What Is an MRI?

So you've just seen your doctor, and he says that he wants you to have an MRI of your knee. You've heard the term MRI before, but you are not quite sure what it involves. Will it be painful? How long will it take? How much radiation will I be exposed to? And, why do I need this test?

Magnetic resonance imaging (MRI) is a relatively new and painless technique that allows doctors to look at the soft tissues of the body. it is different from regular x-rays like you get in a doctor's office because it does not expose you to radiation. Instead, a radio frequency is used to knock your hydrogen atoms out of line. As they move back to their natural alignment, each hydrogen atom in your cells emits a tiny electric signal. The MRI scanner has very strong magnets in special coils to detect the electric signal. A computer uses these signals to create a detailed image of your soft tissues.

Magnetic resonance imaging allows doctors to see the image of many structures inside joints that cannot be felt by direct touch with their fingers. Some of these structures are ligaments (tissues connecting bones), menisci (tissues absorbing shock in the knee), and tendons (tissues connecting muscle to bone). The ability to look at these structures inside an injured joint has greatly improved patient care because, in many instances, it enables doctors to correctly diagnose and treat the injured structures earlier.

Looking inside the knee
The knee is commonly evaluated with MRI. Using this technique to help diagnose knee injuries was first described in 1987, and its use caught on quickly. Most often, doctors take knee MRIs so they can see if your menisci or your cruciate ligaments (ligaments that prevent the upper leg from moving too far forward or too far backward over the lower leg) are damaged. Both of the cruciate ligaments can be seen better on MRI than on any other currently used imaging techniques. Most tears of the medial (inner side) and lateral (outer side) menisci also can be seen on MRI. The MRI does not have the resolution, however, to create a detailed image of every structure in the joint.

In an uninjured knee, menisci and ligaments appear as dark structures that can be seen well (see linked figures). But, because both ligaments and menisci are composed of thick, tough cords (collagen) with little blood supply, there are only a few hydrogen atoms available to emit signals. Therefore, when ligaments are injured or torn, they may not appear at all, and menisci may appear as relatively white areas where joint fluid has entered a torn portion. Orthopaedic surgeons use MRIs also in the shoulder, ankle, elbow, spine, or any area of an arm or leg where they want to look at soft tissues rather than bones.

The main safety issue with MRIs is the strong magnetic field present. People who have metal aneurysm clips, metal shards in their eyes (automotive or factory workers are at risk), or pacemakers should not be examined by MRI. In these people, the magnetic field can displace the metal or cause the pacemaker to malfunction. Other metallic implants in the body generally are not affected by MRI, but they can distort the MRI signal. Remember, if you have any type of implant, you and your doctor should discuss whether an MRI is safe for you.

The other main problem for people who have an MRI is that they must be placed into the "tube" of the scanner and remain perfectly still for 30 to 60 minutes to obtain a good quality MRI. Typically, the scan lasts about 40 minutes. Although the procedure is painless, large people, people with claustrophobia, and small children generally do not tolerate this well. Your doctor may prescribe a sedative for you to help you relax or sleep during the scan if you are claustrophobic. Or, you may be placed in a scanner that is open on both ends.

During the scan, the machine will make knocking sounds. These sounds occur when the magnetic field is changed rapidly.

Magnetic resonance imaging has greatly improved doctors' ability to view soft tissue injuries and has been especially useful in speeding the diagnosis and improving the treatment of athletic injuries. it is often an excellent supplement to your medical history and physical examination. Further advancements in the use of MRI will allow doctors to look at injured structures with even more precision. There is little doubt that MRI will continue to be the imaging technique of choice for soft tissue injuries in sports medicine for the foreseeable future.

Robert F. LaPrade, M.D.
Galveston, Texas