Modeling implanted metals in electrical stimulation applications

Abstract

Background: Metal implants impact the dosimetry assessment in electrical stimulation techniques. Therefore, they need to be included in numerical models. While currents in the body are ionic, metals only allow electron transport. In fact, charge transfer between tissues and metals requires electric fields to drive the electrochemical reactions at the interface. Thus, metal implants may act as insulators or as conductors depending on the scenario. Objective/Hypothesis: The aim of this paper is to provide a theoretical argument that guides the choice of the correct representation of metal implants using purely electrical models but considering the electrochemical nature of the problem in the technology of interest. Methods: We built a simple model of a metal implant exposed to a homogeneous electric field of various magnitudes to represent both weak (e.g., tDCS), medium (TMS) or strong field stimulation. The same geometry was solved using two different models: a purely electric one (with different conductivities for the implant), and an electrochemical one. As an example of application, we also modeled a transcranial electrical stimulation (tES) treatment in a realistic head model with a skull plate using a high and low conductivity value for the plate. Results: Metal implants generally act as electric insulators when exposed to electric fields up to around 100 V/m (tES and TMS range) and they only resemble a perfect conductor for fields in the order of 1000 V/m and above. The results are independent of the implant's metal, but they depend on its geometry. Conclusion(s): Metal implants can be accurately represented by a simple electrical model of constant conductivity, but an incorrect model choice can lead to large errors in the dosimetry assessment. In particular, tES modeling with implants incorrectly treated as conductors can lead to errors of 50% in induced fields or more. Our results can be used as a guide to select the correct model in each scenario.

Biophysical modeling of the electric field magnitude and distribution induced by electrical stimulation with intracerebral electrodes

“…A floating potential boundary condition was imposed at the surface of active contacts with a current of ± 1 mA. Inactive contacts were set as electric insulation, according to a recent study where it was shown that metallic materials act as electric insulators when exposed to electric fields up to 100 V/m [11]. The point and line approximations (0D and 1D, respectively) were set respectively to a point or line current source of ± 1 mA.…”

“…The mesh comprised approximately 4 million tetrahedral finite elements with an average element quality of 0.57. The volume mesh was imported into Comsol, where appropriate electrical conductivities were assigned to each region: the head models were assigned to isotropic conductivity values as mentioned before, whereas the SEEG lead was assigned to a low conductivity, following [11]. Regarding the burr-hole in the skull and scalp, it was assigned to a conductivity of 1.79 S/m (same as CSF).…”

Stereo-EEG based personalized multichannel transcranial direct current stimulation in drug-resistant epilepsy

“…(see again Supplementary Material) show that when these factors are correctly modelled, the montage optimization algorithm can adjust the currents and electrode positions to account in part for the local increases of E-field under the skull defects. Such models can also be extended to include the effects of implants such as Ti plates (Mercadal et al, 2021a(Mercadal et al, , 2021b. It should be noticed, however, that it is hard to accurately segment accurately the scar tissue with MRI data alone, and the incorporation of CT data with robust segmentation algorithms for scar tissue would improve the models.…”