MRI is an abbreviation for "magnetic resonance imaging." This technology allows a detailed image to be created of the internal parts of the human body. An MRI machine uses a strong magnetic force and sophisticated software to create a picture of the body part in question.
MRI is most commonly used to look at "soft tissues" within the body. It has found great success in giving detailed images of the ligaments and tendons of the joints, of the organs, of the abdomen and pelvis, and of the brain.
One key difference from a CT scan or x-ray is that MRI produces no radiation. For this reason, it is often used in children or pregnant women.