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

• Published in: Scientific reports Vol. 9; no. 1; pp. 14196 - 15
• Main Authors: 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
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 02.10.2019; Nature Publishing Group
• Subjects: 631/378; 639/766/25; Equipment Design; Humanities and Social Sciences; Humans; Magnetic Fields; Magnetoencephalography; Magnetoencephalography - instrumentation; multidisciplinary; Neurons - physiology; Permeability; Physical Phenomena; Science; Science (multidisciplinary); Wearable Electronic Devices
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/s41598-019-50697-w

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 × 1.6 m 2 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 six-coil system is achieved with this three-coil system, decreasing the strongest field component B x by a factor of 53, and the strongest gradient dB x /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.