Positron Emission Tomography and Computed Tomography (PET-CT) Scans

Approved by the Cancer.Net Editorial Board, 02/2020

A positron emission tomography scan is known as a PET scan. PET scan is a type of test that may be used in cancer treatment. It can be done along with a CT scan. If so, doctors call it a PET-CT scan. But you might also just hear it called a PET scan.

For some types of cancer, a PET-CT scan is a way to help find cancer and learn its stage. Stage is a way to describe where the cancer is and if it has spread. Doctors also learn information about the stage if and how the cancer is affecting your body's functions. Knowing the stage of cancer helps you and your doctor choose the best treatment. It also helps your doctor predict your chance of recovery.

How is a PET-CT scan different than a CT scan?

A CT scan shows detailed pictures of the organs and tissues inside your body. A PET scan can find abnormal activity and it can be more sensitive than other imaging tests. It may also show changes to your body sooner. Doctors use PET-CT scans to provide more information about the cancer.

In addition to learning your cancer stage, a PET-CT scan can help the doctor:

  • Find the right place for a biopsy.

  • Learn if your cancer treatment is working.

  • Check for new cancer growth after treatment has ended.

  • Plan radiation therapy.

How does a PET-CT scan work?

Before your PET-CT scan, you will get an injection of a small amount of a radioactive sugar called fluorodeoxyglucose-18. This substance is sometimes called FGD-18, radioactive glucose, or a tracer. The cells in your body absorb sugar. Areas that use more energy pick up more of the sugar. Cancer cells tend to use more energy than healthy cells. The PET scan shows where the radioactive tracer is in your body.

The CT scan takes x-rays of your body from different angles. You might get a shot of dye before the x-rays. This helps some of the details show up better. Finally, a computer combines the PET and CT images. Your doctor gets a detailed 3-D result that shows anything abnormal, including tumors.

Are PET-CT scans safe?

PET-CT scans do carry a risk of radiation. This type of scan uses some radiation from x-rays, the substance used in the PET scan, or both. Scanning a smaller body area means less radiation. So does a CT without the dye that helps show details.

The benefits of these tests are usually greater than the risks. During these tests, you will be exposed to small amounts of radiation. This low dose of radiation has not been shown to cause harm. For children or for other people who need multiple PET scans, CT scans, and x-rays, there may be a small potential increased risk of cancer in the future.

Doctors can use lower dose scans or limit the areas that need to be scanned. Make sure all of your doctors know how many imaging scans you have had, including the number and type. This information can help them decide which scans to use in the future to help reduce your risk. If you are concerned about your radiation exposure, talk with your doctor, including asking whether you can have another type of test that uses less radiation instead.

Who will do my PET-CT scan?

A technologist who specializes in doing these types of scans will do your PET-CT scan. After the test is done, a doctor who specializes in looking at the test results will look at your scan. This person is a nuclear medicine specialist or a radiologist. They will decide what the results mean.

PET-CT scans can be done at a hospital or a center that does imaging tests.

Getting ready for a PET-CT scan

When you schedule a PET-CT scan, the staff will tell you how to get ready. Be sure to follow their instructions carefully to avoid affecting your scan results. Talk with the staff about the following topics, and ask questions about any information that is unclear or concerning to you.

What to eat. You may be told to drink only clear liquids after midnight the night before the scan. Depending on what part of your body will be scanned, you may need to stop all eating and drinking 4 hours before the scan. For some scans, you can eat and drink normally.

Your medications and health history. Ask whether you should take your usual medications or supplements on the day of the test. Also, let them know if you have diabetes or other medical conditions. In particular, diabetes can alter your test results and the radioactive tracer can impact your blood sugar. If you are breastfeeding or could be pregnant, tell the staff. These scans can put the baby at risk.

Allergies. Let the staff know about any drug or food allergies you have, including any allergic reactions to iodine you may have had in the past.

What to avoid. Don't do any heavy exercise like running, jogging, or weightlifting 24 hours before your exam. Exercise can make your scans less accurate.

