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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

Authors

Niall Holmes

Tim M. Tierney

James Leggett

Elena Boto

Stephanie Mellor

Gillian Roberts

Ryan M. Hill

Vishal Shah

Gareth R. Barnes

Matthew J. Brookes

Richard Bowtell



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.

Journal Article Type Article
Publication Date Oct 2, 2019
Print ISSN 2045-2322
Publisher Nature Publishing Group
Peer Reviewed Peer Reviewed
Volume 9
Article Number 14196
APA6 Citation Holmes, N., Tierney, T. M., Leggett, J., Boto, E., Mellor, S., Roberts, G., …Bowtell, R. (2019). Balanced, bi-planar magnetic field and field gradient coils for field compensation in wearable magnetoencephalography. Scientific Reports, 9, https://doi.org/10.1038/s41598-019-50697-w
DOI https://doi.org/10.1038/s41598-019-50697-w
Publisher URL https://www.nature.com/articles/s41598-019-50697-w

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