Spring 1995
Mapmaker's tools
Three main tools for brain mapping have individual strengths, and they often are used in combination to identify how the brain functions. Here is how they work and the jobs they do best:
MRI
MRI, or magnetic resonance imaging, produces images of the body's soft tissues such as those in the brain. It can distinguish structures even .05-inch apart. MR imagery uses pulses of radio-frequency energy to create contrasts, and thus identify structures. Varied techniques such as fMRI, or functional MRI, also allow images to show cognitive functions. MRI is commonly used to diagnose diseases or injuries without X-rays or surgery. In brain mapping, MRI helps define and locate parts of the brain most active when the brain performs various tasks.
PET
PET, or positron-emission tomography, can track blood flow, energy metabolism and receptor binding in the brain. In PET studies, the patient is injected with a biochemical, such as glucose, that is "labeled" with a radioactive substance that emits positively charged particles, or positrons. The positrons combine with negatively charged electrons normally found in the body's cells to produce gamma rays. The PET device detects the gamma rays and converts them into color-coded images. PET is used to diagnose epilepsy and tumors and help in planning neurosurgery. In brain mapping, PET gives a working portrayal of many brain functions.
EEG
EEG, or electroencephalography, records the brain's electrical activity. Brain cells produce electricity when they are active. Electrodes placed on a person's scalp record the voltage. An EEG session can help diagnose epilepsy, stroke or head injury. In brain mapping, it is most valuable in identifying how long it takes the brain to perform functions. EEG devices record in intervals of 1 millisecond, the equivalent of 1,000th of a second. The EEG also detects voltage fluctuations common in cognitive exercises and approximate locations of the activity.
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