Distortion of the electric field distribution induced in the brain during transcranial magnetic stimulation

In this study, the effects of various parameters, such as the geometrical head model, conductivity condition and stimulus position, on the electric field induced in the brain during transcranial magnetic stimulation are thoroughly examined. It is revealed that the distortion of the induced field causes the movement of the maximum field point and also leads to the deviation of the field focusing region from the stimulus centre. Numerical results show that the induced field distortion is primarily caused by the spatial asymmetry of the head geometry with respect to the stimulus centre and the induced field distribution is further deformed by imposing the heterogeneous conductivity condition. For verification purposes, an elaborate phantom head model has been constructed and the experimental results have been compared to the predicted fields yielding good agreement.