Why didn't my doctor order an MRI?

If you're wondering why your doctor didn't order an MRI during your medical visit, the most likely answer is you probably don't need one.

Your doctor or health care practitioner isn't trying to shortchange you; he or she is actually trying to practice good medicine. The magnetic resonance imaging is an excellent but expensive diagnostic test, and it isn't always necessary or useful. A doctor's medical judgment and experience are important in knowing when the situation is right to take that extra step.

In routine clinical use since the early 1980s, the MRI is a safe, noninvasive way to evaluate organs and tissues inside the body. The technology uses radio waves and a changing magnetic field to produce very sharp computer-generated images of tissue that can be viewed as slices in any orientation. Unlike plain X-rays or CT scans, MRIs do not emit potentially dangerous ionizing radiation, and they cause no known bodily damage. (As long as you don't carry iron-containing objects into the scanner, such as a paperclip or coins, which have the potential to become catastrophic airborne projectiles under the influence of the MRI's powerful magnetic field.)

An MRI is not always the most accurate test. An MRI of the knee is notoriously poor at diagnosing an anterior cruciate ligament tear and frequently overcalls tears of the rotator cuff, diagnoses that can readily be made by an orthopedic surgeon upon thorough physical examination.

Sometimes MRIs are almost too good at demonstrating anatomical detail. For instance, a spine MRI will definitely show the large herniated intervertebral disc in the lower back that is pressing on a nerve and triggering the pain that sent you to the doctor. However, that same MRI scan also will tend to display all the other herniated discs of your lumbar spine. Some may be even bigger than the one actually causing your pain, yet none of these are responsible for your symptoms. Your doctor now must determine which of those disc abnormalities is the culprit causing your particular problem and how to address it.

This may lead to more invasive and painful tests, such as a myelogram or discogram, requiring injections of dye into the spinal canal or disc. The current clinical recommendation of the North American Spine Society is to perform an MRI only after six weeks of conservative therapy, such as medications and physical therapy, has failed. Based on the natural history of low back pain, many cases tend to resolve by then.

The point is that the MRI can reveal additional deviations from usual anatomy - normal variants, acceptable age-related degeneration, findings not causing any problems, or all of the above. Just knowing other "abnormalities" exist in your spine could lead to excessive and unnecessary worry.

An MRI isn't always the first, best or right diagnostic tool. Your doctor should be able to explain why you need, or do not need, one.