Abstract
One of the primary technical challenges facing magnetoencephalography (MEG) is that the magnitude of neuromagnetic fields is several orders of magnitude lower than interfering signals. Recently, a new type of sensor has been developed – the optically pumped magnetometer (OPM). These sensors can be placed directly on the scalp and move with the head during participant movement, making them wearable. This opens up a range of exciting experimental and clinical opportunities for OPM-based MEG experiments, including paediatric studies, and the incorporation of naturalistic movements into neuroimaging paradigms. However, OPMs face some unique challenges in terms of interference suppression, especially in situations involving mobile participants, and when OPMs are integrated with electrical equipment required for naturalistic paradigms, such as motion capture systems. Here we briefly review various hardware solutions for OPM interference suppression. We then outline several signal processing strategies aimed at increasing the signal from neuromagnetic sources. These include regression-based strategies, temporal filtering and spatial filtering approaches. The focus is on the practical application of these signal processing algorithms to OPM data. In a similar vein, we include two worked-through experiments using OPM data collected from a whole-head sensor array. These tutorial-style examples illustrate how the steps for suppressing external interference can be implemented, including the associated data and code so that researchers can try the pipelines for themselves. With the popularity of OPM-based MEG rising, there will be an increasing need to deal with interference suppression. We hope this practical paper provides a resource for OPM-based MEG researchers to build upon.
Highlights
Wellcome Centre for Human Neuroimaging, UCL Queen Square Institute of Neurology, University College London, London WC1N 3AR, UK article info
optically pumped magnetometer (OPM) face some unique challenges in terms of interference suppression, especially in situations involving mobile participants, and when OPMs are integrated with electrical equipment required for naturalistic paradigms, such as motion capture systems
OPMs are likely to be used as a clinical tool in pre-epilepsy surgery planning (Feys et al, 2021; Mellor et al, 2021b; Vivekananda et al, 2020), and the study of mild traumatic brain injury (Allen et al, 2021)
Summary
Magnetoencephalography (MEG) is a non-invasive neuroimaging technique that measures small magnetic fields outside of the head that originate from current flows throughout the brain (Cohen, 1968). The signal-to-noise ratio (SNR) of gradiometers can be up to 100 times higher than magnetometers (Taulu et al, 2014) For this reason, modern SQUID-MEG systems typically use multichannel arrays of axial/planar gradiometers (e.g. the CTF 275-channel system), or a mixture of magnetometers and gradiometers (e.g. the MEGIN TRIUXTM Neo). Modern SQUID-MEG systems typically use multichannel arrays of axial/planar gradiometers (e.g. the CTF 275-channel system), or a mixture of magnetometers and gradiometers (e.g. the MEGIN TRIUXTM Neo) These hardware-based techniques are complemented by a suite of interference suppression tools that aim to exploit the spatial and temporal properties of MEG data to isolate signals originating from inside the brain whilst suppressing external signals
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