Surgeries and Procedures

What to Expect When Getting an MRI

When a disease affects your nerves, joints or other, more delicate parts of the body, your doctor needs to detect subtle changes that may indicate a serious problem. In these cases, he or she often turns to magnetic resonance imaging, or MRI, to gain a detailed glimpse inside. Here's what you need to know about getting an MRI.

During an MRI, you are usually placed on a flat table and moved into the core of a large machine. The procedure can take quite a long time, ranging from about 10 minutes to a full hour for a comprehensive scan.

The basic idea behind an MRI is that certain atoms in your body move in response to a strong magnetic force. So, inside the MRI machine are strong magnets, which send powerful magnetic waves through your body. You can't feel them, but this force causes most of the hydrogen atoms inside your body to line up, while a few stubborn ones don't budge. A particular radio frequency is then sent through your body that causes these stragglers to spin. When the radio frequency is turned off, the atoms stop spinning and release their extra energy. A metal coil measures this released energy and since atoms in different parts of the body spin at different rates, a computer uses this data to create a detailed picture of all of your tissues.

Because the picture it creates is so detailed, an MRI can be used to diagnose an array of medical problems, including: multiple sclerosis, tumors in the brain, joint injuries, tendonitis, strokes and even some infections of the spine or joints.

Many times a doctor will first inject a special dye into the part of the body he wants to examine. This dye changes the way atoms spin in a magnetic field, making the different tissues in your body appear even more detailed on an MRI screen.

The MRI itself is incredibly safe. However, because it is essentially a big magnet, metal objects cannot be in the same room as an MRI machine. Often MRI centers have very special rules about whom and what can go inside the room to prevent something from "turning into a missile," says Dr. Robert Zimmerman, the executive vice-chair of radiology at New York-Presbyterian Hospital and Weill Cornell Medical Center.

These rules include prohibiting metal objects that may be inside you, such as pacemakers, some aneurysm clips (devices meant to secure a weakened blood vessel) and some orthopedic and dental implants. People with these devices cannot go near an MRI for fear that the magnet will cause them to move around inside your body.

Additionally, the MRI machine is big, noisy and patients can feel very confined during the procedure. Newer models tend to be a bit wider, but still, "about five to ten percent of my patients cannot tolerate them," says Zimmerman.

An MRI is generally not performed on women within the first 12 weeks of pregnancy. As always, if there is any reason to suspect that you may be pregnant, be sure to tell your doctor.