Numerical calculation of eddy currents in transcranial magnetic stimulation for psychiatric treatment

In this study, current distributions in transcranial magnetic stimulation (TMS) were calculated under various conditions and compared with the current distribution in electroconvulsive therapy (ECT) in order to find an optimum condition of TMS as an alternative to ECT. Current distributions in TMS were obtained for the following coil shapes and coil diameters: circular coils of 50 mm, 75 mm, 100 mm, 125 mm, and 150 mm, and figure-eight coils of 50 mm, 75 mm, 100 mm, and 125 mm. Evaluative performance function F increases with the difference in current distributions between ECT and TMS. The minimum value of F decreased with an increase in the coil diameter. In the case of a 150-mm circular coil, the performance function had a minimum value of 0.40 at a coil current of 29 kA, which corresponded to a magnetic flux density of 0.24 T. A coil position on the forehead and the use of a large circular coil gave better results.