Abstracts

Rationale: Transcranial direct current stimulation (tDCS) has proven to be effective in reducing the seizure frequency in patients with epilepsy. tDCS greatly benefits from designing appropriate electrode montages to target the desired areas of the brain. For this purpose, patient-specific models, usually solved with the finite elements method (FEM), are used. It is not uncommon among the potential beneficiaries of tDCS to have skull plates implanted after a craniotomy. These plates alter the current flow, impacting the electric field in the brain. Therefore, to accurately predict the field distribution it is important to model them properly. Since these plates are commonly made of titanium, it may seem straightforward to model them by simply associating to them a large conductivity. However, while currents in the body are ionic, metals only allow electron transport. In fact, charge transfer between tissues and metals requires electrochemical reactions to happen at the interface, and these reactions need a minimum amount of energy (i.e. voltage gradient). Thus, depending on the interface voltage, metal implants may act as insulators or as conductors. In this study, we propose an electrochemical model for such skull plates and quantify the impact of the model choice on the predicted electric fields in the brain. Our analysis applies to transcranial current stimulation (tES) in the quasi-static approximation regime (< 1 kHz), including tDCS and tACS (transcranial alternating current stimulation).

Methods: We built a simple FEM model of a metal implant exposed to a homogeneous electric field. The same geometry was solved using two different models: a purely electric one (with different conductivities for the implant), and an electrochemical one where the electrode-electrolyte interface was modeled using the Butler-Volmer equation. We also modeled a tDCS treatment in a realistic head model with a skull plate to compare the field distribution in the brain when the plate is modeled with high or low conductivity.

Results: Comparing the electrochemical model with simple geometry with its equivalent electric model shows that metal implants act as electric insulators for electric fields up to 200 V/m and they only resemble a perfect conductor for fields above 1000 V/m. We also compared the fields generated at the cortical surface in the realistic model when the skull plate is modeled with high or low conductivity. Our results show that the fields predicted by the two models differ by more than 25% over large regions of the brain.

Conclusions: Due to the low magnitude of the fields generated in tES, there is no charge transfer between the tissues and metallic implants. Therefore, skull plates should be modeled as electrical insulators. The model chosen for the skull plate can have a large impact on the magnitude and distribution of the predicted field in the brain.

Funding: Please list any funding that was received in support of this abstract.: This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 855109).