ADAM Health Encyclopedia

PET scan

Definition

Positron emission tomography (PET) 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 organs, a PET scan shows how the organs and tissues are functioning.

See also:

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

Alternative Names

Nuclear medicine scan; Positron emission tomography

Why the Test is Performed

A PET scan can reveal the size, shape, position, and some function of organs, so your doctor can make sure they are working as well as they should.

This test can be used to diagnose cancer, heart problems, and brain disorders. It can see how far cancer has spread, show areas in which there is poor blood flow to the heart, and check brain function.

Several PET scans may be taken to determine how well you're responding to treatment for cancer or another illness.

How the Test is Performed

PET scans use a small amount of a radioactive substance injected into a vein, usually on the inside of the elbow. The substance travels through the blood and collects in organs or tissues.

You will be scanned approximately 60 minutes after receiving the radioactive substance. You lie 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 converts it into 3-dimensional pictures. The images are sent to a computer, where they are displayed on a monitor for the physician to read.

You must lie still during the PET scan so that the machine can get clear images of your organs. The test takes about 30 minutes.

How to Prepare for the Test

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 doctor if you are pregnant or think you might be pregnant.

Also tell your doctor 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.

How the Test Will Feel

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.

Risks

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 fetuses 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 tracer. Some people have pain, redness, or swelling at the injection site.

Considerations

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.

Normal Results

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.

What Abnormal Results Mean

• 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