Magnetic Resonance Imaging (MRI). Guide. Magnetic resonance imaging (MRI) is a test that uses a magnetic field and pulses of radio wave energy to make pictures of organs and structures inside the body.In many cases, MRI gives different information about structures in the body than can be seen with an X-ray, ultrasound, or computed tomography (CT) scan.n MRI scan can be done for the: 1  Head. 2  MRI can look at the brain for tumors, an aneurysm, bleeding in the brain, nerve injury, and other problems, such as damage caused by a stroke. 3  MRI can also find problems of the eyes and optic nerves, and the ears and auditory nerves.