B0 shimming at 9.4T with multicoil approach: numerical investigation of different coil geometries

Purpose/Introduction: B0 inhomogeneities are caused by susceptibility differences between air and neighboring tissue and lead to image artifacts such as distortions, blurring, and signal loss. Modern scanners are equipped with spherical harmonics (SH) shim coils, generally up to 2nd or 3rd order, but the latter allow mitigating the magnetic field offsets only to a certain degree. A new method based on a multicoil approach has been proposed recently and was shown to be more effective than SH shimming [1–4]. Subjects and Methods: For the simulations, a set of coils was mounted on a cylinder (ø 360 mm), which would be large enough to accommodate inside additional RF transmit and receive arrays. Different coil arrangements and shapes were evaluated by trying to homogenize B0 distributions typically encountered in case of human brain imaging at 9.4 T. The comparison of the different configurations was based on the standard deviation of the resulting magnetic field distributions. Figure 1 shows some of the examined structures. Results: Among all investigated configurations, the best results were obtained for the given B0 field distribution with the 12 channel design (Figs. 1a, 2). Discussion/Conclusion: The simulations of the different configurations demonstrated that the homogeneity of the B0 field can be increased by the use of multiple small shimming coils. Future work will include practical realization of the proposed structures, investigation of their influence on the transmission efficiency of the RF coil and experimental validation of their B0 shimming capabilities