Search the Health Library
Get the facts on diseases, conditions, tests and procedures.
I Want To...
I Want To...
Find Research Faculty
Enter the last name, specialty or keyword for your search below.
School of Medicine
I Want to...
Positron Emission Tomography (PET)/CT
Positron emission tomography (PET), also called PET imaging or PET scan, is a diagnostic examination that involves the acquisition of physiologic images based on the detection of radiation from the emission of positrons. Positrons are tiny particles emitted from a radioactive substance administered to the patient. The subsequent images of the human body developed are used to evaluate a variety of diseases.
- What are some common uses of the procedure?
- How should I prepare for the procedure?
- What does the equipment look like?
- How does the procedure work?
- How is the procedure performed?
- What will I experience during the procedure?
- Who interprets the results and how do I get them?
PET scans most often are used to detect cancer and examine the effects of cancer therapy by characterizing biochemical changes in the cancer. These scans can be performed on the entire body. Scans of the brain are used to evaluate patients who have memory disorders of an undetermined cause, suspected or proven brain tumors or seizure disorders that are not responsive to medical therapy and are therefore candidates for surgery.
PET usually is an outpatient procedure. You should not eat during the six hours leading up to the scan. However, you will be encouraged to drink water. Your doctor will instruct you regarding the use of medications prior to the procedure. Please allow approximately three-and-a-half hours for the entire PET scan. It is important to be on time for your appointment and receive the radioactive substance at the scheduled time.
Note: Diabetic patients should ask for any specific diet guidelines to control glucose levels on the day of the test.
You will be taken to an examination room that houses the PET scanner, which has a hole in the middle and looks like a large donut. Multiple rings of detectors within the machine record the emission of energy from the radioactive substance in your body and create an image. While lying on a cushioned table, you will be moved through the hole of the machine. Images are displayed and recorded on the computer monitor viewed by the technician.
Before the examination begins, a radioactive substance is produced in a machine called a cyclotron and attached, or tagged, to a natural body compound, most commonly glucose. Once this substance is administered to the patient, the radioactivity localizes in the appropriate areas of the body and is detected by the PET scanner.
Different colors or degrees of brightness on a PET image represent different levels of tissue or organ function. For example, because healthy tissue uses glucose for energy, it accumulates some of the tagged glucose, which will show up on the PET images. However, cancerous tissue, which uses more glucose than normal tissue, will accumulate more of the substance and appear brighter than normal tissue on the PET images.
A technologist will take you into an injection room, where the radioactive substance is administered as an intravenous injection. It will take approximately 60 minutes for the substance to travel through your body and accumulate in the tissue under study. During this time, you will be asked to rest quietly and avoid significant movement or talking, which may alter the localization of the administered substance. After that time, the scanning will begin. The procedure usually takes 50 to 60 minutes. For melanoma patients, the scan will last 90 to 120 minutes.
Most PET patients (except brain PET) will be required to have a CT scan immediately following the PET scan. Preparation for the CT scan will be explained at the time of scheduling.
Usually, there are no restrictions on daily routine after the test, although you should drink plenty of fluids to flush the radioactive substance from your body.
The administration of the radioactive substance usually is done via intravenous (IV) injection, in which case you will feel like a slight pinprick. You will then be made as comfortable as possible before you are positioned in the PET scanner for the test. You will be asked to remain still for the duration of the examination. Patients who are claustrophobic may feel some anxiety while positioned in the scanner. Also, some patients find it uncomfortable to hold one position for more than a few minutes. You will not feel anything related to the radioactivity of the substance in your body.
Patients undergo PET because their referring physician has recommended it. A radiologist who has specialized training in PET will interpret the images and forward a report to your referring physician. It usually takes one to three days to interpret, report and deliver the results. Medicare and insurance companies cover many of the applications of PET, and coverage continues to increase.
For more information or to schedule an appointment for a PET or CT scan at Johns Hopkins Bayview, please call 410-550-0211.
Request an Appointment
Already a Patient?
Traveling for Care?
Whether you're crossing the country or the globe, we make it easy to access world-class care at Johns Hopkins.