What a PET scan is and why doctors order it
A PET scan (positron emission tomography) is a medical imaging test that shows how your body's cells are working, not just what they look like. Unlike X-rays or CT scans that take pictures of your organs and bones, a PET scan tracks chemical activity inside your body by detecting radiation from a tracer — a radioactive substance injected into your bloodstream before the test.
Doctors order PET scans most often to look for cancer, check how well cancer treatment is working, or investigate brain problems like dementia or epilepsy. The scan can also help cardiologists see which parts of your heart muscle are still getting blood flow after a heart attack. Because the test shows activity rather than structure, it can sometimes spot disease earlier than other imaging methods.
Key Takeaways
- A PET scan uses a radioactive tracer injected into your vein to show how cells are working, not what they look like.
- The test takes 30 to 60 minutes total, but the actual scanning part lasts only 20 to 30 minutes while you lie still inside a machine.
- You will receive instructions to fast (usually for 4 to 6 hours) before the scan and to avoid strenuous activity the day of the test.
- The radiation dose from a PET scan is small and leaves your body within hours; the tracer is not toxic.
- Results typically come back within a few days, and your doctor will explain what the images show at a follow-up appointment.
How the tracer works and what you need to know before arrival
The radioactive tracer used in a PET scan is usually a form of glucose (sugar) tagged with a radioactive element. Cancer cells and areas of inflammation consume glucose faster than healthy tissue, so they show up as brighter spots on the scan. Your body metabolizes the tracer naturally over a few hours, and the radiation is gone within a day.
Before your appointment, you will receive written instructions that typically include fasting for 4 to 6 hours before the scan. Do not eat or drink anything except water during that time. You should also avoid strenuous exercise for 24 hours before the test, because muscle activity can affect how the tracer distributes through your body and make the images harder to read. Wear comfortable, loose clothing with no metal zippers, snaps, or underwire — metal interferes with the scanner.
Tell the imaging center if you are pregnant, breastfeeding, or diabetic. Pregnant women usually do not have PET scans unless the benefit clearly outweighs the small radiation risk. If you are diabetic, your blood sugar level may affect the tracer's behavior, so the center may ask you to check your glucose before arrival.
What happens during the scan itself
When you arrive, a technician will place an IV line in your arm and inject the radioactive tracer. You will then wait in a quiet room for 30 to 60 minutes while the tracer circulates through your body and concentrates in the areas your doctor wants to examine. This waiting period is important — it allows the tracer to reach the target tissue and clear from areas that are not of interest.
After the waiting period, the technician will guide you into the PET scanner, which looks like a large doughnut or tunnel. You will lie on a table that slides into the opening. The actual scanning takes 20 to 30 minutes, during which you must remain still. The scanner detects radiation coming from the tracer and creates detailed images of the activity inside your body. You will not feel anything during the scan — it is painless and silent.
If you are claustrophobic, tell the technician before the scan starts. Most PET scanners are open enough that people find them less confining than MRI machines, but the center may be able to offer support or adjust the procedure if you are anxious.
Radiation exposure and safety during a PET scan
The radiation dose from a PET scan is small and comparable to the background radiation you are naturally exposed to over several months. The tracer leaves your body through urine and sweat within 24 hours, and the radioactive material decays rapidly. After the scan, you can return to normal activities when ready — there is no lingering radiation risk to you or to people around you.
Pregnant women should not have PET scans except in rare cases where the medical benefit is urgent, because any radiation exposure during pregnancy carries a small risk to the fetus. If you are breastfeeding, ask your doctor whether you should pump and discard milk for a few hours after the scan, though many centers say this precaution is not necessary with the tracer doses used.
If you have a severe allergy to the tracer itself (extremely rare), tell the imaging center before your appointment. Allergic reactions to PET tracers are uncommon, but the center needs to know your history.
How long results take and what comes next
A radiologist will review your PET scan images, usually within 24 to 48 hours, and write a report describing what the scan shows. Your doctor will receive this report and schedule a follow-up appointment to discuss the results with you. Do not expect to hear findings when ready after the scan — the images need to be analyzed carefully, and your doctor will want to explain what they mean in the context of your medical history.
The report will describe areas of high or low activity and what those patterns suggest. If the scan shows something unexpected, your doctor may order additional tests — such as a biopsy, CT scan, or blood work — to confirm the finding. If the scan is normal, your doctor will explain what that means for your diagnosis or treatment plan.
Costs and what insurance typically covers
PET scan costs vary widely depending on where you live, which facility performs the scan, and what body part is being imaged. A single PET scan can range from $3,000 to $6,000 or more before insurance. Most insurance plans, including Medicare, cover PET scans when a doctor orders them for specific medical reasons — such as cancer diagnosis, cancer follow-up, or evaluation of dementia — but coverage rules differ by plan.
Before your appointment, contact your insurance company to confirm coverage and ask about your out-of-pocket costs. If you do not have insurance or your plan does not cover the scan, ask the imaging center about payment plans or reduced-cost options. Some centers offer discounts for uninsured patients who pay upfront.
Frequently Asked Questions
Is a PET scan safe?
Yes. The radiation dose is small and comparable to natural background radiation over several months. The tracer is not toxic and leaves your body within 24 hours. Serious side effects are extremely rare.
Can I eat or drink before my PET scan?
No — you will be instructed to fast for 4 to 6 hours before the scan. Water is usually allowed. Eating or drinking before the test can affect how the tracer distributes and make the images less useful.
How long does a PET scan take?
The entire appointment is usually 1 to 2 hours, but the actual scanning takes only 20 to 30 minutes. Most of the time is spent waiting for the tracer to circulate through your body after injection.
Will I be able to drive home after the scan?
Yes. A PET scan does not use sedation, and the radiation does not impair your ability to drive. You can return to normal activities when ready after the test.
What if I am claustrophobic?
PET scanners are generally more open than MRI machines, but tell the technician about your anxiety before the scan. The center may offer support, allow a companion in the room, or discuss other options with your doctor.