPECT agent is nearly 100% complete within 30 – 60s, reflecting cerebral blood flow (CBF) at the time of injection. These properties of SPECT make it particularly well suited for epilepsy imaging, which is usually made difficult by problems with patient movement and variable seizure types. SPECT provides a "snapshot" of cerebral blood flow since scans can be acquired after seizure termination (so long as the radioactive tracer was injected at the time of the seizure). A significant limitation of SPECT is its poor resolution (about 1 cm) compared to that of MRI.
Like PET, SPECT also can be used to differentiate different kinds of disease processes which produce dementia, and it is increasingly used for this purpose. Neuro-PET has a disadvantage of requiring use of tracers with half-lives of at most 110 minutes, such as FDG. These must be made in a cyclotron, and are expensive or even unavailable if necessary transport times are prolonged more than a few half-lives. SPECT, however, is able to make use of tracers with much longer half-lives, such as technetium-99m, and as a result, is far more widely available.
See also
- Brain mapping
Brain mapping is a set of neuroscience techniques predicated on the mapping of quantities or properties onto spatial representations of the brain resulting in maps.- Overview :...
- Functional neuroimaging
Functional neuroimaging is the use of neuroimaging technology to measure an aspect of brain function, often with a view to understanding the relationship between activity in certain brain areas and specific mental functions...
- functional near-infrared imaging
- History of brain imaging
- Human Cognome Project
The Human Cognome Project seeks to reverse engineer the human brain, paralleling in many ways the Human Genome Project and its success in deciphering the human genome. The HCP is a multidisciplinary undertaking, relevant to, among others: biology, neuroscience, psychology, cognitive science,...
- Magnetic resonance imaging
Magnetic Resonance Imaging , or nuclear magnetic resonance imaging , is primarily a medical imaging technique most commonly used in radiology to visualize the internal structure and function of the body...
- Magnetoencephalography
Magnetoencephalography is an imaging technique used to measure the magnetic fields produced by electrical activity in the brain via extremely sensitive devices such as superconducting quantum interference devices . These measurements are commonly used in both research and clinical settings...
- 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...
- Neuroimaging software
- Statistical parametric mapping
Statistical parametric mapping or SPM is a statistical technique for examining differences in brain activity recorded during functional neuroimaging experiments using neuroimaging technologies such as fMRI or PET...
- Transcranial magnetic stimulation
Transcranial magnetic stimulation is a noninvasive method to excite neurons in the brain: weak electric currents are induced in the tissue by rapidly changing magnetic fields...
- Voxel-based morphometry
- Physioscan
Further reading
- Philip Ball. Brain Imaging Explained.
- J. Graham Beaumont (1983). Introduction to Neuropsychology. New York: The Guilford Press.
- Jean-Pierre Changeux (1985). Neuronal Man: The Biology of Mind. New York: Oxford University Press.
- Malcom Jeeves (1994). Mind Fields: Reflections on the Science of Mind and Brain. Grand Rapids, MI: Baker Books.
- Richard G. Lister and Herbert J. Weingartner (1991). Perspectives on Cognitive Neuroscience. New York: Oxford University Press.
- James Mattson and Merrill Simon (1996). The Pioneers of NMR and Magnetic Resonance in Medicine. United States: Dean Books Company.
- Lars-Goran Nilsson and Hans J. Markowitsch (1999). Cognitive Neuroscience of Memory. Seattle: Hogrefe & Huber Publishers.
- Donald A. Norman (1981). Perspectives on Cognitive Science. New Jersey: Ablex Publishing Corporation.
- Brenda Rapp (2001). The Handbook of Cognitive Neuropsychology. Ann Arbor, MI: Psychology Press.
- Berger, H. (1929). Über das Elektroenkephalogramm des Menschen. European Archives of Psychiatry and Clinical Neuroscience. 87(1), 527:570.
External links