On the ultimate shimming performance in the human brain

Purpose/Introduction: Improved shimming the human brain has attracted increasing interest in the recent years, using both conventional spherical harmonics (SH) of higher orders1 and multiple local coils2-8. Currently, the exact trade-off between the shim coil dimensions, the current and power requirements and the achievable shimming performance remains unclear. In this study we explore the impact of these factors on shimming the human brain. Subjects and Methods: B0 field map of the whole brain of one healthy volunteer was shimmed to the second order to account for the currently available shimming and define a valid starting point. The resultant field map was used as a target magnetic field. A stream function9 on a cylindrical surface of certain dimensions was then optimized to minimize the standard deviation of the residual magnetic field over the whole brain. During the optimization procedure, dissipated power of the coil was constrained by Pmax. All optimization problems were solved with the regularization tools10 for MATLAB (The MathWorks. Natick, USA). Considering the dimensions of a typical RF-shimming array, multicoil 7,8 and whole-body gradient coil, the radii of three cylindrical current-carrying surfaces were selected to be 105, 180 and 309 mm, with the lengths of 230, 300 and 503 mm, respectively. Results: Figure 1 shows the variation of standard deviations (SD) of the residual magnetic field inhomogeneity with regard to Pmax. In order to compare different shimming strategies, SDs achieved with global SH shimming are also marked by horizontal dotted lines. As seen, the variation of SDs for different shim coils present a similar tendency. Discussion/Conclusion: As seen in Fig. 1, for coils of smaller radius less power is required to achieve a certain shimming fideliy. Alternatively, given the power limit, a decision on the coil geometry can be made. The presented coil layouts demonstrate the importance of the certain areas of the current carrying surface, e.g. in the low face region or areas close to ears, which may guide future designs of shim arrays with irregularly-shaped coil elements or combined RF-shimming arrays