positron emission tomography (PET), imaging technique used indiagnosis and biomedical research. It has proved particularly useful for studyingbrain andheart functions and certain biochemical processes involving these organs (e.g.,glucosemetabolism andoxygen uptake). In PET achemical compound labeled with a short-livedpositron-emittingradionuclide ofcarbon, oxygen,nitrogen, orfluorine 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 areannihilated byelectrons, giving rise togamma rays 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 ofcancer and cancermetastasis (spread) and in the evaluation of heart conditions. PET studies have helped scientists understand more about howdrugs affect the brain and what happens duringlearning, when usinglanguage, and in certain brain disorders, such asstroke,depression, andParkinson disease. In addition, scientists are working to find ways to use PET to identify the biochemical nature of neurological disorders andmental disorders 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 specifictreatments.