Positron Emission Tomography (PET)

Test Overview

Positron emission tomography (PET) is a test that uses a special type of camera and a tracer (radioactive chemical) to look at organs in the body.

During the test, the tracer liquid is put into a vein (intravenous, or IV) in your arm. The tracer moves through your body, where much of it collects in the specific organ or tissue. The tracer gives off tiny positively charged particles (positrons). The camera records the positrons and turns the recording into pictures on a computer.

PET scan pictures do not show as much detail as computed tomography (CT) scans or magnetic resonance imaging (MRI) because the pictures show only the location of the tracer. The PET picture may be matched with those from a CT scan to get more detailed information about where the tracer is located.

A PET scan is often used to find cancer, to check blood flow, or to see how organs are working.

See pictures of a PET scanner and PET scans of the brain .

Why It Is Done

A positron emission tomography (PET) scan is done to:

• Study the brain's blood flow and metabolic activity. A PET scan can help a doctor find nervous system problems, such as Alzheimer's disease, Parkinson's disease, multiple sclerosis, transient ischemic attack (TIA), amyotrophic lateral sclerosis (ALS), Huntington's disease, stroke, and schizophrenia.
• Find changes in the brain that may cause epilepsy.
• Find some cancers, especially lymphoma or cancers of the breast, brain, lung, colon, or prostate. In its early stages cancer may show up more clearly on a PET scan than on a CT scan or an MRI.
• See how advanced a cancer is and whether it has spread to another area of the body (metastasized). It is often necessary to do both CT and PET scans to evaluate cancer.
• Help a doctor choose the best treatment for cancer. PET scans may also be done to see whether surgery can be done to remove a tumor.
• Find poor blood flow to the heart, which may mean coronary artery disease.
• Find damaged heart tissue, especially after a heart attack.
• Help choose the best treatment, such as coronary artery bypass graft surgery, for a person with heart disease.

How To Prepare

Stop taking medicines and herbal remedies 24 hours before you have this test. If you take insulin to control diabetes, you may need to take less than your normal dose. Talk with your doctor about how much insulin you should take.

Do not smoke or drink caffeine or alcohol for 24 hours before this test.

Do not eat or drink for 8 hours before this test.

Tell your doctor if you are or might be pregnant or if you are breast-feeding.

Tell your doctor if you have a fear of enclosed spaces or have ever had a panic attack.

You may be asked to sign a consent form for this test. Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done or what the results mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?) .

How It Is Done

A positron emission tomography (PET) scan is done in a hospital nuclear medicine department or at a special PET center by a radiologist or nuclear medicine specialist and a technologist. You will be asked to lie on a table that is hooked to a large scanner, camera, and computer.

The radioactive tracer is usually given in a vein (IV).

The PET scanner, which is shaped like a doughnut, moves around you. The scanned pictures are sent to a computer screen so your doctor can see them. Many scans are done to make a series of pictures. It is very important to lie still while each scan is being done. At some medical centers, a CT scan will be done at the same time.

For a PET scan of the brain, you will lie on a bed. You may be asked to read, name letters, or tell a story, depending on whether speech, reasoning, or memory is being tested. During the scan, you may be given earplugs and a blindfold (if you do not need to read during the test) to wear for your comfort.

If you are having a PET scan of your heart, electrodes for an electrocardiogram (EKG, ECG) will be put on your body.

During the test, you will be alone in the scanner room. The technologist will watch you through a window and you will be able talk to him or her through a two-way intercom at all times.

The test takes 1 to 3 hours.

After the test, drink lots of fluids for the next 24 hours to help flush the tracer out of your body.

How It Feels

You will not feel pain during the test. The table you lie on may be hard and the room may be cool. It may be difficult to lie still during the test.

You may feel a quick sting or pinch when the IV is put in your arm. The tracer may make you feel warm and flushed. Some people feel sick to their stomach or have a headache. Tell your doctor how you are feeling.

You may feel nervous inside the PET scanner.

Risks

There is always a slight chance of damage to cells or tissue from radiation, including the low levels of radiation used for this test. But the chance of damage is usually very low compared with the benefits of the test.

Most of the tracer will be flushed from your body within 6 to 24 hours. Allergic reactions to the tracer are very rare.

In rare cases, some soreness or swelling may develop at the IV site where the radioactive tracer was put in. Apply a moist, warm compress to your arm.

Results

Positron emission tomography (PET) is a test that uses a special type of camera and a tracer (radioactive chemical) to look at organs in the body.

The radiologist may discuss preliminary results of the PET scan with you right after the test. Complete results are usually available in 1 to 2 days.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

• Being pregnant. A PET scan is not usually done during pregnancy because the radiation could harm the unborn baby (fetus).
• Using caffeine, tobacco, or alcohol in the past 24 hours.
• Not being able to lie still for the test.
• Being too anxious.
• Using sedatives.
• Taking medicines, such as insulin, that change your metabolism.

What To Think About

• The radioactive tracer may be passed to a baby through breast milk. Talk to your doctor if you are breast-feeding and need to have a PET scan.
• PET scans are expensive and not yet widely available.
• A CT scan and PET scan may be done at the same time.
• Single photon emission computed tomography (SPECT) imaging is a method that may be used to see whether a person with chest pain is at high risk for a heart attack. SPECT imaging also may be done with cardiac stress testing to check for coronary artery disease (CAD) or to choose the best treatment for it.¹
• You may not be able to have a PET scan if you have recently had surgery, a biopsy, chemotherapy, or radiation therapy.

References

Citations

1. Klocke RJ, et al. (2003). ACC/AHA/ASNC guidelines for the clinical use of cardiac radionuclide imaging-Executive summary: A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. Circulation, 108: 1-15.

Other Works Consulted

• Pagana KD, Pagana TJ (2006). Mosby's Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.

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