Balanced, bi-planar magnetic field and field gradient coils for field compensation in wearable magnetoencephalography

Holmes, Niall; Tierney, Tim M.; Leggett, James; Boto, Elena; Mellor, Stephanie; Roberts, Gillian; Hill, Ryan M.; Shah, Vishal; Barnes, Gareth R.; Brookes, Matthew J.; Bowtell, Richard

Abstract

To allow wearable magnetoencephalography (MEG) recordings to be made on unconstrained subjects the spatially inhomogeneous remnant magnetic field inside the magnetically shielded room (MSR) must be nulled. Previously, a large bi-planar coil system which produces uniform fields and field gradients was used for this purpose. Its construction presented a significant challenge, six distinct coils were wound on two 1.6 x 1.6 m2 planes. Here, we exploit shared coil symmetries to produce coils simultaneously optimised to generate homogenous fields and gradients. We show nulling performance comparable to that of a sixcoil system is achieved with this three-coil system, decreasing the strongest field component Bx by a factor of 53, and the strongest gradient dBx/dz by a factor of 7. To allow the coils to be used in environments with temporally-varying magnetic interference a dynamic nulling system was developed with a shielding factor of 40 dB at 0.01 Hz. Reducing the number of coils required and incorporating dynamic nulling should allow for greater take-up of this technology. Interactions of the coils with the high-permeability walls of the MSR were investigated using a method of images approach. Simulations show a degrading of field uniformity which was broadly consistent with measured values. These effects should be incorporated into future designs.

Citation

Holmes, N., Tierney, T. M., Leggett, J., Boto, E., Mellor, S., Roberts, G., Hill, R. M., Shah, V., Barnes, G. R., Brookes, M. J., & Bowtell, R. (2019). Balanced, bi-planar magnetic field and field gradient coils for field compensation in wearable magnetoencephalography. Scientific Reports, 9, Article 14196. https://doi.org/10.1038/s41598-019-50697-w

• Journal Article Type: Article
• Acceptance Date: Sep 17, 2019
• Online Publication Date: Oct 2, 2019
• Publication Date: Oct 2, 2019
• Deposit Date: Sep 24, 2019
• Publicly Available Date: Oct 3, 2019
• Journal: Scientific Reports
• Electronic ISSN: 2045-2322
• Publisher: Nature Publishing Group
• Peer Reviewed: Peer Reviewed
• Volume: 9
• Article Number: 14196
• DOI: https://doi.org/10.1038/s41598-019-50697-w
• Public URL: https://nottingham-repository.worktribe.com/output/2654790
• Publisher URL: https://www.nature.com/articles/s41598-019-50697-w
• Additional Information: Received: 17 January 2019; Accepted: 17 September 2019; First Online: 2 October 2019; : V.S. is the founding director of QuSpin, the commercial entity selling optically pumped magnetometers (OPMs). QuSpin built the OPMs used here and advised on the system design and operation, but played no part in the subsequent measurements or data analysis. This work was funded by a Wellcome award which involves a collaboration agreement with QuSpin.
• Contract Date: Oct 3, 2019