Brain imaging
Brain imaging
Individuals with brain damage have been studied extensively to provide information about the role of different areas of the brain.
Recent advances in technology allow us to identify similar information on individuals who have not suffered brain injury by using imaging techniques that allow us to gain insights on brain structure and function.
Techniques involving radiation
A computerised tomography (CT) scan involves taking a number of x-rays of a particular section of a person's body or brain. The x-rays pass through tissues of different densities at different rates, allowing a computer to construct an overall image of the area of the body being scanned.
A CT scan is often used to determine whether someone has a tumour, or significant brain atrophy.
The following CT scans show healthy brain (A) and a brain with a tumour in the left frontal lobe (B).
Image credit: (A) A modification of work by "Aceofhearts1968"/Wikimedia Commons. (B) A modification of work by Roland Schmitt et al.
Positron emission tomography
Positron emission tomography (PET) scans create pictures of the living, active brain. An individual receiving a PET scan drinks or is injected with a mildly radioactive substance, called a tracer. Once in the bloodstream, the amount of tracer in any given region of the brain can be monitored.
As brain areas become more active, more blood flows to that area. A computer monitors the movement of the tracer and creates a rough map of active and inactive areas of the brain during a given behaviour.
PET scans show little detail, are unable to pinpoint events precisely in time, and require that the brain be exposed to radiation. Therefore, this technique has been replaced by the fMRI as an alternative diagnostic tool.
However, combined with CT, PET technology is still being used in certain contexts. For example, CT/PET scans allow better imaging of the activity of neurotransmitter receptors and open new avenues in schizophrenia research.
Using a hybrid approach with CT/PET technology, CT contributes clear images of brain structures, while PET shows the brain's activity.
Image credit: Health and Human Services Department, National Institutes of Health.
Techniques involving magnetic fields
In magnetic resonance imaging (MRI), a person is placed inside a machine that generates a strong magnetic field. The magnetic field causes the hydrogen atoms in the body's cells to move. When the magnetic field is turned off, the hydrogen atoms emit electromagnetic signals as they return to their original positions.
Tissues of different densities give off different signals, which a computer interprets and displays on a monitor. Functional magnetic resonance imaging (fMRI) operates on the same principles, but it shows changes in brain activity over time by tracking blood flow and oxygen levels.
The fMRI provides more detailed images of the brain's structure, as well as better accuracy in time, than is possible in PET scans.
With their high level of detail, MRI and fMRI are often used to compare the brains of healthy individuals to the brains of individuals diagnosed with psychological disorders. This comparison helps determine what structural and functional differences exist between these populations.
The following image represents a single frame from an fMRI.
Image credit: A modification of work by Kim J, Matthews NL, Park S.
Techniques involving electrical activity
In some situations, it is helpful to gain an understanding of the overall activity of a person's brain, without needing information on the actual location of the activity.
Electroencephalography (EEG) serves this purpose by providing a measure of a brain's electrical activity. An array of electrodes is placed around a person's head. Using caps with electrodes, modern EEG research can study the precise timing of overall brain activities.
Image credit: SMI Eye Tracking.
The signals received by the electrodes result in a printout of the electrical activity of his or her brain, or brain waves, showing both the frequency (number of waves per second) and amplitude (height) of the recorded brainwaves, with an accuracy within milliseconds.
This information is especially helpful to researchers studying sleep patterns among individuals with sleep disorders.
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