A 7-channel high-$T_{\text{c}}$ SQUID-based on-scalp MEG system

Objective: To present the technical design and demonstrate the feasibility of a multi-channel on-scalp magnetoencephalography (MEG) system based on high critical temperature (high- $T_{\text{c}}$ ) superconducting quantum interference devices (SQUIDs). Methods: We built a liquid nitrogen-cooled cryostat that houses seven YBCO SQUID magnetometers arranged in a dense, head-aligned array with minimal distance to the room-temperature environment for all sensors. We characterize the performance of this 7-channel system in terms of on-scalp MEG utilization and present recordings of spontaneous and evoked brain activity. Results: The center-to-center spacing between adjacent SQUIDs is 12.0 and 13.4 mm and all SQUIDs are in the range of 1-3 mm of the head surface. The cryostat reaches a base temperature of ~70 K and stays cold for >16 h with a single 0.9 L filling. The white noise levels of the magnetometers is 50-130 $\text{fT/Hz}^{1/2}$ at 10 Hz and they show low sensor-tosensor feedback flux crosstalk (<0.6%). We demonstrate evoked fields from auditory stimuli and single-shot sensitivity to alpha modulation from the visual cortex. Conclusion: All seven channels in the system sensitively sample neuromagnetic fields with mmscale scalp standoff distances. The hold time of the cryostat furthermore is sufficient for a day of recordings. As such, our multi-channel high- $T_{\text{c}}$ SQUID-based system meets the demands of on-scalp MEG. Significance: The system presented here marks the first high- $T_{\text{c}}$ SQUID-based on-scalp MEG system with more than two channels. It enables us to further explore the benefits of on-scalp MEG in future recordings.