Magnetic Field Mapping and Correction for Moving OP-MEG

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

Abstract

Background: Optically 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. Objective: We model the spatial variation in the magnetic field and use the model to predict the movement artefact found in a dataset. Methods: We demonstrate a method for modelling this field with a triaxial magnetometer, then show that we can use the same technique to predict the movement artefact in a real OPM-based MEG (OP-MEG) dataset. Results: Using 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. Conclusion: The 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. Significance: Real-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.

Citation

Mellor, S., Tierney, T. M., O'Neill, G. C., Alexander, N., Seymour, R. A., Holmes, N., …Barnes, G. R. (2022). Magnetic Field Mapping and Correction for Moving OP-MEG. IEEE Transactions on Biomedical Engineering, 69(2), 528-536. https://doi.org/10.1109/TBME.2021.3100770

• Journal Article Type: Article
• Acceptance Date: Jul 7, 2021
• Online Publication Date: Jul 29, 2021
• Publication Date: Feb 1, 2022
• Deposit Date: Mar 21, 2022
• Publicly Available Date: Mar 21, 2022
• Journal: IEEE Transactions on Biomedical Engineering
• Print ISSN: 0018-9294
• Electronic ISSN: 1558-2531
• Publisher: Institute of Electrical and Electronics Engineers
• Peer Reviewed: Peer Reviewed
• Volume: 69
• Issue: 2
• Pages: 528-536
• DOI: https://doi.org/10.1109/TBME.2021.3100770
• Public URL: https://nottingham-repository.worktribe.com/output/7352161
• Publisher URL: https://ieeexplore.ieee.org/document/9501491
• Additional Information: © 2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted; component of this work in other works.