ABSTRACT
The noninvasive study of biochemical and physiological processes in organ systems with in vivo radiotracers has been made possible by the development of the medical cyclotron in conjunction with quantitative gamma ray scintigraphy, particularly, positron emission tomography (PET) 1–10 The medical cyclotron has been used for the production of various shortlived, positron emitting radionuclides including carbon-11 (t1/2 = 20.4 min), nitrogen-13 (t1/2 = 9.9 min), oxygen-15 (t1/2 = 2.0 min), and fluorine-18 (t1/2 = 110 min).4 This has led to the production of a wide variety of biological molecules labeled with such isotopes by either organic or enzymatic procedures. PET provides cross-sectional images of the tissue distributions of biomolecules labeled with a positron emitting radionuclide. 5 , 7–9 The basis of the imaging technique is the coincidental detection of two oppositely directed 511 keV photons by external gamma ray detectors. The 511 keV photons are formed by a mass-energy transmutation of an positron emitted from the nucleus of the radioactive atom with a nearby electron. The detectors are arranged circumferentially around the subject, with each opposing pair forming a line of coincidence. An event is recorded by a given set of detectors if the resulting photon pair falls into that line of coincidence. The data from many events are collected, corrections made for attenuation of the photons within the subject and cross-sectional images, then reconstructed in an analogous fashion to X-ray computed tomography (CT).
