Positron Emission Tomography
What is a PET scan?
A positron emission tomography (PET) scan is a type of nuclear medicine imaging test. It is used to examine various body tissues to identify certain conditions by looking at blood flow, metabolism, and oxygen use. PET scans may also be used to see how well the treatment of certain diseases is working.
For a PET scan, a tiny amount of a radioactive substance, called a radioactive tracer is used to show the metabolism of a particular organ or tissue. This test gives the doctor information about the function and structure of the organ or tissue, as well as information about its biochemical properties. A PET scan may detect biochemical changes in an organ or tissue that are signs of a disease process before physical changes related to the disease can be seen with other imaging tests, such as computed tomography (CT) or magnetic resonance imaging (MRI).
PET scans are often done along with CT scans (called a PET/CT scan) to provide more definitive information about metabolism changes and exactly where they are happening in the body.
PET works by using a special camera that detects positrons emitted by the radioactive tracer in the organ or tissue being examined.
The radioactive tracers are attached to a chemical substance that a particular organ or tissue uses during metabolism. These substances include glucose, carbon, or oxygen. For example, in PET scans of the brain, a radioactive substance is applied to glucose to create a radionuclide called fluorodeoxyglucose (FDG), because the brain uses glucose for its metabolism. FDG is widely used in PET scanning.
Other substances may be used for PET scanning, depending on the purpose of the scan. If blood flow and perfusion of an organ or tissue is of interest, the radionuclide may be a type of radioactive oxygen, carbon, nitrogen, or gallium.
The radioactive tracer/sugar is put into a vein through an intravenous (IV) line. It moves through the blood and collects in areas with a lot of cell activity. During this time, the PET scanner slowly moves over the body. A computer creates a map of the body. The amount of the radionuclide collected in the tissue affects how brightly the tissue appears on the image, and indicates the level of sugar uptake or cell activity in that organ or tissue.
For example, cancer cells use a lot of sugar and will show up as bright spots (called “hot spots”) on a PET scan. Damaged heart tissue will be less active and use less sugar. It would be seen as a darker spot compared to the normal heart tissue.
Why might I need a PET scan?
In general, PET scans are used to evaluate organs and/or tissues for the presence of disease or other conditions. More specific reasons for PET scans include:
- To diagnose neurological conditions such as:
- Alzheimer’s disease and other dementias
- Parkinson's disease (a progressive disease of the nervous system in which a fine tremor, muscle weakness, and a peculiar type of gait are seen)
- Huntington's disease (a hereditary disease of the nervous system which causes increasing dementia, bizarre involuntary movements, and abnormal posture)
- Epilepsy (a brain disorder involving recurrent seizures)
- To locate the specific area to be reached during brain surgery
- To evaluate the brain after injury to look for a blood clot or bleeding in, or blood and oxygen flow to the brain tissue
- To detect the spread of cancer to other parts of the body from the original cancer site
- To see how well cancer treatment is working
- To evaluate blood flow heart muscle to determine if treatment is needed to improve blood flow to the heart and to determine the effects of a heart attack
- To further identify lung lesions or masses seen on chest X-ray and/or chest CT
- To look for cancers that have come back after treatment and find them earlier than can be done with other diagnostic tests
There may be other reasons for your doctor to recommend a PET scan.
What are the risks of a PET scan?
The amount of the radionuclide injected into your vein for the procedure is very small, and there is no need for precautions against radiation exposure. The injection of the radionuclide may cause some slight discomfort. Allergic reactions to the radionuclide are rare, but may occur. Tell your doctor if you are allergic to or sensitive to medications, contrast dyes, iodine, or latex.
For some people, having to lie still on the scanning table for the length of the procedure may cause some discomfort or pain.
Tell your healthcare provider if you are pregnant, or think you may be pregnant, are lactating, or breastfeeding.
There may be other risks depending on your specific medical condition. Be sure to discuss any concerns with your doctor prior to the procedure.
