Towards a 384-channel magnetoencephalography system based on optically pumped magnetometers

Magnetoencephalography using optically pumped magnetometers (OPM-MEG) is gaining traction as a neuroimaging tool, with potential for improved performance and practicality compared with conventional instrumentation. However, OPM-MEG has so far lagged conventional-MEG in terms of the number of independent measurements of the MEG signal that can be made across the scalp (i.e., the number of channels). This is important since increasing the channel count offers improvements in sensitivity, spatial resolution, and coverage. Unfortunately, increasing channel count also poses significant technical and practical challenges. Here, we describe a new OPM-MEG system which exploits 3-axis sensors and integrated miniaturised electronic control units to measure MEG data from up to 384 channels. We also introduce a high-speed calibration method to ensure that the fields measured are high fidelity. We initially validate this system using a phantom: specifically, we localise a set of magnetic dipoles and compare the MEG-derived distances between them with the ground truth, demonstrating an accuracy of ~1 mm. We further show that the correlation between a mathematical model of the phantom and the magnetic fields measured is >0.998. Secondly, we demonstrate utility of our system for human MEG acquisition. Via measurement of visual cortex activity, we demonstrate improvements in sensitivity afforded by a higher channel count, and via a movie-watching experiment we show increased spatial resolution. In sum, we report the first OPM-MEG system with a channel count larger than that of typical conventional MEG devices. This represents a significant step towards OPMs becoming the sensor of choice for MEG.