A positron emission tomography (PET) scan is a type of imaging test. It uses a radioactive substance called a tracer to look for disease in the body.
A PET scan shows how organs and tissues are working. This is different than magnetic resonance imaging (MRI) and computed tomography (CT), which show the structure of, and blood flow to and from organs. Many places have machines that combine the PET and CT images, so that only one exam is performed.
Related tests include:
Positron emission tomography
How the Test is Performed
A PET scan uses a small amount of radioactive material (tracer). The tracer is given through a vein (IV). The needle is most often inserted on the inside of your elbow. The tracer travels through your blood and collects in organs and tissues. This helps the radiologist see certain areas more clearly.
You will need to wait as the tracer is absorbed by your body. This takes about 1 hour.
Then, you will lie on a narrow table that slides into a large tunnel-shaped scanner. The PET detects signals from the tracer. A computer changes the signals into 3D pictures. The images are displayed on a monitor for your health care provider to read.
You must lie still during the 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 to 6 hours before the scan. You will be able to drink water. If you have diabetes, your health care provider will tell you not to take your diabetes medicine before the test because it will interfere with the results.
Tell your 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 provider about the medicines you are taking, including those bought without a prescription. Sometimes, medicines may interfere with the test results.
How the Test will Feel
You may feel a sharp sting when the needle with 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
- Determine if a mass in your lung is cancerous or harmless
Several PET scans may be taken over time to check how well you are responding to treatment for cancer or another illness.
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
The amount of radiation used in a PET scan is about the same amount as used in most CT scans. These scans use short-lived tracers, so the radiation is gone from your body in about 2 to 10 hours.
Tell your doctor before having this test if you are pregnant or breastfeeding. Infants and babies developing in the womb are more sensitive to radiation because their organs are still growing.
Rarely, people may have an allergic reaction to the tracer material. 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. This helps find the exact location of the tumor.
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Reviewed By: Jason Levy, MD, Northside Radiology Associates, Atlanta, GA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.