Recent advances have led to the increased use of Optically Pumped Magnetometers (OPMs) for magnetoencephagraphy (MEG), which improve adaptability of systems by operating without cryogenics. Our lab features a cylindrical magnetic alloy shield to house our OPM-MEG system, as opposed to a magnetically shielded room. This human-sized shield significantly reduces the material costs and footprint required for MEG, increasing global accessibility to the technology. However, OPMs require near-zero background magnetic field to operate with MEG sensitivity. Residual (post-shielding) spatially varying fields thus both hinder OPM operation and degrade signal-to-noise ratio (SNR). While current-carrying electromagnetic coils have been used to actively compensate fields in shielded rooms, they require many (e.g. 3-15) layers of coil windings. This work simulates a single coil, designed using an average of 11 individual residual field maps (collected on different days) to compensate for the temporally stable field within our cylindrical shield. We compared our solution to the standard approach to minimize specific spatial variations with 12 coils. Coils were simulated based on the stream function method (using bfieldtools) and measured fields were decomposed via spherical harmonics. Our simulated single-coil solution, using regularization of 10,000, reduces the peak field magnitude from 28 ± 2 nT to 5 ± 1 nT across the 11 field maps and reduces spatially varying fields by 15 ± 1 dB whilst requiring 100 dB, but only reduced spatially varying fields by 9.6 ± 0.1 dB and would require > 800 m of wire and > 0.18 mW. Future work will experimentally verify the effectiveness of our coil solutions and quantify the impact in human recordings. Reducing the burden of both passive and active magnetic shielding will aid in the wider dissemination of MEG for clinical use. • Multi-coil systems are used to compensate residual fields in OPM-MEG. • Tailored single-coil solutions can address temporally stable fields. • Our tailored single-coil solution uses 800 m multi-coil solution. • Field inhomogeneity reduced by 15 dB with tailored single-coil solution in simulation.
Leslie et al. (Sun,) studied this question.