What’s the difference between a CT scan and an MRI?
Most people are very knowledgeable about what an X-ray is, but not everyone knows exactly what some other imaging tests, such as CT scans and MRIs, actually do.
“X-rays are more common and widely available,” said Alexander Adduci, MD, PhD, MBA, the chief of Radiology at Cape Cod Hospital. “They are often used as a first step in working up a problem. They do involve radiation, but the overall dose is lower than a CT. They provide less information than CTs and MRIs.”
CT and MRI are both cross-sectional imaging exams that are used to further investigate some symptoms or some finding in the body, he said.
One of the biggest differences is how the two scans work. A CT scan uses X-ray radiation and an MRI uses magnetic fields and radio waves to generate images so doctors can see what is going on inside our bodies. The other big difference is the amount of time each test takes.
“Generally, a CT is a quick exam,” Dr. Adduci said. “It’s just a few minutes, whereas an MRI could be 30 or 40 minutes.”
The different scans are good for different things. Because the results are so quick, a CT scan is the most widely used scan for emergency situations. It’s good for seeing the details of bones and fractures and is an all-around good scan for most things.
“Usually, we don’t go to MRI unless we need to look at something better or there is a specific question,” Dr. Adduci said. “MRI is also better at looking at the inside of joints and cartilage and ligaments. It’s also much better at looking at the spine and we use it a lot for the brain as well.”
Most healthcare providers have a lot of experience ordering the tests and are very familiar with what they are looking for and which test is best for which ailment. For many indications there isn’t anything the patient needs to do to prep for the test, but the person who sets up the appointment for the scan will inform you if there are any special instructions.
For example, for abdominal imaging, the patient may need to refrain from eating or drinking for several hours beforehand. With some CT scans and MRIs, a contrast agent is injected through a vein to add extra information. With a CT, the intravenous contrast is iodine based and sometimes the patient will also need to drink an iodine-based contrast before the scan. The contrast that is used for some MRIs is gadolinium and it is given by injection.
“We’re always trying to find the best test for the patient for the indication they have,” Dr. Adduci said. “A CT is good for a lot of different indications and an MRI is more of a specialized test.”
