High order passive shimming assembly for MRI magnets

A magnetic resonance imaging apparatus comprising:

• a bore for receiving a subject to be imaged, the bore having a diameter and an axial length;
• an annular magnet surrounding the bore to generate a temporally constant magnetic field axially through the bore, the temporally constant magnetic field having non-uniformity components;
• a radio frequency coil disposed around the bore and positioned to broadcast cast radio frequency signals into the bore and receive radio frequency signals from the bore;
• a self-shielded gradient coil assembly disposed between the radio frequency coil and the bore, the self-shielded gradient coil assembly including a primary gradient coil assembly having a smaller diameter and disposed adjacent the radio frequency coil and a secondary gradient coil assemblies defining a shim tray receiving annular region therebetween;
• a plurality of segmented rings of ferrous material extending circumferentially around the examination region adjacent the primary gradient coil assembly for shimming the magnetic field;
• at least 32 shim trays disposed circumferentially around the primary gradient coil assembly in the shim tray receiving region and mounted to the primary gradient coil assembly, each of the shim trays including at least 20 of the shim receiving pockets, the shims held in the shim trays adjusting the magnetic field shimming performed by the segmented rings.

A method of shimming a magnetic resonance apparatus which includes a bore having a diameter and an axial length, an annular magnet surrounding the bore to generate a temporally constant magnetic field axially through the bore, a radio frequency coil disposed to broadcast radio frequency signals into the bore and receive radio frequency signals from the bore, the method comprising:

• (a) measuring magnetic field uniformity including measuring for harmonic distortion components from Z6 at least through Z12;
• (b) placing ferrous shim members in annular rings within and around the bore to make an initial coarse compensation at least for the measured Z12 harmonic distortion component;
• (c) measuring the magnetic field at least at 20 axial positions along a surface of an imaging volume and at a plurality of rotationally displaced positions around the imaging volume;
• (d) mounting ferrous plates selectively in an array of shim receiving pockets of shim trays which are disposed axially along and circumferentially around the bore in accordance with the measurement of step (c) to refine the compensation for the Z12 harmonic distortion component and for other harmonic distortion components;
• (e) selectively repeating steps (c) and (d) until harmonic distortion is minimized.