PET Scan (Positron Emission Tomography)

Medically Reviewed by Sabrina Felson, MD on March 12, 2023
5 min read

A PET scan (also known as positron emission tomography and PET/CT) is a type of imaging study that can show doctors what’s happening in your body and how it’s working. It’s different from an X-ray, CT, or MRI. They all provide images, but a PET scan shows how your body’s working.

It provides information about blood flow and how your body’s using oxygen and sugar. That can give important clues about how a disease is unfolding.

When you get a PET scan, your doctor first gives you a radioactive substance called a radiotracer (or just “tracer”). The tracer gives off radiation, which the PET scan machine picks up on. The images you get show where in your body the tracer goes. If it builds up in certain areas, that could be a sign of disease.

A PET scan can help doctors test for disease, prepare for surgery, and see how well treatments are working. You might get one for several reasons, but they’re most often used with cancer, heart disease, and brain conditions.

Your doctor may use a PET scan to:

  • Find cancer
  • See if cancer has spread
  • Check if cancer treatment is working
  • Determine if cancer came back after treatment

With heart disease, your doctor might use a PET scan to:

Your doctor may also use it to check for brain conditions, such as:


Doctors use different types of imaging for different reasons. Often, you start with an X-ray because it’s a quick way to get basic information. But if you need finer detail, you might then get a CT scan or MRI.

Many doctors use MRI/PET and CT/PET hybrid scanners, which combines the two tools into a single scan. This allows doctors to do either a CT or MRI scan in combination with a PET scan all at once.

A PET scan can show what’s actually happening in your cells. One reason that’s important is because early on, some diseases don’t cause changes you can see with an MRI or CT scan. But they do cause changes in how your cells are working. That means a PET scan might help your doctor find a disease that other types of imaging can’t.

First, you’ll need to tell your doctor about any of the following:

  • Allergies, especially to contrast dye, iodine, or seafood
  • Health conditions, like diabetes, or any illnesses you’ve had recently
  • Medicines, herbs, and supplements you take

If you’re a woman, tell your doctor if you’re:

  • Breastfeeding -- you may need to pump milk because you can’t breastfeed until the tracer is out of your body. Check with your doctor to see how long you should wait.
  • Pregnant or think you might be -- the tracer can harm your baby, so talk to your doctor about the best options for you.

Your doctor will give you specific directions to prepare for your scan. Be sure to follow them closely. Often you’ll need to:

  • Avoid intense physical activity for 24 hours before the scan
  • Drink only water and avoid eating for several hours prior to the scan
  • Remove all piercings, jewelry, and metal objects from your body

It depends where and why you get the scan, but typically, you:

  • Change into a hospital gown
  • Go to the bathroom
  • Get the tracer -- depending on the type, you’ll either swallow it, breathe it in, or get it through a needle
  • Wait 30 minutes to an hour for your body to absorb the tracer
  • Lie very still on your back while images are taken. It’s important not to move or talk during the scan, which may last up to an hour.

The PET scan machine is a big, open circle -- like a standing donut -- with a table that moves in and out of it. If you have a fear of tight, closed spaces, you may get a drug to help keep you calm. You’ll hear the machine buzz and click as it takes images.

The scan itself is painless. For some people, staying still for so long is the hardest part and may cause some aches or discomfort.

After the scan, drink plenty of fluids to help flush the tracer out of your body. Your doctor may suggest you avoid close contact with pregnant women, kids, or babies for a few hours since you’ll be radioactive for a short time. Within a few hours or days after the PET scan, the radioactive material within the tracer will decay and will no longer be radioactive. You’ll pass it out of your body in urine and stool.

A PET scan is painless and has few risks and side effects. But you may have discomfort or problems, such as:

  • Pain or redness where a tracer is injected
  • A hard time fitting into the PET/CT machine if you are overweight
  • Claustrophobia, if you are unable to be in enclosed spaces
  • Allergic reactions to a tracer, although this rarely happens and may be a mild reaction
  • Inaccurate test results among people with diabetes whose blood sugar levels or insulin levels aren’t within the right range during the test
  • Your exposure to radioactive material is very low but non nonexistent.


A PET scan shows bright areas where there’s heavy activity in your cells, which may be a sign of disease. To get a more complete picture of what’s going on, your doctor may compare your PET scan with results from other imaging you’ve had. Test results are very accurate, but a combined PET/CT tends to be more accurate than the results of either test on its own. You can get results within 24 hours, but it depends on where you have the scan done.