PET scans: What are they and what to expect

A positron emission tomography (PET) scan is an imaging exam that shows how your tissues and organs are working. PET scans can cause anxiety for some people. But knowing what to expect and following the recommended prep can make a big difference.

Here, I’ll answer common questions about PET scans, including how they work, how to prepare for one and their role in cancer treatment.

What is a PET scan?

A PET scan is an imaging test that uses a radiotracer to look at things like sugar metabolism or tumor markers.

A PET scan captures images of your tissues and organs to see how they are working. PET scans can be used to help diagnose or guide the treatment of cancer and other medical conditions like Alzheimer’s disease.

How are PET scans used in cancer care?

While PET scans cannot confirm cancer, they can help get to a diagnosis. If your PET scan reveals something concerning, your doctor may order a biopsy to confirm whether you have cancer.

Once a cancer diagnosis has been made, a PET scan can help determine how large the tumor is and how far it has spread in the body.

PET scans can also let your doctors know if cancer treatment is working. Your doctor may order multiple PET scans throughout treatment to see how the tumor is responding. A PET scan can show whether the cancer has stayed the same, grown, spread or shrunk. This can help guide treatment.

What is the difference between a CT scan and a PET scan?

CT scans and PET scans are both imaging tests that look at what’s happening inside the body.

A CT scan uses X-rays that allow doctors to see what certain organs look like. For example, a lung cancer screening CT scan can show us a nodule in the lung. We can see how big it is and what it’s made of (fat, soft tissue, etc.), but we don’t know if the tumor is active.

PET scans look for metabolic activity within cells. This helps us know if the tumor is growing or the cancer has spread.

Modern PET scans include a CT scan, or in some cases an MRI. These are known as a PET-CT or PET-MRI. They can help doctors better understand the cancer.

How do you prepare for a PET scan?

It's important to prepare for your PET scan, so your doctor gets the information they need. Preparation is different for each radiotracer used, so be sure to follow your doctor’s instructions.

The most common type of PET scan uses FDG (a radioactive sugar) as a radiotracer, which looks at sugar metabolism. So, eating or drinking anything with calories will affect your scan results and may require you to do a repeat scan.

That’s why we ask that for six hours before your PET scan, you don’t eat or drink anything except for plain, unflavored, non-carbonated water or prescribed medicines that do not contain sugar and are not for diabetes. Your last meal before your scan should include foods high in protein and plenty of water. Avoid carbohydrates and foods with sugar.

It’s also important that you don’t exercise for 24 hours before your PET scan. That’s because exercise affects how the radiotracer works and may affect the results of your test.

Contact the PET scan facility if you are:

• Diabetic
• Pregnant or breastfeeding
• Taking steroid medication
• Over 500 pounds
• Claustrophobic
• Unable to lie flat on your back for 15 to 45 minutes

Additionally, let the facility know if you:

• Have an on-body injector, pump or continuous glucose monitor
• Have a PEG or nasogastric feeding tube
• Recently received a high-density contrast procedure, such as a barium swallow
• Must be transported by stretcher or ambulance

If any of these apply to you, the PET care team can make adjustments to keep you comfortable and ensure accurate PET scan results.

What happens during a PET scan?

When you arrive for your PET scan, your care team will start an IV line, and a radiotracer will be injected into your body. If the radiotracer is FDG and you are diabetic, your care team will check your blood sugar before starting. During this time, you may read, listen to music with headphones or read while the tracer goes through your bloodstream. You will be asked to empty your bladder right before the PET scan because having a full bladder can affect the quality of the images.  

Once you’re ready to start the scan, you’ll be asked to lie on your back on a padded table. The table will move into a large, tunnel-shaped scanner where the PET scan is performed. It’s very important to remain still during the test. That’s because movement could affect the results of your PET scan.

How long does a PET scan take?

It takes about 60 to 90 minutes for the PET scan’s radiotracer to start working. The scan can take anywhere from 10 to 40 minutes.

In total, you should plan to be in the PET department for 2 to 3 hours.

What happens after a PET scan?

After your PET scan, a very small amount of the radiotracer will remain in your body. So, be sure to drink plenty of water to help flush it out of your system. You may eat and use the restroom as usual after your scan.

A specialist will review your PET scan and share the results with your doctor. Your doctor will discuss the results and answer any questions you may have at your next appointment.

Aaron Jessop, M.D., is a diagnostic imaging specialist at MD Anderson.

Request an appointment at MD Anderson online or call 1-833-216-7301.