A lightweight magnetically shielded room with active shielding

TLDR

Magnetically shielded rooms use layers of MuMetal to block external magnetic fields that interfere with high‑precision measurements such as MEG, and optically pumped magnetometers enable wearable MEG but require stringent shielding within ±5 nT; current OPM‑optimized MSRs are costly, heavy, and difficult to install. The study aims to develop a lightweight MSR that reduces weight by 30 % and external dimensions by 40–60 % while maintaining a low remnant magnetic field for OPM‑MEG. The design incorporates electromagnetic coils, including a window‑coil active shielding system of simple rectangular coils mounted on the walls, to cancel the remnant field and allow participant movement. Mapping the remnant field and coil‑generated field shows the active shielding can reduce the central cubic metre field to |B| = 670 ± 160 pT, thereby cutting cost, installation time, and siting constraints for OPM‑MEG.

Abstract

Magnetically shielded rooms (MSRs) use multiple layers of materials such as MuMetal to screen external magnetic fields that would otherwise interfere with high precision magnetic field measurements such as magnetoencephalography (MEG). Optically pumped magnetometers (OPMs) have enabled the development of wearable MEG systems which have the potential to provide a motion tolerant functional brain imaging system with high spatiotemporal resolution. Despite significant promise, OPMs impose stringent magnetic shielding requirements, operating around a zero magnetic field resonance within a dynamic range of ± 5 nT. MSRs developed for OPM-MEG must therefore effectively shield external sources and provide a low remnant magnetic field inside the enclosure. Existing MSRs optimised for OPM-MEG are expensive, heavy, and difficult to site. Electromagnetic coils are used to further cancel the remnant field inside the MSR enabling participant movements during OPM-MEG, but present coil systems are challenging to engineer and occupy space in the MSR limiting participant movements and negatively impacting patient experience. Here we present a lightweight MSR design (30% reduction in weight and 40-60% reduction in external dimensions compared to a standard OPM-optimised MSR) which takes significant steps towards addressing these barriers. We also designed a 'window coil' active shielding system, featuring a series of simple rectangular coils placed directly onto the walls of the MSR. By mapping the remnant magnetic field inside the MSR, and the magnetic field produced by the coils, we can identify optimal coil currents and cancel the remnant magnetic field over the central cubic metre to just |B|= 670 ± 160 pT. These advances reduce the cost, installation time and siting restrictions of MSRs which will be essential for the widespread deployment of OPM-MEG.