Comparison of rectangular and damped sinusoidal dB/dt waveforms in magnetic stimulation

In magnetic (eddy-current) stimulation, sensory and motor nerve are stimulated by a time-varying magnetic field (dB/dt) produced in an appropriate coil. In this study, magnetic stimulation thresholds for a solenoidal coil encircling the forearm were determined in 14 healthy human volunteers, 11 males and 3 females. Damped sinusoidal dB/dt pulses with durations (onset to zero crossing) ranging from 120 μs to 2500 μs were produced by discharging a capacitor bank into the coil. Rectangular dB/dt pulses with durations ranging from 50 μs to 2500 μs were generated by a Gradient Ramp Accelerator Module (GRAM) in series with a linear amplifier. The threshold data were fitted to an equivalent circuit underlying the hyperbolic form of the strength-duration curve. Average electric field rheobases for the sinusoidal and rectangular pulses were 5.87 V/m and 5.36 V/m, respectively; chronaxies for these waveforms were 393 μs and 395 μs, respectively. This study shows that stimulation thresholds obtained with damped sinusoidal dB/dt pulses can be used to accurately predict the thresholds that would be obtained for rectangular waveforms