An ultra-sensitive multi-channel MEG system for the non-invasive single-trial detection of cortical population spikes

We describe the design and performance of a highly sensitive 12-channel magnetoencephalography (MEG) system to be operated in a magnetically shielded room. The aim of the recording system is to measure spatially resolved high frequency (450–850 Hz) somatosensory evoked responses (hfSERs), as elicited by electrostimulation of the median nerve, at a single-trial level. The system uses current sensing superconducting quantum interference devices (SQUIDs) operated in an ultra-low noise dewar. The sensor head consists of 9 multi-turn, hexagonal signal magnetometers, 2 single-turn, circular reference magnetometers and one circular axial gradiometer. Given limitations in the dewar’s size, a balance was found between minimising the magnetic field noise and the spatial distribution of the individual MEG channels. Using SQUIDs currently made at the Physikalisch-Technische Bundesanstalt, the minimum white noise performance is expected to be ≈ 380~aT/Hz1/2 , with the achieved noise performance in the frequency band of interest of ≈ 500~aT/Hz1/2 . Additionally, simulations of the neuronal magnetic field of the hfSERs predicted a peak-to-peak strength of 35 fT for our system, in accordance with previous recordings. We also present measures to increase the dynamic range of our MEG device and show in vivo measurement results which demonstrate spatially resolved detection of the hfSERs with amplitudes of ≈ 50~fTₚₚ at the single-trial level.