B0 Shimming using Pyrolytic Graphite

摘要

B0 magnetic field non-uniformity is the cause of a large amount of image artifacts in MRI. B0 inhomogeneities arise due to magnetic susceptibility differences between tissues. In particular, the 9 ppm magnetic susceptibility difference between air and tissue generate disturbances in the B0 main field near the skin. We study the B0 passive shimming approach of covering the skin with a susceptibility-matching material from both an experimental and a mathematical viewpoint. In the experimental study, a lightweight and simple to shape pyrolytic graphite composite foam is used to compensate for the field inhomogeneities in the region of the neck. We experimentally demonstrate that the pyrolytic graphite foam improves the uniformity of the static field in a phantom and in vivo at 3T. In the numerical study, we aim for a design of a neck shim which efficiently homogenizes the B0 field while being practically implementable. We propose a level set optimization method as an approach to find the optimum design for a neck shim. Simulations prove that the proposed method is able to solve the topological optimization problem while preserving the imposed constraints.