History of neuroimaging
Encyclopedia
The history of neuroimaging
, began in the early 1900s with a technique called pneumoencephalography
. This process involved draining the cerebrospinal fluid
from around the brain and replacing it with air, altering the relative density of the brain and its surroundings, to cause it to show up better on an x-ray
. It was considered to be incredibly unsafe for patients (Beaumont 8). A form of magnetic resonance imaging
(MRI) and computed tomography (CT) were developed in the 1970s and 1980s. The new MRI and CT technologies were considerably less harmful and are explained in greater detail below. Next came SPECT and PET
scans, which allowed scientists to map brain function because, unlike MRI and CT, these scans could create more than just static images of the brain's structure. Learning from MRI, PET and SPECT scanning, scientists were able to develop functional MRI (fMRI) with abilities that opened the door to direct observation of cognitive activities.
has been one of the main desires of philosophers throughout the ages. Questions about thoughts, desires, etcetera have drawn psychologists, computer scientists, philosophers, sociologists and the like together into the new discipline
of cognitive science. Non-invasive imaging of the human brain has proven invaluable in this context.
Structural imaging began with early radiographic
techniques to image the human brain
. Unfortunately, because the brain is almost entirely composed of soft tissue that is not radio-opaque, it remains essentially invisible to ordinary or plain x-ray examination. This is also true of most brain abnormalities, though there are exceptions such as a calcified tumour (e.g.meningioma
, craniopharyngioma
, some types of glioma
); whilst calcification in such normal structures as the pineal body, the choroid plexuses, or large brain arteries may indirectly give important clues to the presence of structural disease in the brain itself.
In 1918 the American neurosurgeon Walter Dandy
introduced the technique of ventriculography whereby images of the ventricular system
within the brain were obtained by injection of filtered air directly into one or both lateral ventricles of the brain via one or more small trephine holes drilled in the skull under local anaesthesia. Though not usually a painful procedure, ventriculography carried significant risks to the patient under investigation, such as haemorrhage, infection, and dangerous changes in intracranial pressure. Nevertheless the surgical information given by this method was often remarkably precise and greatly enlarged the capabilities and accuracy of neurosurgical treatment. 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 pneumoencephalography
. It further extended the scope for precise intracranial diagnosis, but at a similar cost of risks to the patient as well as being, in itself, a most unpleasant and often painful ordeal.
, professor of neurology in Lisbon
and Nobel Prize for Physiology or Medicine winner in 1949, introduced cerebral angiography, whereby both normal and abnormal blood vessel
s in and around the brain could be visualized with great accuracy. In its early days this technique likewise carried both immediate and long-term risks, many of them referable to deleterious effects of the positive-contrast substances that were used for injection into the circulation. Techniques have become very refined in the past few decades, with one in 200 patients or less experiencing ischemic sequelae from the procedure. As a result, cerebral angiography remains an essential part of the neurosurgeon's diagnostic imaging armamentarium and, increasingly, of the therapeutic armamentarium as well, in the neurointerventional management of cerebral aneurysm
s and other blood-vessel lesions and in some varieties of brain tumor
.
(in 1961) Godfrey Newbold Hounsfield and Allan McLeod Cormack
(in 1973) are associated with this revolutionary innovation, which enabled much easier, safer, non-invasive, painless and (to a reasonable extent) repeatable neuro-investigation. Cormack and Hounsfield won the Nobel Prize in Physiology or Medicine
in 1979 for this work.
inhalation provided the first blood flow maps of the brain.
Developed in the early 1960s by Niels A. Lassen
, David H. Ingvar and Erik Skinhøj in southern Scandinavia
it used the isotope xenon-133. Later versions would have 254 scintillator
s so a two-dimensional image could be produced on a color monitor. It allowed them to construct images reflecting brain activation from speaking, reading, visual or auditory perception and voluntary movement.
