The electric field induced in the brain by magnetic stimulation: a 3-D finite-element analysis of the effect of tissue heterogeneity and anisotropy

Abstract—We investigate the effect of tissue heterogeneity and anisotropy on the electric field and current density distribution in duced in the brain during magnetic stimulation. Validation of the finite-element (FE) calculations in a homogeneous isotropic sphere showed that the magnitude of the total electric field can be cal culated to within an error of approximately 5 % in the region of interest, even in the presence of a significant surface charge con tribution. We used a high conductivity inclusion within a sphere of lower conductivity to simulate a lesion due to an infarct. Its ef fect is to increase the electric field induced in the surrounding low conductivity region. This boost is greatest in the vicinity of inter faces that lie perpendicular to the current flow. For physiological values of the conductivity distribution, it can reach a factor of 1.6 and extend many millimeters from the interface. We also show that anisotropy can significantly alter the electric field and current de