Realising the potential of wearable magnetoencephalography

Functional brain imaging has pushed the field of neuroscience towards unprecedented insights into neural dynamics. These advances have come in tandem with developments across many areas of physics, particularly quantum mechanics, which has been instrumental in the creation of quantum sensors that enable high-precision neuroimaging techniques like magnetoencephalography (MEG)—a non-invasive method for measuring the brain’s tiny magnetic fields (~10-12 to 10-15 Tesla) generated by neural activity. Conventional MEG systems rely on superconducting quantum interference devices (SQUIDs) cooled to near absolute zero temperatures using liquid helium, which necessitates a complex and expensive cryogenic infrastructure. These systems require subjects to remain almost completely immobile within a fixed, bulky scanner, severely restricting natural movement and potentially introducing artifacts related to head positioning. Sensor-to-scalp distance is also limited due to the insulation surrounding the sensors, substantially reducing the spatial resolution and signal-to-noise ratio of the recordings. In recent years, the miniaturisation of optically pumped magnetometers has enabled the development of novel MEG systems, capable of overcoming the technical challenges of cryogenic MEG. Their ability to work at room temperature and small footprint, has made OPMs an ideal candidate for revolutionising MEG hardware. Recent work has not only demonstrated the feasibility of such systems but has also demonstrated some of the advantages of OPM-MEG. Notably, OPM-MEG has enabled neuroimaging of previously challenging populations, including infants and young children, by overcoming traditional constraints of movement and scanning environment. This is particularly important for advancing our understanding of neurodevelopmental disorders such as autism spectrum disorder (ASD), where early brain development and functional connectivity are key areas of investigation. It also offers brain imaging of participants in standing positions or during ambulatory movement. This capability is vital for investigating the neural basis of movement and its disruption in neurodegenerative conditions such as Parkinson’s disease or multiple sclerosis, where motor disfunction is a core symptom. However, the versatility of OPMs does not come without constraints. Their dynamic range is limited by the inherent sensitivity of the optically pumped alkali vapor sensors, which can become saturated when exposed to magnetic fields exceeding their linear detection range. This strict requirement for a low-field environment means that, like SQUID-MEG, the system must be housed in a magnetically shielded room (MSR). Additionally, remnant fields inside the MSR must be cancelled out using active magnetic shielding from electromagnetic coil systems to obtain optimal OPM performance. In this thesis, the sensitivity of OPM-MEG is demonstrated with a study on modulation of the beta band during visual attentional cues and tactile stimuli. Our results reveal significant differences in beta band power that highlight the ability of OPM-MEG to measure subtle neurophysiological effects. We observed distinct patterns of beta band (13-30 Hz) oscillatory activity in sensorimotor cortical regions, with notable decreases during visual cues indicating participants to ignore tactile stimuli on either one of their hands. Specifically, the OPM-MEG recordings demonstrated a temporally precise desynchronization of beta band activity immediately preceding and during sensory integration tasks, providing valuable insights into the neural mechanisms of attentional modulation. While previous OPM-MEG studies focused on robust, repeatable induced effects, this work shows that subtle attentional signals can not only be detected but also used to probe the individual trial dynamics underlying them. Secondly, we developed a novel active shielding system for OPM-MEG. Borrowing ideas from magnetic resonance imaging, we designed and constructed a multi-coil system for active magnetic field cancellation. The design follows a matrix pattern that strategically distributes compensation currents to create a low magnetic field nulling environment. Unlike traditional passive shielding methods, our active shielding approach can dynamically respond to changes in the background, significantly reducing background noise and enhancing the signal-to-noise ratio of OPM-MEG recordings. The system was also employed to null the field during ambulatory movement of the participant, via optical tracking of the sensor positions. These results open up a wealth of possibilities for neuroimaging experiments, especially for studies on neurological conditions affecting motor function. Lastly, we employed a similar matrix coil system to improve the calibration of the OPMs. Co-registration errors of the sensor array to the anatomy and gain changes can significantly impact the accuracy of source reconstruction in MEG. Here, we compared two different calibration methods: the matrix coil system and a head mounted system designed by the sensor manufacturer to generate dipole-like fields from a set of small coils. By producing known fields, we were able to calibrate and localise the sensors relative to each other. Our findings revealed that both calibration approaches improved the signal-to-noise of reconstructed sources, suggesting that the use of external known fields enables more accurate determination of sensor locations, orientations, and gain. This technical improvement represents a critical step towards enhancing the precision of OPM-MEG source reconstruction techniques. Together with the cognitive demonstrations presented, these technical advances mark an important step in the maturation of OPM-MEG as a neuroimaging tool, helping to pave the way toward realising its full potential in both research and clinical contexts.