Transcranial current stimulation focality using disc and ring electrode configurations: FEM analysis

Abstract

We calculated the electric fields induced in the brain during transcranial current stimulation (TCS) using a finite-element concentric spheres human head model. A range of disc electrode configurations were simulated: (1) distant-bipolar; (2) adjacent-bipolar; (3) tripolar; and three ring designs, (4) belt, (5) concentric ring, and (6) double concentric ring. We compared the focality of each configuration targeting cortical structures oriented normal to the surface ('surface-radial' and 'cross-section radial'), cortical structures oriented along the brain surface ('surface-tangential' and 'cross-section tangential') and non-oriented cortical surface structures ('surface-magnitude' and 'cross-section magnitude'). For surface-radial fields, we further considered the 'polarity' of modulation (e.g. superficial cortical neuron soma hyper/depolarizing). The distant-bipolar configuration, which is comparable with commonly used TCS protocols, resulted in diffuse (un-focal) modulation with bi-directional radial modulation under each electrode and tangential modulation between electrodes. Increasing the proximity of the two electrodes (adjacent-bipolar electrode configuration) increased focality, at the cost of more surface current. At similar electrode distances, the tripolar-electrodes configuration produced comparable peak focality, but reduced radial bi-directionality. The concentric-ring configuration resulted in the highest spatial focality and uni-directional radial modulation, at the expense of increased total surface current. Changing ring dimensions, or use of two concentric rings, allow titration of this balance. The concentric-ring design may thus provide an optimized configuration for targeted modulation of superficial cortical neurons.

Technique and Considerations in the Use of 4x1 Ring High-definition Transcranial Direct Current Stimulation (HD-tDCS)

“…As a general rule, increasing 4x1 ring diameter will increase the depth of penetration and maximum intensity under the ring 28 . Conversely, reducing ring radius increases focality but decreases induced brain electric field.…”

Measurements and models of electric fields in the in vivo human brain during transcranial electric stimulation

“…Current-flow modeling approaches range from spherical models (Rush and Driscoll, 1969;Ferdjallah et al, 1996;Stecker, 2005;Miranda et al, 2006;Datta et al, 2008;Dmochowski et al, 2012) to more realistic individualized models derived from MRI (Wagner et al, 2004;Datta et al, 2009;Sadleir et al, 2010;Parazzini et al, 2011;Minhas et al, 2012;Datta et al, 2010;Wagner et al, 2007). Individualized modeling implies the need to consider the anatomy of individual subjects , in particular for patient populations that may have abnormal brain anatomy Dmochowski et al, 2013).…”

Transcranial Current Brain Stimulation (tCS): Models and Technologies

“…The current direction in the brain was predominantly radial under the electrode and tangential between electrodes. Datta also used a FE spherical head model to study the focality of different combinations of disc and ring electrodes [35] and found that decreasing electrode separation improved focality but increased shunting through the scalp. Use of more than two disc electrodes or of ring electrodes allowed increasing focality in the sense that the current density could be made higher under one electrode and lower under the others.…”