Soon after the invention of CAT, the development of radioligand
s started the functional imaging revolution. Radioligands either remain within the blood stream or enter the brain and bind to receptors. Radioligands are either single photon or positron emitters. This is how single photon emission computed tomography (SPECT) and positron emission tomography
(PET) got their names. Edward J. Hoffman
and Michael Phelps developed the first human PET scanner in 1973.
Functional imaging took a large step forward with the development of oxygen-15 labelled water (H215O, or H20-15) imaging. H20-15 emits positrons and creates images based on regional blood flow within the brain. Since active neurons recruit a robust blood supply, H20-15 PET allowed investigators to make regional maps of brain activity during various cognitive tasks. Later, a more common sort of functional imaging based on PET scans used FDG, a positron-emitting sugar-derivative which is distributed in the brain according to local metabolic activity. Unlike the short half-life of oxygen-15 (2.25 minutes), the 110 minute half-life of FDG allowed PET scans by machines physically distant from the cyclotron producing the isotope (in this case fluorine-18).
(MRI or MR scanning) was developed. Rather than using ionizing or x-radiation, MRI uses the variation in signals produced by proton
s in the body when the head is placed in a strong magnetic field
. Associated with early application of the basic technique to the human body are the names of Jackson (in 1968), Damadian (in 1972), and Abe and Paul Lauterbur
(in 1973). Lauterbur and Sir Peter Mansfield
were awarded the 2003 Nobel Prize in Physiology or Medicine
for their discoveries concerning MRI. At first, structural imaging benefited more than functional imaging from the introduction of MRI. During the 1980s a veritable explosion of technical refinements and diagnostic MR applications took place, enabling even neurological tyros to diagnose brain pathology that would have been elusive or incapable of demonstration in a living person only a decade or two earlier.
Scientists soon learned that the large blood flow changes measured by H20-15 PET were also imaged by MRI. Functional magnetic resonance imaging
(fMRI) was born. 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.
Physicists have also developed other MRI-based techniques such as magnetic resonance spectroscopy (for measuring some key metabolites such as N-acetylaspartate and lactate within the living brain) and diffusion tensor imaging (for mapping white matter tracts within the living brain). Whereas structural MRI and CAT scanning have a large place in medicine, fMRI and its brethren techniques are still largely devoted to neuroscience research. However, very recently neurologists have started to use fMRI to begin to answer clinical questions, such as how long after thrombotic stroke is it safe and effective to give clot-dissolving drug like tissue plasminogen activator
(TPA). Similarly, PET and SPECT have moved out of neuro-research and are increasingly being used clinically to help diagnose and differentiate types of dementing illnesses (dementia
).
Besides fMRI, another example of technology allowing relatively older brain imaging techniques to be even more helpful is the ability to combine different techniques to get one brain map. This happens quite frequently with MRI and EEG
scans. The electrical diagram of the EEG provides split-second timing while the MRI provides high levels of spatial accuracy.
Anatomically-constrained Magnetoencephalography (aMEG) is a relatively new technique which was first employed in 2000. It combines the spatial resolution of a structural MRI scan with the temporal resolution of the MEG
. Often the non-uniqueness of the MEG source estimation problem (inverse problem
) can be alleviated by incorporating information from other imaging modalities as an a priori constraint. aMEG uses anatomical MRI data as a geometrical or location constraint and as a medium for visualization of MEG results. MEG does not provide structural or anatomical information. Therefore, MEG data is often combined with MR data into a composite image whereby functional information is overlaid on the corresponding anatomy to produce an activation map.
Advances have been made in a number of ways regarding neuroimaging, and this section will cover some of the more prominent improvements including computational advances, transcranial magnetic stimulation, and nuclear magnetic resonance
.
To begin with, much of the recent progress has had to do not with the actual brain imaging methods themselves but with our ability to utilize computers in analyzing the data. For example, substantial discoveries in the growth of human brains from age three months to the age of fifteen have been made due to the creation of high-resolution brain maps and computer technology to analyze these maps over various periods of time and growth (Thompson, UCLA). This type of breakthrough represents the nature of most breakthroughs in neuroscience
today. With fMRI technology mapping brains beyond what we are already understanding, most innovators time is being spent trying to make sense of the data
we already have rather than probing into other realms of brain imaging and mapping.
