Spatial sampling for triaxial OPM-MEG: understanding neurodevelopmental trajectory

Magnetoencephalography (MEG) measures magnetic fields generated by current flow in neural assemblies, providing a non-invasive means to track brain function. In recent years, the MEG field has been revolutionised by a new class of field sensor called an Optically Pumped Magnetometer (OPM). OPMs are small, lightweight, and allow construction of wearable and reconfigurable arrays of 'triaxial' field sensors (that measure magnetic fields along three axes simultaneously). These systems can be used to capture MEG data across the life span, from infancy to adulthood. With such multi-axis sensing arrays coming into routine use, here we aimed determine how dense a sensor array is required to adequately capture the spatial variation of neuromagnetic fields across the scalp. Approach: We simulated neuromagnetic fields based on realistic head geometries of individuals aged 15 months to 34 years. We then used two independent approaches to determine the sensor spacing required to capture the highest spatial variation of the field across the scalp surface: one method was based on spherical harmonic decomposition of neuromagnetic fields, and the other based on evaluating sensor redundancy in densely sampled arrays. Main results: Both methods agree in showing that the required sensor separation increases with age, from ~40 mm in a 1-year-old to ~55 mm in an adult. Significance: The sensor spacings obtained in our simulations represent upper bounds in the required values: separations above these bounds will not adequately capture spatial field variation; however, continuing to reduce sensor separation below these limits will continue to improve performance. These findings are important for the future development of OPM-MEG technology, particularly the construction of systems for neurodevelopmental applications.