Hybrid suppression of ferromagnetic boundary coupling combining the self-shielding and image methods for magnetoencephalography measurements

Wearable magnetoencephalography (MEG) systems require compensating coils to establish an ultra-weak magnetic field environment in a magnetically shielded room (MSR). Ferromagnetic boundary coupling between compensating coils and the passive shielding layer presents a major technical challenge. In order to reduce the coupling of ferromagnetic boundaries and improve the uniformity and size of uniform regions, this paper proposes a novel hybrid suppression method that combines the self-shielding method (S-SM) with imaging method (IM). The proposed method utilizes S-SM to create a suppression area thus cutting off the coupling pathway, while applying the IM to calculate and eliminate the remaining unsuppressed boundary coupling magnetic fields. This paper has described the design theory of the proposed method and verified its feasibility through simulation and experiment of three-axis magnetic field coils. As a representative case, a quantitative analysis of the Bx coil, designed to generate a transverse magnetic field relative to the coil system axis, was performed. The magnetic field uniformity of the IM&S-SM coils are demonstrated 64.9% and 31.9% improvements in magnetic field uniformity compared to only-IM and only-S-SM coils within equivalently sized uniform regions. After the three-axis closed-loop control of the magnetic field fluctuations, evoked MEG by periodical eye opening and closing has been measured successfully, further demonstrating the practical application of the method.