This can be seen more clearly in the fact that brain imaging archives are catching on and neuroinformatics
is allowing researchers to examine thousands of brains rather than just a few (Lynch). Also, these archives are universalizing and standardizing formats and descriptions so that they are more searchable for everyone. For the past decade we have been able to get data and now our technology allows us to share findings and research much easier. This has also allowed for "brain atlases" to be made. Brain atlases are simply maps of what normal functioning brains look like (Thompson, Bioinformatics).
Transcranial magnetic stimulation
(TMS) is a recent innovation in brain imaging. In TMS, a coil is held near a person's head to generate magnetic field impulses that stimulate underlying brain cells to make someone perform a specific action. Using this in combination with MRI, the researcher can generate maps of the brain performing very specific functions. Instead of asking a patient to tap his or her finger, the TMS coil can simply "tell" his or her brain to tap his or her finger. This eliminates many of the false positives received from traditional MRI and fMRI testing. The images received from this technology are slightly different from the typical MRI results, and they can be used to map any subject's brain by monitoring up to 120 different stimulations. This technology has been used to map both motor processes and visual processes (Potts link at bottom of TMS). In addition to fMRI, the activation of TMS can be measured using electroencephalography
(EEG) or near infrared spectroscopy
(NIRS).
Nuclear magnetic resonance
(NMR) is what MRI and fMRI technologies were derived from, but recent advances have been made by going back to the original NMR technology and revamping some of its aspects. NMR traditionally has two steps, signal encoding and detection, and these steps are normally carried out in the same instrument. The new discovery, however, suggests that using laser-polarized xenon
gas for "remembering" encoded information and transporting that information to a remote detection site could prove far more effective (Preuss). Separating the encoding and detection allows researchers to gain data about chemical, physical
, and biological
processes that they have been unable to gain until now. The end result allows researchers to map things as big as geological core samples or as small as single cell
s.
It is interesting to see how advances are split between those seeking a completely mapped brain by utilizing single neuron
imaging and those utilizing images of brains as subjects perform various high-level tasks. Single neuron imaging (SNI) uses a combination of genetic engineering and optical imaging techniques to insert tiny electrodes into the brain for the purpose of measuring a single neuron's firing. Due to its damaging repercussions, this technique has only been used on animals, but it has shed a lot of light on basic emotional and motivational processes. The goal of studies in higher-level activities is to determine how a network of brain areas collaborates to perform each task. This higher-level imaging is much easier to do because researchers can easily use subjects who have a disease such as Alzheimer's. The SNI technology seems to be going after the possibility for AI while the network-probing technology seems to be more for medical purposes.
Neuroimaging
Neuroimaging includes the use of various techniques to either directly or indirectly image the structure, function/pharmacology of the brain...
, began in the early 1900s with a technique called pneumoencephalography
Pneumoencephalography
Pneumoencephalography is a medical procedure in which most of the cerebrospinal fluid is drained 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 image...
. This process involved draining the cerebrospinal fluid
Cerebrospinal fluid
Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...
from around the brain and replacing it with air, altering the relative density of the brain and its surroundings, to cause it to show up better on an x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
. It was considered to be incredibly unsafe for patients (Beaumont 8). A form of magnetic resonance imaging
Magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
(MRI) and computed tomography (CT) were developed in the 1970s and 1980s. The new MRI and CT technologies were considerably less harmful and are explained in greater detail below. Next came SPECT and PET
Positron emission tomography
Positron emission tomography is nuclear medicine imaging technique that 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...
scans, which allowed scientists to map brain function because, unlike MRI and CT, these scans could create more than just static images of the brain's structure. Learning from MRI, PET and SPECT scanning, scientists were able to develop functional MRI (fMRI) with abilities that opened the door to direct observation of cognitive activities.
