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A low residual environmental magnetic field is required for the proper operation of most optically pumped magnetometers (OPMs). This is achieved using a combination of passive and active magnetic shielding. Passive magnetic shielding often uses multiple layers of highly permeable materials. A realistic two-layer magnetically shielded room (MSR) for biomagnetic measurements is numerically studied using the finite element method. A measured B-H characteristic of Mumetal is used, which is linearly extrapolated from the Rayleigh region (RR) into the low magnetic field range. This extension of the magnetization curve yields no significant differences compared to using a constant permeability value for the low magnetic field range. Furthermore, we model the MSR with only one shield and apply a simple analytical method of images (MOI). The MOI yields on average a 100-fold reduction in computation time. The relative difference of the magnetic flux density computed with MOI and finite element method (FEM) is smaller than < 1% at the center of a spherical region of interest (ROI) with a radius of 0.3 m desired from the point of magnetoencephalography (MEG) using OPMs. The achieved accuracy of the MOI makes it suitable for the optimization of active shielding coils. [ABSTRACT FROM AUTHOR]

While optically pumped magnetometers (OPMs) can be attached to the head of a person and allow for highly sensitive recordings of the human magnetoencephalogram (MEG), they are mostly limited to an operational range of approximately 5 nT. Consequently, even inside a magnetically shielded room (MSR), movements in the remnant magnetic field disable the OPMs. Active suppression of the remnant field utilizing compensation coils is therefore essential. We propose 8 compensation coils on 5 sides of a cube with a side length of approximately 2 m which were optimized for operation inside an MSR. Compared to previously built bi-planar compensation coils, the coils proposed in this report are more complex in geometry and achieved smaller errors for simulated compensation fields. The proposed coils will allow for larger head movements or smaller movement artifacts in future MEG experiments compared to existing coils. [ABSTRACT FROM AUTHOR]