Patent ID: 7439836

Claim:
A Magnetic Resonance Imaging (MRI) system, comprising: a cryogenic magnet assembly comprising a cryogenic vessel and a superconducting magnet disposed within the vessel; a gradient coil module disposed radially inboard of the cryogenic magnet assembly; and a plurality of room temperature steel rings proximate to the gradient coil module and disposed radially inboard of the cryogenic magnet assembly; wherein at least the superconducting magnet, the gradient coil module, and the plurality of room temperature steel rings define a field of view (FOV); wherein in response to the superconducting magnet being energized, the plurality of room temperature steel rings create high order harmonics that serve to expand the magnetic field homogeneity FOV, and low order harmonics that tend to degrade the magnetic field homogeneity within the FOV; and wherein the low order harmonics are compensated for by the cryogenic magnet assembly.