Early uses of brain imaging
The desire to understand the human mindMind
The concept of mind is understood in many different ways by many different traditions, ranging from panpsychism and animism to traditional and organized religious views, as well as secular and materialist philosophies. Most agree that minds are constituted by conscious experience and intelligent...
has been one of the main desires of philosophers throughout the ages. Questions about thoughts, desires, etcetera have drawn psychologists, computer scientists, philosophers, sociologists and the like together into the new discipline
Discipline
In its original sense, discipline is referred to systematic instruction given to disciples to train them as students in a craft or trade, or to follow a particular code of conduct or "order". Often, the phrase "to discipline" carries a negative connotation. This is because enforcement of order –...
of cognitive science. Non-invasive imaging of the human brain has proven invaluable in this context.
Structural imaging began with early radiographic
Radiography
Radiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....
techniques to image the human brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...
. Unfortunately, because the brain is almost entirely composed of soft tissue that is not radio-opaque, it remains essentially invisible to ordinary or plain x-ray examination. This is also true of most brain abnormalities, though there are exceptions such as a calcified tumour (e.g.meningioma
Meningioma
The word meningioma was first used by Harvey Cushing in 1922 to describe a tumor originating from the meninges, the membranous layers surrounding the CNS ....
, craniopharyngioma
Craniopharyngioma
Craniopharyngioma is a type of brain tumor derived from pituitary gland embryonic tissue, that occurs most commonly in children but also in men and women in their 50s and 60s....
, some types of glioma
Glioma
A glioma is a type of tumor that starts in the brain or spine. It is called a glioma because it arises from glial cells. The most common site of gliomas is the brain.-By type of cell:...
); whilst calcification in such normal structures as the pineal body, the choroid plexuses, or large brain arteries may indirectly give important clues to the presence of structural disease in the brain itself.
In 1918 the American neurosurgeon Walter Dandy
Walter Dandy
Walter Edward Dandy, M.D. was an American neurosurgeon and scientist. He is considered one of the founding fathers of neurosurgery, along with Victor Horsley and Harvey Cushing...
introduced the technique of ventriculography whereby images of the ventricular system
Ventricular system
The 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 via one or more small trephine holes drilled in the skull under local anaesthesia. Though not usually a painful procedure, ventriculography carried significant risks to the patient under investigation, such as haemorrhage, infection, and dangerous changes in intracranial pressure. Nevertheless the surgical information given by this method was often remarkably precise and greatly enlarged the capabilities and accuracy of neurosurgical treatment. 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 pneumoencephalography
Pneumoencephalography
Pneumoencephalography is a medical procedure in which most of the cerebrospinal fluid is drained 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 image...
. It further extended the scope for precise intracranial diagnosis, but at a similar cost of risks to the patient as well as being, in itself, a most unpleasant and often painful ordeal.
Development of modern techniques
In 1927 Egas MonizEgas Moniz
António Caetano de Abreu Freire Egas Moniz , known as Egas Moniz , was a Portuguese neurologist and the developer of cerebral angiography...
, professor of neurology in Lisbon
Lisbon
Lisbon is the capital city and largest city of Portugal with a population of 545,245 within its administrative limits on a land area of . The urban area of Lisbon extends beyond the administrative city limits with a population of 3 million on an area of , making it the 9th most populous urban...
and Nobel Prize for Physiology or Medicine winner in 1949, introduced cerebral angiography, whereby both normal and abnormal blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s in and around the brain could be visualized with great accuracy. In its early days this technique likewise carried both immediate and long-term risks, many of them referable to deleterious effects of the positive-contrast substances that were used for injection into the circulation. Techniques have become very refined in the past few decades, with one in 200 patients or less experiencing ischemic sequelae from the procedure. As a result, cerebral angiography remains an essential part of the neurosurgeon's diagnostic imaging armamentarium and, increasingly, of the therapeutic armamentarium as well, in the neurointerventional management of cerebral aneurysm
Aneurysm
An aneurysm or aneurism is a localized, blood-filled balloon-like bulge in the wall of a blood vessel. Aneurysms can commonly occur in arteries at the base of the brain and an aortic aneurysm occurs in the main artery carrying blood from the left ventricle of the heart...
s and other blood-vessel lesions and in some varieties of brain tumor
Tumor
A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...
