Multi-Channel SQUID Magnetospinogram System With Closed-Cycle Helium Recondensing

We developed a magnetospinogram (MSG) system equipped with a closed-cycle helium recondensing unit. The MSG system detects weak magnetic fields evoked by neural activity of spinal cords using superconducting quantum interference devices (SQUIDs) and provides functional images effective for the diagnosis of spinal cord diseases. Thus far, the cost of liquid helium (LHe) required to maintain the superconductivity of the SQUID magnetometers has prevented the MSG from being used widely. Here, we combine an LHe recycling system using a pulse tube cryocooler and a low-loss transfer tube for transporting LHe to the system. This allows the superconducting state of SQUIDs to be maintained for half a year without refilling the LHe. The frequency band of noise from the recondensing system was different from that of the MSG signals. Therefore, the MSG signals were obtained at a sufficient signal-to-noise ratio even when the recondensing system, which is located just beside the magnetically shielded room, was in operation. In this paper, the details of the developed MSG system and the results from preliminary measurements using the system are reported.