Shimming Design of Magnetic Shield Room Using Ferromagnetic Plates

Abstract

Recent wearable ultrasensitive magnetometers opened novel applications of magnetoencephalography (MEG) to brain machine interface. In a recent study, the subject was allowed to move the head in a limited area of approximately 40 x 40 x 40 cm3 having a uniform magnetic field produced by a set of active shimming coils(1). The significant challenge of such wearable MEG systems is to improve the uniformity of magnetic field in the entire space in the magnetic shield room (MSR) and to suppress motion artifacts in MEG signals. A typical approach of realizing high field uniformity has been to make MSRs with highly symmetric shapes such as cylinder and polyhedron. However, these MSRs still require additional adaptive techniques to improve residual non-uniformity. The purpose of this study is to develop a new method of passive shimming for an MSR using ferromagnetic plates attached to its walls.The concept of this study is to optimize the spatial distribution of the magnetic field inside MSR with combination of ferromagnetic plates attached to the surface of MSR. The method mainly consists of two steps. In the forward problem, firstly, we calculated the spatial distributions of magnetic fields with the ferromagnetic plates of 601 mm (1 mm thickness) attached at 16 locations on the surface of a downsized magnetic shield box (MSB) of 200350 mm (1 mm thickness) (Fig. 1(a)). The uniformity was evaluated by obtaining the standard deviation for 9 measurement points inside the MSB. We established the relationship between the individual shimming plates and the resulting magnetic field uniformity inside MSR. Secondly, by solving the inverse problem based on the forward solution, we identified the optimum number of plates to be attached at each location. We used the Tikhonov regularization2) to solve the ill-posed inverse problem, and the L-curve method3) to determine its optimal parameters. In the experiments, we measured and evaluated the spatial distribution of magnetic fields using nine magneto-impedance (MI) sensors (ROHM BM1422AGMV), as shown in Figs. 1(b) and (c).We found one shimming steel plate caused magnetic field variation of 50 ~ 300 nTinside MSB in the forward problem. And then we let the value of magnetic field of 9 measurement points equal to zero and solve the inverse problem to find the number of shimming plates. Figure 2(a) shows the combination of the shimming plates optimized by the inverse problem. Figures 2(b) and (c) show the magnetic field distribution in the area of 120 x 120 mm inside MSB with and without the combination of shimming plates, respectively. The shielding factor increased 131% of MSB, from ~5 to ~8, by the proposed method. We also evaluated magnetic field uniformity by calculating the standard deviation of 9 measurement points inside MSB. The standard deviation is 0.211 and 0.379 with and without the shimming method, indicating the magnetic field uniformity inside MSB has improved 44% by the shimming method.Figures 2(d) and(e) show the magnetic field distribution measured by nine MI sensors in the area of 160 x 160 mm without MSB and with MSB, respectively. The experimental shielding factor is ~7, which is comparable to the simulation results. In conclusion, the proposed methods of passive shimming for an MSR using ferromagnetic plates demonstrates the feasibility of improving the magnetic field uniformity and is applicable to wearable MEG system in the future.This work was supported by MEXT Q-LEAP with the Grant Number JPMXS0118067395. **