Passive shimming technique for MRI magnets

摘要

A subject receiving bore (12) of a magnetic resonance apparatus has an axial length to diameter ratio of less than 1.75:1 and preferably about 2:1. The temporally constant magnetic field generated by superconducting magnets (10) surrounding the bore have various magnetic field harmonic distortions generally in the Z1-Z18 range. Shim trays (80) are disposed longitudinally around the bore (12). Each shim tray contains a number of shim pockets (84) which receive ferrous shims for shimming the magnetic field in the bore (12). An initialization system (60) calibrates the initial magnetic field within the bore (12). An initial shim distribution is determined which shims the inhomogeneous magnetic field toward a target more homogeneous magnetic field. An optimization system (66) determines a residual magnetic field from a difference between the present inhomogeneous magnetic field of the bore and the target magnetic field. The amounts of ferrous material assigned to each shim pocket are optimized in fractional groups to determine an optimal shim settings to produce a shimming magnetic field equal to a negative of the residual magnetic field. In this manner, the net magnetic field is substantially equal to the target homogeneous magnetic field.