Role of Electrical Conductivity & Electrode Configurations in the Forward Model of High Definition Transcranial Direct Current Stimulation (Hd-tDCS)

Abstract

Background: High definition Transcranial Direct Current Stimulation (HD-tDCS) is one of the most recent developments in neuro-modulation which finds its applications in treatment and research of many neurological and psychiatric disorders. HD-tDCS employs arrays of small concentric scalp electrodes for targeted, safe and cost effective stimulation of brain. HD-tDCS is an emerging technology and in-depth apprehension of the biophysical interactions is needed. Since in-vivo observations are quite difficult during performance of the clinical trials, computational modeling has played a vital role in the understanding and optimizing stimulation therapies. Objective: The aim of the study is to address the shortcomings of existing HD tDCS computational models by constructing realistic three dimensional models with more layers and different electrode placements. Methodology: Finite Element (FE) models were developed from the averaged and subject specific radiological images (Magnetic Resonance Imaging (MRI)). Using Maxwell’s, Laplace’s and Gauss’s equation the Electric field (E-field) distribution has been assessed. Under the influence of different electrode configurations (anode fixed at C3) and biological tissue conductivities in a multilayer (19 layers) brain model, the electric field maps have been examined. Results: Most of the current was shunted in non-cortical areas due to the large impedance of the scalp. In addition, the peak value of the induced E-field was spotted beneath the anode and the current was mainly distributed underneath the circumference of stimulation montage. As the current density in a brain layer tends to stay constant, the inclusion of directional conductivity caused significant changes in E-field (Topological ~ 40% and Magnitude ~ 0.7) as compared to isotropic models. Variations in the radius and shape of the montage brought noticeable changes in the E-field maps. The intensity and the depth of penetration amplified when the radius was increased. However, the spreading of fields over larger area of Head lessened their focality. The fields became more uniform and hence the skewness decreased. The same trends were observed when the electrode configuration was given a 450 shift in the orientation as the area under electrodes has increased Conclusion: The accuracy of the predicted region of interest and the dosage parametersof stimulation can be controlled by using effective modeling and simulation approaches. Based on individual anatomy and pathological conditions, this study would be beneficial to the clinicians for planning customized HD-tDCS treatments of the patients.