Characterization of a flexible cap for simultaneous OPM-MEG and EEG measurements

Abstract Objective. Combining magnetoencephalography (MEG) and electroencephalography (EEG) can lead to more accurate source reconstruction results. Previous efforts of simultaneous EEG and MEG using optically pumped magnetometers (OPM-MEG) used a two-layer setup, where a helmet or cap with mounted OPMs was worn on top of an EEG cap, resulting in decreased OPM signal amplitudes. This is inevitable for rigid OPM-MEG helmets used by most research groups, which are often heavy and need fixation, require time-consuming customization or do not adapt to different head shapes, diminishing the advantages of OPM-MEG. In this study, we therefore aimed to create a flexible cap that enables fully integrated simultaneous OPM-MEG and EEG measurements, which are suitable for source reconstruction and yield the same signal quality as each individual modality. Approach. We integrated OPM mounts into a commercial EEG cap and implemented a sensor localization pipeline based on co-registering the known OPM mount geometry to 3D scans of participants wearing the cap. In a multi-paradigm experiment with five volunteers, we investigated how much OPMs shift during an experiment due to head movements of the participant. We also compared signal-to-noise ratios and artifact strengths between single-modality and multimodal measurements. Main results. The distribution of changes in OPM positions and orientations are within acceptable limits for source reconstruction as published in a simulation study. Occipital OPMs experienced larger shifts. No systematic difference in signal-to-noise ratios or artifact strengths between single-modality and multimodal acquisition could be observed, indicating no signal degradation due to the presence of the other modality. Significance. Measuring simultaneous OPM-MEG and EEG using a flexible cap could improve source reconstruction results and help to bring OPM-MEG to clinical settings due to easier comparison and verification with the established EEG, which is relevant for presurgical epilepsy evaluation.