Passive Shimming of Magnetically Shielded Room Using Ferromagnetic Plates

Although recent wearable devices for magnetoencephalography (MEG) facilitate measurements of unrestrained subjects, a significant challenge has arisen due to large signal perturbations, caused by human movements under a nonuniform magnetic field inside large magnetically shielded rooms (MSRs). This study proposes and demonstrates a novel method for improving the magnetic field uniformity of an MSR by placing ferromagnetic plates on its surfaces. We solved an inverse problem to identify the appropriate locations of shimming plates for a given target distribution of a magnetic field. We used the Tikhonov regularization and the L-curve method to determine the optimal parameters. By solving the inverse problem, we obtained the locations and quantities of steel plates that could reduce the magnetic field non-uniformity relative to the original one without shimming. In a 2-D numerical simulation, we evaluated the uniformity by calculating the standard deviation of the magnetic field at nine measurement points inside an MSR. The standard deviations were 0.21 and 0.38 with and without shimming, indicating that the magnetic field uniformity has improved by 44% after shimming. An experimental study on a prototype MSR also indicated a 9% improvement. These results suggest that the proposed shimming method has the potential for improving the magnetic field uniformity inside large MSRs for the MEG of freely moving subjects and for fundamental physical measurements.