Brain tests medical

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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.

Alternative Names

Brain nuclear medicine scan; Brain positron emission tomography

How the test is performed

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.

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).

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.

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.

Why the test is performed

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.

Normal Values

There are no problems detected in the size, shape, or function of the brain. There are no areas in which the radiotracer has abnormally collected.

What abnormal results mean

Abnormal results may be due to:

  • Alzheimer's disease or dementia
  • Brain tumors (cancerous and non-cancerous)
  • Epilepsy
  • Movement disorders (such as Parkinson's disease)

What the risks are

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.

Before receving the contrast, tell your health care provider if you take the diabetes medication metformin (Glucophage) because you may need to take extra precautions.

It is possible, although very unlikely, to have an allergic reaction to the radioactive tracer. Let your doctor know if you have ever had an allergic reaction to injected contrast dye. Some people have pain, redness, or swelling at the injection site.

Special 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.


Foldvary-Schaefer N, Wyllie E. Epilepsy. In: Goetz GC. Textbook of Clinical Neurology. 3rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 52.

Small GW, Bookheimer SY, Thompson PM, Cole GM, Hung SC, Kepe V, et al. Current and future uses of neuroimaging for cognitively impaired patients. Lancet Neurol. 2008;7:161-172.

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