A positron emission tomography (PET) scan is an imaging test that uses a radioactive substance (called a tracer) to look for disease in the body.

Unlike magnetic resonance imaging (MRI) and computed tomography (CT) scans, which reveal the structure of and blood flow to and from organs, a PET scan shows how organs and tissues are working.

See also:

• Bone scan
• Nuclear ventriculography
• Pulmonary ventilation/perfusion scan
• Renal scan
• Thyroid scan

The health care provider will inject a small amount of a radioactive material into one of your veins, usually on the inside of the elbow. The substance travels through the blood and collects in organs and tissues.

You'll be asked to wait nearby as the radioactive substance is absorbed by your body. This usually takes about 1 hour.

Then, you will lie down on a table that slides into a tunnel-shaped hole in the center of the PET scanner.

The PET machine detects energy given off by the radioactive substance and changes it into 3-dimensional pictures. The images are sent to a computer, where they are displayed on a monitor for the health care provider to read.

You must lie still during the PET scan so that the machine can produce clear images. How long the test takes depends on what part of the body is being scanned.

You must sign a consent form before having this test. You will be told not to eat anything for 4 - 6 hours before the PET scan, although you will be able to drink water.

Tell your health care provider if you are pregnant or think you might be pregnant.

Also tell your health care provider about any prescription and over-the-counter medicines that you are taking, because they may interfere with the test.

Be sure to mention if you have any allergies, or if you've had any recent imaging studies using injected dye (contrast).

During the test, you may need to wear a hospital gown. Take off any jewelry, dentures, and other metal objects because they could affect the scan results.

You will feel a sharp prick when the needle with the radioactive substance is inserted into your vein. You shouldn't feel anything during the actual PET scan.

A PET scan can reveal the size, shape, position, and some function of organs.

This test can be used to:

• Check brain function
• Diagnose cancer, heart problems, and brain disorders
• See how far cancer has spread
• Show areas in which there is poor blood flow to the heart

Several PET scans may be taken over time to determine how well you are responding to treatment for cancer or another illness.

There are no problems detected in the size, shape, or position of an organ. There are no areas in which the radiotracer has abnormally collected.

• Abnormal heart function
• Abnormal size, shape, or position of an organ
• Alzheimer's disease
• Cancer (presence and how far it has spread)
• Change in organ function
• Growths
• Infection

The amount of radiation used in a PET scan is low. It is about the same amount of radiation as in most CT scans. Also, the radiation doesn't last for very long in your body.

However, women who are pregnant or are breastfeeding should let their doctor know before having this test. Infants and babies developing in the womb are more sensitive to the effects of radiation because their organs are still growing.

It is possible, although very unlikely, to have an allergic reaction to the radioactive subtance. Some people have pain, redness, or swelling at the injection site.

It is possible to have false results on a PET scan. Blood sugar or insulin levels may affect the test results in people with diabetes.

Most PET scans are now performed along with a CT scan. This combination scan is called a PET/CT.

Beller GA, Kramer CM. Nuclear cardiology and computed tomography. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 54.

Ettinger DS. Lung cancer and other pulmonary neoplasms. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 201.

Hackney D. Radiologic imaging procedures. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 419.