The Foundation of EGI's new GTEN Technology:
Advances in Transcranial Direct Current Stimulation
New computational simulations have shown that it is feasible to achieve dense array tDCS (transcranial Direct Current Stimulation) of the brain with the 256-channel Geodesic Sensor Net. The precise targeting should allow important advances in the new approaches now being explored for treating a variety of neurological and psychiatric disorders, from tinnitus to depression.
These simulations have been completed in different computational environments by a team of investigators including Pavel Govyadinov, Sergei Turovets, Phan Luu, and Don Tucker at EGI; Adnan Salman, Daniel Ellsworth, and Allen Malony at the University of Oregon; Moritz Dannhauer and Rob McLeod at the University of Utah; Burak Erem, Sehymus Guler, and Dana Brooks at Northeastern University. The EGI and Oregon teams used a Finite Difference Method (FDM) implemented on the UO 
Neuroinformatics Center’s ACISS supercomputer. The Utah and Northeastern teams also used EGI’s electrical head model, developed from registered MRI and CT images, with conductivity specified through measurement with bounded electrical impedance tomography (bEIT). The improved precision of this model was aided by not only by inclusion of diffusion imaging data, but also by inclusion of bone density (X-ray attenuation) estimates of skull conductivity in the EGI head model (protected by US Patent 6,529,759).
Importantly, the Utah and Northeastern scientists solved the electrical current flow with a Finite Element Model (FEM), which allowed a high resolution analysis of flow at the electrode surface. What was unexpected was the simulations of current flow with the conventional large sponge electrodes (Figure 1). Rather than being distributed over the surface of these large electrodes, the results from the FEM simulations showed that the current is exclusively focused 
on the perimeter of the electrodes. In hindsight, the perimeter current flow is predicted by well known physical principles, but the majority of tDCS researchers will now have to reexamine the rationale for the large electrodes.
Once the high resolution Electrical Head Model was created (Figure 2), both FDM and FEM simulations converged to suggest that reasonably focal (~ 1 cm square) patches of the cortex can be targeted with an effective current level (surface anodal or surface cathodal depending on polarity) when current delivery is optimized with the dense array. Importantly, unlike conventional tDCS with two large sponge electrodes, carefully patterned tDCS with multiple electrodes of the dense array can target focal sites both at the outer (gyral) surface of the cortex as well as deep sites, such as the anterior cingulate cortex (Figure 3). Integration of the individual’s cortex model from MRI (Figure 4) allows highly unique targeting patterns to be fit to that person’s brain.
References
Dannhauer, M., D. Brooks, D. Tucker and R. MacLeod (2012). "A pipeline for the simulation of transcranial direct current stimulation for realistic human head models using SCIRun/BioMesh3D." Conf Proc IEEE Eng Med Biol Soc 2012: 5486-5489.
Guler, S., Dannhauer, M., Macleod, R., Erem, B, Tucker, D. M., Turovets, S., Mattson, C. and Brooks, D. Optimized Current Stimulus Patterns for Targeted tDCS with Flexible Objectives and Constraints. Manuscript in preparation, 2014.
Salman, A., Malony, A., Turovets, S., Volkov, V., Ozog D., and Tucker, D. M. Concurrency and Computation: Practice and Experience, in press.
