PositronPositron emission tomographyPositron emission tomography (Pet scan) is a beautiful example as to how a typical nuclear process, ‘positronAnnihilation annihilation’, may be used to identify human body parts of higher metabolic activityMetabolic activity. It has been observed that the metabolic activityMetabolic activity (sugar to energy conversion pathways) of abnormal body parts (like the sites of cancer, brain parts with abnormal neuroactivity, etc.) consumes sugar at a faster rate than the corresponding normal part. As a result if sugar is injected in the body, it accumulates at these abnormal activity locations in the body. Sugar loaded with some short-lived radioactive (tracer) material that emits positronsPositron in decay is injected in the body through an intravenous line. The externally injected tracer sugar accumulates at sites of abnormal metabolic activities in about half an hour. The body is then scanned for the pair of annihilationAnnihilation gamma rays of 511 keV energy emitted in opposite direction in the process of electron—positron annihilation at the site of the abnormal activity. All details of the method are given in this chapter.