Automatic High-Order Shimming by Sampling Columns in the Cartesian Coordinates

Yan Zhang¹, Shizhe Li¹, Jun Shen¹

A new automatic high-order shimming method is presented which performs the field mapping by sampling a group of columns in the Cartesian coordinate system. It was found that a minimum of a pair of four columns in two separate slices could unambiguously determine an optimal correction field that comprises the spherical harmonic terms up to the third-order. The technique of multiple stimulated echoes was incorporated into the method, allowing the use of eight or more shots to accomplish field mapping. The shim currents were first determined in the logic frame by assuming 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.