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.

Positron Emission Tomography (PET) provides physicians with unique diagnostic information which may alter patient management and reduce the total cost of patient care. It produces images of molecular-level physiological function which can be used to measure many vital processes, including glucose metabolism, blood flow and perfusion, and oxygen utilization. With these images, physicians can identify normal and abnormal states.

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 ringed-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 performed immediately because the positron-emitting tracers usually decay (lose their positrons) rather quickly.

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A PET scan may be used to evaluate patients with the following illnesses:

• Cancer — PET scans can be used to detect cancerous tumors, to determine cancer stage (extent of spread) and to judge the effectiveness of cancer treatment. They are used most often in patients with brain cancer, colorectal cancer, lymphoma, melanoma or lung cancer.
• Brain diseases — PET scans can be used to evaluate neurological illnesses, especially epilepsy, and Alzheimer's disease and other dementias.
• Cardiac illnesses — PET scans can be used to evaluate how well the heart muscle is functioning in patients with coronary artery disease or cardiomyopathy.

Pet Scan