- (written as positron emission tomography)Its abbreviation is PET. A medical imaging technique which detects the movement and concentration of an injected radioactive tracer, to monitor and study metabolic activity in a patient. The tracer emits positrons, which collide with electrons in the body and produce gamma radiation which is then detected and quantified. PET is especially useful in examining and diagnosing brain function and capabilities, in addition to tumors and strokes. Also called positron emission tomography scan.