A PET scan (positron emission tomography) is an imaging test that shows how your organs and tissues are functioning at a cellular level. Unlike X-rays or CT scans, which show structure, a PET scan reveals metabolic activity, making it especially useful for detecting cancer, evaluating heart function, and diagnosing brain disorders like Alzheimer’s and Parkinson’s disease.
If you searched “pep scan,” you’re almost certainly looking for information about a PET scan. Here’s how it works, what to expect, and what the results mean.
How a PET Scan Works
Before the scan, you receive an injection of a small amount of radioactive material called a radiotracer through an IV line. The most common tracer is a modified form of glucose (sugar). Because cancer cells, active brain tissue, and working heart muscle all consume more glucose than surrounding tissue, the tracer naturally concentrates in those areas.
As the tracer breaks down inside your body, it releases tiny particles called positrons. When a positron collides with a nearby electron, the collision produces a pair of gamma rays that shoot out in exactly opposite directions. The PET scanner, a large doughnut-shaped machine, detects these paired gamma rays and feeds the data to a computer. The computer then builds a detailed image map showing where metabolic activity is highest. Areas that absorbed more tracer glow brighter on the image, while less active areas appear dimmer.
What PET Scans Are Used For
PET scans have three major clinical roles:
- Cancer: Diagnosing, staging, and monitoring treatment for many cancers, including breast, lung, and thyroid. Because cancer cells divide rapidly and burn through glucose faster than normal cells, they light up on PET images. Follow-up scans can show whether a tumor is shrinking in response to treatment.
- Heart disease: Evaluating blood flow to the heart muscle and identifying areas of damaged tissue after a heart attack or in cases of coronary artery disease.
- Brain disorders: Detecting early signs of Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and other forms of dementia by mapping which brain regions have reduced metabolic activity.
Different tracers can target different biological processes. The standard glucose-based tracer works well for most cancers, but specialized tracers exist for specific situations. For prostate cancer, a tracer that binds to a protein found on over 90% of prostate cancer cells provides superior sensitivity compared to the standard glucose tracer, which prostate tumors don’t absorb as readily.
What to Expect During the Scan
Most PET scans follow the same general sequence. You’ll be asked to fast for several hours beforehand, typically 4 to 6 hours, because eating raises your blood sugar and can interfere with the tracer’s distribution. You can usually take your regular medications with water.
After the tracer injection, you’ll wait roughly 60 minutes in a quiet room while it circulates and gets absorbed. During this time, you’ll be asked to sit or lie still and avoid talking, since even muscle activity from speaking can cause the tracer to collect in areas like the vocal cords, potentially creating misleading bright spots on the image.
The scan itself typically takes 20 to 45 minutes. You lie on a narrow table that slides slowly through the scanner. The machine is open on both ends, so it feels less enclosed than an MRI. The process is painless beyond the initial IV needle stick.
PET/CT: Why Both Are Often Done Together
Most modern PET scans are performed as PET/CT scans, combining the metabolic images from PET with the structural detail of a CT scan in a single session. The CT portion shows the exact anatomy (bones, organs, blood vessels), while the PET portion reveals which tissues are metabolically active. Overlaying both images lets doctors pinpoint exactly where abnormal activity is located.
The combined radiation exposure depends on how the CT component is used. The PET portion delivers roughly 8 millisieverts (mSv) for a standard adult dose. If the CT is used only to help locate abnormalities seen on the PET images, it adds about 7 mSv. A full diagnostic-quality CT scan can add up to 30 mSv. For context, the average person absorbs about 3 mSv per year from natural background radiation.
Understanding Your Results: SUV Numbers
PET scan reports often include a number called the SUV, or standardized uptake value. Think of it as a measurement of how brightly a particular spot glows on the scan. Higher SUV numbers mean more tracer was absorbed in that area, which can indicate a cancerous growth.
However, cancer isn’t the only thing that lights up. Infections, inflammation, and autoimmune reactions can all create bright spots and elevated SUV numbers. Even talking or humming before the scan can cause high uptake in the vocal cords with no tumor present. That’s why SUV numbers are never interpreted in isolation. Your doctor considers them alongside your symptoms, other imaging, biopsy results, and medical history.
On follow-up scans during cancer treatment, a dropping SUV number is generally a good sign. It suggests the tumor’s metabolic activity is decreasing, meaning treatment is working. A rising number may prompt your care team to reassess the current approach.
After the Scan
The radiotracer breaks down quickly and leaves your body primarily through urine. Drinking extra water after the scan helps flush it out faster. Most people can return to normal activities immediately. The radioactive exposure from the tracer itself is short-lived, with most of it cleared within a few hours.
Results are typically read by a nuclear medicine specialist or radiologist and sent to your referring doctor within a few days, though timelines vary by facility.