Towards a 384-channel magnetoencephalography system based on optically pumped magnetometers
Authors:
Holly Schofield,
Ryan M. Hill,
Lukas Rier,
Ewan Kennett,
Gonzalo Reina Rivero,
Joseph Gibson,
Ashley Tyler,
Zoe Tanner,
Frank Worcester,
Tyler Hayward,
James Osborne,
Cody Doyle,
Vishal Shah,
Elena Boto,
Niall Holmes,
Matthew J. Brookes
Abstract:
Magnetoencephalography using optically pumped magnetometers (OPM-MEG) is gaining significant traction as a neuroimaging tool, with the potential for improved performance and practicality compared to conventional instrumentation. However, OPM-MEG has so far lagged conventional-MEG in terms of the number of independent measurements of magnetic field that can be made across the scalp (i.e. the number…
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Magnetoencephalography using optically pumped magnetometers (OPM-MEG) is gaining significant traction as a neuroimaging tool, with the potential for improved performance and practicality compared to conventional instrumentation. However, OPM-MEG has so far lagged conventional-MEG in terms of the number of independent measurements of magnetic field that can be made across the scalp (i.e. the number of channels). This is important since increasing channel count offers improvements in sensitivity, spatial resolution and coverage. Unfortunately, increasing channel count also poses significant technical and practical challenges. Here, we describe a new OPM-MEG array which exploits triaxial sensors and integrated miniaturised electronic control units to measure MEG data from up to 384 channels. We also introduce a high-speed calibration method to ensure the that the fields measured by this array are high fidelity. The system was validated using a phantom, with results showing that dipole localisation accuracy is better than 1 mm, and correlation between the measured magnetic fields and a dipole model is >0.998. We then demonstrate utility in human MEG acquisition: via measurement of visual gamma oscillations we demonstrate the improvements in sensitivity that are afforded by high channel density, and via a moviewatching paradigm we quantify improvements in spatial resolution. In sum, we show the first OPM-MEG system with a channel count larger than that of typical conventional MEG devices. This represents a significant step on the path towards OPMs becoming the sensor of choice for MEG measurement.
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Submitted 3 September, 2025;
originally announced September 2025.
Determining sensor geometry and gain in a wearable MEG system
Authors:
Ryan M. Hill,
Gonzalo Reina Rivero,
Ashley J. Tyler,
Holly Schofield,
Cody Doyle,
James Osborne,
David Bobela,
Lukas Rier,
Joseph Gibson,
Zoe Tanner,
Elena Boto,
Richard Bowtell,
Matthew J. Brookes,
Vishal Shah,
Niall Holmes
Abstract:
Optically pumped magnetometers (OPMs) are compact and lightweight sensors that can measure magnetic fields generated by current flow in neuronal assemblies in the brain. Such sensors enable construction of magnetoencephalography (MEG) instrumentation, with significant advantages over conventional MEG devices including adaptability to head size, enhanced movement tolerance, lower complexity and imp…
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Optically pumped magnetometers (OPMs) are compact and lightweight sensors that can measure magnetic fields generated by current flow in neuronal assemblies in the brain. Such sensors enable construction of magnetoencephalography (MEG) instrumentation, with significant advantages over conventional MEG devices including adaptability to head size, enhanced movement tolerance, lower complexity and improved data quality. However, realising the potential of OPMs depends on our ability to perform system calibration, which means finding sensor locations, orientations, and the relationship between the sensor output and magnetic field (termed sensor gain). Such calibration is complex in OPMMEG since, for example, OPM placement can change from subject to subject (unlike in conventional MEG where sensor locations or orientations are fixed). Here, we present two methods for calibration, both based on generating well-characterised magnetic fields across a sensor array. Our first device (the HALO) is a head mounted system that generates dipole like fields from a set of coils. Our second (the matrix coil (MC)) generates fields using coils embedded in the walls of a magnetically shielded room. Our results show that both methods offer an accurate means to calibrate an OPM array (e.g. sensor locations within 2 mm of the ground truth) and that the calibrations produced by the two methods agree strongly with each other. When applied to data from human MEG experiments, both methods offer improved signal to noise ratio after beamforming suggesting that they give calibration parameters closer to the ground truth than factory settings and presumed physical sensor coordinates and orientations. Both techniques are practical and easy to integrate into real world MEG applications. This advances the field significantly closer to the routine use of OPMs for MEG recording.
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Submitted 11 October, 2024;
originally announced October 2024.