Sources and effects of electrode impedance during deep brain stimulation

Abstract

Objective: Clinical impedance measurements for deep brain stimulation (DBS) electrodes in human patients are normally in the range 500-1500 Omega. DBS devices utilize voltage-controlled stimulation; therefore, the current delivered to the tissue is inversely proportional to the impedance. The goals of this study were to evaluate the effects of various electrical properties of the tissue medium and electrode-tissue interface on the impedance and to determine the impact of clinically relevant impedance variability on the volume of tissue activated (VTA) during DBS.

Methods: Axisymmetric finite-element models (FEM) of the DBS system were constructed with explicit representation of encapsulation layers around the electrode and implanted pulse generator. Impedance was calculated by dividing the stimulation voltage by the integrated current density along the active electrode contact. The models utilized a Fourier FEM solver that accounted for the capacitive components of the electrode-tissue interface during voltage-controlled stimulation. The resulting time- and space-dependent voltage waveforms generated in the tissue medium were superimposed onto cable model axons to calculate the VTA.

Results: The primary determinants of electrode impedance were the thickness and conductivity of the encapsulation layer around the electrode contact and the conductivity of the bulk tissue medium. The difference in the VTA between our low (790 Omega) and high (1244 Omega) impedance models with typical DBS settings (-3 V, 90 mus, 130 Hz pulse train) was 121 mm3, representing a 52% volume reduction.

Conclusions: Electrode impedance has a substantial effect on the VTA and accurate representation of electrode impedance should be an explicit component of computational models of voltage-controlled DBS.

Significance: Impedance is often used to identify broken leads (for values > 2000 Omega) or short circuits in the hardware (for values < 50 Omega); however, clinical impedance values also represent an important parameter in defining the spread of stimulation during DBS.

Figures

Fig. 1

Axisymmetric model of DBS. The right side shows the DBS electrode (centered on the z-axis) surrounded by a volume conductor that includes an electrode encapsulation layer (Ec), bulk tissue medium (T), and IPG encapsulation layer (Ea). FEM mesh and voltage solution are shown as colored area adjacent to electrode for −1 V stimulus. Model parameters were: volume conductor height (H) and length (L); encapsulation thickness around the electrode lead (tEc) and IPG (tEa); electrode contact height (h) and radius (r); bulk tissue conductivity (σT), electrode encapsulation conductivity (σEc) and IPG encapsulation conductivity (σEa). The left side shows an equivalent circuit model of DBS system including −1 V source, extension wire (Rext, 40 Ω), lead wire (RLead, 40 Ω), electrode contact capacitance (C), electrode encapsulation layer (REc), bulk tissue (RT) and IPG encapsulation layer (REa).

Fig. 2

Impedance sensitivity. (A) Impedance as a function of the volume conductor dimensions (height and length). (B) Impedance as a function of the electrode contact surface area resulting from changes in both height and diameter. (C) Impedance as a function of conductivity for the bulk tissue medium, electrode encapsulation, and IPG encapsulation relative to the standard model geometry. Inset shows impedance versus resistivity for the same data. D) Impedance as a function of the encapsulation thickness on the IPG or electrode. In each plot, all model parameters were defined in Table 1, and only the listed parameter was changed.

Fig. 3

Volume of tissue activated. (A) spatial plot of axisymmetric stimulation spread for common DBS stimulus settings (−1.5, −2, −2.5 or −3 V pulse amplitude; 130 Hz; 90 μs pulse width) using the low, medium and high impedance models. (B) 3D model of Medtronic 3387 DBS electrode implanted in thalamus shown relative to sagittal and axial MRI slices. VTAs are shown for the high (top right) and low (bottom right) impedance models.