Electric field calculations in brain stimulation: The importance of geometrically accurate head models

Introduction Numerical field calculations for brain stimulation based on realistic head models have the potential to contribute to better predictions of the position and size of stimulated brain areas. Here, we discuss the impact of geometrical features such as the gyrification pattern and local skull shape on the estimated fields in transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS). Methods The reconstruction of the head models from structural MR data and their subsequent usage in finite-element simulations of the electric field distribution in TMS and tDCS was performed using the software pipeline described in [3] (www.simnibs.de). Estimates of the conductivity anisotropy of white matter were included, derived from high resolution (1mm³) DTI data acquired at 7T. Results For a figure-8 coil in TMS, we demonstrate a systematic dependency of the electric field strength in the brain on the local gyral orientation relative to the induced currents and on the anisotropy of WM. Importantly, these effects can result in a spatial offset of the electric field peak from a position directly underneath the coil centre. Initial results for tDCS hint towards a prominent role of the local skull geometry underneath the electrodes in shaping the field distribution in the brain. Discussion Our results demonstrate that individual anatomical features can significantly influence the estimated fields, suggesting that our understanding of brain stimulation might substantially benefit from using individualized head models. Future research should therefore test how well interindividual differences in the estimated field distributions predict interindividual variations in the physiological stimulation effects