Evaluation of methodologies for computing the deep brain stimulation volume of tissue activated

Abstract

Objective. Computational models are a popular tool for predicting the effects of deep brain stimulation (DBS) on neural tissue. One commonly used model, the volume of tissue activated (VTA), is computed using multiple methodologies. We quantified differences in the VTAs generated by five methodologies: the traditional axon model method, the electric field norm, and three activating function based approaches—the activating function at each grid point in the tangential direction (AF-Tan) or in the maximally activating direction (AF-3D), and the maximum activating function along the entire length of a tangential fiber (AF-Max). Approach. We computed the VTA using each method across multiple stimulation settings. The resulting volumes were compared for similarity, and the methodologies were analyzed for their differences in behavior. Main results. Activation threshold values for both the electric field norm and the activating function varied with regards to electrode configuration, pulse width, and frequency. All methods produced highly similar volumes for monopolar stimulation. For bipolar electrode configurations, only the maximum activating function along the tangential axon method, AF-Max, produced similar volumes to those produced by the axon model method. Further analysis revealed that both of these methods are biased by their exclusive use of tangential fiber orientations. In contrast, the activating function in the maximally activating direction method, AF-3D, produces a VTA that is free of axon orientation and projection bias. Significance. Simulating tangentially oriented axons, the standard approach of computing the VTA, is too computationally expensive for widespread implementation and yields results biased by the assumption of tangential fiber orientation. In this work, we show that a computationally efficient method based on the activating function, AF-Max, reliably reproduces the VTAs generated by direct axon modeling. Further, we propose another method, AF-3D as a potentially superior model for representing generic neural tissue activation.

Defining the impact of deep brain stimulation contact size and shape on neural selectivity

“…In this work, we focused exclusively on monopolar stimulation modeled with the axon model method. We have previously shown that all standard VTA methods for monopolar stimulation yield similar results 18 — however, these similarities break down with bipolar stimulation. Thus, if one were to choose to expand our work to bipolar stimulation, it may also be necessary to test several methods of VTA computation.…”

“…Notably, we repeated experiments with different conductivities, including and excluding an encapsulation layer and found that while activation thresholds changed in response to changes in conductivity layer, the conclusions were similar regardless of conditions. We implemented a Dirichlet boundary condition at the outer edge of the model to serve as a distant ground 1,8,18 (100 mm x 100 mm x 100 mm) and solved for the electric potential solution.…”

“…We generated volumes based on previous methodology 8,12,18 , in which we isosurfaced the maximum eigenvalue of the Hessian matrix of second spatial derivatives of the electric potential based on second difference vs. stimulation threshold fit functions. For each fiber diameter, we generated the fit functions using a two-point Power law based on the firing threshold and maximum second derivative for tangential neurons spaced 0.2 mm apart along an orthogonal line radiating from the contact center up to 10 mm away.…”

Steering the Volume of Tissue Activated With a Directional Deep Brain Stimulation Lead in the Globus Pallidus Pars Interna: A Modeling Study With Heterogeneous Tissue Properties

“…The direct bounding VTA method used here therefore resulted in VTAs that were larger at the anode than previously reported. There was evidence that previously reported method likely underestimated the activations near the anode (Slopsema et al, 2018;Anderson et al, 2019;Duffley et al, 2019), therefore, the method used here that incorporates multiple orientations orthogonal to the lead offered a more directly interpretable VTA activation profile, especially for a larger target such as the globus pallidus, and especially with a lead with larger vertical spacing (1.5 mm). However, a bigger VTA produced by bipolar stimulations does not directly translate to better therapy or equate with lower side effect threshold-the shape of the VTA matters more in terms of overlapping with therapy regions and side effect pathway activations.…”

Deep Brain Stimulation: Imaging on a group level

“…Lead-DBS uses a VTA model that thresholds the E-Field magnitude as this practice was suggested to yield good approximations by others (Åström et al, 2015). However, recent results by leading groups in this field suggested significant limitations of the approach (Duffley et al, 2019;Gunalan et al, 2017). Moreover, limitations apply to the concept of the VTA itself.…”