CT Scan vs. MRI: How They Work and What They Show

Some injuries or conditions you can see with the naked eye. Others require a more in-depth look. And if your healthcare provider suspects a condition like internal bleeding, tumors or muscle damage, they may order a CT scan or an MRI.

The choice to use a CT scan vs. an MRI is a call your provider makes, primarily based on what they suspect they’ll find.

What are these tests?

CT scans and MRIs are both imaging technologies, not treatments. That means they help your provider make diagnoses, plan your treatment and monitor changes in your body.

“CTs and MRIs are two ways of looking inside your body, beyond the skin, and seeing what’s going on,” explains radiologist Albert Parlade, MD. “MRIs and CTs are able to see different things inside your body to help your provider make a diagnosis.”

When it comes to a CT scan vs. an MRI, which one is better for you depends on certain factors, like what exactly your provider is searching for, as well as your health status.

Difference between CT scan and MRI

CT scans and MRIs use different technologies to see what’s happening inside your body.

For a quick look, here are some differences between CT and MRI:

What a CT scan is used for

CT stands for “computed tomography scan.” Some people also call it a CAT scan. You can think of a CT scan as a 3D X-ray machine.

Because CT scanners use X-rays, they can show the same things as an X-ray but with much more precision. An X-ray is a flat look at the area being imaged, whereas a CT gives a more complete, in-depth picture.

CT scans are used to look at things like:

• Bones
• Stones
• Blood
• Organs
• Lungs
• Cancer stages
• Abdominal emergencies

“CTs are good at looking at ‘bones and stones’ — like kidney stones and gallstones,” Dr. Parlade says. “Also, surgeons commonly use CTs to plan surgeries. For example, an orthopaedic surgeon may want a 3D image of a bone before repairing a complex fracture.”

Getting a CT scan should be a quick and painless process. You lie on a table that moves slowly through a doughnut-shaped scanner. Depending on what your provider is looking for, you may also need to have an IV injection of a contrast dye. Each scan takes less than a minute.

Considerations

The biggest concern some people have with CT scans (and X-rays for that matter) is the potential for radiation exposure. Some experts suggest that the ionizing radiation given off by CT scans can slightly increase cancer risk in some people. But the exact risk is a matter of controversy. The U.S. Food and Drug Administration (FDA) says the risk of cancer from CT radiation is “of statistical uncertainty” based on current scientific knowledge.

But because there may be some risk from CT radiation, you’re not a good candidate for a CT scan when you’re pregnant (but some exceptions may be made if the scan is deemed absolutely necessary).

What an MRI is used for

MRI stands for “magnetic resonance imaging.” In short, MRIs use magnets and radio waves to create images of the inside of your body.

MRIs are used to look at things like:

• Tissue
• Nerves
• Organs
• Spine and spinal cord
• Blood vessels
• Lymph nodes
• Brain
• Joints

MRIs are really good at differentiating tissue. For example, a provider may use a full-body CT to spot tumors. Then, order an MRI to better understand any masses found in the CT.

“Also, you can see some nerves with MRI. You can see if there’s injury or inflammation in a nerve in certain parts of the body,” Dr. Parlade illustrates. “We don’t directly see nerves on a CT scan.”

A typical MRI scan takes about 30 to 50 minutes, and you have to lie very still during the procedure. The machines can be loud, and some people may benefit from wearing earplugs or using headphones to listen to music during their scan. And depending on what your provider is looking for, they may use IV contrast dye.

Considerations

While an MRI may be a better technique to see certain structures in your body, not everyone is a good candidate.

If you have certain types of metal in your body, you can’t get an MRI. That’s because an MRI is essentially a magnet, so it can interfere with some metal implants. That includes some pacemakers, defibrillators or shunts.

Is one better than the other?

Neither a CT nor an MRI is always better — it’s a matter of what you’re looking for and how well you can tolerate each, Dr. Parlade emphasizes.

“A lot of times, people think one is better than the other,” he notes. “But it really depends on what your doctor’s question is.”

Whether your provider orders a CT or an MRI, the goal is to see what’s happening inside your body to get you the best treatment.