What is the Difference Between an MRI and a CT Scan?

Although the terms are often used interchangeably by some patients, an MRI and a CT scan are two entirely different diagnostic tests. While they are both medical imaging tests that doctors may use to diagnose a problem inside the body, an MRI and a CT scan use different methods to form images. Other differences between an MRI and a CT scan include the quality of certain types of images, length of the tests, contrasting agents used during a procedure, and safety.

An MRI, or magnetic resonance imaging, test uses magnets to create images. During an MRI, a patient lies down on a table and is inserted into a long cylinder, which is essentially a large electromagnetic field. Radio waves inside the tube produce the internal images.

A CT, or computed tomography, test is an imaging technique that uses radiation to produce internal images. The CT scanner rotates around the patient's body. While this happens, x-rays are passed through the scanner to create the images.

Another major difference between an MRI and a CT scan is the quality of certain images. While an MRI scan will typically produce much clearer images, it is usually used to get images of tumors and soft tissues, as well as brain and spinal cord injuries. Unlike a CT scan, it usually does not do well when trying to get images of body cavities, such as the chest or abdomen. A CT scan is also considered to be the best way to get accurate images of bones.

Historically, both an MRI and a CT scan took up to an hour or more to complete. Today, however, both of these procedures are often completed much quicker. An MRI scan does take a little longer to complete, though.

During both an MRI and a CT scan, contrast agents may be used. This is a type of dye that can be used to enhance visibility in certain areas of the body, such as the blood vessels or gastrointestinal tract. The contrast agent used during an MRI, gadolinium, often causes less adverse reactions than the barium or iodine typically used during a CT scan.

Since it uses radiation as a means to produce an image, there is some concern about the safety of CT scans. Some research suggests that getting these types of scans can possibly increase a patient's risk of getting cancer. Since an MRI uses no radiation, it is considered much safer. Those with an artificial pacemaker, though, should avoid getting an MRI, since it could possibly cause the pacemaker to malfunction.