MRI is an abbreviation of 'magnetic resonance imaging'.
A magnetic coil which produces the magnetic field that influences the alignment of molecules in the tissues, so that the various structures can be seen.
Results are read as body 'slices'.
Value of knee MRI in the diagnosis and management of knee disorders. Subhas N, Patel SH, Obuchowski NA and Jones MH. Orthopedics. 2014 Feb;37(2):e109-16. doi: 10.3928/01477447-20140124-11.