Journal Article

Real-time, model-based magnetic field correction for moving, wearable MEG

O'Neill, George C., Barnes, Gareth R., Mellor, Stephanie, Timms, Ryan C., Seymour, Robert A., Payne, Heather, Alexander, Nicholas, Spedden, Meaghan E., Tierney, Tim M.

2023

Overview

•Zero-field OPMs operate within a limited magnetic field range.•We correct for background field changes in real-time using coils on-board the OPMs.•We used a model of the background field based on low-order regular solid harmonics.•We were able to record auditory evoked fields during movements of 1.5m - 2m. Most neuroimaging techniques require the participant to remain still for reliable recordings to be made. Optically pumped magnetometer (OPM) based magnetoencephalography (OP-MEG) however, is a neuroimaging technique which can be used to measure neural signals during large participant movement (approximately 1 m) within a magnetically shielded room (MSR) (Boto et al., 2018; Seymour et al., 2021). Nevertheless, environmental magnetic fields vary both spatially and temporally and OPMs can only operate within a limited magnetic field range, which constrains participant movement. Here we implement real-time updates to electromagnetic coils mounted on-board of the OPMs, to cancel out the changing background magnetic fields. The coil currents were chosen based on a continually updating harmonic model of the background magnetic field, effectively implementing homogeneous field correction (HFC) in real-time (Tierney et al., 2021). During a stationary, empty room recording, we show an improvement in very low frequency noise of 24 dB. In an auditory paradigm, during participant movement of up to 2 m within a magnetically shielded room, introduction of the real-time correction more than doubled the proportion of trials in which no sensor saturated recorded outside of a 50 cm radius from the optimally-shielded centre of the room. The main advantage of such model-based (rather than direct) feedback is that it could allow one to correct field components along unmeasured OPM axes, potentially mitigating sensor gain and calibration issues (Borna et al., 2022).

• Publisher: Elsevier Inc,Elsevier Limited,Academic Press,Elsevier
• Subject: Auditory evoked field / Brain / Closed loop systems / Control algorithms / Experiments / Humans / Magnetic field correction / Magnetic Fields / Magnetoencephalography / Magnetoencephalography - methods / MEG / Movement / Neuroimaging / Optically pumped magnetometer / Sensors / Walking OP-MEG / Wearable Electronic Devices