The short answer
An MRI uses a powerful magnet and radio waves — not radiation — to take detailed pictures of the body in slices. These images can show places where there may be tumors. During an MRI you lie still inside a long chamber, and the machine makes loud thumping noises.
An MRI uses a powerful magnet and radio waves to take pictures of the body in slices.
MRI does not use ionizing radiation, unlike CT scans and X-rays.
The detailed images can show places where there may be tumors.
During the scan you lie still inside a long chamber, and the machine makes loud thumping and rhythmic sounds.
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The full explanation.
The simple version
An MRI is short for magnetic resonance imaging. It uses a powerful magnet and radio waves, to take pictures of your body in slices. Those slices are combined into detailed images of the inside of your body, which can show places where there may be tumors.
One important difference from a CT scan or X-ray: an MRI does not use ionizing radiation. It makes its pictures with a magnet and radio waves instead. That is part of why it can sometimes be repeated more often than CT, without added radiation exposure being a concern.
Before the scan
Tell your team about any metal in or on your body before scheduling. This includes pacemakers, certain implants, metal fragments, or a cochlear implant. The strong magnet can affect metal objects, so this safety check happens before you're anywhere near the machine, not after. You'll also remove jewelry, watches, and anything with metal, and change into a gown if needed.
What the scan is actually like
You lie still on a table. It slides into a long chamber, surrounding part or all of your body. The machine makes loud thumping noises and rhythmic beats while it works, so most centers offer earplugs or headphones with music. The scan itself does not hurt. But holding still matters, since movement can blur the images, and sometimes means part of the scan has to be redone. A full MRI often takes 30 to 60 minutes, depending on how many areas are being imaged.
If you feel closed in
Some people feel anxious inside the chamber, sometimes called claustrophobia. Tell your team ahead of time if this concerns you. Options can include a more open-style MRI machine, or a mild relaxation medicine before the scan. Simply talking through what to expect beforehand can help too. Mentioning this early, rather than during the scan, gives your team time to plan around it.
Contrast dye
Sometimes a special dye, called a contrast agent, is injected into your vein before or during the exam. This dye can make tumors show up more brightly in the pictures, helping your care team see them more clearly. Tell your team about any kidney problems or prior reactions to contrast dye beforehand, since this can affect whether it's used.
What the images can and cannot tell you
An MRI can show the size, location, and some features of an abnormal area very clearly. But it usually cannot say for certain whether that area is cancer. In most cases, only a biopsy, examining actual cells under a microscope, can confirm a cancer diagnosis. Think of the MRI as pointing to where to look closely, not as the final answer by itself.
How long results usually take
Many MRI results are available within a few days. Some centers can share preliminary results sooner, if the situation is urgent. Ask when to expect a full report and who will walk you through what it means.
What to ask your team
- Why is an MRI the right scan for me, rather than another test?
- Will I need a contrast dye, and are there any concerns for me?
- Is there anything about metal implants or devices I should mention first?
- What can help me stay comfortable and still during the scan?
- When will I get results, and who will explain them?
When to get help sooner
- Call 911 or go to an emergency department if your lips, tongue or throat begin to swell after a contrast injection, or you start to wheeze, or hives spread across your body. Serious reactions to gadolinium are rare, but they need treatment straight away.
- Call the imaging department or your care team the same day if you feel sick, vomit, or come out in a mild rash in the hours after the scan. Tell them too if the arm where the dye went in becomes painful, puffy or tight, since dye can leak outside the vein.
Sources
Words to know
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Common questions
What is an MRI?
An MRI (magnetic resonance imaging) uses a powerful magnet and radio waves to take pictures of your body in slices. These slices are combined to create detailed images of the inside of your body, which can show places where there may be tumors.
Does an MRI use radiation?
No. Unlike CT scans and X-rays, an MRI does not use ionizing radiation. It creates its images using a strong magnet and radio waves instead.
What does an MRI feel like?
When you have an MRI, you lie still on a table that is pushed into a long chamber that surrounds part or all of your body. The machine makes loud thumping noises and rhythmic beats during the scan. It does not cause pain, but staying still is important for clear pictures.
Will I be given a contrast dye?
Sometimes. A special dye, called a contrast agent, may be injected into your vein before or during the exam. This dye can make tumors show up more brightly in the pictures. Your care team will let you know if you need it.
How is an MRI different from a CT scan?
Both create detailed pictures of the inside of the body, but they work differently. A CT scan uses X-rays (a form of radiation) and a computer, while an MRI uses a magnet and radio waves and no radiation. Your doctor chooses the scan that will best answer the question at hand.
Questions to ask your doctor
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Last updated: 2026-08-18Next planned review: 2027-07-03
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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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