The Collaborative International Dictionary of English v.0.48:

magnetic resonance imaging \mag*net"ic res"on*ance im"ag*ing\ n.
   (Medicine)
   a medical diagnostic procedure utilizing the phenomenon of
   nuclear magnetic resonance to generate images of internal
   parts of the body. It depends on the differential absorption
   of electromagnetic radiation by different types of living
   tissue in a magnetic field. It is complementary to X-ray
   imaging in that the softer tissue show more prominently in
   magnetic resonance images, rather than bone, as with X-rays.
   It is a non-invasive procedure, allowing such images to be
   obtained without penetration of the tissue by objects. It is
   abbreviated MRI. As with computerized tomography, the
   results are usually presented as images of sequential planar
   sections of that part of the body of concern to the
   physician.
   [PJC]

WordNet (r) 3.0 (2006):

MRI
    n 1: the use of nuclear magnetic resonance of protons to produce
         proton density images [syn: magnetic resonance imaging,
         MRI]

The Free On-line Dictionary of Computing (30 December 2018):

MRI

   1.  Magnetic Resonance Imaging.

   2. Measurement Requirements and Interface.