Benchmarking transcranial electrical stimulation finite element models: a comparison study

Abstract

Objective: To compare field measure differences in simulations of transcranial electrical stimulation (tES) generated by variations in finite element (FE) models due to boundary condition specification, use of tissue compartment smoothing filters, and use of free or structured tetrahedral meshes based on magnetic resonance imaging (MRI) data. Approach: A structural MRI head volume was acquired at 1 mm3 resolution and segmented into ten tissue compartments. Predicted current densities and electric fields were computed in segmented models using modeling pipelines involving either an in-house (block) or a commercial platform commonly used in previous FE tES studies involving smoothed compartments and free meshing procedures (smooth). The same boundary conditions were used for both block and smooth pipelines. Differences caused by varying boundary conditions were examined using a simple geometry. Percentage differences of median current density values in five cortical structures were compared between the two pipelines for three electrode montages (F3-right supraorbital, T7-T8 and Cz-Oz). Main results: Use of boundary conditions commonly used in previous tES FE studies produced asymmetric current density profiles in the simple geometry. In head models, median current density differences produced by the two pipelines, using the same boundary conditions, were up to 6% (isotropic) and 18% (anisotropic) in structures targeted by each montage. Tangential electric field measures calculated via either pipeline were within the range of values reported in the literature, when averaged over cortical surface patches. Significance: Apparently equivalent boundary settings may affect predicted current density outcomes and care must be taken in their specification. Smoothing FE model compartments may not be necessary, and directly translated, voxellated tissue boundaries at 1 mm3 resolution may be sufficient for use in tES FE studies, greatly reducing processing times. The findings here may be used to inform future current density modeling studies.

Modeling transcranial electrical stimulation in the aging brain

“…Where {Volume interest } is either WM, GM, or both WM and GM (brain) volume. Isotropic conductivity values for all tissue types except air were modified from the default setting to the values listed in Table 3 [13]. WM isotropic conductivity was computed as an average of the longitudinal and transverse measured values [14,29].…”

“…Therefore, finite element modeling (FEM) based computational studies have been employed to predict electrical current distribution in the human head during stimulation [9e12]. Individual head models are typically converted from T1-weighted magnetic resonance (MR) images following a standard modeling workflow [13]. Prior research shows that variation in brain anatomy e.g., gyrus morphology have considerable effects on modeled field measures in the brain, and thus warrant the need for individualized head models [12,14,15].…”

“…Tissue conductivity values for six tissue types. White matter, gray matter, bone and skin values were sourced from the literature for measurement frequency up to 20 kHz[13] and within the range of reported values in typical volume conductor models[29]. Bone conductivity (combined) was computed as s ¼ can was cancellous bone and s cor was the average of cortical bone conductivity reported in the reference.…”

A Systematic Review and Meta-Analysis of Transcranial Direct Current Stimulation to Remediate Age-Related Cognitive Decline in Healthy Older Adults

“…Recent tDCS modeling studies have shown that interindividual variability in anatomy has an important role in altering delivered tDCS current in the brain. 88,[96][97][98][99][100] In older adults, age-related effects such as brain atrophy and white matter hyperintensities (WMH) were found as important factors that affect modeled field distribution following tDCS. 77,101 Brain atrophy is commonly described as a shrinkage of cortical structures that occurs with healthy aging.…”

White matter hyperintensities affect transcranial electrical stimulation in the aging brain

“…Transcranial electrical stimulation (tES) applies mild current (1e2 mA) to the scalp to improve brain functions and has shown promising results in older adults [10,11]. MRI-derived finite element models (FEM) have been used to predict tES current flow inside human's head and showed that individual brain morphology critically affects distributed current flow in the brain [12,13]. Given that the prevalence of WMH increases with age and the presence of WMH alters individual brain anatomy, WMH commonly found in older adults may affect tES current flow in the aging brain.…”