What to wear. Wear loose, comfortable clothing without metal zippers or buttons. You will need to remove any clothing that includes metal because metal can affect the scan. This includes belts, earrings, shirts with snaps or zippers, bras, and glasses. If your clothing cannot be worn during the scan, you can wear a hospital gown. You will be asked to remove any jewelry, so you may want to leave it at home the day of your exam.

Insurance, costs, and consent. If you are concerned about the cost of your PET-CT scan, find out what your insurance provider will cover before the scan. Ask how much of the cost you will have to pay. Once you get to the doctor's office or hospital, the staff will ask you to sign a consent form. This form states that you understand the benefits and risks of the scan and agree to have it.

During the PET-CT scan

A staff member will put an IV into one of your veins. This is a thin tube with a needle attached. The IV feels like a pinprick. Once the IV is in place, you will get the radioactive substance for the scan. You will not feel anything from the radioactive substance.

After the substance has been injected, you must limit your movement and avoid activity, but you can sit in a chair comfortably. Moving too much can make the substance move into areas that are not being studied. This makes it harder for doctors to read the scan. The radioactive substance takes 30 to 90 minutes to reach the body parts that will be scanned.

You may also be asked to drink a contrast liquid. You might also get contrast liquid through your IV. This contrast liquid helps make the pictures clearer. Right before the test begins, you will be asked to go to the bathroom to empty your bladder.

This liquid can make your IV area feel hot or itchy. You may also have a metallic taste in your mouth. These feelings should go away in a few minutes. If you have a more serious reaction, such as trouble breathing, say something right away.

When it is time for the scan to begin, the technologist will help you position your body comfortably on a table. You will likely lie on your back, but you may need to lie on your stomach or side. This depends on what part of your body needs to be scanned.

Sometimes a PET-CT scan is used to plan radiation therapy to treat the cancer. In this case, your body position will be very important. The technologist position you with a mask or cast. These tools help you keep your body very still during the scan.

The PET-CT machine looks like a large donut. When it starts, the table slides quickly through the hole in the center. This helps show if you are in the right position. Then the table slides slowly back and forth. A staff member will watch the test from a nearby room. You can talk to them and they can talk to you.

Will I be comfortable during the scan?

The staff will make you as comfortable as possible. A PET-CT scan does not hurt. But some positions might be uncomfortable or tiring. You need to lie still for the entire scan. You might also need to keep your arms above your head. The staff member might ask you to hold your breath sometimes. Motion from breathing can cause blurry pictures.

The staff member might also raise, lower, or tilt the table during the scan. This gets pictures from different angles. Ask them to tell you when the table will move.

You can expect to hear whirring or clicking sounds from the machine. Some machines are noisier than others.

Your appointment will last 1 hour to 3 hours. Once the radioactive substance gets to the right area through the IV, the scan itself usually only takes about 30 minutes. If the machine scans a large area, the test might take longer. The staff member can tell you about how long it will take.

When the scan is finished, you might need to stay on the table while a doctor looks at the images to check to make sure the images are not blurry. If they are not clear, you might need another scan.

After the PET-CT scan

You can do normal activities after the scan. This includes driving. The staff will tell you to drink several glasses of water. This helps wash the radioactive substance and dye out of your body.

Questions to ask your health care team

You might want to ask your health care team the following questions before a PET-CT scan.

  • Where will I have the PET-CT scan?

  • How long will it take?

  • What are the benefits and risks?

  • Will I need dye for the CT? How will I get it? What happens if I am allergic to the dye?

  • What can I eat or drink before the scan? What about taking my usual medications?

    What should I avoid?

  • When will I learn the test results?

  • Who will tell me the results, and how?

  • Will I need other tests

Related Resources

CT Scans and Cancer Risk

Preparing for Your PET-CT Scan

Expert Podcast: Coping with "Scanxiety"

More Information

RadiologyInfo.org: Positron Emission Tomography - Computed Tomography (PET/CT)