Spinal cord evoked magnetic field measurement using a magnetospinography system equipped with a cryocooler

We have developed a magnetospinography (MSG) system that detects weak magnetic fields associated with spinal cord neural activity using an array of low-temperature superconducting quantum interference device (SQUID)-based magnetic flux sensors. A functional image of the spinal cord can be obtained noninvasively by using this system, and it is effective for precise lesion localization in the diagnosis of spinal cord diseases. The running cost of the developed MSG system mainly depends on liquid helium (LHe) consumption, which is required to maintain the superconducting state of the SQUID sensors. To reduce the LHe consumption, we incorporate a pulse-tube-refrigerator-based cryocooler into the MSG system. Cold gaseous helium is circulated between the cryocooler and the MSG system for cooling the thermal radiation shield of the dewar vessel. Consequently, we achieved a 46% decrease in the LHe consumption rate. Conventional biomagnetic field detection such as magnetoencephalography is often hindered by severe low-frequency band noise from the cryocooler. However, in the case of MSG measurements, such noise can be filtered out because the band of the signal is much higher than that of the cryocooler noise. We demonstrated that the signal-to-noise ratio of the cervical spinal cord evoked magnetic field measurement performed with a working cryocooler is comparable to that of the measurement without a cryocooler.