a technique in nuclear medicine for cross-sectional imaging that enables a noninvasive assessment and localization of metabolic activity to be made. Emission of a positron by a radioisotope results in annihilation of the positron on collision with an electron, and the creation of two gamma rays of known energy travelling in exactly opposite directions. The PET scanner has detectors on each side of the patient to detect the simultaneous arrival of the gamma rays. Images are created using reconstruction algorithm similar to CT scanning. Fluorodeoxyglucose (FDG), using fluorine-18, is used to examine glucose metabolism, and ammonia, using nitrogen-13, gives information on perfusion. Carbon-11 and oxygen-15 can also be used as radioisotopes for PET scanning. Some diseases result in decreased uptake of the radio-labelled material due to decreased function; others, including many tumours, show increased glucose metabolism and concentrate the isotope avidly. In this way functional activity of the tissues can be compared with anatomical images obtained by CT or MRI scanning. Originally used to study activity in the brain, PET is now also used for investigating the chest and abdomen. See also tomography. Compare computerized tomography.
Medical dictionary. 2011.