imaging technique used in diagnosis and biomedical research. It has proved particularly useful for studying brain and heart functions and certain biochemical processes involving these organs (e.g., glucose metabolism and oxygen uptake). In PET a chemical compound labeled with a short-lived positron-emitting radionuclide of carbon, oxygen, nitrogen, or fluorine is injected into the body. The activity of such a radiopharmaceutical is quantitatively measured throughout the target organs by means of photomultiplier-scintillator detectors. As the radionuclide decays, positrons are annihilated by electrons (electron), giving rise to gamma rays (gamma ray) that are detected simultaneously by the photomultiplier-scintillator combinations positioned on opposite sides of the patient. The data from the detectors are analyzed, integrated, and reconstructed by means of a computer to produce images of the organs being scanned.

PET has become a valuable tool in the detection of cancer and cancer metastasis (spread) and in the evaluation of heart conditions. PET studies have helped scientists understand more about how drugs (drug) affect the brain and what happens during learning, when using language, and in certain brain disorders, such as stroke, depression, and Parkinson disease (parkinsonism). In addition, scientists are working to find ways to use PET to identify the biochemical nature of neurological disorders and mental disorders (mental disorder) and to determine how well therapy is working in patients. PET has revealed marked changes in the depressed brain, and knowing the location of these changes helps researchers understand the causes of depression and monitor the effectiveness of specific treatments.