PET scan

Definition

A positron emission tomography scan is a type of imaging test. It uses a radioactive substance called a tracer to look for disease in the body.

A positron emission tomography (PET) scan shows how organs and tissues are working.

• This is different than MRI and CT scans. These tests show the structure of, and blood flow to and from organs.
• Machines that combine the PET and CT images, called a PET/CT, are commonly used.

Alternative Names

Positron emission tomography; Tumor imaging - PET; PET/CT

How the Test is Performed

A PET scan uses a small amount of radioactive 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 but no other beverages including coffee. If you have diabetes, your provider will tell you not to take your diabetes medicine before the test. These medicines 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. Let your provider know about the medicines you 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

The most common use for a PET scan is for cancer, when it may be done:

• To see how far cancer has spread. This helps to select the best treatment approach.
• To check how well your cancer is responding, either during treatment or after treatment is completed.

This test can also be used to:

• Check brain function
• Identify the source of epilepsy in the brain
• Show areas in which there is poor blood flow to the heart
• Determine if a mass in your lung is cancerous or harmless

Normal Results

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:

• Cancer
• Infection
• Problem with organ function

Risks

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. Having many x-rays, CT or PET scans over time may increase your risk for cancer. However, the risk from any one scan is small. You and your doctor should weigh this risk against the benefits of getting a correct diagnosis for a medical problem.

Tell your provider 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.

Considerations

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.

References

Hutton BF, Segerman D, Miles KA. Radionuclide and hybrid imaging. In: Adam A, Dixon AK, Gillard JH, Schaefer-Prokop CM, eds. Grainger & Allison's Diagnostic Radiology. 6th ed. New York, NY: Elsevier Churchill Livingstone; 2015:chap 6.

Meyer PT, Rijintjes M, Hellwig S, Kloppel S, Weiller C. Functional neuroimaging: functional magnetic resonance imaging, positron emission tomography, and single-photon emission computed tomography. In: Daroff RB, Jankovic J, Mazziotta JC, Pomeroy SL, eds. Bradley's Neurology in Clinical Practice. 7th ed. Philadelphia, PA: Elsevier; 2016:chap 41.

Khuri FR. Lung cancer and other pulmonary neoplasms. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 191.