Visualization of conductive spinal cord activity using a biomagnetometer

Study Design. The authors measured conductive cervical spinal cord evoked magnetic fields (SCEFs) after thoracic spinal cord stimulation in cats and visualized spinal cord activities. Objective. To evaluate the usefulness of magnetic field measurement. Summary of Background Data. Magnetic field measurement has several theoretical advantages compared with electric potential measurement. Although biomagnetometers for the brain and heart are already on the market and are widely used, methods for magnetic field measurement of the spinal cord have not been established. Method. Cervical laminectomy was performed on adult cats under anesthesia and the dural tube was exposed. Electrical stimuli were applied to the lower thoracic spinal cord by a catheter epidural electrode. SCEFs were recorded using a biomagnetometer specially designed for recording spinal cord action potentials. SCEFs were measured at 35 different points over the cervical spine and isomagnetic field maps of SCEFs were constructed. Thereafter, the spinal cord was transected completely at C5 and SCEFs were measured again. Results. The detected SCEFs showed a clear biphasic configuration. The first deflection of the magnetic fields from the left side was directed outward, but the right-side deflection was directed inward. The second deflection showed reversed polarity. The isomagnetic field maps of SCEFs clearly demonstrated the quadrupolar pattern and propagated at a conduction velocity of 80-120 m/s. After spinal cord transection, the propagation of SCEFs stopped at the transection site, and the SCEFs could not be obtained above the site. Conclusions. The authors concluded that magnetic field measurement is useful for evaluation of spinal cord function. Moreover, it was apparent that SCEFs could indicate conduction block in the spinal cord.