PET-CT Scanning

Reviewed March 31, 2026

Positron emission tomography (PET) and computerized tomography (CT) are two imaging tools that allow doctors to pinpoint the location of cancer within the body. PET-CT combines the two techniques to more accurately diagnose and locate disease.

The PET scan detects the metabolic signal of actively growing cancer cells in the body and the CT scan provides a detailed picture of the internal anatomy that reveals the location, size and shape of cancerous growths. When the results of PET and CT scans are combined, the image provides complete information on cancer location and metabolism.

PET-CT scans are used for cancers such as:

  • Breast cancer. Fluorodeoxyglucose positron emission tomography (FDG PET) may show breast cancer cells not detected by other methods.
  • Colorectal cancer. Regular monitoring with PET scans, which detects changes in cell function, can identify recurrences of colorectal cancer earlier than conventional scanning.
  • Ovarian cancer. PET-CT may improve the accuracy of ovarian cancer staging and help doctors determine whether cancer has spread to nearby lymph nodes without the need for surgery.

How to Prepare

Clothing. Wear comfortable clothes, without metal such as zippers and snaps.

Food and drink. Do not consume anything except water for six hours prior to the scan.

Medications. You may take your regularly scheduled medications before arriving for your scan, if you can tolerate taking them on an empty stomach. Bring a list of your medications to your appointment.

Procedure. You will receive an injection of a radioactive tracer, which takes about an hour to disperse through your body. You will then lie down on the scanner bed. The scan itself takes about 30 to 45 minutes and the entire procedure takes about two hours.

After your scan. Drink plenty of fluids after your scan. A radiologist will interpret your PET-CT scan and send the results to your doctor within one to two days. Please contact your doctor to discuss the results.

UCSF Health medical specialists have reviewed this information. It is for educational purposes only and is not intended to replace the advice of your doctor or other health care provider. We encourage you to discuss any questions or concerns you may have with your provider.