Positron emission tomography (PET), also known as emission computed tomography, is a noninvasive procedure that combines the tomographic capabilities of computed tomography scanning with the use of radiopharmaceuticals to provide clinical information about physiological organ function and structure.5 The physiological organ activities that are studied include tissue metabolism; structure or density; and body fluid volume or flow, or both.
These studies involve the IV administration of tracers tagged with a radioactive isotope designed to measure biochemical activities. This isotope emits positrons (positive electrons) as it deteriorates or decays; the positrons combine with electrons (negative electrons) normally found in specific tissue cells. The combination then emits 
-rays that can be detected by a PET scanner or camera that transmits the information to a computer.6 The computer determines the location and distribution of the radioactivity and translates the emissions as color-coded images on a screen. The images can be viewed immediately, and the information and images are stored for future use.
The radionuclides used for PET are produced by a cyclotron and are different from those produced by a nuclear reactor and used in conventional scanning. They have a short half-life and are free of contaminants.7 Only institutions that have access to this capability and have a team of professionals and a facility to accommodate this technology can perform PET studies because the radionuclide must be used immediately after its production. The isotopes used are oxygen 15 (15O), nitrogen 13 (13N), fluorine 18 (18F), and carbon 11 (11C), which are the main body constituents.
Although PET is more expensive than are conventional nuclear scans, it is becoming a widely used diagnostic method, especially for analysis of brain and heart pathology, staging of disease, and determining the effectiveness of therapy protocols.8 It also has diagnostic value in the study of the function of other regions or organs of the body through its ability to obtain physiological and biochemical information not possible with other scanning methods.