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Positron Emission Tomography (PET Scan): Purpose, Procedure, and Results

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A positron emission tomography (PET) scan 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.

Positron Emission Tomography (PET Scan)

Why do healthcare providers use PET scans?

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

  • Cancer, including breast cancer, lung cancer and thyroid cancer.
  • Coronary artery disease, heart attack or other heart problems.
  • Brain disorders, such as brain tumors, epilepsy, dementia and Alzheimer’s disease.

What does a PET scan show?

A PET scan can:

  • Measure vital functions, such as blood flow, oxygen use and blood sugar (glucose) metabolism.
  • Identify organs and tissues that aren’t working as they should.
  • Detect cancerous tumor cells to help gauge cancer spread (metastasis).
  • Evaluate how well a treatment plan is working and help your healthcare provider adjust treatment, if needed.

How is a PET scan different from a CT or MRI scan?

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 and can primarily be used for diagnosing and monitoring cancers of the soft tissues (brain, head and neck, liver and pelvis).

How does a PET scan work?

PET scans detect diseased cells in organs and tissues. You receive an intravenous (IV) injection of a safe amount of a radioactive drug. This substance is called a radiotracer.

Diseased cells in your body absorb more of the radiotracer than healthy ones do. 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 should 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 and supplements you take.
  • Alert your provider if you think you could be pregnant.
  • 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.

What should I expect during a PET scan?

The following steps occur during a PET scan:

  • You 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 sit in a chair for about an hour while the radiotracer moves through your bloodstream. 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.
  • In approximately one hour, your organs and tissues absorb the radiotracer.
  • If you are getting a PET/CT scan, you may also get an IV injection of a contrast dye. This dye helps produce sharper CT images.
  • You 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, you must remain still. Movement can blur the images.
  • You’ll hear buzzing and clicking sounds as the scanner takes images.
  • 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.
  • 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 45 minutes. After the test, you’ll need to wait while the technologist reviews the scans to ensure the images are clear.

Are there any risks to getting a PET scan?

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. In general, PET scans are safe and rarely cause problems. Exceptions include:

  • Women who are pregnant or breastfeeding should not get PET scans. The radiation may be harmful to an unborn baby 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. Your medical team can give you medication to quickly slow and stop this response.
  • 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.
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