Improvement of SQUID Magnetometer System for Extending Application of Spinal Cord Evoked Magnetic Field Measurement

Superconducting quantum interference device (SQUID)-based spinal cord evoked magnetic field (SCEF) measurement systems are being developed for the diagnosis of spinal cord diseases such as myelopathy. Previous papers have reported several SQUID magnetometers optimized for the detection of the magnetic field evoked by axonal nerve activity at the neck of subjects in sitting or supine positions, thus enabling noninvasive functional imaging of the cervical spinal cord. In this study, we implemented two improvements in the SQUID SCEF system to extend its application range and to make it suitable for practical use in hospitals. One was a newly designed cryostat that was implemented in order to apply the SCEF measurement not only to the cervical region but also to wider regions such as the lumbar or sacral regions. The other improvement was the implementation of SQUID driving electronics to detect not only conventional fast SCEF components accompanied by action potentials but also slow SCEF components resulting from postsynaptic activity. We also implemented direct flux feedback using a set of reference sensors composed of three SQUID magnetometers arranged in an orthogonal orientation in order to reduce noise. We then performed an SCEF measurement using the improved system and successfully observed the distribution of the magnetic field from the sacral region with a peak amplitude of less than 10 fT.