MRI

I have rheumatoid arthritis. My joints are all fine except for one knee. Why should I have an MRI scan instead of an X-ray?

The knee is composed of several bones joined to form a hinge-shaped joint which is held together inside and at the edges by fibrous soft tissues called ligaments.

The bone ends are covered by a tough and slippery protective layer called cartilage. Muscles which move the joint and tendons which attach the muscles to bone also cross the knee joint.

After a fall, muscles, ligaments and cartilages around the knee joint can be damaged leading to swelling, pain and sometimes instability. Sometimes part of the joint cartilage can become separated and float free within the joint which can cause a feeling of locking of the joint.

Although X-rays of joints are often useful, they are best for showing bony injuries such as a fracture (broken bone) or problems with the bone around the joint such as osteoarthritis (wear and tear).

X-rays do not show soft tissues in any detail whereas MRI (magnetic resonance imaging) is excellent for this and can reveal torn, worn or damaged cartilages, ligaments or muscles as well as bony problems. MRI can also detect any increased fluid within the joint, which can occur after an injury or in conditions such as rheumatoid arthritis and sometimes osteoarthritis.

The MRI scan procedure

An MRI scan involves lying on a table surrounded by a strong magnet for between 20 and 45 minutes. The scan is painless, but the magnetic field can be harmful for certain patients including those with a heart pacemaker, so you will be asked some questions to make sure that the scan is safe for you.

Sometimes if the lining of the joint needs to be examined in more detail, you will need to have an injection of “contrast” which will show up on the MRI pictures if the joint lining is inflamed (synovitis).

You will need to undress and wear a gown and to remove all jewelery which could affect the magnetic field.
The scan can be quite noisy and claustrophobic for some people and you will need to lie still while the machine is working. You may be offered headphones through which music is played to help you relax.

If you feel nervous during the scan, you can speak to the radiographers controlling the machine and they can talk to you and help if necessary.