Do I Need An MRI?

By: Kathleen A. Hogan, MD

When I see patients in the office with knee or hip pain, I am often asked, “Do I need an MRI?”  and “I had an MRI, why do I need X-rays?”

There are many different imaging studies that physicians use to evaluate painful or injured joints.   An X-ray is typically the first study that is obtained.  Why do we get X-rays if other tests, like an MRI, show more detail?  Different images give different information.   X-rays are best for looking bones.  Fractures can be seen in more detail on an Xray than on an MRI, for example.  Moderate and severe arthritis can also be better visualized on X-rays.    A CT scan is a special type of X-ray which gives three dimensional cross sectional images.  It is excellent at evaluating bony anatomy.   However, it does expose patients to quite a bit of radiation.

In contrast, an MRI images soft tissues (such as ligaments and meniscus) more clearly than X-rays.   No radiation is used to create the images obtained by an MRI.   A very strong magnetic field is used to obtain the images.  An MRI takes about an hour to perform and requires the patient to lay very quietly in a narrow tube.  Unlike an X-ray, only one body part can be imaged at a time.  It takes twice as long to do an MRI of two knees compared to only one.  Some people are unable to have an MRI because of metal implants (stents in heart or brain), pacemakers or claustrophobia.

When do you need an MRI?  An MRI should be used to look for a specific abnormality suspected based on history or physical exam.  Examples include rotator cuff tears, meniscal tears, labral tears, avascular necrosis, or ligament tears.

Many patients with arthritis wonder, ‘Why I do not need an MRI to diagnosis arthritis or to plan a knee or hip replacement?’   X-rays are better at evaluating end stage arthritis than MRIs.  If you have arthritis on an x-ray, an MRI will likely not change the treatment recommendations and therefore is not needed.  It is also important to know that just because a test is abnormal does not mean that the pain is caused by the abnormality.   For example, if an MRI of the knee shows a torn meniscus on the outside of the knee but the patient’s pain is on the inside part of the knee, that torn meniscus is not causing the pain.

There are many imaging tests available to help physicians diagnose the cause of joint pain.   Usually there is not one definitive test that gives an answer, but a combination of physical exam and diagnostic studies which leads to a diagnosis.  While MRI can be helpful, it is not always needed.