Dynamic Shimming of the Human Brain at 7 T

Dynamic shim updating (DSU) of the zero- to second-order spherical harmonic field terms has previously been shown to improve the magnetic field homogeneity in the human brain at 4 T. The increased magnetic field inhomogeneity at 7 T can benefit from inclusion of third-order shims during DSU. However, pulsed higher order shims can generate a multitude of temporally varying magnetic fields arising from eddy currents that can strongly degrade the magnetic field homogeneity. The first realization of zero- to third-order DSU with full pre-emphasis and B(0) compensation enabled improved shimming of the human brain at 7 T not only in comparison with global (i.e. static) shimming but also when compared to state-of-the-art zero- to second-order DSU. Temporal shim-to-shim interactions were measured for each of the 16 zero- to third-order shim coils along ID column projections on a spherical phantom. The decomposition into up to three exponentials allowed full pre-emphasis and B(0) compensation of all 16 shims covering 67 potential shim-to-shim interactions. Despite the significant improvements achievable with DSU, the magnetic field homogeneity is still not perfect even when updating all zero- through third-order shims. This is because DSU is still inherently limited by the shallowness of the low-order spherical harmonic fields and their inability to compensate the higher order inhomogeneities encountered in vivo. However, DSU maximizes the usefulness of conventional shim coil systems and provides magnetic field homogeneity that is adequate for a wide range of applications. (C) 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 37B: 116-128, 2010