A positron emission tomography (PET) scan is an imaging test that uses a radioactive substance called a tracer to look for disease in the body.
Unlike magnetic resonance imaging (MRI) and computed tomography (CT), which reveal the structure of and blood flow to and from organs, a PET scan shows how organs and tissues are working.
Positron emission tomography
How the Test is Performed
A PET scan requires a small amount of radioactive material (tracer). This tracer is given through a vein (IV), usually on the inside of your elbow. It travels through your blood and collects in organs and tissues. The tracer helps the radiologist see certain areas or diseases more clearly.
You will need to wait nearby as the tracer is absorbed by your body. This usually takes about 1 hour.
Then, you will lie on a narrow table, which slides into a large tunnel-shaped scanner. The PET scanner detects signals from the tracer. A computer changes the results into 3-D pictures. The images are displayed on a monitor for your doctor to read.
You must lie still during test. Too much movement can blur images and cause errors.
How long the test takes depends on what part of the body is being scanned.
How to Prepare for the Test
You may be asked not to eat anything for 4 - 6 hours before the scan. You will be able to drink water.
Tell your health care provider if:
- You are afraid of close spaces (have claustrophobia). You may be given a medicine to help you feel sleepy and less anxious.
- You are pregnant or think you might be pregnant.
- You have any allergies to injected dye (contrast).
Always tell your health care provider about the medicines you are taking, including those bought without a prescription. Sometimes, medications may interfere with the test results.
How the Test Will Feel
You may feel a sharp sting when the needle containing the tracer is placed into your vein.
A PET scan causes no pain. The table may be hard or cold, but you can request a blanket or pillow.
An intercom in the room allows you to speak to someone at any time.
There is no recovery time, unless you were given a medicine to relax.
Why the Test is Performed
A PET scan can reveal the size, shape, position, and some function of organs.
This test can be used to:
- Check brain function
- Diagnose cancer, heart problems, and brain disorders
- See how far cancer has spread
- Show areas in which there is poor blood flow to the heart
Several PET scans may be taken over time to determine how well you are responding to treatment for cancer or another illness.
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A normal result means there were no problems seen in the size, shape, or position of an organ. There are no areas in which the tracer has abnormally collected.
What Abnormal Results Mean
Abnormal results depend on the part of the body being studied. Abnormal results may be due to:
- Change in the size, shape, or position of an organ
- Problem with organ function
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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.
Women who are pregnant or are breastfeeding should let their doctor know before having this test. Infants and babies developing in the womb are more sensitive to the effects of radiation because their organs are still growing.
It is possible, although very unlikely, to have an allergic reaction to the radioactive substance. Some people have pain, redness, or swelling at the injection site.
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.
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David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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