The term CAT and MRI refer to two different types of medical scans. A CAT scan, or Computed Axial Tomography scan, is a type of imaging test that uses X-rays to create detailed pictures of the inside of the body. An MRI, or Magnetic Resonance Imaging scan, is a type of imaging test that uses powerful magnets and radio waves to create detailed pictures of the inside of the body. Both scans are used to diagnose and monitor a variety of medical conditions.