What a PET scan is and why doctors order them
A PET scan (positron emission tomography) is a medical imaging test that shows how your organs and tissues are working at the cellular level, not just their shape or size. Unlike CT or X-ray scans that take pictures of structure, a PET scan tracks activity — it detects where your body is using glucose (sugar) at higher or lower rates than normal.
Doctors order PET scans most often to look for cancer, because cancer cells typically consume glucose much faster than healthy cells do. They also use PET scans to evaluate how well the heart is pumping blood, to check for Alzheimer's disease or other forms of dementia, and to assess brain disorders like epilepsy or Parkinson's disease. Your doctor will tell you specifically why they are ordering the scan and what they are looking for.
Key Takeaways
- A PET scan uses a radioactive tracer injected into your bloodstream to show which parts of your body are using the most energy.
- The entire appointment usually lasts two to four hours, though the actual scanning time is only 20 to 40 minutes.
- You will need to fast for four to six hours before the scan and avoid strenuous activity for 24 hours after it.
- PET scans use a small amount of radiation, but the dose is considered safe for diagnostic purposes and is cleared from your body within hours.
- Results typically come back within a few days, and your doctor will review them with you and explain what they mean for your next steps.
How the PET scan procedure actually works
The scan begins with an injection of a radioactive tracer — most commonly a glucose-based compound called FDG (fluorodeoxyglucose) — into a vein in your arm. This tracer travels through your bloodstream and collects in areas where cells are using energy rapidly. You then wait 30 to 90 minutes in a quiet room while the tracer circulates and concentrates in the target organs.
After the waiting period, a technician will position you on a table and slide you into the PET scanner, which looks like a large tube with a ring-shaped camera inside. The camera detects the radiation emitted by the tracer and creates detailed images of activity throughout your body. You need to lie still during the scan, which typically takes 20 to 40 minutes depending on which parts of your body are being scanned. The machine is not loud, and you can communicate with the technician through an intercom if you need anything.
Many PET scans are combined with a CT scan in a single machine (called a PET-CT), which gives your doctor both the activity information from the PET and the structural detail from the CT. This combination scan takes longer but provides more complete information in one appointment.
What you need to do before your PET scan
Your doctor's office will give you specific instructions, but standard preparation includes fasting for four to six hours before the scan — usually no food or drinks except water. This matters because food raises your blood glucose level, which can interfere with how the tracer distributes in your body and make the images less clear.
Wear comfortable, loose-fitting clothing with no metal zippers, snaps, or underwire, since metal can interfere with the scanner. Remove jewelry, watches, and hearing aids before the scan. Tell the technician if you are pregnant, breastfeeding, or have diabetes, as these situations may change how the scan is performed or how results are interpreted.
Avoid strenuous exercise for 24 hours before the scan. Physical activity causes your muscles to use more glucose, which can create false signals in the images. If you take insulin or diabetes medication, ask your doctor whether to take your usual dose on the morning of the scan, since fasting may affect your blood sugar.
What happens after the scan and when you get results
After the scan is complete, you can return to your normal activities when ready. The radioactive tracer leaves your body naturally through urine and sweat over the next several hours. Drink plenty of water after the scan to help flush the tracer out faster, though this is not medically urgent — the amount of radiation is small and clears on its own.
A radiologist (a doctor who specializes in medical imaging) will review your scan images, usually within one to three business days. Your referring doctor will then contact you to discuss the results. If the scan shows something unexpected or concerning, your doctor may want to order additional tests or schedule a follow-up appointment to talk through next steps.
Results are not always definitive on their own. A PET scan showing high activity in an area might suggest cancer, infection, inflammation, or normal variation depending on the context. Your doctor will combine the PET results with your symptoms, medical history, and any other test results to reach a diagnosis.
Radiation safety and side effects
The radioactive tracer used in a PET scan delivers a small dose of radiation — roughly equivalent to the background radiation you receive naturally over several months, or similar to a CT scan of the chest. The dose is considered safe for diagnostic purposes and is approved by the FDA. The radiation does not stay in your body; it decays and is eliminated within hours.
Side effects from the tracer injection itself are rare. Some people experience mild bruising or soreness at the injection site. Allergic reactions to the tracer are extremely uncommon. If you are pregnant or breastfeeding, tell your doctor before the scan, as they may recommend waiting or may use a different imaging method instead.
If you are claustrophobic, let the technician know before the scan begins. The PET scanner is more open than an MRI machine, but if anxiety is a concern, your doctor may discuss options like sedation or rescheduling with you beforehand.
Cost, insurance, and what to bring
PET scan costs vary widely depending on your location, the facility, and what is being scanned. A single PET scan typically ranges from $3,000 to $6,000 before insurance, though combined PET-CT scans may cost more. Most insurance plans cover PET scans when they are ordered by a doctor for a medically necessary reason, but your out-of-pocket cost depends on your deductible, copay, and coinsurance.
Call your insurance company before your appointment to confirm coverage and ask what your responsibility will be. Bring your insurance card and photo ID to the appointment. If cost is a barrier, ask the facility's billing department whether they offer payment plans or whether you may be may be able to access for financial information programs.
Arrive 15 to 30 minutes early to complete paperwork. Bring a list of all medications and supplements you take, as some can interfere with scan results. If you have had previous imaging scans (CT, MRI, X-rays), bring those images or records if you have them, as they help the radiologist compare changes over time.
Frequently Asked Questions
Is a PET scan painful?
No. The only potentially uncomfortable part is the needle stick when the tracer is injected into your vein, which feels like a standard blood draw. The scanning itself is painless — you lie still while the camera takes pictures, and you do not feel the radiation.
Can I drive home after a PET scan?
Yes. The tracer does not make you drowsy or impair your ability to drive. You can return to normal activities when ready after the scan ends. If your doctor gave you sedation to help you relax during the scan, you would need someone else to drive you home.
How long does it take to get PET scan results?
The radiologist typically reviews the images within one to three business days. Your doctor will then contact you, usually by phone or at a follow-up appointment, to discuss what the scan shows. If results are urgent (for example, if cancer is suspected), your doctor may prioritize the review and call you sooner.
What if I am claustrophobic or anxious about the scan?
Tell your doctor and the technician before the appointment. The PET scanner is more open than an MRI, but if you are still uncomfortable, your doctor may recommend a mild sedative to help you relax. Some facilities also allow a family member or friend to stay in the scanning room with you for reassurance.
Can I eat or drink anything before my PET scan?
You should fast for four to six hours before the scan, which means no food and no sugary drinks. Water is usually allowed. If you have diabetes or take medications that require food, ask your doctor for specific instructions, as they may adjust your fasting time or medication schedule.