S 1 New methods for electrode optimization for high-definition transcranial electric stimulation (hd-tES)

Publication date: October 2017
Source:Clinical Neurophysiology, Volume 128, Issue 10
Author(s): C.H. Wolters
Background/problem descriptiontES is a non-invasive brain stimulation technique which modifies neural excitability by providing weak currents through scalp electrodes. The aim of my presentation is to introduce and analyze new forward and inverse simulation methods for safe and well-targeted multi-electrode tES.MethodsContinuous Galerkin Finite Element Method (CG-FEM) simulations will be reviewed as current standard tES forward simulation approach (Wagner et al., 2014). The tES forward problem is then related to the Electroencephalography (EEG) forward problem using Helmholtz principle of reciprocity (Wagner et al., 2016) and a new Unfitted Discontinuous Galerkin FEM (UDG-FEM) approach for both EEG and tES forward problem will be presented (Nüßing et al., 2016). UDG-FEM combines the advantages of a FEM approach on a structured hexahedral mesh with implicit surface representations given by level set functions. Finally, a new multi-electrode tES optimization approach is presented that uses the alternating direction method of multipliers (ADMM) for optimizing the focality, orientation and intensity of current density at the target location, while minimizing current density in the remaining brain (Wagner et al., in press).ResultsCG-FEM simulations show that standard bipolar electrode montages induce a very widespread current flow field with the strongest intensities often located in non-target brain regions. The results will also point out the limitations of such standard CG-FEM simulation approaches with regard to so-called skull leakage problems, which are alleviated using UDG-FEM to appropriately model smooth tissue surfaces. The electrode optimization results in a highly realistic six-compartment geometry-adapted hexahedral head model reveal that the optimized current flow fields show significantly higher focality and, in most cases, higher directional agreement to the target vector in comparison to standard bipolar electrode montages. The new ADMM optimization approach will also be compared to other optimization approaches from the literature.Discussion/conclusionThe new ADMM hd-tES electrode optimization approach based on UDG-FEM realistic head modeling offers an innovative new approach to calculate individually optimized electrode currents. In combination with hd-tES hardware, it bears the potential to significantly increase effects of non-invasive tES stimulation.



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