Passive shimming of MR magnets: algorithm, hardware, and results

An improved method for allowing a magnetic resonance (MR) magnet to meet its stringent field homogeneity requirements has been developed. The method is called passive shimming and involves the placement of pieces of ferromagnetic material in the magnet bore at locations which are determined from an initial field map. Software and hardware which facilitate this shimming procedure have been developed at GE and successfully implemented in MR scanners around the world. The software uses a linear programming algorithm to minimize the thicknesses of shims at each of a number of allowable locations which will bring the peak-to-peak field error in the volume of interest to its minimum level. The hardware consists of arc-shaped drawers on each of which shim packages may be built up to a predetermined maximum thickness. This shimming technology represents a major cost reduction from the superconducting or resistive electromagnetic correction coils which are typically used to shim a magnet´s field.<>