Active and Passive Shielding for OPM MEG

Recently we have shown that optically pumped magnetometers (OPMs) can be used to build a wearable magnetoencephalography (MEG) system, that can be used to measure brain activity with a sensitivity similar to that of cryogenic systems, but which allows a seated subject to make head movements during data recording (Boto et al, Nature 555,657-661,2018). This new approach to MEG is based on the use of lightweight, miniaturised OPMs that can be placed on the surface of the scalp. Excellent magnetic shielding is also a pre-requisite for OPM-MEG because the current generation of OPMs will only operate in an ambient magnetic field that is <1.5 nT. Our work to-date was carried out in a three-layer magnetically shielded room (MSR), formed from two layers of mu-metal and one layer of aluminium, and involved using an array of bi-planar coils to attenuate the ~25 nT residual Earth's field within the MSR to <1 nT over a central head-sized volume (Holmes et al, NeuroImage,181,760-774,2018). To extend the capability of OPM-MEG to allow implementation of experiments in which subjects make larger movements (e.g. when standing or walking) or undergo social interactions we need to be able to cancel the residual field over a larger volume of the MSR, and to facilitate wider take-up of OPM-MEG, cheaper and easier-to-site MSRs are required.

The proposed project will underpin these developments by focusing on full integration of passive (mu-metal and Al/Cu) and active (coils) shielding technologies, involving theory, simulation and experimental measurements using small-scale mu-metal structures. This will be facilitated by a 4-month placement with MSL, the industrial partner. Ultimately, we aim to construct coil systems that will allow ambulatory measurements to be made in a new three-layer MSR at the SPMIC and which will make possible a new cheaper design of MSR that uses only a single layer of mu-metal and can fit into a normal-height room. Assuming successful implementation of the lighter shield, we will install a system at our clinical partner site - Young Epilepsy -which will be used in research, scanning children with drug-resistant epilepsy.