PET scan or positron emission tomography, a medical imaging technique that monitors metabolic, or biochemical, activity in the brain and other organs by tracking the movement and concentration of a radioactive tracer injected into the bloodstream. The technique uses special computerized imaging equipment and rings of detectors surrounding the patient to record gamma radiation produced when positrons (positively charged particles) emitted by the tracer collide with electrons.

PET scans are especially valuable in imaging the brain. They are used in medicine to diagnose brain tumors and strokes, and to locate the origins of epileptic activity; in psychiatry to examine brain function in schizophrenia, bipolar disorder, and other mental illnesses; and in neuropsychology to study such brain functions and capabilities as speech, reading, memory, and dreaming.