Three-Dimensional Head Model Simulation of Transcranial Magnetic Stimulation

TLDR

The study introduces a finite element method to evaluate induced current density in a realistic human head model exposed to time‑varying magnetic fields. By varying tissue electric properties, permittivity, and source frequency, the authors generated current density magnitude and vector plots across tissue layers to assess boundary effects and the influence of alpha‑dispersion theories. They found that current density magnitude correlates with conductivity, but when permittivity is high displacement currents dominate, normal components at cortical interfaces appear contrary to symmetric models, and cortical geometry changes can shift activation sites.

Abstract

This paper presents a finite element method used to evaluate the induced current density in a realistic model of the human head exposed to a time varying magnetic field. The tissue electric properties were varied to ascertain their influence on the induced currents. Current density magnitude and vector plots were generated throughout the tissue layers to determine the effects of tissue boundaries on the field. The current density magnitude correlated to the conductivity of the tissue in all the cases tested except where the tissue permittivity was raised to a level to allow for displacement currents. In this case, the permittivity of the tissue was the dominant factor. Current density components normal to the tissue interface were shown to exist in all solutions within the cortex contrary to the predictions of present models that rely on symmetrical geometries. Additionally, modifications in the cortical geometry were shown to perturb the field so that the site of activation could be altered in diseased patient populations. Finally, by varying the tissue permittivity values and the source frequency, we tested the effects of alpha dispersion theories on transcranial magnetic stimulation.