A lightweight magnetically shielded room with active shielding

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.

Recording of Ictal Epileptic Activity Using on‐Scalp Magnetoencephalography

“…Twenty‐four triaxial OPMs (QZFM‐G3, QuSpin; sampling frequency: 1200 Hz) were used alongside video recording (LifeCam Cinema, Microsoft Corporation) within a magnetically shielded room (OPM‐Compact MuRoom, Cerca Magnetics Limited; remnant field <1‐2 nT after degaussing and active field nulling 4 ). MRI/OPM co‐registration was based on 3D optical scanning (EinScan, Shining 3D; Fig1A,B).…”

From cryogenic to on‐scalp magnetoencephalography for the evaluation of paediatric epilepsy

“…activation procedure]) was demonstrated in one school‐age child with RFE despite spontaneous and seizure‐related movements, with a localization of the seizure‐onset zones in agreement with intracranial EEG 42 . This successful ictal recording, despite movements, was obtained thanks to the use of additional field‐nulling coils that reduce the remnant magnetic field within the MSR to render OPM‐MEG movement‐compliant 43 . Such reduction is mandatory when OPMs are used in an open‐loop setup, as large magnetic deviations induced by movements lead to magnetic signal deformation and even saturation 40 .…”

“…42 This successful ictal recording, despite movements, was obtained thanks to the use of additional field-nulling coils that reduce the remnant magnetic field within the MSR to render OPM-MEG movement-compliant. 43 Such reduction is mandatory when OPMs are used in an open-loop setup, as large magnetic deviations induced by movements lead to magnetic signal deformation and even saturation. 40 The development of closed-loop OPMs should further improve the movement compliance of OPMs: by continuously zeroing the magnetic field around the sensor, closed-loop OPMs never undergo large magnetic deviations and avoid signal deformation and saturation, thereby increasing the dynamic range of OPM-MEG.…”

Simultaneous whole-head electrophysiological recordings using EEG and OPM-MEG

“…To achieve a magnetically “quiet” environment, the system is housed in a magnetically-shielded room (MSR) (Holmes, et al, 2022) comprising multiple layers of high permeability / conductivity metal which reduce both static and time-varying magnetic fields. The room is equipped with degaussing coils and these were used to demagnetise the innermost metal layer, which results in a reduction of static field beyond that afforded by the room itself.…”