Constrained image-based B0 shimming accounting for "local minimum traps" in the optimization and field inhomogeneities outside the region of interest.

PURPOSE: To improve B0 shimming for applications in high- and ultrahigh-field magnetic resonance imaging and magnetic resonance spectroscopy.
METHODS: An existing image-based constrained B0 shimming algorithm was enhanced using two techniques: (1) A region of less interest was introduced to control B0 field inhomogeneities in the vicinity of the region of interest; (2) multiple sets of starting values were used for the fitting routine, to avoid "getting trapped" in a local minimum of the optimization function. The influence of constraints during the fitting procedure, due to hardware limitations, on the B0 shim result was investigated. The performance of this algorithm was compared to other B0 shim algorithms for typical shim problems in head and body applications at 3T and 7T.
RESULTS: Utilization of a weighted region of less interest lead to a significant gain in B0 homogeneity adjacent to the region of interest. The loss of B0 quality due to the enlarged total shim volume within the region of interest remained minimal, allowing for improved artifact reduction in magnetic resonance spectroscopic imaging. Multiple sets of starting values and consideration of shim field constraints led to an additional gain in B0 shim quality.
CONCLUSION: The proposed algorithm allows for more flexible control of B0 inhomogeneities and, hence, enables gains in image and spectral quality for MR applications. RO1AR050597