Physiological effects of fast oscillating magnetic field gradients

To evaluate the physiological thresholds of neuromuscular stimulation relevant to very fast NMR imaging studies that use gradient switching at frequencies of 1-2 kHz and a maximum magnetic field of up to 10 mT, a series of studies were done with human volunteers using an experimental echo planar gradient coil set. The threshold for induction of localized and momentary sensations in the human back and abdomen for 10 subjects is 60 T/s for sinusoidally oscillating magnetic fields at 1.27 kHz. The threshold relates to an E field of 6 V/m and is shown to vary with number of oscillations and frequency in accord with theory. Using a simple model of E field induction, the threshold for stimulation of cardiac electrical events should be greater than 4 times this value.