The short answer
CT uses special x-ray equipment to make detailed slice pictures and is fast and widely available. MRI uses strong magnets and radio waves and often shows soft tissue in more detail, with no x-rays. PET shows how active tissues are, using a small amount of a radioactive tracer. They answer different questions, and are sometimes combined.
CT uses special x-ray equipment to make detailed 'slice' pictures; it is fast and widely available.
MRI uses strong magnets and radio waves, uses no x-rays, and often shows soft tissue in more detail.
PET uses a small amount of a radioactive tracer to show how active tissues are, not just their shape.
PET and CT are often combined in one scan to show both a tumor's location and its activity.
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The full explanation.
The short version
CT, MRI, and PET are all imaging tests, but they work differently and show different things:
- CT uses special x-ray equipment to make detailed "slice" pictures. It is fast and widely available.
- MRI uses strong magnets and radio waves instead of x-rays. It often shows soft tissue in more detail.
- PET uses a small amount of a radioactive tracer to show how active tissues are, not just their shape.
Neither is simply "best." Each answers a different question.
What CT shows
A CT scan sends x-rays through the body, and a computer builds detailed cross-section pictures, like slices of a loaf of bread. CT is good at showing the structure of organs, bones, and other tissues, and it is used to help find tumors, guide biopsies, plan treatment, and check whether a cancer is responding. It is usually quick and available in many places.
What MRI shows
An MRI machine surrounds you with a strong magnetic field and uses radio waves; a computer turns the signals into images. Because MRI does not use x-rays, it does not expose you to that type of radiation, and it often gives sharper detail of soft tissue such as the brain and spinal cord. MRI usually takes longer than a CT and the machine is noisy, so you are given ear protection.
What PET shows
PET is different: it shows function, not just structure. You are given a small amount of a radioactive tracer. The most common tracer is a radioactive form of sugar. Because cancer cells often use sugar faster than normal cells, they can take up more of the tracer and appear different on the scan. Other PET tracers can show other features, such as whether cancer may have spread.
Why PET and CT are often combined
CT and PET are frequently done together in one procedure called combined PET/CT. The CT part makes clear pictures of the organs and structures, and the PET part shows which areas are active. Together they can show both a tumor's location and its activity, which may improve the ability to diagnose cancer, see how far it has spread, plan treatment, and check response.
The bottom line
Your care team picks the scan — or the combination — based on what they need to see. Ask why a particular scan is being recommended, whether you will need a contrast agent or a tracer, how to prepare, and how long it should take. For MRI, tell your team about any metal implants or devices, and for any scan, mention if there is a chance you are pregnant.
Words to know
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Common questions
What is the difference between CT, MRI, and PET?
CT uses special x-ray equipment to make detailed pictures, or slices, of the body. MRI uses strong magnets and radio waves instead of x-rays and often shows soft tissue in more detail. PET uses a small amount of a radioactive tracer to show how active tissues are. Each answers a different question.
What does a PET scan show that a CT does not?
CT mainly shows the structure of organs and tissues. PET shows function — how active tissues are. Cancer cells often use energy faster than normal cells, so they can take up more of the PET tracer and show up differently. Combining PET and CT can show both location and activity.
Which scan has no radiation?
MRI does not use x-rays or the tracer used in PET, so it does not involve that type of radiation. CT uses x-rays, and PET uses a small amount of a radioactive tracer. 'Safer' depends on the situation, and your care team weighs the trade-offs.
Why might I need more than one type of scan?
Each scan shows something different, so your team may use more than one to get a fuller picture. Sometimes PET and CT are done together in a single procedure. Your care team chooses based on what they need to learn about your situation.
Questions to ask your doctor
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Last updated: 2026-07-14Next planned review: 2027-07-14
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How this page was created
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. We do not employ clinicians and do not intend to — our work 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 checked line by line against the sources listed on it. That confirms the sources support what the page says. 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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