Magnetic resonance imaging (MRI) is a diagnostic technique that uses a magnetic field to produce pictures of structures inside the body.
During an MRI, your body is in a very strong magnetic field. The MRI machine also uses pulses of radio waves. The machine creates an image based on the way hydrogen atoms in your body react to the magnetic field and the radio waves. MRI signals can give an image of a single slice of any part of the body, much like a slice of bread in a loaf. Usually, images are created of several "slices" of an organ or part of the body. The MRI's computer also can combine these slices into three-dimensional (3-D) images.
Because water molecules are especially sensitive to the forces used in this technique, MRI scans are very good at showing differences in water content between different body tissues. This is particularly important in detecting tumors and in checking for problems in the body's soft tissues, such as the brain, spinal cord, heart and eye.