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.
Choose how you want to understand this
The full explanation.
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 and makes the pictures clearer. 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, often a form of sugar. It collects in tissue that is using a lot of energy. Areas that are very active "light up" on the pictures. That shows where activity is happening. But a bright spot is not proof of cancer. Infection, inflammation and healing tissue can look active too.
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. In many cases a biopsy is needed too, because it examines actual cells under a microscope. Whether you need a PET, a CT, or both is a decision your care team makes for your situation.
Questions worth asking
Ask your team what the scan is meant to show in your case. Ask whether you need to prepare, since some PET scans require fasting. Ask when you will go over the results together. Knowing the plan ahead of time makes the appointment feel more predictable.
When to get help sooner
Both scans are safe for most people. The warning signs below are about the contrast agent used with some CT scans, and they 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.
- 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. Kidney function can be checked with a simple blood test first.
Sources
Words to know
Tap any term to see what it means.
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?
No. PET tracer collects in any tissue that is very active, which includes infection, inflammation, and healing as well as some cancers. That is one reason a PET result is read alongside 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
Being prepared helps you get the most out of your appointments. Save or print these questions.
Tap a question to save it to your list (kept on this device).
Your next step
Plain-language definitions for both sides of the comparison.
Speak With Trained Specialists & Human Navigators
Cancer Explained provides educational guidance, but does not replace trained specialists, social workers, or your medical team.
Talk to a trained cancer information specialist
Free, confidential assistance from NCI Cancer Information Service via phone, chat, or email.
Contact your oncology team
Locate after-hours contact numbers, portal messages, or urgent triage phone lines.
Find a patient navigator
Get one-on-one help with appointments, logistics, translation, and care coordination.
Find a genetic counselor
Discuss inherited mutation risk, family history, and genetic testing options.
Find an oncology social worker
Access emotional counseling, family support groups, and mental health resources.
Find a financial navigator
Locate copay assistance foundations, grant programs, and lodging/travel support.
Find a clinical-trial specialist
Search matching studies and speak with NCI trial information specialists.
Get urgent help
Immediate emergency guidance for fever (>100.4°F during chemo), severe pain, or shortness of breath.
Help Us Improve This Guide
Did this explanation answer your question and help you determine your next step?
Know someone who needs this?
Plenty of people are looking for something like this and do not know where to start. If this would help a friend or someone you love, send it on — we have written an opening line so you do not have to stare at an empty message. You can change every word of it.
Your message is written and sent in your own email or messaging app — we never see who you send it to, and nothing is added to any list.
Plain-language explanation of the published sources cited on this page. AI-assisted, source-checked, not clinician-reviewed.
Last updated: 2026-08-11Next planned review: 2027-07-14
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.
General education — varies by person. Answers genuinely differ between people. This page explains what commonly varies and points you to your care team 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.
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.
Read more about our editorial process, our use of AI, and our corrections policy.
Spotted a problem? Report an error — a factual mistake, broken or outdated source, confusing wording, or anything that seems unsafe. Please do not include names, medical record numbers, dates of birth, addresses, or other identifying medical information in your report.
After using this page, do you understand what to do next?
Anonymous — we only record the answer, never who gave it.
Related articles
Still have questions?
Educational answers, plain language
Free to print and share