B inhomogeneity: Causes and coping strategies

We review the artifacts that arise in ultra-high field brain imaging and spectroscopy due to subject-specific B0 field perturbations in vivo. We then describe how to map static B0 fields and track dynamic field fluctuations using field probes or navigators. We then explore hardware and software approaches for correcting B0-induced artifacts either prospectively (“B0 shimming”) or retrospectively. Recent developments in the shim coil design including high-order spherical harmonic insert coils, multicoil shim arrays, integrated RF/B0 shim arrays, and target-field-based “optimal” shim coil windings are then described. The ultimate bounds on the shim coil performance for a given coil geometry are then discussed. Finally, we highlight emerging applications of local shim coil technology that go beyond conventional B0 shimming.