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Abstract:
High magnetic fields increase are advantageous for MRS, since sensitivity and spectral dispersion are increased, however, spectral peaks are also broadened in vivo due to stronger susceptibility-induced effects. The shim requirements on our 7T monkey system exceeded the capabilities for active shimming tenfold, which were overcome by the optimized combination of passive and active shimming. Simple ferro-shim geometries of only two pieces per shim term were numerically optimized to generate the second order spherical harmonic shim fields. The removal of large scale field inhomogeneities was achieved by mounting the ferro-shims on a cylinder surface that surrounded the monkey head and the RF coils. Subsequently, high accuracy adjustment of the active shims was done with FASTMAP. The experimental effort for the passive shimming was acceptable, the shimming was reproducible between
sessions; no eddy current or spike artifacts were introduced. The presented methods are of high practical relevance for shimming on high field systems and/or shimming of anatomical regions which inherently require high shim fields.
