Magnetic stimulation in humans by MRI pulsed gradient fields

The strength-duration relationship for stimulation by pulsed magnetic fields was measured in 84 human subjects. Spatial and temporal characteristics of dB/dt are similar to those in whole-body MRI y and z gradient coils. Pulse durations ranged from 0.1 ms to 1 ms. Strength response ranging from no sensation to intolerable was classified by the subject on a 0 to 10 scale. The thresholds were fit to the hyperbolic strength-duration equation with parameters of rheobase b and chronaxie c. For induction of onset of perception (score=1) by the y-coil, (b,c)=(14.9 T/s, 0.365 ms) and for the z-coil (b,c)=(26.2 T/s, 0.378 ms). Induction of an intolerable sensation (10) requires dB/dt approximately twice that required to achieve perception