What to Expect During a PET or CT Scan

A patient undergoing a CT scan.
A patient undergoing a CT scan. Morsa Images/Getty Images

Positron Emission Tomography, or a PET scan, is an imaging test that shows the metabolic activity levels of the cells and helps to identify cancer cells. The most common type of PET scan used today is the FDG-PET, and it is often used along with CT, in which case the scans are called FDG-PET/CT scans.

The present article refers to this type of scan.

PET scans help your healthcare team to determine the best treatment options for you and to evaluate response to therapy that you've received.​

Of note, other types of PET scans are in use. For instance, the Gallium-68 DOTATOC PET-CT scan is used for certain types of less common cancers, known as neuroendocrine tumors. It is also worth noting that not all cancers are visualized by and FDG-PET-CT scans.

Many of the common types of lymphoma can be visualized using FDG-PET/CT.

How Does a PET Work?

During an FDG-PET scan, a slightly radioactive sugar, or radiotracer, is introduced into your body. This is usually done through an intravenous injection, but in some cases, you might be asked to drink or inhale the radiotracer. This sugar will accumulate in your tissues and show the activity levels of the cells. Emissions from the tracer are picked up by imaging equipment, providing a picture of the metabolic activity within.

Since some types of cancer cells have a higher metabolic rate than normal cells and therefore take more of the radioactive sugar, clinicians can identify them by the amount of radiotracer they take up. There are types of cancer cells with low uptake of sugar that are not seen on a routine FDG-PET scan.

Basically, The FDG-PET scan is able to locate cancer cells based on their uptake of sugar.

What is PET/CT Scan?

Computerized Tomography, or CT, is able to provide very detailed information about structures inside your body. These days, many healthcare centers use a combination of PET and CT scans to get the most precise and accurate images. Combining these tests has a number of benefits:

  • a PET scan identifies the abnormal cells in your body, and the CT scan pinpoints their exact location
  • a PET scan can help to determine if masses seen on a CT scan contain cancer cells or not
  • a CT scan can help determine if the cells that show high metabolic activity on a PET scan are actually abnormal cells, or if they're normal cells that just use more sugar as fuel—like muscle or white blood cells

Preparing for Your Exam

In order for technicians to get the most precise images, they will provide you with detailed information about preparing for your PET scan. Often these instructions will indicate that you should plan to:

  • Fast (have nothing to eat) for 4-6 hours before your test to ensure that your cells are hungry enough to ingest the sugar radiotracer. This includes having no coffee or tea, chewing gum or candy.
  • Drink plenty of plain water leading up to your test.
  • Take pain medications if you need them before the test. You will be required to lie still for long periods during your scan, so you'll want to be sure that any discomfort is adequately controlled beforehand.
  • Take any medications as usual on the day of your exam, except for diabetic medications or drug syrups.
  • Leave any metal objects such as jewelry, or removable dental appliances, at home. At the time of the test, you may be asked to remove eyeglasses or hearing aides since metals can interfere with the images.

You must inform the nuclear medicine department if you:

  • Have had a biopsy, surgical procedure, or radiation therapy in the 10 days prior to your scan appointment
  • Develop an infection of any kind before your appointment
  • Change your mind and decide not to have the test
  • Are claustrophobic or become nervous in small spaces. You may need a sedative or anti-anxiety medication to get through the scan comfortably.
  • Have any allergies or previous reactions to contrast dyes.
  • Could be pregnant or if you are breastfeeding
  • Are diabetic

What Happens During the Exam?

When you arrive for a PET scan, your history and information will be reviewed, and you'll have a chance to ask any additional questions that you may have. You may also be asked to sign a consent form before the exam. An intravenous (IV) site may be started, especially if you're having a CT scan along with your PET scan.

Your blood glucose levels may be checked at this time. If this is the case, you will have a finger poke, and your blood sugars will be checked using a handheld machine called a glucometer. If your levels are too high, you may need to reschedule your test.

At this point, you'll be made comfortable in a quiet, warm room, and the radiotracer will be administered to you. It may take some time for the radiotracer to travel through your body and accumulate in your tissues, so you will be asked to lie or sit quietly during this time. Depending on the area of your body being scanned, and the guidelines followed by your health care center, you might be asked not to read, listen to music, or talk in order to avoid all movement.

When the time comes, you will lie on a moveable table that slides you through the ring-like scanner that takes the images. You'll need to be very still during the scan; it usually takes between 20 and 60 minutes for both the PET and CT scans to be performed.

The technicians will ensure that they got all the images that they needed, and if so, they'll remove your IV site and you'll be free to leave following the exam. If you took a sedative prior to the scan, it may not have worn off, so you'll need to have somebody else drive you home.

Altogether, you should expect to be at the imaging department for about 2 hours, from start to finish.

After the Exam

After the PET scan is complete, you'll be able to eat and drink. In fact, drinking lots of fluids is recommended to help flush the radiotracer from your system. The amount of radiation you'll receive during a PET scan is similar to that of a CT or bone scan.

This radiation will naturally lose its activity in your body and may also be passed in your stool and/or urine in the hours following your PET scan. Some sources recommend staying away from babies or pregnant women for a few hours after you leave. The radiotracer should be completely out of your system after a day or so.

The nuclear medicine specialist will review the images from your scan, and forward their interpretation to your referring doctor. Ask if you should contact your doctor for the results, or if they will contact you.

Summing It Up

PET scans are able to detect abnormal cells just by the way they absorb and metabolize sugar. Particularly when used in combination with a CT scan, a PET scan can provide precise details about cancers such as lymphoma and myeloma, and help doctors plan and evaluate treatment.

Before your scheduled PET scan, ensure that you have been given, and understand, all the instructions given to you to help you prepare.


Bredella, M., Steinbach, L., Caputo, G., Segall, G., Hawkins, R. Value of FDG PET in the Assessment of Patients with Multiple Myeloma. American Journal of Roentgenology 2005. 184: 1199- 1204.

Burton, C., Ell, P., Linch, D. The role of PET imaging in lymphoma. British Journal of Haematology 2004. 126:772-84.

Seam, P., Juweid, M., Cheson, B. The role of FDG-PET scans in patients with lymphoma. Blood. 2007. 110: 3507-3516.

American College of Radiology Imaging Network. About Pet Scans. Accessed April 2016.