A 75-ch SQUID Biomagnetometer System for Human Cervical Spinal Cord Evoked Field

A 75-ch SQUID biomagnetometer system for the measurement of the cervical spinal cord evoked magnetic field (SCEF) was developed for the purpose of the noninvasive functional diagnosis of the spinal cord. The sensor array has 25 SQUID vector sensors arranged along the cylindrical surface to fit to the shape of the subject's neck. The magnetic fields, not only in the direction radial to the subject's body surface but also in the tangential direction, are observed in the area of 80 mm times 90 mm at one time. The dewar has a unique shape with a cylindrical main body and a protrusion from its side surface. The sensor array is installed in the protruded part. This design is optimized to detect magnetic signals at the back of the neck of the subject sitting in a reclining position. We applied the developed SQUID system to the cervical SCEF measurement of normal subjects who were given electric pulse stimulation to their median nerves at the wrists. The evoked magnetic signals were successfully detected at the cervixes of all subjects. A characteristic pattern of transition of the SCEF distribution was observed as a reproducible result and the signal components propagating along the spinal cord were found in the time varying SCEF distribution. We expect that the investigation of the propagating signal components would help to establish a noninvasive functional diagnosis of the spinal cord.