Sensory and motor stimulation thresholds of the ulnar nerve from electric and magnetic field stimuli: implications to...

Rapidly changing magnetic fields from gradient coils induce electric fields in the individual being imaged, which can potentially result in peripheral nerve stimulation (PNS). This is a safety concern in MRI. Nerves exposed to either electric fields or time-varying magnetic fields are presumed to display equivalent stimulation threshold characteristics. This assumption has motivated the use of electric stimulation literature to be applied to gradient field safety standards. The consistency of peripheral nerve stimulation thresholds were compared by measuring chronaxie times for electric and magnetic stimulation for both motor and sensory fibers in the ulnar nerve for a group of healthy volunteers. Thresholds were determined with both electromyography and also by having the subjects report stimulation onset. Chronaxie times measured between motor and sensory fibers were statistically different. However, this difference does not account for the substantial discrepancy reported between measured electric and magnetic stimulation chronaxie times. We further establish that sensation threshold as defined perceptually by the subject volunteer is adequate as a simple and reliable measurement tool. Based on these observations, significant adjustments may need to be made to nerve parameters taken from the electric field stimulation literature prior to applying them directly to gradient induced stimulation in MRI.