Abstract
There are individual variations on the motor threshold (MT) and therapeutic effect in clinical treatment using transcranial magnetic stimulation (TMS)．These variations may result from the difference of individual brain anatomies. In this study, we built numerical brain models individually from six subjects, and calculated the distributions of eddy currents induced by TMS. The brain models were built from individual MRI data with segmenting into gray matter, white matter, and cerebrospinal fluid. The location of the figure-eight stimulator coil was recorded using a binocular infrared camera when the stimulation response of twitch observed over 50 % of trials. The eddy current distributions were obtained using an originally developed solver based on the scalar potential finite difference (SPFD) method. The results showed different distributions of the eddy current density between each brain models. The average eddy current density in the primary motor cortex was 17±6.9 A/m2 for the stimulus intensity corresponding to the MT. Assessment of the relationship between the eddy current density, stimulus conditions, and brain anatomy would help understanding of the mechanism of the varying MT. The developed model enabled us to compare the numerical results with experiments. Experiments have shown that a displacement of stimulator coil from the appropriate location causes an increase in the MT. This phenomenon was observed also in our simulations.
