CT scan vs. MRI: What’s the difference?

You may already know that MRIs and CT scans are two of the imaging methods doctors use to diagnose and stage cancer. 

But do you also understand what the difference is between them? And, how physicians decide which one is most appropriate for you?

To answer these and six other questions, we went to Melissa Chen, M.D., a neuroradiologist who specializes in the diagnosis of brain tumors and head and neck cancers.

What are MRIs and CT scans?

A CT scan is like a series of X-rays taken very quickly in a circle around you. When combined and looked at together, they provide a detailed, three-dimensional image of your body.

MRIs use a large, powerful magnet and radio waves to create a similar picture. The radio waves cause the molecules in your body to line up in a certain way, and they send out signals when they revert to their normal positions. This gives us information about the different types of tissue in your body.

Both scans require patients to lie on a movable table that passes through a big, donut-shaped machine.

Is one type of scan better or more detailed than the other?

No. That’s like comparing apples to oranges. They’re both great, just in different ways. I think of them as complementary, because they give us different types of information. 

Generally, CT scans are better at spatial resolution, while MRIs are better at contrast resolution. That means CT scans are good at showing us where the edges of things are — where this structure ends and that other one begins. MRIs are good at showing us the differences between various parts of the body and can help cancer tissue stand out from normal tissue.

How do doctors decide which scan to use?

That depends on the patient, their particular type of cancer and what question the doctor is trying to answer. Everyone’s case is unique.

If your doctor wants to assess a bony structure, for instance, then a CT scan could be good for that. But if they’re trying to distinguish between normal tissue and cancerous tissue, an MRI is probably a better choice. If someone has a lot of ascites, though, or fluid-filled pockets, they can distort an MRI and make it difficult to get a good, clear image.

So, often, it boils down to: what’s the best picture I can get based on the patient’s condition?

What are some of the advantages and disadvantages of MRIs and CT scans?

A CT scan is much faster than an MRI. It’s super-quick. The preparation usually takes longer than the scan itself, which lasts a minute or less. If someone is in a lot of pain, or if they find it hard to hold still for long periods of time, then a CT scan is often your best option.

We can see things more clearly sometimes with an MRI, but those take anywhere from 30 minutes to an hour to obtain, depending on how much of your body is being scanned. If someone moves during the scan, the images will be distorted.

Still, if doctors see something on a CT scan they’re unsure about, they may order an MRI to get a better look at it and figure out what it is. That’s why I think of MRIs as more of a problem-solving tool. 

Are CT scans or MRIs ever the preferred scan for a particular cancer?

Yes. CT scans are really good at showing lung cancer, for instance. But you’re going to want an MRI for anything related to the spinal canal. MRIs are also the preferred scan for looking at brain tumors.

Is there any reason why someone should not have an MRI or a CT scan?

Yes. Since a very powerful magnet is involved, let your doctor know if you have any metal implants, pacemakers or prosthetics before having an MRI. Also let them know if you have a history of metal-working (like welding) or have any type of foreign body embedded in your tissues, such as bullet fragments, metal flakes or shrapnel.

If something is ferro-magnetic (iron-based), it could become dislodged and move around inside your body during an MRI.

Why does the room tend to be cold when you get an MRI or CT scan?

A lot of patients comment about how cool it is in the room while they’re getting an MRI. That’s because MRI machines have a special cooling process, and we have to keep the room cold to prevent them from overheating. We offer patients heated blankets to stay warm.

Should patients be concerned about radiation from scans?

Some patients worry about exposure to ionizing radiation, which is used during X-rays, mammograms and CT scans. But we do everything we can to minimize the amount of radiation being used, while still obtaining the level of detail we need in an image. And, we wouldn’t recommend a particular scan if we weren’t totally convinced that the benefits of having it far outweighed any risks.

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