A PET scan (positron emission tomography) is an imaging test that reveals how your tissues and organs are functioning, not just what they look like. Unlike X-rays or CT scans that show structure, a PET scan highlights areas of high or low metabolic activity, making it especially useful for detecting cancer, evaluating brain disorders, and assessing heart health. Most PET scans today are performed as combined PET/CT scans, pairing functional and structural imaging in a single session.
How a PET Scan Works
Before the scan, you receive an injection of a small amount of radioactive material called a tracer. The most common tracer is a modified form of glucose tagged with a radioactive fluorine atom. Because cancer cells, inflamed tissue, and highly active brain regions consume more glucose than surrounding tissue, the tracer concentrates in those areas.
Once inside your cells, the tracer emits tiny particles called positrons. Each positron almost immediately collides with a nearby electron, and that collision produces two gamma rays that shoot off in exactly opposite directions. The PET scanner’s ring of detectors picks up both gamma rays arriving at the same instant, and a computer uses millions of these paired signals to build a detailed 3D map of tracer activity throughout your body. Areas that absorb more tracer appear as bright “hot spots” on the image.
What PET Scans Are Used For
Cancer
Oncology is the most common reason for a PET scan. After a cancer diagnosis, the scan helps determine whether the disease has spread to lymph nodes or distant organs, a process called staging. For lung cancer, PET/CT reaches about 90% accuracy for detecting lymph node involvement, with significantly better performance than a standard CT scan, which relies on node size alone. Beyond initial staging, PET scans track how a tumor responds to chemotherapy or radiation. A conventional CT can show whether a tumor has physically shrunk, but that doesn’t always reflect what’s actually happening. PET imaging shows whether the remaining tissue is still metabolically active, which is often a better indicator of whether treatment is working.
Brain Disorders
PET scans play a growing role in evaluating neurological conditions. In Alzheimer’s disease, specialized tracers bind to the amyloid plaques that are a hallmark of the disease. A negative amyloid PET scan can effectively rule out Alzheimer’s as the cause of cognitive decline, while a positive result increases diagnostic confidence and can guide treatment decisions. This is particularly valuable in complex cases where someone has both epilepsy and memory problems, since the cause of cognitive decline may not be obvious. Other brain-specific tracers can measure dopamine activity in Parkinson’s disease, map blood flow and oxygen use, and even track serotonin pathways in mood disorders.
Heart Health
In cardiology, PET scans evaluate blood flow to the heart muscle and determine whether areas of damaged tissue are still viable. This distinction matters when deciding whether a patient would benefit from a procedure to restore blood flow. If the heart muscle in a particular area is alive but underfed, reopening a blocked artery could help. If the tissue is already dead, the risk of intervention may not be worthwhile.
Preparing for Your Scan
Preparation for a standard glucose-based PET scan centers on controlling your blood sugar, since the tracer competes with regular glucose for entry into cells. You’ll need to fast for about 12 hours beforehand, drinking only plain water. No gum, mints, candy, vitamins, or syrup-based medications during the fast. On the day of the scan, your blood glucose needs to fall between 80 and 150 mg/dL. If it’s above 150, the scan may need to be rescheduled because elevated blood sugar interferes with tracer uptake and can produce unreliable images.
If you have diabetes, your care team will typically have you do a practice fasting run beforehand to make sure your blood sugar stays in range overnight. If your glucose drops below 70 mg/dL or you feel symptoms of low blood sugar, you should treat it as you normally would and call to reschedule.
What Happens During the Scan
The tracer is injected into a vein in your arm, and then you wait. Guidelines recommend starting the scan about 60 minutes after injection, with an acceptable window of 55 to 75 minutes. During this uptake period, you’ll sit or lie quietly. Physical activity, talking, or even chewing can redirect the tracer to muscles instead of the areas your doctor needs to see.
Once the uptake period is over, you lie on a narrow table that slides into the scanner, which looks like a large doughnut. The actual scanning portion typically takes 20 to 40 minutes depending on how much of your body needs to be imaged. The machine is quiet compared to an MRI, and the process is painless. If you’re having repeat scans to monitor treatment, the timing between injection and scanning should stay consistent (within 10 to 15 minutes of prior scans) so results can be accurately compared.
How Results Are Interpreted
Radiologists use a measurement called the standardized uptake value (SUV) to quantify how much tracer a particular area has absorbed, adjusted for your body weight and the amount of tracer injected. A higher SUV means more metabolic activity. You may have heard that an SUV above 2.5 suggests malignancy, but this threshold is unreliable in practice. Infections and inflammation can produce high SUV readings that mimic cancer, while slow-growing tumors sometimes show minimal uptake.
When an area shows no more tracer uptake than the normal tissue around it and the overall suspicion for cancer is low, that’s generally reassuring and can help avoid unnecessary biopsies. But the SUV number alone never makes a diagnosis. Your doctor interprets PET findings alongside your symptoms, lab results, and other imaging.
Radiation Exposure and Safety
A PET scan does involve radiation. The tracer itself delivers an average effective dose of roughly 3.4 millisieverts (mSv), comparable to about one year of natural background radiation. However, because most PET scans are done as PET/CT combos, the CT portion adds its own dose. A combined PET/CT can deliver around 8 to 10 mSv or more depending on the CT settings, which is higher than a standard CT scan on its own.
For context, a single chest X-ray delivers about 0.04 mSv, roughly 27 times less than a CT exam. The radiation from a PET/CT is considered low-risk for adults, but cumulative exposure matters for patients who need repeated scans over time, especially children. Facilities typically use the lowest CT dose that still produces diagnostic-quality images.
Pregnancy is not an absolute barrier to PET imaging. The American College of Obstetricians and Gynecologists states that when a PET scan is medically necessary, it should not be withheld from a pregnant patient, since the radiation dose is well below levels associated with fetal harm. For breastfeeding mothers, some radioactive materials are excreted into breast milk, so a temporary pause in nursing may be recommended depending on the specific tracer used.