A positron emission tomography, or PET, scan is an imaging technique that uses positively charged particles (radioactive positrons) to detect subtle changes in the body's metabolism and chemical activities. A PET scan provides a color-coded image of the body's function, rather than its structure.
During a PET scan, a substance called a tracer that produces radioactive positrons either is injected into a vein or inhaled as a gas. This tracer is typically a chemical that is normally found in the body (carbon, nitrogen, oxygen) that has been altered to allow it to emit positrons. Once the tracer enters the body, it travels through the bloodstream to a specific target organ, such as the brain or heart. There the tracer emits positrons, which collide with electrons (negatively charged particles), producing gamma rays (similar to X-rays). These gamma rays are detected by a ring-shaped PET scanner and analyzed by a computer to form an image of the target organ's metabolism or other functions.
A PET scan is painless, except for a mild skin prick if the tracer is injected. Once the tracer is given, the PET scan must be done immediately because the positron-emitting tracers usually decay (lose their positrons) rather quickly.