Reliable in Vivo Shimming for Solving Shim Degeneracy and Incorporating System Constaints at High B0 Fields

Iulius Dragonu¹, Nicoleta Baxan¹, ², Markus Wick², Jeff Snyder³, Franek Hennel², Jrgen Hennig¹, Dominik von Elverfeldt¹, Maxim Zaitsev¹

Discontinuities in the magnetic susceptibility generate induction fields which perturb the static B0 field. At high B0 fields the susceptibility difference effects are enhanced leading to potential shimming problems. The problem of finding the correct shim values can become ill-conditioned for very thin or off-centred volumes. In such cases, the required linear independence of shim functions is not maintained leading to shim degeneracy. In this work, we propose a new method for solving the shim degeneracy problem in order to calculate the correct shim values. In addition, this technique will optimize the shim currents taking into account the limitations of the maximum current available for each shim channel.