The short answer
A PET scan is a type of nuclear scan that makes 3-D pictures of areas where cells take up a radioactive form of sugar. Because cancer cells often take up more sugar than healthy cells, they can show up on the scan. PET is frequently combined with CT to show both where a tumor is and how active it is.
A PET scan is a type of nuclear scan that shows areas in the body where cells take up glucose (sugar).
Cancer cells often take up more glucose than healthy cells, so they can stand out on a PET scan.
Before the scan, you receive an injection of a radioactive tracer, then lie on a table that moves through the scanner.
Combined PET/CT pairs PET's activity information with CT's detailed anatomy in a single procedure.
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This page explains what a PET scan is, how to prepare, what happens during the scan, and what the results can and cannot show.
The simple version
A PET scan, short for positron emission tomography, is a type of nuclear scan. It makes detailed 3-D pictures of areas inside your body where cells take up glucose, a sugar. Cancer cells often take up more glucose than healthy cells, so those areas can show up on the scan, helping doctors find cancer and see how it behaves.
How to prepare
MedlinePlus says you may be asked not to eat for 4 to 6 hours before the scan. Follow the exact instructions your imaging team gives you. If you have diabetes, tell your team well ahead of time. Blood sugar or insulin levels can affect the results, so ask what to do about your diabetes medicines that day. Do not change them without that advice.
Tell your team about all medicines you take. Tell them if you are pregnant or might be, because developing organs are more sensitive to radiation.
What happens during the scan
Before the scan, you get an injection of a tracer. Most often, it is FDG, a radioactive form of glucose. MedlinePlus says it takes about an hour for the tracer to be absorbed before scanning starts. During the scan, you lie still on a table that moves through the scanner. A PET/CT combines two scans in one visit. How long the scan takes depends on which part of your body is studied. Ask your team how long to plan for.
Is the radiation safe?
A PET or PET/CT scan uses a small, carefully controlled amount of radioactive tracer. A combined PET/CT also adds the CT's own low-dose X-ray radiation. MedlinePlus says the radiation is about the same as in most CT scans. It leaves your body within about 2 to 10 hours. A single scan carries minimal risk, but repeated imaging over time may add a small increase in cancer risk. Ask your doctor how the scan will help your care.
Combined PET/CT
PET is frequently paired with CT in a single procedure. The CT creates detailed "anatomic" pictures of your organs and structures. The PET adds "functional" information. It shows where cells are more active, since cancer cells often use energy differently from normal ones. Combining the two gives a fuller picture of a tumor's location, and whether it is growing or spreading, than either test alone.
According to the National Cancer Institute, PET/CT may improve the ability to diagnose cancer. It can show how far a tumor has spread, help plan treatment, and track how well treatment is working. It may also reduce how many extra imaging tests you need.
What a PET scan cannot tell you
A PET scan is not perfect. MedlinePlus notes that it can give false results, for example when blood sugar or insulin levels are off. An active area on a PET scan is not automatically cancer.
Like all imaging, a PET scan is one part of the overall picture. Your doctor reads the results alongside lab tests and, in most cases, a biopsy (removing a small sample of tissue to check under a microscope). Then your doctor explains what the findings mean for your care and what, if anything, happens next.
After the scan
Ask your team what to do after the scan. A small amount of tracer stays in your body for a few hours as it clears. Ask whether they recommend any precautions for that time, and for how long.
Questions to ask your team
- What do you hope to learn from this scan?
- How should I prepare, including fasting and timing of diabetes medicines?
- What tracer will be used?
- How long will the appointment take?
- How does this scan fit with my other tests and treatment plan?
- When will results be ready, and who will walk me through them?
Sources
Common questions
What is a PET scan?
A PET scan is a type of nuclear scan that makes detailed 3-D pictures of areas inside your body where glucose (a sugar) is taken up. Because cancer cells often take up more glucose than healthy cells, the pictures can help find cancer in the body.
What happens during a PET scan?
Before the scan, you receive an injection of a tracer — often a radioactive form of the sugar glucose, called FDG. During the scan, you lie still on a table that moves back and forth through a scanner while it makes the pictures.
What is a combined PET/CT scan?
A PET/CT combines two imaging methods in one procedure. The CT is done first to create detailed pictures of the organs and structures in your body, and the PET shows areas where cells are more active. Cancer cells often behave differently from normal cells, so combining the two can give a more complete picture.
Why would I have a PET/CT instead of just one scan?
Combining CT and PET may show a tumor's location and its growth or spread more completely than either test alone. NCI notes it may improve the ability to diagnose cancer, determine how far a tumor has spread, plan treatment, and monitor response — and it may reduce the number of additional tests needed.
Does a PET scan involve radiation?
Yes. A PET scan uses a small amount of radioactive tracer, and a combined PET/CT also includes the CT's X-ray radiation. The amounts are carefully controlled. Your doctor can explain the benefits and any risks for your situation.
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Last updated: 2026-08-05Next planned review: 2027-07-03
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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