Brain imaging is a fairly recent discipline within medicine and neuroscience. Brain imaging falls into two broad categories -- structural imaging and functional imaging. The former deals with the overall structure of the brain and the precise diagnosis of intracranial disease and injury. The latter is used for neurological and cognitive science research and building brain-computer interfaces. It enables, for example, the processing of sensory information coming to the brain and of commands going from the brain to the organism to be "lit up" or visualized directly instead of by simple clinical inference.
Types of brain imaging
Computed axial tomography (CT or CAT) scanning uses a series of x-rays of the head taken from many different directions. Typically used for quickly viewing brain injuries, CT scanning has a computer program that uses a set of algebraic equations to estimate how much x-ray is absorbed in a small area within a cross section of the brain (Jeeves 21). In the final analysis, the harder a material is, the whiter it will appear on the scan. CT scans are primarily used for evaluating swelling from tissue damage in the brain and in assessment of ventricle size. Modern CT scanning exposes the subject to about as much radiation as a single x-ray and can provide reasonably good images in a matter of minutes.
Magnetic Resonance Imaging (MRI) uses magnetic fields and radio waves to produce high quality two- or three-dimensional images of brain structures without injecting radioactive tracers. During an MRI, a large cylindrical magnet creates a magnetic field 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 frequency 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 anatomical 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 of the brain, it fails to provide information about how well the brain is working at the time of imaging. The distinction is now made between MRI imaging and functional imaging since the brain's function rather than the brain's structure is of interest.
Electroencephalography (EEG) is the oldest of the modern brain imaging techniques and uses electrodes placed on the scalp to detect and measure patterns of electrical activity coming from the brain. There have been many recent developments regarding EEG's ability to read brain activity data from the entire head simultaneously (Thompson, Bioinformatics). Using scale electrodes, EEG can determine the relative strengths and positions of electrical activity in different brain regions by measuring electrical activity on the outside of the brain. EEG records timing of activity very precisely but resolution is poor and does not directly record interior brain activity. As a result, researchers often use EEG images of brain electrical activity in combination with MRI scans to better pinpoint the location of the activity in the brain.
Positron Emission Tomography (PET) measures emissions from radioactively labeled chemicals that have been injected into the bloodstream and uses the data to produce two or three-dimensional images of the distribution of the chemicals throughout the brain (Nilsson 57). PET scans involve the use of a machine called a cyclotron to label chemicals with small amounts of radioactivity. The labeled compound, called 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 different regions of the brain. A computer uses the data gathered by the sensors to create multicolored two or three-dimensional images that show where the compound acts in the brain.
The greatest benefit of PET scanning is that different compounds can show blood flow and oxygen and glucose metabolism in the tissues of the working brain. These measurements reflect the amount of brain activity in the various regions of the brain and allow us to learn more about how the brain works. PET scans were superior in terms of resolution and speed of completion (as little as 30 seconds) when they first came online. The improved resolution permitted better judgments 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 (Nilsson 60). Before fMRI technology came online, PET scanning was the preferred method of brain imaging, and it still continues to make large contributions to neuroscience.
Similar to PET, single photon emission computed tomography (SPECT) uses radioactive tracers and a scanner to record data that a computer uses to construct two- or three-dimensional images of active brain regions (Ball). SPECT tracers are considered to be more limited than PET scanners in the kinds of brain activity they have the ability to monitor. The tracers of SPECT are longer lasting than those of PET, which allows for different, longer lasting brain functions to be examined, but this also requires more time for the SPECT to be completed. The resolution of a SPECT is poor (about 1 cm) compared to that of PET. SPECT is often chosen over PET simply as a cost issue, for less equipment is involved and fewer staff is required to perform the tests.
Magnetoencephalography (MEG) is similar to EEG, but magnetic fields are measured instead of electric fields.
Functional MRI (fMRI) relies on the magnetic properties of blood to enable scientists to see images of blood flow in the brain as it occurs. This mapping of blood flow allows for dynamic brain mapping to take place (Shorey). During the test, the subject is normally asked to perform a repetitive motion like tapping a finger or tapping a foot. FMRI has taken the place of PET scanning as the king of brain imaging because fMRI can produce images of the brain every second, and scientists can determine with great precision when brain regions become active and for how long. Also, fMRI has such high resolution that it can distinguish structures less than a millimeter apart. This allows scientists to know exactly which areas of the brain are being activated. PET, however, retains the significant advantage of being able to identify which brain receptors are being activated by neurotransmitters, drugs, and potential treatment compounds.
Drawbacks of fMRI are few but substantial at this point. First, it takes quite a bit of time to perform the procedure and the patient needs to be completely still for often more than twenty minutes at a time. Second, and more importantly, interpretations of fMRI results are still vague. It is difficult to determine if the subject was thinking about something that caused certain parts of the brain to activate, if the scanner picked up real data or noise, and so on (Shorey). For these and other reasons, fMRI technology has begun to be combined with EEG technology.
See main article History of brain imaging
In 1918 the American neurosurgeon Walter Dandy introduced the technique of ventriculography whereby X-ray 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. 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.
In 1927 Egas Moniz, professor of neurology in Lisbon, 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 Cormack and Godfrey Newbold Hounsfield brought about the use 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, the development of radioligands allowed single photon emission computed tomography (SPECT) and positron emission tomography (PET).
More or less concurrently, magnetic resonance imaging (MRI or MR scanning) was developed by researchers including Peter Mansfield and Paul Lauterbur, who were awarded the Nobel Prize for Physiology or Medicine in 2003. 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 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.
In early 2000s the field of brain imaging reached the stage where limited practical applications of functional brain imaging became feasible. The main application area is crude forms of brain-computer interface.
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