A flexible optimization framework for transcranial electric stimulation, temporal interference stimulation and tumor treating fields

I will introduce a novel flexible optimization framework that is designed to optimize electrode configurations for various treatment modalities which employ scalp electrodes to induce electric fields in the brain. Unlike existing optimization methods that require multi-channel systems and rely on a predetermined discretization of the electrode positions on the scalp, the proposed leadfield-free algorithm is aimed at electrodes of arbitrary shape that are placed freely on the head surface. It is based on a geodetic coordinate system for the efficient parametrization of electrode positions, shapes and configurations, and a finite-element method optimized for the repeated fast evaluation of varying electrode configurations on the head surface. It ensures absence of spatial intersections between the electrode shapes and electrode arrays, and their placement within a suited head region avoiding the face and ears. The new optimization was validated on various application examples, including standard two-electrode Transcranial Electrical Stimulation (TES), focal center-surround TES (4x1-TES), Temporal Interference Stimulation (TIS) and Tumor Treating Fields (TTF), demonstrating reliability and robustness. It supports several quantities of interest (QoI), including the electric field strength in a brain ROI, the field component normal to the cortical surface orientation, and the TI envelope. It enables both the maximization of the QoI in the ROI, and the optimization of the intensity-focality tradeoff between the QoI in the ROI and the rest of the brain. Despite its flexibility, the computational requirements are kept moderate, allowing it to run on regular desktop computers and laptops, promoting widespread use in basic and clinical research. Overall, the presented optimization framework offers high flexibility to optimize several practically employed electrode configurations. It will be distributed as part of SimNIBS, including example scripts and documentation for each modality to assist users in implementing this approach as part of their experimental studies.