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Positron Emission Tomography
(PET Scan)


Positron emission tomography (PET) combines the techniques of nuclear scanning and biochemical analysis to assess body processes like blood flow and metabolism in certain organs. In this test, a radioactive tracer is tagged to a biologically active molecule—such as glucose, oxygen, carbon monoxide, hormones, or neurotransmitters—and introduced into the body, usually by injection. After you enter a special scanning machine, radiation detectors record the emissions of the radioactive material. This information is relayed to a computer, which constructs color-coded, cross-sectional images depicting which areas of the organ are active. PET scanning can provide valuable information not only about the structure of particular organs, but also about how they work.


Purpose of the Test

To assess various body processes—particularly brain activity, but also activity in the heart, lungs, and other tissues and organs.

To evaluate neurologic illnesses, including epilepsy, Alzheimer disease, and other types of dementia.

To help determine heart muscle function in patients with heart disease and distinguish between viable and dead cardiac tissue during the early stages of a heart attack.

To detect cancerous tumors, determine the stage of cancer, and evaluate the effectiveness of cancer therapy.

Who Performs It

A specially trained radiologist or technician.

Special Concerns

PET scanning is very costly (because the positron-emitting radiotracers used for this test must first be generated by a particle accelerator, or cyclotron), and is not routinely performed outside of major medical institutions.

This test should not be performed in pregnant or breastfeeding women because of possible risks to the fetus or infant.

Extreme obesity may limit the accuracy of PET scans of the heart or lung.

Drugs such as tranquilizers and sedatives as well as recent use of caffeine, alcohol, or tobacco may alter the test results.

When radiolabeled glucose is used, the presence of diabetes may affect the results. Blood sugar levels must be monitored during testing in most patients.

Before the Test

Do not ingest alcohol, caffeine, tobacco, sedatives, or tranquilizers for 24 hours before the procedure.

If you have diabetes, you will be asked to take your pretest dose of insulin at a meal 3 to 4 hours before the test.

Two intravenous (IV) lines may be placed in veins in your arms, one for infusing the radioactive tracer and the other for taking a series of blood samples.

Empty your bladder before the test.

What You Experience

You will lie down on a table.

The radioactive tracer is either injected through one of the IV lines or inhaled in the form of a radioactive gas.

You enter into the PET scanning machine, and the gamma rays emitted by the radiotracer are recorded by a circular array of detectors. The resulting images are displayed on a computer.

You must lie very still during the procedure.

If you are undergoing a PET scan of the brain, special cushions may be placed against your head to hold it in place. You may be asked to perform various cognitive activities, such as doing a mathematical calculation or remembering a sequence of words. To minimize external stimuli, you may be asked to wear a blindfold and earplugs.

The procedure can take from 1 to 2 hours.

Risks and Complications

The radioactive tracers used in PET scans are short-lived and rapidly cleared from the body. They are not associated with any significant risks or complications.

After the Test

You may be advised to stand up slowly after the procedure to avoid feeling faint or dizzy.

You are free to leave the testing facility and resume your normal activities.

Drink plenty of fluids to help flush the radioactive material from your body.

Blood may collect and clot under the skin (hematoma) at the IV needle insertion site(s); this is harmless and will resolve on its own. For a large hematoma that causes swelling and discomfort, apply ice initially; after 24 hours, use warm, moist compresses to help dissolve the clotted blood.


A radiologist reviews the PET scan data for evidence of abnormalities.

If a definitive diagnosis can be made, appropriate therapy will be initiated.

In some cases, additional tests, such as magnetic resonance angiography, may be needed to establish a diagnosis and determine the extent of the problem.


From The Johns Hopkins Consumer Guide to Medical Tests. You can order this book now on our secure server.


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