.
Computerized tomography
With the advent of computerized axial tomography (CAT or CT scanning), ever more detailed anatomic images of the brain became available for diagnostic and research purposes. The names of OldendorfWilliam H. Oldendorf
William Henry Oldendorf was an American neurologist, physician, researcher, medical pioneer, founding member of the American Society for Neuroimaging , and originator of the technique of computed tomography....
(in 1961) Godfrey Newbold Hounsfield and Allan McLeod Cormack
Allan McLeod Cormack
Allan 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 ....
(in 1973) are associated with this revolutionary innovation, which enabled much easier, safer, non-invasive, painless and (to a reasonable extent) repeatable neuro-investigation. Cormack and Hounsfield won the Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine administered by the Nobel Foundation, is awarded once a year for outstanding discoveries in the field of life science and medicine. It is one of five Nobel Prizes established in 1895 by Swedish chemist Alfred Nobel, the inventor of dynamite, in his will...
in 1979 for this work.
Radioactive neuroimaging
Early techniques such as xenonXenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
inhalation provided the first blood flow maps of the brain.
Developed in the early 1960s by Niels A. Lassen
Niels A. Lassen
Niels Alexander Lassen was a Danish medical doctor and pioneer of neuroimaging.His father, H.C.A...
, David H. Ingvar and Erik Skinhøj in southern Scandinavia
Scandinavia
Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...
it used the isotope xenon-133. Later versions would have 254 scintillator
Scintillator
A scintillator is a special material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light...
s so a two-dimensional image could be produced on a color monitor. It allowed them to construct images reflecting brain activation from speaking, reading, visual or auditory perception and voluntary movement.
Soon after the invention of CAT, the development of radioligand
Radioligand
A radioligand is a radioactive biochemical substance that is used for diagnosis or for research-oriented study of the receptor systems of the body....
s started the functional imaging revolution. Radioligands either remain within the blood stream or enter the brain and bind to receptors. Radioligands are either single photon or positron emitters. This is how single photon emission computed tomography (SPECT) and positron emission tomography
Positron emission tomography
Positron emission tomography is nuclear medicine imaging technique that 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) got their names. Edward J. Hoffman
Edward J. Hoffman
Edward Joseph Hoffman helped invent the first human PET scanner, a commonly used whole-body scanning procedure for detecting diseases like cancer. Hoffman, with Michael Phelps, developed the Positron Emission Tomography scanner in 1973.Hoffman was born in St. Louis, Missouri. He earned a BS...
and Michael Phelps developed the first human PET scanner in 1973.
Functional imaging took a large step forward with the development of oxygen-15 labelled water (H215O, or H20-15) imaging. H20-15 emits positrons and creates images based on regional blood flow within the brain. Since active neurons recruit a robust blood supply, H20-15 PET allowed investigators to make regional maps of brain activity during various cognitive tasks. Later, a more common sort of functional imaging based on PET scans used FDG, a positron-emitting sugar-derivative which is distributed in the brain according to local metabolic activity. Unlike the short half-life of oxygen-15 (2.25 minutes), the 110 minute half-life of FDG allowed PET scans by machines physically distant from the cyclotron producing the isotope (in this case fluorine-18).
Magnetic resonance imaging
More or less concurrently, magnetic resonance imagingMagnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
(MRI or MR scanning) was developed. Rather than using ionizing or x-radiation, MRI uses the variation in signals produced by proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s in the body when the head is placed in a strong magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. Associated with early application of the basic technique to the human body are the names of Jackson (in 1968), Damadian (in 1972), and Abe and Paul Lauterbur
Paul Lauterbur
Paul Christian Lauterbur was an American chemist who shared the Nobel Prize in Physiology or Medicine in 2003 with Peter Mansfield for his work which made the development of magnetic resonance imaging possible.Dr...
