A positron emission tomography (PET) scan is a medical imaging technique that allows doctors to observe metabolic processes and chemical activity in the body. Unlike some other imaging tests, such as CT scans, which show only the structure of the body, PET scans provide information on how well your tissues and organs are working.
The technique uses safe radioactive tracers attached to sugar molecules that are injected into the bloodstream, but they may also be given orally or via inhalation. The PET scanner detects cells that absorb large amounts of the tracer (by using the sugar), indicating a potential health problem, particularly cancer, heart disease, and various brain conditions.
Healthcare providers use PET scans to evaluate organs and tissues for the presence of disease or other conditions and to assess the function of organs, including the heart or brain. Here are some specific reasons why you might have a PET scan.
Cancer Diagnosis and Management
With a higher metabolic rate than normal healthy cells, cancer cells show up on PET scans as bright spots, allowing providers to:
Detect cancerous cells
Assess the spread of cancer to other parts of the body from the original site (metastasis)
Monitor the effectiveness of treatments
Determine if cancer has recurred or returned after treatment
Some cancers do not appear on PET scans.
The technique is most often used to detect brain, colorectal, and lung cancers, lymphoma, and melanoma.
Neurological Conditions
PET scans are useful for assessing neurological conditions because they can reveal blood flow throughout the brain (more blood flow is correlated with areas of higher brain activity), glucose and oxygen metabolism, and amyloid plaques (harmful deposits of beta-amyloid protein that can disrupt cell function and communication).
They are used to diagnose and evaluate brain conditions such as:
Additionally, a PET scan is required to determine eligibility for some treatments. For example, you need to undergo a PET scan or lumbar puncture (a procedure to examine cerebrospinal fluid) that confirms you have beta-amyloid plaques before your provider can prescribe you lecanemab (Leqembi), a drug that can slow Alzheimer’s progression by attacking the proteins in the brain.
Cardiovascular Issues
PET scans help providers assess heart health by imaging blood flow in the heart and identifying both dead and injured-but-still-functioning tissue. Providers use this information to:
Detect areas in your heart with poor blood flow
Diagnose coronary artery disease, diseased heart muscle, and cardiac sarcoidosis (inflammatory disease that causes scarring of heart tissue)
Determine if certain cardiac treatments are right for you and how well you are responding to cardiac treatment
PET scans are also useful for detecting infection or inflammation in the heart and other areas of the body.
PET scans work using radioactive tracers that differ depending on the purpose of the scan, though a radionuclide called fluorodeoxyglucose (FDG) is the most commonly used substance.
When cells break down the tracer, positrons (positively charged, low energy radiation) are emitted. These positrons react with nearby electrons, producing gamma rays. The PET scanner detects and analyzes the gamma rays, producing image maps of organs or tissues. The more of the tracer that tissues take up, the brighter those areas appear on the image.
Before a PET scan, you will need to remove any jewelry and metallic objects, which interfere with the scan, and you may be asked to put on a hospital gown and empty your bladder. A PET scan involves several steps:
You will receive an IV injection of the radioactive tracer and be asked to sit still for about an hour while the tracer distributes throughout your body and is absorbed by your organs and tissues.
You will then lie on your back on a flat, padded table that slides into the PET scanner, a large machine shaped like a doughnut, and remain still during the scan.
The table will guide you through the PET scanner as it captures images of your body, a process that takes about half an hour.
A computer processes the signals into detailed 3D images of your internal body structures, which your provider will review for clarity before removing your IV line and discharging you.
To prepare for a PET scan:
Pause or adjust medications, as instructed by your provider.
Avoid caffeine for 24 hours before the procedure if you’re being scanned for heart concerns.
Avoid strenuous exercise for 24 hours before your scan.
Follow a low-carbohydrate, no-sugar diet for 24 hours before the scan.
Avoid eating anything for four to six hours before the scan, and make sure your last meal includes high-protein foods.
Wear comfortable, loose-fitting clothing and leave metallic accessories at home.
Inform the medical team of any existing medical conditions, allergies, if you’re pregnant or breastfeeding, or if you’re claustrophobic.
For a PET scan appointment, expect to:
Spend about three hours to complete the test, with much of that time spent sitting or lying as still as possible
Hear noise from the machine, which may resemble whirring or clicking sounds, during the scan
Go home and resume your normal activities after your procedure is finished and you’re cleared to leave (you may be advised to drink plenty of fluids to flush the tracer from your system)
Avoid close contact with pregnant women, babies, and young children for six hours after the scan while the radioactive substances are still in your body
Wait at least a day to get your results, though it may take longer.
While PET scans involve a small amount of radiation exposure (similar to several years’ worth of natural radiation exposure from the environment), they are safe and not associated with any side effects. But the radiation may be harmful to an unborn baby or infant, making PET scans a risk to people who are pregnant or breastfeeding (the radiation can contaminate breast milk).
Though rare, some people have a mild allergic reaction to the tracer, potentially causing weakness, sweating, and difficulty breathing. A small bruise and swelling may also develop around the injection site.
Minimal care is required after a PET scan. Make sure to drink plenty of water to flush your system and monitor your health for any allergic or injection-site reactions.
Complications from PET scans are exceedingly rare. Given that radiation exposure from all sources add up over a lifetime, it’s possible that even the low doses of radiation in PET scans can increase your cancer risk very slightly.
Another potential complication from the scan is anxiety and discomfort from being in the enclosed space of the PET scanner.
A PET scan helps doctors look at how your tissues and organs are functioning by spotting areas of higher chemical or metabolic activity that might signal diseases like cancer or heart problems. If your doctor recommends a PET scan, you'll be asked to avoid sugary foods and drinks beforehand and might need to take a break from certain medications. Drink lots of water after your scan to help flush out the radioactive tracer from your system.
