Magnetic Field Mapping and Correction for Moving OP-MEG.

Stephanie Mellor,Gareth R Barnes,George C Oaneill,Nicholas Alexander,Molly Rea,Matthew J Brookes,James Leggett,Robert A Seymour,Ryan M Hill,Gillian Roberts,Tim M Tierney,Eleanor A Maguire,Niall Holmes,Elena Boto,Jose D Lopez,Matthew C Walker,Richard Bowtell

doi:10.1109/tbme.2021.3100770

Abstract

BackgroundOptically pumped magnetometers (OPMs) have made moving, wearable magnetoencephalography (MEG) possible. The OPMs typically used for MEG require a low background magnetic field to operate, which is achieved using both passive and active magnetic shielding. However, the background magnetic field is never truly zero Tesla, and so the field at each of the OPMs changes as the participant moves. This leads to position and orientation dependent changes in the measurements, which manifest as low frequency artefacts in MEG data.ObjectiveWe modelled the spatial variation in the magnetic field and used the model to predict the movement artefact found in a dataset.MethodsWe demonstrate a method for modelling this field with a triaxial magnetometer, then showed that we can use the same technique to predict the movement artefact in a real OPM-based MEG (OP-MEG) dataset.ResultsUsing an 86-channel OP-MEG system, we found that this modelling method maximally reduced the power spectral density of the data by 27.8 ± 0.6 dB at 0 Hz, when applied over 5 s non-overlapping windows.ConclusionThe magnetic field inside our state-of-the art magnetically shielded room can be well described by low-order spherical harmonic functions. We achieved a large reduction in movement noise when we applied this model to OP-MEG data.SignificanceReal-time implementation of this method could reduce passive shielding requirements for OP-MEG recording and allow the measurement of low-frequency brain activity during natural participant movement.

Highlights

MAGNETOENCEPHALOGRAPHY (MEG) is a noninvasive functional neuroimaging technique, which can be used to localize neuronal current flow with high spatial and temporal resolution
superconducting quantum interference devices (SQUIDs)-based MEG systems consist of a large vacuum flask with a helmet shaped recess for the head that is surrounded by superconducting coils
We focus on optically pumped magnetometers (OPMs) that operate in the Spin Exchange Relaxation Free (SERF) regime, but the methods outlined below would be common to many magnetometers

Summary

Introduction

MAGNETOENCEPHALOGRAPHY (MEG) is a noninvasive functional neuroimaging technique, which can be used to localize neuronal current flow with high spatial and temporal resolution. In MEG, the magnetic field due to current flow across active neuronal populations is recorded outside of the head At the scalp, this magnetic field is in the range of femto- to pico-Tesla [1]. SQUID-based MEG systems consist of a large vacuum flask with a helmet shaped recess for the head that is surrounded by superconducting coils These systems are very sensitive and have excellent dynamic range, but are stationary, expensive and require participants to remain still during the recording. Compact optically pumped magnetometers (OPMs) have been developed [2]–[8] These devices can be worn directly on the scalp and so enable participant movement during scanning [9]. Participants who struggle to remain still, such as children or people with movement disorders [11], can be more studied with OP-MEG

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Discussion

Conclusion