A 7-Channel High-${T}_\text{c}$ SQUID-Based On-Scalp MEG System

<italic>Objective:</italic> To present the technical design and demonstrate the feasibility of a multi-channel on-scalp magnetoencephalography (MEG) system based on high critical temperature (high-<inline-formula><tex-math notation="LaTeX">${T}_\text{c}$</tex-math></inline-formula>) superconducting quantum interference devices (SQUIDs). <italic>Methods:</italic> 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. <italic>Results:</italic> 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 <inline-formula><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula>70 K and stays cold for <inline-formula><tex-math notation="LaTeX">$>$</tex-math></inline-formula>16 h with a single 0.9 L filling. The white noise levels of the magnetometers is 50–130 fT/Hz<sup>1/2</sup> at 10 Hz and they show low sensor-to-sensor feedback flux crosstalk (<inline-formula><tex-math notation="LaTeX">$< $</tex-math></inline-formula>0.6%). We demonstrate evoked fields from auditory stimuli and single-shot sensitivity to alpha modulation from the visual cortex. <italic>Conclusion:</italic> All seven channels in the system sensitively sample neuromagnetic fields with mm-scale scalp standoff distances. The hold time of the cryostat furthermore is sufficient for a day of recordings. As such, our multi-channel high-<inline-formula><tex-math notation="LaTeX">${T}_\text{c}$</tex-math></inline-formula> SQUID-based system meets the demands of on-scalp MEG. <italic>Significance:</italic> The system presented here marks the first high-<inline-formula><tex-math notation="LaTeX">${T}_\text{c}$</tex-math></inline-formula> 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.

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