A PET scan creates images of how your body is using energy at the cellular level

A PET scan (positron emission tomography) is a medical imaging test that shows where cells in your body are most active. Unlike X-rays or CT scans that show what something looks like, a PET scan shows how hard your cells are working. Cancer cells, inflamed tissue, and areas with reduced blood flow all use energy differently than healthy tissue, so a PET scan can reveal these differences.

The test involves injecting a small amount of radioactive tracer — usually a sugar-based substance — into your bloodstream. Your body's cells absorb this tracer in proportion to how much energy they are using. A scanner then detects the radiation and creates a map of activity throughout your body. The whole process takes about 30 to 60 minutes from injection to the end of scanning.

Key Takeaways

  • A PET scan shows cellular activity by tracking how your body uses a radioactive tracer, not by taking a picture of your organs the way an X-ray does.
  • The test is most commonly used to detect cancer, evaluate how well cancer treatment is working, and assess brain disorders like dementia and epilepsy.
  • You will need to fast for several hours before the scan and may need to stop certain medications, so ask your doctor what to do before your appointment.
  • The radiation dose from a PET scan is small and leaves your body within hours, so the risk is generally considered low for the information it provides.
  • Your doctor will receive detailed images and a written report within a few days, which they will discuss with you at a follow-up visit.

Why doctors order a PET scan

Doctors use PET scans most often to look for cancer or to see whether cancer treatment is working. Because cancer cells use energy much faster than normal cells, they show up as bright spots on a PET scan. This helps doctors find tumors that might be too small to see on other imaging tests and can reveal whether cancer has spread to other parts of the body.

PET scans are also used to evaluate brain disorders. Dementia, Alzheimer's disease, epilepsy, and Parkinson's disease all change how the brain uses energy, and a PET scan can show these patterns. Cardiologists sometimes order PET scans to check blood flow to the heart muscle, particularly when someone has had a heart attack or has chest pain.

What to do before your PET scan appointment

Your doctor or the imaging center will give you specific instructions, but most PET scans require you to fast — eat and drink nothing except water — for 4 to 6 hours before the test. This is because food affects how your body uses glucose, which would interfere with the scan results.

Tell your doctor about any medications you take, especially diabetes medications. If you have diabetes, you may need to adjust your insulin or other drugs before the scan. Wear comfortable, loose clothing without metal zippers, snaps, or underwire, because you may be asked to change into a hospital gown. Let the imaging center know if you are pregnant, think you might be pregnant, or are breastfeeding, because the radioactive tracer can affect a developing baby.

Arrive 15 to 30 minutes early so staff can check your blood sugar (if you have diabetes) and place an IV line for the tracer injection.

What happens during the scan

A technician will insert an IV into your arm and inject the radioactive tracer. You will then wait in a quiet room for 30 to 90 minutes while your body absorbs the tracer. During this time, try to stay still and relaxed — movement and talking use energy and can affect the results.

When it is time for the scan, you will lie on a table that slides into a large, doughnut-shaped machine. The scanner detects radiation coming from the tracer in your body and creates images. You will hear soft humming or clicking sounds. The actual scanning usually takes 20 to 30 minutes. You will not feel pain, heat, or any sensation from the radiation.

If you are claustrophobic, let the technician know beforehand. The machine is open on both ends, and you can communicate with the technician through a speaker system if you become uncomfortable.

After your PET scan

You can return to your normal activities when ready after the scan. The radioactive tracer leaves your body naturally through urine and sweat over the next several hours. Drink plenty of water to help flush it out faster. There are no lasting side effects from the radiation dose used in a PET scan.

Your doctor will receive the images and a written report from the radiologist within a few days. The radiologist is a doctor who specializes in reading medical images. Your own doctor will review the report and discuss the results with you, usually at a follow-up appointment. Ask your doctor what the results mean for your diagnosis and what happens next.

Understanding the radiation in a PET scan

The radioactive tracer used in a PET scan exposes you to a small amount of radiation. The dose is comparable to the radiation you receive from a CT scan or from natural background radiation over the course of a year. The tracer is designed to break down quickly, so most of it leaves your body within hours.

For most people, the benefit of the information a PET scan provides outweighs the small radiation risk. However, if you are pregnant or breastfeeding, the risk is different, which is why you must tell your doctor about your pregnancy status. People who have had multiple imaging tests involving radiation may want to discuss cumulative exposure with their doctor.

PET scan versus other imaging tests

Different imaging tests show different things. A CT scan or MRI takes detailed pictures of the structure of your organs — what they look like. A PET scan shows function — how hard your cells are working. Often, doctors order both tests together because they provide different information. A CT scan might show the size and location of a tumor, while a PET scan shows whether the tumor is active.

An ultrasound uses sound waves instead of radiation and is often used first for certain organs like the heart or abdomen. An X-ray is quick and uses less radiation than a CT scan but shows less detail. Your doctor will choose the test that best answers the medical question they are trying to solve.

Frequently Asked Questions

Is a PET scan safe?

Yes. The radiation dose is small and leaves your body quickly. The main precaution is to avoid the test if you are pregnant or breastfeeding, because the tracer can reach a developing baby or pass into breast milk. Tell your doctor if either applies to you.

How long does it take to get results?

The radiologist typically completes the report within one to three days. Your doctor will then review it and schedule a time to discuss the findings with you. Do not assume no news is bad news — ask your doctor's office when you can expect to hear back.

Will a PET scan show if my cancer is cured?

A PET scan can show whether a tumor is still active after treatment, but it cannot prove you are cured. Your doctor uses PET results along with other tests, your symptoms, and your medical history to assess your status. Some scans show no activity in a tumor, which is a good sign, but your doctor will explain what it means for your specific situation.

Can I drive home after my PET scan?

Yes, you can drive yourself home. The radioactive tracer does not affect your alertness or ability to drive. You do not need someone to pick you up unless your doctor has given you sedation, which is rare for PET scans.

What if I am claustrophobic?

Tell the imaging center before your appointment. The PET scanner is more open than an MRI machine, and you can communicate with the technician at any time. If you are very anxious, ask your doctor whether a mild sedative might help.