A comparison between human magnetostimulation thresholds in whole-body and head/neck gradient coils.

Gradient coil magnetostimulation thresholds were measured in a group of 20 volunteers in both a whole-body gradient coil and a head/neck gradient coil. Both coils were operated using both x and y axes simultaneously (xy oblique mode). The waveform applied was a 64-lobe trapezoidal train with 1-ms flat-tops and varying rise times. Thresholds were based on the subjects' perception of stimulation, and painful sensations were not elicited. Thresholds were expressed in terms of the total gradient excursion required to cause stimulation as a function of the duration of the excursion. Thresholds for each subject were fit to a linear model, and values for the threshold curve slope (SR(min)) and vertical axis intercept (DeltaG(min)) were extracted. For the body coil, the mean values were: SR(min) = 62.2 mT/m/ms, DeltaG(min) = 44.4 mT/m. For the head/neck coil, the mean values were: SR(min) = 87.3 mT/m/ms, DeltaG(min) = 78.9 mT/m. These curve parameters were combined with calculated values for the induced electric field as a function of position within the coil to yield the tissue specific parameters E(r) (electric field rheobase) and tau(c) (chronaxie). For tissue stimulated within the body coil, the mean values were: E(r) = 1.8 V/m, tau(c) = 770 micros. For tissue stimulated within the head/neck coil, the mean values were: E(r) = 1.3 V/m, tau(c) = 1100 micros. Scalar potential contributions were not included in the calculation of induced electric fields. The mean threshold curves were combined with the gradient system performance curves to produce operational limit curves. The operational limit curves for the head/neck coil system were verified to be higher than those of the whole-body coil; however, the head/neck system was also found to be physiologically limited over a greater range of its operation than was the body coil. Subject thresholds between the two coils were not well correlated.