What a PET scan of the brain is

A PET scan of the brain is an imaging test that shows how your brain is using energy and blood flow, rather than just its physical structure. The letters stand for positron emission tomography. A doctor injects a small amount of radioactive tracer — usually a sugar-based substance — into your bloodstream, and a scanner detects where that tracer collects in your brain. Areas using more energy light up on the image.

This is different from an MRI or CT scan, which photograph the brain's anatomy — its size, shape, and whether anything is obviously blocking or swelling. A PET scan shows function: which parts of your brain are active, and which are not. That difference matters because some conditions damage how the brain works before they show up as physical changes.

Key Takeaways

  • A PET scan detects how your brain is using energy and blood flow by tracking a radioactive tracer injected into your bloodstream.
  • Doctors order PET scans when they suspect dementia, epilepsy, brain tumors, or certain psychiatric conditions, because these show up as abnormal energy patterns before structural damage appears.
  • The scan itself takes 30 to 60 minutes and involves lying still in a scanner; the radioactive tracer leaves your body naturally within hours.
  • A radiologist interprets the images and sends a report to your doctor, who will discuss what the findings mean for your diagnosis and next steps.

Why a doctor would order a PET scan of the brain

Doctors order brain PET scans when they need to see how your brain is functioning, not just its shape. The most common reasons are to investigate memory loss or confusion that might point to Alzheimer's disease or another dementia, to locate the source of seizures in epilepsy, to check for cancer that has spread to the brain, or to evaluate certain psychiatric conditions like depression or schizophrenia when the diagnosis is unclear.

A PET scan can also help doctors plan brain surgery by showing which areas are most active and which should be avoided. In some cases, it helps distinguish between different types of brain tumors or between a tumor and scar tissue from an old injury or infection.

What happens during a PET scan of the brain

You will arrive at an imaging center or hospital and have an IV placed in your arm. A technician injects the radioactive tracer through the IV. You then wait 30 to 60 minutes while the tracer circulates and collects in your brain — this waiting period lets the tracer reach the areas your brain is actively using.

After the waiting period, you lie on a table that slides into a large, doughnut-shaped scanner. You need to stay very still for 20 to 30 minutes while the scanner detects the radiation from the tracer. The scanner is not loud like an MRI, and you will not feel anything during the scan itself. Once the scan is complete, the table slides back out and you can go home or to your next appointment.

Radiation exposure and safety

The amount of radiation in a PET scan is small and leaves your body naturally within a few hours as you urinate and breathe. The dose is considered safe for diagnostic purposes — meaning the information gained from the scan outweighs the small radiation risk. However, if you are pregnant or breastfeeding, tell your doctor before the scan, because the tracer can cross to a fetus or into breast milk.

If you have diabetes, let the imaging center know before your appointment. The tracer is often a radioactive form of glucose (sugar), and your blood sugar level can affect how the scan looks. You may be asked to fast or adjust your insulin before the test.

How the results are interpreted

A radiologist — a doctor who specializes in reading medical images — reviews your PET scan and writes a report describing what they see. They note which areas of your brain show normal, high, or low energy use, and compare your scan to what a healthy brain typically looks like. The report goes to your doctor, who ordered the scan.

Your doctor will discuss the results with you and explain what they mean for your condition. A PET scan result is one piece of information; your doctor combines it with your symptoms, your medical history, blood tests, and sometimes other imaging to reach a diagnosis. One abnormal PET scan does not automatically mean you have a specific disease — your doctor interprets it in context.

Cost and insurance coverage

A brain PET scan typically costs between $3,000 and $6,000 before insurance, though the price varies by location and imaging center. Most insurance plans cover a brain PET scan when a doctor orders it for a medically necessary reason — such as investigating dementia, seizures, or cancer — but coverage depends on your specific plan and whether your doctor's order meets the insurance company's criteria.

Before your scan, ask the imaging center to check with your insurance about coverage. If you do not have insurance or your plan does not cover it, ask whether the center offers a cash discount or payment plan. Some academic medical centers or research programs may offer PET scans at reduced cost if you meet certain criteria.

Alternatives to a brain PET scan

Depending on what your doctor is investigating, other imaging tests might provide useful information. An MRI of the brain shows detailed pictures of brain structure and can detect tumors, bleeding, or scarring. A CT scan is faster and less expensive than an MRI but gives less detail. An EEG (electroencephalogram) records electrical activity in the brain and is often used to diagnose epilepsy without radiation.

For dementia evaluation, some doctors use a combination of cognitive testing, blood tests for biomarkers, and structural imaging before ordering a PET scan. Your doctor will recommend the test or tests that best fit your situation and the question they are trying to answer.

Frequently Asked Questions

Is a PET scan painful?

No. The only discomfort is usually the IV needle when it is placed in your arm. The scan itself is painless — you will not feel the radiation or the scanner working. You may feel slight discomfort from lying still for 20 to 30 minutes, especially if you have back or neck pain.

Can I eat or drink before a PET scan?

This depends on the type of tracer used and your doctor's instructions. For many brain PET scans, you can eat and drink normally, but you should avoid caffeine and strenuous exercise for 24 hours before the scan, as these can affect blood flow patterns. Always follow the specific instructions your imaging center gives you.

How long does it take to get results?

The scan itself takes 30 to 60 minutes, but the radiologist's report usually takes one to three business days. Your doctor will contact you to discuss the results and what they mean. Do not assume no news is good news — always follow up with your doctor's office if you have not heard back within a week.

Will a PET scan show if I have Alzheimer's disease?

A PET scan can show a pattern of low brain activity that is consistent with Alzheimer's disease, but it cannot diagnose Alzheimer's on its own. Doctors use PET scans along with cognitive testing, memory evaluations, and sometimes blood tests to reach a diagnosis. A normal PET scan does not rule out early dementia.

Do I need to do anything special after the scan?

No special precautions are needed. The radioactive tracer leaves your body naturally. You can resume normal activities, eat, drink, and be around other people. There is no lingering radiation risk to you or anyone around you after the scan is complete.