PET Scan

Positron emission tomography (PET) scans detect early signs of cancer, heart disease and brain conditions. It involves an injection of a safe radioactive tracer that helps detect diseased cells.


A PET scan machine, which has a donut-shaped hole your body passes through while laying on a motorized exam table.
A positron emission tomography (PET) scan uses a safe injectable radioactive chemical called a radiotracer and a device called a PET scanner to show images of your organs and tissues at work.

What is a PET scan?

A positron emission tomography (PET) scan is an imaging test that produces images of your organs and tissues at work. The test uses a safe, injectable radioactive chemical called a radiotracer and a device called a PET scanner.

The scanner detects diseased cells that absorb large amounts of the radiotracer, which indicates a potential health problem.

Healthcare providers frequently use PET scans to help diagnose cancer and assess cancer treatment. They can also assess certain heart and brain issues with the scan.


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What’s the difference between a PET scan, CT scan and MRI?

Computed tomography (CT) scans use X-rays. Magnetic resonance imaging (MRI) scans use magnets and radio waves. Both produce still images of organs and body structures.

PET scans use a radioactive tracer to show how an organ is functioning in real time. PET scan images can detect cellular changes in organs and tissues earlier than CT and MRI scans. Your healthcare provider may perform a PET scan and CT scan at the same time (PET-CT). This combination test produces 3D images that allow for a more accurate diagnosis.

Some hospitals now use a hybrid PET/MRI scan. This new technology creates extremely high-contrast images. Providers mainly use this type of scan for diagnosing and monitoring cancers of the soft tissues (brain, head and neck, liver and pelvis).

What does a PET scan check for?

Your healthcare provider may order a PET scan to check for signs of:


When would I need a PET scan?

In general, a PET scan can measure vital functions, such as blood flow, oxygen use and blood sugar (glucose) metabolism. It can also identify organs and tissues that aren’t working as they should.

If your healthcare provider suspects you may have cancer, they’ll likely recommend a PET scan, which can detect cancer and/or make a diagnosis.

If you’ve already been diagnosed with cancer, your provider may recommend more than one PET scan throughout your treatment to:

  • Determine whether the cancer has spread in your body (metastasized).
  • Assess the effectiveness of treatment.
  • Determine if the cancer has returned after treatment (recurred).
  • Evaluate the prognosis (outlook) of the cancer.

If you’re having heart issues, your provider may recommend a PET scan to:

If you’re experiencing neurological symptoms, your provider may recommend a PET scan to evaluate possible brain abnormalities, such as tumors, seizures and other central nervous system conditions.

Test Details

How does a PET scan work?

A PET scan is a type of nuclear medicine imaging. Nuclear medicine uses small and safe amounts of radioactive material, called radiotracers, given through an IV.

Unlike other imaging techniques, PET scans focus on processes and molecular activity within your body. This gives them the potential to find disease in its earliest stages.

Diseased cells in your body absorb more of the radiotracer than healthy ones do. These are called “hot spots.” The PET scanner detects this radiation and produces images of the affected tissue. A PET/CT scan combines X-ray images from a CT scan with PET scan images.


How do I prepare for a PET scan?

PET scans are an outpatient procedure, which means you go home the same day. Your healthcare provider will give you detailed instructions on how to prepare for the scan. In general, you should:

  • Make sure your provider has a current list of all medications, vitamins and supplements you take, as well as any allergies you have.
  • Alert your provider if you think you could be pregnant or if you’re breastfeeding (chestfeeding).
  • Not eat anything for six hours before the test. Your healthcare provider may change this direction if you have diabetes.
  • Drink only water.
  • Avoid caffeine for 24 hours before the test if you’re being tested for a heart problem.
  • Wear comfortable clothes and leave metal accessories, such as jewelry, eyeglasses, dentures and hairpins at home.
  • Tell your healthcare provider if being in an enclosed space makes you anxious. You may be able to take a mild sedative to help you relax during the procedure.

What should I expect during a PET scan?

You can expect the following during a PET scan:

  • You’ll receive an IV injection of a radiotracer that contains a safe amount of a radioactive drug. The most commonly used radiotracer is fluorodeoxyglucose (FDG).
  • You’ll sit in a chair for about an hour while the radiotracer moves through your bloodstream and gets absorbed by your organs and tissues. Too much activity can send the radiotracer to areas of your body that your healthcare provider isn’t testing. You won’t be able to feel the radiotracer.
  • If you’re getting a PET/CT scan, you may also get an IV injection of a contrast dye. This dye helps produce sharper CT images.
  • You’ll lie on an exam table that slides in and out of the PET/CT scanner. This scanner is shaped like a doughnut. The doughnut or tunnel opening is about 30 inches in diameter.
  • During the scan, which usually takes about 30 minutes, you must remain still. Movement can blur the images.
  • You’ll hear buzzing and clicking sounds as the scanner takes images.
  • A technologist will review the scans before you leave to ensure the images are in focus.

How long does a PET scan take?

The entire PET scan process takes about two hours.

It can take up to 60 minutes for your body to absorb the injected radiotracer. During this time, you’ll need to sit quietly and limit your movements. The actual PET scan takes about 30 minutes. After the test, you’ll need to wait while the technologist reviews the scans to ensure the images are clear.

What are the risks and side effects of a PET scan?

In general, PET scans are safe and rarely cause problems. The amount of radiation in the radioactive tracer is very low. It doesn’t stay in your body for long. You should drink lots of water after a PET scan to help flush the radioactive drug from your body.

PET scans generally only pose risks in the following situations:

  • People who are pregnant, breastfeeding or chestfeeding should not get PET scans. The radiation may be harmful to a fetus and can pass to an infant in breast milk.
  • Some people have an allergic reaction to PET scan radioactive tracers or CT scan contrast dyes. These allergic reactions are extremely rare and usually mild. Your medical team can give you medication to quickly slow and stop this response if it happens.
  • People with diabetes may not absorb the sugar in the radiotracer, which can affect scan results. Your healthcare provider will offer suggestions to modify your diet and medications before the test.
Care at Cleveland Clinic

Results and Follow-Up

When should I get my PET scan results?

A radiologist with specialized training in PET scans will review the images, write a report and send it to your healthcare provider. This process usually takes 24 hours.

A note from Cleveland Clinic

A positron emission tomography (PET) scan is a very useful and generally safe imaging test that healthcare providers use to assess cancer, heart issues and brain conditions. If you need a PET scan and are worried about the exam or have questions about it, don’t be afraid to ask your healthcare provider. They’re available to help and support you.

Medically Reviewed

Last reviewed on 10/19/2022.

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