[Studies on transcranial magnetic stimulation--characteristics of the magnetic coil]

I measured the characteristics of several different magnetic coils. In a round coil and a doughnut-shaped coil, induced voltages measured by a longitudinal probe and compound muscle action potentials (CMAPs) recorded from the abductor pollicis brevis showed two peak voltages in the former at a point halfway between the coil center and the outer edge, and in the center of the windings in the latter. On the other hand, in a double coil, the maximum induced voltage showed a peak at the center of the coil, and when the coil current was antidromically charged against the median nerve, the distribution of CMAPs was more focal. When the distance from the coil surface was disregarded, the maximum induced voltage spot in the double coil did not move. However, in the other coils, it moved closer to the outer edge. From these findings, it was confirmed that a double coil is the most suitable for focal stimuli. In conclusion, when using magnetic coils, the peak spot of various coils and the direction of the coil current against the target nerve are important.