(in 1973). Lauterbur and Sir Peter Mansfield
Peter Mansfield
Sir Peter Mansfield, FRS, , is a British physicist who was awarded the 2003 Nobel Prize in Physiology or Medicine for his discoveries concerning magnetic resonance imaging . The Nobel Prize was shared with Paul Lauterbur, who also contributed to the development of MRI...
were awarded the 2003 Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine administered by the Nobel Foundation, is awarded once a year for outstanding discoveries in the field of life science and medicine. It is one of five Nobel Prizes established in 1895 by Swedish chemist Alfred Nobel, the inventor of dynamite, in his will...
for their discoveries concerning MRI. At first, structural imaging benefited more than functional imaging from the introduction of MRI. During the 1980s a veritable explosion of technical refinements and diagnostic MR applications took place, enabling even neurological tyros to diagnose brain pathology that would have been elusive or incapable of demonstration in a living person only a decade or two earlier.
Scientists soon learned that the large blood flow changes measured by H20-15 PET were also imaged by MRI. Functional magnetic resonance imaging
Functional magnetic resonance imaging
Functional magnetic resonance imaging or functional MRI is a type of specialized MRI scan used to measure the hemodynamic 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. 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.
Physicists have also developed other MRI-based techniques such as magnetic resonance spectroscopy (for measuring some key metabolites such as N-acetylaspartate and lactate within the living brain) and diffusion tensor imaging (for mapping white matter tracts within the living brain). Whereas structural MRI and CAT scanning have a large place in medicine, fMRI and its brethren techniques are still largely devoted to neuroscience research. However, very recently neurologists have started to use fMRI to begin to answer clinical questions, such as how long after thrombotic stroke is it safe and effective to give clot-dissolving drug like tissue plasminogen activator
Tissue plasminogen activator
Tissue plasminogen activator is a protein involved in the breakdown of blood clots. It is a serine protease found on endothelial cells, the cells that line the blood vessels. As an enzyme, it catalyzes the conversion of plasminogen to plasmin, the major enzyme responsible for clot breakdown...
(TPA). Similarly, PET and SPECT have moved out of neuro-research and are increasingly being used clinically to help diagnose and differentiate types of dementing illnesses (dementia
Dementia
Dementia is a serious loss of cognitive ability in a previously unimpaired person, beyond what might be expected from normal aging...
).
Multimodal neuroimaging
Multimodal imaging combines existing brain imaging techniques in synergistic ways which facilitate the improved interpretation of data.Besides fMRI, another example of technology allowing relatively older brain imaging techniques to be even more helpful is the ability to combine different techniques to get one brain map. This happens quite frequently with MRI and EEG
Electroencephalography
Electroencephalography is the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain...
scans. The electrical diagram of the EEG provides split-second timing while the MRI provides high levels of spatial accuracy.
Anatomically-constrained Magnetoencephalography (aMEG) is a relatively new technique which was first employed in 2000. It combines the spatial resolution of a structural MRI scan with the temporal resolution of the MEG
Magnetoencephalography
Magnetoencephalography is a technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain, using arrays of SQUIDs...
. Often the non-uniqueness of the MEG source estimation problem (inverse problem
Inverse problem
An inverse problem is a general framework that is used to convert observed measurements into information about a physical object or system that we are interested in...
) can be alleviated by incorporating information from other imaging modalities as an a priori constraint. aMEG uses anatomical MRI data as a geometrical or location constraint and as a medium for visualization of MEG results. MEG does not provide structural or anatomical information. Therefore, MEG data is often combined with MR data into a composite image whereby functional information is overlaid on the corresponding anatomy to produce an activation map.
Recent breakthroughs
Recent breakthroughs in non-invasive brain imaging have been somewhat limited because most of them have not been completely novel; rather, they are simply refining existing brain imaging techniques. fMRI is a perfect example of this from the early 1990s, and it still remains the most popular brain imaging technique available today.Advances have been made in a number of ways regarding neuroimaging, and this section will cover some of the more prominent improvements including computational advances, transcranial magnetic stimulation, and nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
.
