Automatic high-order shimming using parallel columns mapping (PACMAP)

Abstract

This work presents a new automatic high-order shimming method that maps the B0 field using a group of parallel columns. We found that a pair of four columns in two separate slices could determine an optimal correction field comprising the spherical harmonic terms up to the third-order. The technique of multiple stimulated echoes was incorporated into the method, allowing the use of at least eight shots to accomplish field mapping. The shim currents were first determined in the logic frame by assuming that the slices were in axial planes, and then uniquely converted into the physical frame where the slices could be at any oblique angle, by using a spherical harmonics rotation transformation. This method thus works regardless of slice orientation. It was demonstrated on a 3T scanner equipped with a complete set of second-order harmonic shim coils. Both phantom and in vivo experiments showed that this newly introduced high-order shimming method is an effective and efficient way to reduce field inhomogeneity for a region of imaging slices. © 2009 Wiley-Liss, Inc.