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

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 × 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 six-coil 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.

Pragmatic spatial sampling for wearable MEG arrays

“…These include the advent of high critical temperature SQUIDs (high T c -SQUIDS), nitrogen vacancy magnetometers and optically pumped magnetometers (OPMs) 16 – 21 . These new sensors allow for the construction of MEG arrays that can be brought closer to the scalp (within a few mm) and in the case of OPMs, offer the flexibility to image human brain function during subject movement 22 – 24 ( with the crucial addition of field nulling coils 25 – 27 ). From here on we refer to these methods collectively as on-scalp MEG as opposed to typical helium-based cryogenic systems, which are often displaced some distance (~ 20 mm) from the scalp (off-scalp system).…”

“…When operating in the spin exchange relaxation Free (SERF) regime these sensors are only linear within a few nT of zero field 22 . These gain errors can be caused by subject movement through a non-zero background field or by the change in the ambient magnetic field over time 26 , 27 . Some form of active shielding 27 , 46 or closed-loop feedback would typically be required to minimize these issues.…”

Multi-Channel Whole-Head OPM-MEG: Helmet Design and a Comparison with a Conventional System

“…Even though OPMs remain operational in the low background field inside our MSR, head movement within this field still generates artefactual signals which can distort measured brain activity. For this reason, background field and gradients were further controlled using a set of bi-planar coils placed either side of the participant Holmes et al, 2019). These coils, which are wound on two 1.6 m square planes separated by a 1.5 m gap in which the participant is placed, generate 3 orthogonal magnetic fields and linear gradients within a (hypothetical) 40 cm cube inside which the participant's head is positioned.…”

“…Flexibility of placement means an OPM array can, in principle, be adapted to any head size . In addition, when background fields are controlled Holmes et al, 2019) it is feasible to collect data whilst a participant moves Hill et al, 2019). It is therefore possible that the coming years could see a shift in MEG technology, away from fixed cryogenic systems and towards wearable, adaptable, motion-robust systems which provide high quality data.…”

Compensation System for Biomagnetic Measurements with Optically Pumped Magnetometers inside a Magnetically Shielded Room

“…If dynamic changes in magnetic fields inside MSR affect measurements, a dynamic compensation can be used [ 42 , 43 ]. Even moving magnetoencephalography with OPMs was proven to be possible when complex bi-planar coils setup for nulling the magnetic field and a magnetic field gradient was engaged [ 44 ].…”