Magnetic resonance imaging (MRI) is a test that produces very clear pictures, or images, of the human body without the use of x-rays. MRI uses a large magnet, radio waves and a computer to produce these images. An MRI exam may provide further insight into an abnormality determined previously by an x-ray, ultrasound, or CT scan. The advantage of MRI over past imaging techniques is that by using magnetism instead of X-rays, MRI avoids exposing patients to ionizing radiation.