The short answer
A CT scan makes detailed pictures of the body's structure using x-rays. A PET scan uses a small amount of radioactive tracer to show how active tissue is. They answer different questions and are often done together as a PET-CT.
A CT scan shows detailed structure — the size, shape, and location of tissues and any masses.
A PET scan shows function — how active tissue is, using a small amount of radioactive tracer.
The two are often combined into a single PET-CT so activity can be matched to a precise location.
Neither test alone diagnoses cancer; results are read alongside your history and, often, a biopsy.
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This page is for anyone who has been booked for a PET scan, a CT scan, or both. It explains what each test shows, why they are often combined, and when to get help after a scan.
The short version
PET scans and CT scans are both imaging tests used in cancer care. They answer different questions. A CT scan shows structure — the size, shape and location of tissues and any masses. A PET scan shows function — how active tissue is. The two kinds of information fit together, so the tests are often combined into a single PET-CT.
What a CT scan shows
A CT scan uses x-ray equipment and a computer. Together they build detailed pictures, or "slices," of the inside of your body. A CT is good at showing the exact size, shape and position of organs and masses. On its own, though, it usually cannot tell how active a spot is.
You may be given a contrast agent, sometimes called dye, before a CT. It highlights certain areas. It can be given by mouth, by injection into a vein, or by enema.
What a PET scan shows
A PET scan works differently. You are given a small amount of a radioactive tracer. NCI notes that cancer cells often use sugar differently than normal cells, which tracers such as FDG can show. The tracer collects in areas of high chemical activity, which appear as bright spots. That shows where activity is happening. A bright spot is not a diagnosis on its own, so your team reads it with your other results.
Why they are often combined
A PET is good at showing activity. A CT is good at showing exactly where something is. Combining them into a PET-CT lets the care team match an active area to a precise spot, in one session. That can make the images easier to read than either test alone.
Neither test diagnoses cancer by itself
Imaging is one piece of a larger picture. Results are read alongside your medical history, your physical exam and other tests. A biopsy, which examines cells under a microscope, may also be needed. Whether you need a PET, a CT, or both is a decision your care team makes for your situation.
Questions worth asking
- What is the scan meant to show in my case?
- Do I need to prepare before the scan?
- When will we go over the results together?
Knowing the plan ahead of time makes the appointment feel more predictable.
When to get help sooner
The warning signs below are about the contrast agent used with some CT scans. NCI says serious reactions are rare.
- Call 911 or go to an emergency department if you have trouble breathing, or swelling of the throat, lips, tongue or face after a scan with contrast. Do not drive yourself.
- Tell the technologist straight away if you feel itchy, break out in a rash or hives, or feel short of breath during or just after the scan. These reactions can be treated on the spot.
- Call your care team the same day if you notice a rash, hives or itching in the hours after you get home.
Tell your team before the scan if you have kidney problems, or have reacted to contrast before. NCI notes contrast can cause kidney problems, mainly in people whose kidneys already work poorly.
Sources
Common questions
What is the main difference between a PET scan and a CT scan?
A CT scan mainly shows structure — the size, shape, and location of tissues. A PET scan mainly shows function — how active or 'busy' tissue is, using a small amount of radioactive tracer. They answer different questions.
Why are PET and CT often done together?
Combining them into a PET-CT lets the care team see an area of activity on the PET and match it to a precise location on the CT in one session, which can make the pictures easier to interpret.
Does a bright spot on a PET scan always mean cancer?
Not on its own. A PET image shows activity, not a diagnosis, so the result is read alongside your other tests.
Which test is better?
Neither is simply 'better' — they show different things. The right test depends on what your care team is trying to learn. Sometimes one is enough; sometimes both are used.
Questions to ask your doctor
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Last updated: 2026-10-02Next planned review: 2027-07-14
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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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