To begin with, much of the recent progress has had to do not with the actual brain imaging methods themselves but with our ability to utilize computers in analyzing the data. For example, substantial discoveries in the growth of human brains from age three months to the age of fifteen have been made due to the creation of high-resolution brain maps and computer technology to analyze these maps over various periods of time and growth (Thompson, UCLA). This type of breakthrough represents the nature of most breakthroughs in neuroscience
Neuroscience
Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...
today. With fMRI technology mapping brains beyond what we are already understanding, most innovators time is being spent trying to make sense of the data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
we already have rather than probing into other realms of brain imaging and mapping.
This can be seen more clearly in the fact that brain imaging archives are catching on and neuroinformatics
Neuroinformatics
Neuroinformatics is a research field concerned with the organization of neuroscience data by the application of computational models and analytical tools. These areas of research are important for the integration and analysis of increasingly large-volume, high-dimensional, and fine-grain...
is allowing researchers to examine thousands of brains rather than just a few (Lynch). Also, these archives are universalizing and standardizing formats and descriptions so that they are more searchable for everyone. For the past decade we have been able to get data and now our technology allows us to share findings and research much easier. This has also allowed for "brain atlases" to be made. Brain atlases are simply maps of what normal functioning brains look like (Thompson, Bioinformatics).
Transcranial magnetic stimulation
Transcranial magnetic stimulation
Transcranial magnetic stimulation is a noninvasive method to cause depolarization or hyperpolarization in the neurons of the brain...
(TMS) is a recent innovation in brain imaging. In TMS, a coil is held near a person's head to generate magnetic field impulses that stimulate underlying brain cells to make someone perform a specific action. Using this in combination with MRI, the researcher can generate maps of the brain performing very specific functions. Instead of asking a patient to tap his or her finger, the TMS coil can simply "tell" his or her brain to tap his or her finger. This eliminates many of the false positives received from traditional MRI and fMRI testing. The images received from this technology are slightly different from the typical MRI results, and they can be used to map any subject's brain by monitoring up to 120 different stimulations. This technology has been used to map both motor processes and visual processes (Potts link at bottom of TMS). In addition to fMRI, the activation of TMS can be measured using electroencephalography
Electroencephalography
Electroencephalography is the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain...
(EEG) or near infrared spectroscopy
Near infrared spectroscopy
Near-infrared spectroscopy is a spectroscopic method that uses the near-infrared region of the electromagnetic spectrum...
(NIRS).
Nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
(NMR) is what MRI and fMRI technologies were derived from, but recent advances have been made by going back to the original NMR technology and revamping some of its aspects. NMR traditionally has two steps, signal encoding and detection, and these steps are normally carried out in the same instrument. The new discovery, however, suggests that using laser-polarized xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
gas for "remembering" encoded information and transporting that information to a remote detection site could prove far more effective (Preuss). Separating the encoding and detection allows researchers to gain data about chemical, physical
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, and biological
Biological process
A biological process is a process of a living organism. Biological processes are made up of any number of chemical reactions or other events that results in a transformation....
processes that they have been unable to gain until now. The end result allows researchers to map things as big as geological core samples or as small as single cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s.
It is interesting to see how advances are split between those seeking a completely mapped brain by utilizing single neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
imaging and those utilizing images of brains as subjects perform various high-level tasks. Single neuron imaging (SNI) uses a combination of genetic engineering and optical imaging techniques to insert tiny electrodes into the brain for the purpose of measuring a single neuron's firing. Due to its damaging repercussions, this technique has only been used on animals, but it has shed a lot of light on basic emotional and motivational processes. The goal of studies in higher-level activities is to determine how a network of brain areas collaborates to perform each task. This higher-level imaging is much easier to do because researchers can easily use subjects who have a disease such as Alzheimer's. The SNI technology seems to be going after the possibility for AI while the network-probing technology seems to be more for medical purposes.