Proton MRS only benefits from ultra-high field strengths if good B0 homogeneity is achieved. For this, projection-based B0 shim techniques, e.g. FASTMAP, are commonly used in single-voxel acquisitions. However, FASTMAP and its variants typically only use up to 2nd order shim coils and only provide shim values for a specific voxel position. In contrast, fieldmap-based approaches for B0 shimming have no shim order restrictions and can be used to calculate multiple shim settings over different ROIs from a single acquisition. Here we compare projection and fieldmap-based shim approaches in brain areas known to be problematic in terms of B0 homogeneity.
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