Certain factors or conditions may interfere with the accuracy of a PET scan including:
- High blood glucose levels in diabetics
- Caffeine, alcohol, or tobacco consumed within 24 hours of the procedure
- Excessive anxiety (may affect brain function)
- Medications, such as insulin, tranquilizers, and sedatives
Tell your doctor if any of the above situations may apply to you.
How do I get ready for a PET scan?
- Your doctor will explain the procedure and you can ask questions.
- You will be asked to sign a consent form that gives your permission to do the procedure. Read the form carefully and ask questions if anything is not clear.
- Be sure to tell your doctor, the radiologist, or the technologist if you are allergic to latex and/or sensitive to medications, contrast dyes, and/or iodine.
- Fasting (not eating) for a certain period before the procedure is required, usually for at least four hours. Your doctor will give you special instructions ahead of time so you know how long you are to withhold food and drink. Your doctor will also talk to you about using your regular medicines before the PET scan.
- Tell your doctor if you are pregnant or think you may be.
- Make sure your doctor has a list of all medications (prescribed and over-the-counter) and all herbs, vitamins, and supplements that you are taking.
- You should not consume any caffeine or alcohol, or use tobacco, for at least 24 hours before the procedure.
- If you have diabetes and use insulin, you may be instructed to take your pre-procedure insulin dose with a meal 3 to 4 hours before the procedure. Your doctor will give you specific instructions based on your situation. Also, a fasting blood sugar may be taken prior to the procedure. If your blood sugar is elevated, you may be given insulin to lower it.
- Based on your medical condition, your doctor may give you other instructions on what to do before the procedure.
What happens during a PET scan?
PET scans may be done on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your doctor's practices.
Generally, a PET scan follows this process:
- You will be asked to remove any clothing, jewelry, or other objects that may get in the way of the scan. A bracelet with your name and an identification number may be put on your wrist. You may get a second bracelet if you have allergies.
- If you are asked to remove your clothing, you will be given a gown to wear.
- You will be asked to empty your bladder before starting the procedure.
- An intravenous (IV) line will be started in your hand or arm for injection of the radioactive tracer. (Depending on the type of PET scan you may inhale or swallow the tracer.)
- Certain types of scans of the abdomen (belly) or pelvis may require that a urinary catheter be put into the bladder to drain urine during the procedure.
- In some cases, an initial scan may be done before the radioactive tracer is injected, depending on the type of study being done.
- You will be in a chair or on a table in a quiet room.
- The radioactive tracer will be injected into your vein. The tracer will be allowed to concentrate in the organ or tissue for about 45 to 60 minutes. You will stay in the facility during this time and will be asked to rest quietly. You will not be hazardous to other people, as the tracer emits less radiation than a standard X-ray.
- After the tracer has been absorbed for a certain length of time, you will be positioned on a padded table inside the scanner ring and the scan will begin. The scanner will move slowly over the body part being studied.
- When the scan has been completed, the IV line will be removed. If a urinary catheter has been inserted, it will be removed, too.
While the PET scan itself causes no pain, having to lie still for the length of the procedure might cause some discomfort or pain, particularly if you have recently had surgery or a joint injury. The technologist will use all possible comfort measures and complete the procedure as quickly as possible to minimize any discomfort or pain.
What happens after a PET scan?
Be sure to move slowly when getting up from the scanner table to avoid any dizziness or lightheadedness.
You will be instructed to drink plenty of fluids and empty your bladder often for 24 to 48 hours after the test. This will help flush the remaining radioactive tracer from your body.
The IV will be removed and the site will be checked for any signs of redness or swelling. Tell your doctor if you notice any pain, redness, and/or swelling at the IV site after you go home. This may be a sign of infection or other type of reaction.
Your doctor may give you additional or alternate instructions after the procedure, depending on your particular situation.
Next stepsBefore you agree to the test or the procedure make sure you know:
- The name of the test or procedure
- The reason you are having the test or procedure
- The risks and benefits of the test or procedure
- When and where you are to have the test or procedure and who will do it
- When and how will you get the results
- How much will you have to pay for the test or procedure
- MMI board-certified, academically affiliated clinician
- Moloney Johns, Amanda, PA-C, MPAS, BBA