Unified approach in TES and IES optimization applied to realistic head models

FERNANDEZ CORAZZA, MARIANO; TUROVETS, SERGEI; MURAVCHIK, CARLOS

Montreal (Virtual)

Conferencia; OHBM 2020; 2020

OHBM

Introduction:TranscranialElectrical Stimulation (TES) is a fast-growing therapeutic method topotentially treat different neurological disorders. In multiple-electrode TES,a current injection pattern is applied to the electrode array to stimulate somebrain region of interest (ROI). In a recent work, we proved that many patternoptimization methods proposed so far, are in fact specific solutions of thesame mathematical formulation (Fernández-Corazzaet al., 2019). There, we focused in showing and provingthe theoretical links, but illustrated our findings in an atlas head model. Thenovelty in this work is the application of the same approach to detailed headmodels and realistic targets of clinical interest. Moreover, we apply it tointracranial electrical stimulation (IES) with depth electrodes in the contextof epilepsy pre-surgical planning, where optimal targeting has not been fullyexplored (Trébuchon andChauvel, 2016). Methods:Headmodels: Based on structural MRI and CTimages we built three realistic head models, two for TES and one for IES. Forthe TES models, we used BrainK (Li et al.,2016) to segment the tissues and iso2mesh(Fang and Boas,2009) to mesh the volumes. Theelectromagnetic problem was solved using our in-house tetrahedral FEM solver.For the IES model, Simnibs (Nielsen et al.,2018) was used to segment and iElectrodes(Blenkmann etal., 2017) to determine the depth-electrodepositions. In IES, a unique ROI was marked by a clinical expert.Optimizationmethods: The constrained maximizing intensitymethod (Guler et al.,2016) can be stated as follows: maximizethe directional current density at the ROI subject to preset limits in: (i)non-ROI brain energy (α); (ii) total injected current; (iii) injected current per electrode. Inour previous work (Fernández-Corazzaet al., 2019), we demonstrated that by varying thenon-ROI energy constraint, a continuous set of optimal solutions can beobtained numerically (Grant and Boyd,2014) showing a focality-intensity tradeoff with two extreme analytical solutions: the maximum directional intensity(lowest focality), which is equivalent to the reciprocity optimization method (Fernández-Corazzaet al., 2016), and the maximum focality (lowestintensity), which is equivalent to the weighted least squares (WLS) solution (Dmochowski etal., 2011). We defined directional intensityas the mean ROI normal-to-cortex current density, and the focality as the ROI intensitydivided by the square root of α (Fernández-Corazzaet al., 2019).Results:The maximumintensity maps (Fig. 1A,C) show the maximum possible directional intensity ateach point of the cortex (each mesh element of the cortex is assumed as a ROIat a time), whereas the maximum focality maps (Fig. 1B,D) show the maximumpossible focality for each cortical element. These figures provide a clearvisualization of which cortical regions can be targeted with stronger intensity(or better focality) than others. Fig. 1E shows, the focality-intensitytrade-off curves for four representative targets located at deep structures ofinterest. In case of the deep targets (Fig. 1E), the best focality solutionshave a focality of 10 times lower than in case of the cortical targets (compareto Fig. 1B), and the maximum directional intensity is around 3-4 times lower(compare to Fig. 1A). These curves allow to determine, for a specific target,an optimal solution based on its focality-intensity values. For IES, Fig.3A shows the resulting intensity-focality trade-off curve for the ROI, whereasFigs. 3B and 3C show some current injection patterns (Fig. 3B) and currentdensity distributions (Fig. 3B). Note how the WLS solution (left) is lesssparse than the reciprocity solution (right) that only engages two electrodes,following the results found shown in (Fernández-Corazzaet al., 2019) for TES.Conclusions:Largerintensities can be produced at the walls of cortical sulci, whereas most focalsolutions can be obtained when targeting the tips of the gyri. Some deepsources can be targeted with good intensity compared to cortical ROIs, but thefocality decreases more strongly for deep targets. The unifiedapproach can also be applied to IES. The whole range of optimal solutions canbe obtained if current injection through contacts from different electrodes isallowed, which is not typically done in clinical practice yet.