Within the past few years a more precise localization of the emitted radiation has been made possible by the development of a technique analogous to that of computerized axial tomography (CAT), which constructs an image by rotating a source of X rays completely around a human subject. A detector on the other side of the subject from the X-ray source rotates synchronously with the source to record variations in the absorption of the radiation as it passes through the intervening tissues. A computer reconstructs the image in the form of an axial cross section. It is evident that by the time a pathological condition shows up either in a conventional X-ray image or in a CAT scan the underlying biochemical abnormality is usually well advanced. The technique that combines early biochemical assessment of pathology achieved by nuclear medicine with the precise localization achieved by computerized image reconstruction is positron-emission tomography (PET). In this technique a chemical compound with the desired biological activity is labeled with a radioactive isotope that decays by emitting a positron, or positive electron. The emitted positron almost immediately combines with an electron, and the two are mutually annihilated with the emission of two gamma rays. The two gamma rays fly off in very nearly opposite directions, penetrate the surrounding tissue and are recorded outside the subject by a circular array of detectors. A mathematical algorithm applied by computer rapidly reconstructs the spatial distribution of the radioactivity within the subject for a selected plane and displays the resulting image on a cathode-ray screen. The images are recorded at intervals after the administration of the labeled compound and can be color-coded to show differences in the level of activity from point to point. With suitable interpretation PET images can provide a noninvasive, regional assessment of many biochemical processes that are essential to the functioning of the organ that is being visualized.
|Number of pages||12|
|State||Published - Jan 1 1980|