MRI stands for magnetic resonance imaging (often shortened to MR scans). This type of scan has caused a revolution in medical diagnosis in recent years. They combine a powerful magnetic field, radio waves and a sophisticated computer to produce astonishingly detailed pictures of the inside of the body.

The technique doesn't use x-rays and there are no known side-effects or complications from the magnetic fields and radio waves.

How an MRI works

MRI scans depend on nuclear magnetic resonance, a process where atoms are made to vibrate and then give off a tiny electrical signal.

The human body is mostly composed of water and so is full of hydrogen atoms, which are in constant motion. The powerful magnet in the MRI scan makes these hydrogen nuclei line up, just like iron filings on a magnet. The scanner then sends out a pulse of radio waves, which makes the nuclei shift a little bit.

Different parts of the body allow different magnetic fields through. In addition, the hydrogen nuclei in diseased tissue respond to the radio wave in a slightly different way to normal cells.

The computer picks up all these different signals and turns them into detailed pictures of the body. These can show bones, muscles, joints, blood vessels, nerves and other structures in great detail.

What could it show in a painful weak knee?

There are a range of problems that could show up on an MRI scan, from torn and damaged cartilage to scarred and thin muscle or tendon. The scan might show changes in the bone of the knee, a reduced volume of fluid in the joint, or abnormalities or disease in any of the tissues.

A few very sophisticated MRI scanners can even be used to study the knee in motion, to see how everything moves and changes when the knee bends or you walk. (These are only available in one or two large teaching hospitals.)

Painless but not always pleasant

MRI scans are a marvellous non-invasive way to get a diagnosis, but they don't pick up every problem and there are some disadvantages. For example, you may have to remain motionless in a narrow scanning tunnel for as long as an hour, which can be uncomfortable and impossibly claustrophobic for some people. It can also be very noisy.

This article was last medically reviewed by Dr Trisha Macnair in May 2008