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Quick Facts

Positron Emission Tomography (PET)


The Manual's Editorial Staff

Last full review/revision Oct 2019| Content last modified Oct 2019
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What is positron emission tomography (PET scan)?

A PET scan is a test doctors use to take pictures of your organs and tissues. First, doctors inject you with a tiny amount of a radioactive substance (a tracer). The tracer is attached to a chemical your body uses, such as sugar (glucose). A large donut-shaped machine then takes scans of your body as it uses the tracer. A computer takes these scans and creates many detailed pictures of how the inside of your body works. Each picture looks like a slice taken from one part of your body. The computer can also create a 3-D image of the inside of your body.

  • PET scans are good at showing how well your brain or heart is working

  • Doctors do PET scans to look for cancer or track if the cancer has spread

  • You get more radiation than from a plain x-ray

  • PET scans are expensive and aren't available in some places

Why would I need a PET scan?

Doctors do PET scans to look for many kinds of problems, including:

  • Problems with your blood flow

  • Problems with your brain or heart function

  • Finding cancer

  • Checking how well your cancer treatment is working or if your cancer has spread

What happens during a PET scan?

Before the test

Your doctor will tell you not to have any alcohol, caffeine, tobacco, or certain medicines for a period of time before the test. Tell your doctor if you're pregnant or breastfeeding.

During the test

Doctors give you the tracer by injecting it into your vein. It takes between 30 and 60 minutes for the tracer to move through your body.

You'll lie on a narrow, padded table. Doctors will slide the table into the center of the donut-shaped scanner. You'll lie still for about 45 minutes while the machine scans you. You may hear clicks or whirring sounds. Sometimes doctors ask you to do certain activities, such as answer questions, while they scan your brain.

After the test

You can go back to your usual activities.

What are the risks of having a PET scan?


A PET scan exposes you to more radiation than a plain x-ray. Doctors try to limit the total amount of radiation you're exposed to over your lifetime. Too much radiation can raise your chance of getting cancer.

Reactions to the tracer

An allergic reaction to the tracer used during a PET scan is rare but can happen to some people.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
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