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.
A PET scan may be used to evaluate people with the following illnesses:
- Cancer. PET scans can be used to detect cancerous tumors, to determine how much cancer has spread, and to determine how well cancer treatment is working. 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 scans also are used for research in other areas, including drug addiction, psychiatric illnesses and stroke. Medical specialists are just beginning to discover how PET scans can be used to evaluate a wide range of patients. New uses are being found every year. Because some types of PET scans still are considered experimental by some medical insurers, check with your doctor and your medical insurer before your scan to verify your coverage for this procedure.
Because a PET scan involves radioactivity, tell your doctor if you are pregnant or if there is a possibility that you might be pregnant. Tell your doctor if you think you will be unable to lie very still for 30 minutes to two hours during the PET scan.
A PET scan usually is done as an outpatient test in a major medical center that has a small cyclotron, very advanced nuclear medicine equipment used to make the PET tracer.
The PET scanner is a ring-shaped apparatus with an attached table. You will lie on the scanning table, and the table will slide slowly through the opening in the scanner ring. One or two scans might be taken before the tracer is administered. After this initial scanning, either you will inhale the tracer or it will be injected into one of your veins, usually in your arm. Additional scans will be taken while the tracer is in your body.
During the scanning procedure, you must lie very still. The scanning table will glide you through the PET scanner, so you won't need to move. If your head is being scanned, special cushions may be placed against your head to hold it in place. The entire scan should take 30 minutes to 2 hours. Afterward, you can go home and resume your normal activities.
Ask personnel at the scan facility about when you should call your doctor for the official scan report.
The radioactive tracers used in PET scans are considered to be safe, and because they are short-lived, they are quickly cleared from the body.
If the tracer was injected, call your doctor if you have pain, redness or swelling at the injection site.
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