Dynamic B(0)Shimming For Multiband Imaging Using High Order Spherical Harmonic Shims

Purpose To describe and implement a strategy for dynamic slice-by-slice and multiband B(0)shimming using spherical harmonic shims in the human brain at 7T. Theory For thin axial slices, spherical harmonic shims can be divided into pairs of shims (z-degenerate and non-z-degenerate) that are spatially degenerate, such that only 1/2 of the shims (non-z-degenerate) are required for single slice optimizations. However, when combined, the pairs of shims can be used to simultaneously generate the same in-plane symmetries but with different amplitudes as a function of their z location. This enables multiband shimming equivalent to that achievable by single slice-by-slice optimization. Methods All data were acquired at 7T using a spherical harmonic shim insert enabling shimming up through 4th order with two additional 5th order shims (1st-4th+). Dynamic shim updating was achieved using a 10A shim power supply with 2 ms ramps and constrained optimizations to minimize eddy currents. Results In groups of eight subjects, we demonstrated that: 1) dynamic updating using 1st-4th+ order shims reduced the SD of the B(0)field over the whole brain from 32.4 +/- 2.6 and 24.9 +/- 2 Hz with 1st-2nd and 1st-4th+ static global shimming to 15.1 +/- 1.7 Hz; 2) near equivalent performance was achieved when dynamically updating only the non-z-degenerate shims (14.3 +/- 1.5 Hz), or when a using multiband shim factor of 2, MBs= 2, and all shims (14.4 +/- 2.0 Hz). Conclusion High order spherical harmonics provide substantial improvements over static global shimming and enable dynamic multiband shimming with near equivalent performance to that of dynamic slice-by-slice shimming. This reduces distortion in echo planar imaging.