Positron emission tomography scan
Medically reviewed by Drugs.com. Last updated on Aug 10, 2021.
A positron emission tomography (PET) scan is an imaging test that can help reveal the metabolic or biochemical function of your tissues and organs. The PET scan uses a radioactive drug (tracer) to show both normal and abnormal metabolic activity. A PET scan can often detect the abnormal metabolism of the tracer in diseases before the disease shows up on other imaging tests, such as computerized tomography (CT) and magnetic resonance imaging (MRI).
The tracer is most often injected into a vein within your hand or arm. The tracer will then collect into areas of your body that have higher levels of metabolic or biochemical activity, which often pinpoints the location of the disease. The PET images are typically combined with CT or MRI and are called PET-CT or PET-MRI scans.
During a positron emission tomography (PET) scan, you lie on a narrow table that slides into a doughnut-shaped hole. The scanner takes about 30 minutes to produce detailed images of metabolic activity in your tissues and organs.
Why it's done
A PET scan is an effective way to help identify a variety of conditions, including cancer, heart disease and brain disorders. Your doctor can use this information to help diagnose, monitor or treat your condition.
Cancer cells show up as bright spots on PET scans because they have a higher metabolic rate than do normal cells. PET scans may be useful in:
- Detecting cancer
- Revealing whether your cancer has spread
- Checking whether a cancer treatment is working
- Finding a cancer recurrence
PET scans must be interpreted carefully because noncancerous conditions can look like cancer, and some cancers do not appear on PET scans. Many types of solid tumors can be detected by PET-CT and PET-MRI scans, including:
- Head and neck
- Lymphatic system
PET scans can reveal areas of decreased blood flow in the heart. This information can help you and your doctor decide, for example, whether you might benefit from a procedure to open clogged heart arteries (angioplasty) or coronary artery bypass surgery.
PET scans can be used to evaluate certain brain disorders, such as tumors, Alzheimer's disease and seizures.
Combining a PET scan with an MRI or CT scan can help make the images easier to interpret. At left is a CT scan, while the center image is a PET scan. The image on the right is a combined PET-CT scan. The bright spot in the chest, seen best on the PET and PET-CT scans is lung cancer.
This PET image shows an area of reduced blood flow from one of the arteries that feeds the heart. This information may help doctors decide whether to suggest bypass surgery or angioplasty to restore that blood flow.
A PET scan can compare a normal brain (left) with one affected by Alzheimer's disease (right). The loss of red color with an increase in yellow, blue and green colors shows areas of decreased metabolic activity in the brain due to Alzheimer's disease.
For your PET scan, a radioactive drug (tracer) will be injected into a vein. Because the amount of radiation you're exposed to in the tracer is small, the risk of negative effects from the radiation is low. But the tracer might:
- Expose your unborn baby to radiation if you are pregnant
- Expose your child to radiation if you are breastfeeding
- Cause an allergic reaction, although this is rare
Talk with your doctor about the benefits and risks of a PET scan.
How you prepare
Tell your doctor:
- If you've ever had a bad allergic reaction
- If you've been sick recently or you have another medical condition, such as diabetes
- If you're taking any medications, vitamins or herbal supplements
- If you're pregnant or you think you might be pregnant
- If you're breastfeeding
- If you're afraid of enclosed spaces (claustrophobic)
Your doctor will give you detailed instructions on how to prepare for your scan. A general rule is to avoid strenuous exercise for a couple of days before the study and to stop eating four hours before the scan.
What you can expect
The PET-CT or PET-MRI scanner is a large machine that looks a little like a giant doughnut standing upright, similar to CT or MRI scanners.
From start to finish, the procedure takes about two hours to complete and typically does not require an overnight hospital stay. When you arrive for your scan, you may be asked to:
- Change into a hospital gown
- Empty your bladder
A member of your health care team injects the radioactive drug (tracer) into a vein in your arm or hand. You may briefly feel a cold sensation moving up your arm. You rest and remain silent in a reclining chair for 30 to 60 minutes while the tracer is absorbed by your body.
During the procedure
When you are ready, you lie on a narrow, padded table that slides into the part of the scanner that looks like a doughnut hole. During the scan you must be very still so that the images aren't blurred. It takes about 30 minutes to complete a PET-CT scan and 45 minutes for a PET-MRI scan. The machine makes buzzing and clicking sounds.
The test is painless. If you're afraid of enclosed spaces, you may feel some anxiety while in the scanner. Be sure to tell the nurse or technologist about any anxiety causing you discomfort. He or she may give you a drug to help you relax.
After the procedure
After the test you can carry on with your day as usual, unless your doctor tells you otherwise. You'll need to drink plenty of fluids to help flush the tracer from your body.
A doctor specially trained to interpret scan images (radiologist) will report the findings to your doctor.
The radiologist may compare your PET images with images from other tests you've undergone recently, such as MRI or CT. Or the PET images may be combined to provide more detail about your condition.