Master's Theses

Date of Award

2013

Document Type

Thesis

Department

Biomedical Engineering

First Advisor

Marom Bikson

Keywords

tDCS, modeling, FEM

Abstract

Transcranial Direct Current Stimulation (tDCS) is a non-invasive neuromodulation technique that applies low amplitude current via electrodes placed on the scalp. Rather than directly eliciting a neuronal response, tDCS is believed to modulate excitability – encouraging or suppressing activity in regions of the brain depending on the polarity of stimulation. The particular application of tDCS is often determined by the electrode configuration and intensity of stimulation. MRI-derived finite element models have been developed to analyze the effect of these parameters allowing novel electrode configurations to be tested in subject specific models. By creating a subject specific model of an obese subject, the effect of fat on tDCS was examined. The inclusion of fat into the model led to an increase in cortical electric field intensity. To further investigate the influence of fat the conductivity was varied from that of skull to that of skin. Cortical electric field intensity did not change monotonically with fat conductivity. It was postulated that this may be due to a shunting effect both when the shell of fat surrounding the skull is too resistive for penetration and when the fat is so conductive as to lead current around rather than through the head. The effect of electrode positioning was then examined in a new 2x1 Hybrid montage utilizing both HD electrodes and sponge pads. Systematically varying the location of both the anode and cathode led to changes in the electric field distribution. This is in contrast to the old heuristic convention of placing the “active” electrode over a region of interest and neglecting the influence of the “return” electrode. Lastly the radial directionality of electric field was examined in a 4x1 ring configuration. Previous models have predicted the spatial focality of the 4x1 ring configuration. Polarity specificity, the ability to selectively apply either anodal or cathodal stimulation, was demonstrated in a 4x1 montage over the motor strip. The customization of models for specific populations and montages provides new avenues for clinical practice.

 
 

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