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

Unlike magnetic resonance imaging (MRI) and computed tomography (CT) scans, which reveal the structure of the brain, a PET scan shows how the brain and its tissues are working.

A PET scan can reveal the size, shape, and function of the brain, so your doctor can make sure it is working as well as it should. It is most often used when other tests, such as MRI scan or CT scan, do not provide enough information.

This test can be used to:

• Diagnose cancer
• Evaluate a patient with epilepsy who may need surgery
• Help diagnose dementia if other tests and exams do not provide enough information

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

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. Or you may inhale the radioactive material as a gas. The substance travels through the blood and collects in the tissues of the brain.

You will 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 of your brain. You may be asked to read or name letters if your memory is being tested.

The test takes between 30 minutes and 2 hours.

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).

People with diabetes who take insulin injections will need special preparation. Call the PET scan office the day before the study for instructions.

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.