by Rosalyn Carson-DeWitt, MD
PET scans use a radioactive tracer that is introduced into your body to measure the cellular activity of the cell type or body part being scanned. A CT scan takes a large number of x-rays. These are analyzed by a computer to create a 3-dimensional image of the body part being studied. When both tests are performed at the same time, the information about function and structure is integrated through computer models.
Reasons for Test TOP
Because PET/CT scans provide a combination of information about the function and structure of a body part, they are useful for the early diagnosis of cancer. Not only can an abnormal tumor be seen, but the function of the cells that make up the tumor can be analyzed as well. This can help to differentiate between cancerous and noncancerous growths. PET/CT can also be used to see if cancer has spread into other areas of the body.
Brain, endocrine, and heart disorders are also studied using PET/CT scans.
Possible Complications TOP
Problems from the procedure are rare, but all procedures have some risk. Your doctor will review potential problems, like:
What to Expect TOP
Prior to Test
Prepare a list of medications you are taking and bring the list with you to the test. If you have diabetes, discuss taking your diabetic medications and/or insulin with your doctor prior to the test. An abnormal blood glucose level may interfere with the tests results.
Let your doctor know if you have kidney disease. The doctor may need to take steps to avoid kidney injury during the test.
To prepare for your test, you may need to do the following several hours in advance:
If you are breastfeeding, talk to your doctor before you go for your test. Your doctor may recommend that you pump breast milk ahead of time and use it until the contrast materials are no longer in your body.
At the test center, the staff will ask if you have or ever have had:
Description of Test
After Test TOP
How Long Will It Take? TOP
A PET/CT scan takes about a total of 2 hours to complete. The injection occurs about an hour prior to the start of the scan. The scan itself takes about 35 minutes.
Will It Hurt? TOP
The placement of the IV may give you some discomfort, but there should be no other pain involved. You may feel some flushing when the tracer material is injected.
Based on the results, your doctor will decide if any further tests or treatments are needed.
Call Your Doctor TOP
Call your doctor if any of the following occur:
If you think you have an emergency, call for emergency medical services right away.
NIH Clinical Center
Radiology Info—Radiological Society of North America
Canadian Association of Radiologists
Canadian Radiation Protection Association
PET/CT scan. UPMC website. Available at:
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Accessed March 14, 2018.
Positron emission tomography-computed tomography (PET/CT). Radiological Society of North America Radiology Info website. Available at: https://www.radiologyinfo.org/en/info.cfm?pg=pet. Updated January 23, 2017. Accessed March 14, 2018.
Schidt GP, Kramer H, Reiser MF, Glaser C. Whole-body magnetic resonance imaging and positron-emission tomography-computed tomography in oncology. Top Magn Reson Imaging. 2007;18(3):193-202.
Last reviewed March 2018 by EBSCO Medical Review Board Marcie L. Sidman, MD
Last Updated: 3/18/2013
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