A 90‐channel triaxial magnetoencephalography system using optically pumped magnetometers

Abstract

Magnetoencephalography (MEG) measures the small magnetic fields generated by current flow in neural networks, providing a noninvasive metric of brain function. MEG is well established as a powerful neuroscientific and clinical tool. However, current instrumentation is hampered by cumbersome cryogenic field‐sensing technologies. In contrast, MEG using optically pumped magnetometers (OPM‐MEG) employs small, lightweight, noncryogenic sensors that provide data with higher sensitivity and spatial resolution, a natural scanning environment (including participant movement), and adaptability to any age. However, OPM‐MEG is new and the optimum way to design a system is unknown. Here, we construct a novel, 90‐channel triaxial OPM‐MEG system and use it to map motor function during a naturalistic handwriting task. Results show that high‐precision magnetic field control reduced background fields to ∼200 pT, enabling free participant movement. Our triaxial array offered twice the total measured signal and better interference rejection compared to a conventional (single‐axis) design. We mapped neural oscillatory activity to the sensorimotor network, demonstrating significant differences in motor network activity and connectivity for left‐handed versus right‐handed handwriting. Repeatability across scans showed that we can map electrophysiological activity with an accuracy ∼4 mm. Overall, our study introduces a novel triaxial OPM‐MEG design and confirms its potential for high‐performance functional neuroimaging.

Test-Retest Reliability of the Human Connectome: An OPM-MEG study

“…The best ways to design OPMs, sensor arrays, and magnetic shielding (which controls background fields) are not yet settled, and there is relatively little open-source data available from OPM-MEG systems. In this paper, we aimed to evaluate a recently developed triaxial OPM-MEG instrument (Boto et al, 2022; Rea et al, 2022) via quantitative assessment of test-retest reliability for the measurement of human connectomics. We further intended to generate an open-source dataset to allow other researchers to assess OPM-MEG capabilities.…”

Improved non-invasive detection of ictal and interictal epileptiform activity using Optically Pumped Magnetometers

“…For three patients the SNRs of IEDs were (slightly) lower in the OPM data than in the SQUID data, but with negligible effect sizes for two of these patients. Recording along two axes with the OPMs could be a contributing factor here, as this results in a slight reduction in sensitivity (∼30% reported by Osborne et al (2018), but less by Rea et al (2022)). A further possible explanation for this finding could be an insufficient coverage by the limited number of OPMs used in this study.…”

“…Recording along two axes with the OPMs could be a contributing factor here, as this results in a slight reduction in sensitivity (~30% reported by Osborne et al (2018), but less by Rea et al (2022)). A further possible explanation for this finding could be an insufficient coverage by the limited number of OPMs used in this study.…”

“…Moreover, reducing the field gradients in custom-designed MSRs with degaussing coils (Altarev et al, 2015), or by using field-nulling ‘matrix-coils’ that allow for accurate field control in a large volume (Holmes et al, 2021), would reduce the amplitude of movement-related artefacts to begin with. Finally, closed-loop designs can be used to further reduce movement-related artefacts, either at the OPM-level using a feedback signal to drive the on-board coils (requiring tri-axial sensitivity (Rea et al, 2022)), or through the field-nulling coils by ‘real-time’ updating of the coil calibration matrix, which would ensure optimal field cancellation during movements. With such technical developments, the increased SNR of OPMs (Boto et al, 2016; Boto et al, 2017; Iivanainen et al, 2017) could be utilised to its full extent to accurately localise the SOZ non-invasively.…”

“…Six, commercially available Gen2.0 OPMs (QuSpin Inc, Louisville, CO, USA; Zero field magnetometer, 2 nd Generation, dual-axis measurement), with sensitivities of 7-13 fT/√Hz and a dynamic range of ±5 nT (Osborne et al, 2018), were used. We recorded simultaneously along both the radial and a tangential axis to increase the number of measurements and to increase the separability of neuronal and noise signals (Brookes et al, 2021; Nurminen et al, 2013; Rea et al, 2022; Tierney et al, 2021a; Tierney et al, 2022), at a cost of a slight reduction in sensitivity (Osborne et al, 2018). The sensors operate in the spin exchange relaxation-free regime, and the (near) zero-field environment is achieved through ‘on-sensor’ electromagnetic coils wrapped around the vapour cell, that can compensate for remnant fields in the magnetically shielded room (MSR) of up to 50 nT.…”