URL of this page: https://medlineplus.gov/lab-tests/pet-scan/

PET Scan

What is a PET scan?

A PET (positron emission tomography) scan is an imaging test that looks at how well your tissues and organs are working. It also checks for signs of cancer. The scan uses a small amount of a radioactive substance called a tracer.

The tracer settles in areas of your body that have high levels of chemical activity. This activity can be a sign of cancer or other diseases. The tracer can also help measure blood flow, oxygen use, and changes in the metabolism of a particular tissue or organ. Metabolism is a chemical process that your body uses to change food into energy.

Chemical changes in the body can show up before symptoms of disease appear. So a PET scan can find signs of disease at an early stage, often before problems can be seen on other imaging tests.

Other names: positron emission tomography

What is it used for?

A PET scan is most often used to:

A PET scan is often done along with a CT scan, a type of x-ray that takes a series of pictures as it rotates around you.

Why do I need a PET scan?

You may need a PET scan to find out if you have cancer. If you've already been diagnosed with cancer, you may need this test to see if your cancer treatment is working.

A PET scan also helps diagnose and monitor heart and brain diseases. So you may need this test if you:

What happens during a PET scan?

Before the scan, you will change into a hospital gown. You may be asked to empty your bladder. During the scan:

  • A health care provider will inject the radioactive tracer into your vein through an intravenous (IV) line. The tracer sends out a form of energy called gamma rays. The rays are picked up by the scanner to create images of the inside of your body.
  • You will need to wait for the tracer to be absorbed by your body. This takes 45 to 60 minutes.
  • You will then lie on a narrow, padded table, which will slide into a large, tunnel-shaped scanner.
  • The scanner will move slowly across your body to capture images. You will need to be very still as this happens.
  • The scanner will send images to a computer monitor for the provider to review.
  • Your provider will then remove the IV line.

Will I need to do anything to prepare for the test?

You may be asked to not eat or drink for four to six hours before the test.

If you have diabetes and use insulin, you may need to change the timing of your regular dose. Your provider will give you specific instructions about adjusting your insulin.

Also, tell your provider if you have claustrophobia (fear of enclosed spaces). Your provider may decide to give you a medicine before the test to help you relax.

Are there any risks to the test?

There is very little exposure to radiation in a PET scan. Only a small amount of radioactive substance is used, and all of the radiation leaves the body within 2 to 10 hours.

While radiation exposure in a PET scan is safe for most adults, it can be harmful to an unborn baby. So be sure to tell your provider if you are pregnant or think you may be pregnant. Also, tell your provider if you are breastfeeding, because the tracer may contaminate your breast milk.

Allergic reactions to the tracer are rare and usually mild.

What do the results mean?

Depending on which part of the body was being scanned, your results may show:

  • Cancer. Cancer cells show up as bright spots on a PET scan.
  • Heart disease. The scan can show decreased blood flow to the heart.
  • A brain disorder. The scan may show changes in certain brain chemicals that can indicate disease.

If you also had a CT scan, your provider will review the results of the two scans to help make a diagnosis.

If you have questions about your results, talk to your health care provider.

Is there anything else I need to know about a PET scan?

Your PET scan results will be looked at by a radiologist, a doctor who specializes in diagnosing and treating medical conditions using imaging technologies. He or she will share the results with your health care provider.


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The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.