Abstract: The aim of this study was to examine the effect of dispersive tissue properties on the volume conducted voltage waveforms and volume of tissue activated during deep brain stimulation. Inhomogeneous finite-element models were developed, incorporating a distributed dispersive electrode-tissue interface and encapsulation tissue of high and low conductivity, under both current-controlled and voltage-controlled stimulation. The models were used to assess the accuracy of capacitive models, where material properties were estimated at a single frequency, with respect to the full dispersive models. The effect of incorporating dispersion in the electrical conductivity and relative permittivity was found to depend on both the applied stimulus and the encapsulation tissue surrounding the electrode. Under currentcontrolled stimulation, and during voltage-controlled stimulation when the electrode was surrounded by high resistivity encapsulation tissue, the dispersive material properties of the tissue were found to influence the voltage waveform in the tissue, indicated by RMS errors between the capacitive and dispersive models of 20% to 38% at short pulse durations. When the dispersive model was approximated by a capacitive model, the accuracy of estimates of the volume of tissue activated was very sensitive to the frequency at which material properties were estimated. When material properties at 1 kHz were used, the error in the volume of tissue activated by capacitive approximations was reduced to -4.33% and 11.10% respectively for current-controlled and voltage-controlled stimulation, with higher errors observed when higher or lower frequencies were used.
Abstract: This study presents a whole-head finite element model of deep brain stimulation to examine the effect of electrical grounding, the finite conducting volume of the head, and scalp, skull and cerebrospinal fluid layers. The impedance between the stimulating and reference electrodes in the whole-head model was found to lie within clinically reported values when the reference electrode was incorporated on a localized surface in the model. Incorporation of the finite volume of the head and inclusion of surrounding outer tissue layers reduced the magnitude of the electric field and activating function by approximately 20% in the region surrounding the electrode. Localized distortions of the electric field were also observed when the electrode was placed close to the skull. Under bipolar conditions the effect of the finite conducting volume was shown to be negligible. The results indicate that, for monopolar stimulation, incorporation of the finite volume and outer tissue layers can alter the magnitude of the electric field and activating function when the electrode is deep within the brain, and may further affect the shape if the electrode is close to the skull.
Abstract: The aim of this study was to investigate the interaction of the electrode-tissue interface and dispersive tissue properties on waveforms used for deep brain stimulation. A finite element model with a distributed impedance electrical double layer was developed. Bulk tissue capacitance and dispersion were found to alter the voltage waveform under constant current stimulation. When the electrode was surrounded by conductive saline or white matter tissue, the electrical double layer was dominant under voltage controlled stimulation. However, as encapsulation tissue resistivity was increased, to emulate chronic stimulation, the voltage waveform approached that observed during constant current stimulation and the influence of the frequency dependent material properties again became dominant.
