PET Scan combines the techniques of nuclear scanning and biochemical analysis to assess body processes like blood flow and metabolism in certain organs. In PET scan, 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, two-dimensional (2D) 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 PET Scan

  • 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, transient ischemic attacks (“mini-strokes”), Alzheimer’s disease, Parkinson’s disease, multiple sclerosis and other types of dementia or various psychiatric disorders that may produce altered patterns of glucose metabolism in the brain
  • To help determine heart muscle function in patients with heart disease, distinguish between viable and dead cardiac tissue during the early stages of a heart attack and evaluate the amount of muscle damage after a heart attack
  • To detect cancerous tumors, determine the stage of cancer, and evaluate the effectiveness of chemotherapy drugs on specific tissues

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 PET Scan

  • Do not ingest alcohol, caffeine, tobacco, sedatives, or tranquilizers for 24 hours before the procedure.
  • Do not eat or drink anything for 4 hours.
  • 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.
  • Do not exercise heavily for 1 to 2 days prior.
  • Remove jewelry and any items containing metal, including clothing with metal buttons.
  • Inform your doctor if you are afraid of close spaces. A mild sedative may be given to help you relax so you can undergo the test.
  • Tell your doctor if you are or could be pregnant.
  • Inform your doctor if you have any allergies to injected dye.

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.
  • In extremely rare cases, patients may be hypersensitive to the radiochemical and experience an adverse reaction.

After the PET Scan

  • 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.


The Johns Hopkins Consumer Guide to Medical Tests

Simeon Margolis, M.D., Ph.D., Medical Editor

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 17 Jan 2012

Last Modified: 19 Mar 2015