CHARACTERIZATION OF THE ENCAPSULATION PROCESS OF DEEP BRAIN STIMULATION ELECTRODES USING IMPEDANCE SPECTROSCOPY IN A RODENT MODEL
Deep brain stimulation (DBS) is effective for the treatment of patients with Parkinson’s disease (PD), especially in advanced stages which are refractory to conventional therapy. Despite of the regular use in clinical therapy, rodent models for basic research into DBS are not routinely available. The main reason is the geometry difference from rodents to humans, imposing larger problems in the transfer of the stimulation conditions than from primates to humans. For rodents, the development of miniaturized mobile stimulators and stimulation parameters, as well as improved electrode materials and geometry are desirable. The impedance of custom made, cylindrical (contact diameter 200 µm, length 100 µm), platinum/iridium electrodes has been measured in vivo for two weeks to characterize the influence of electrochemical processes and of the adherent cell growth at the electrode surface. During the encapsulation process, the real
part of the electrode impedance at 10 kHz doubled with respect to its initial value after a characteristic decrease by approximately one third at the second day. An outlook is given on further investigations with different electrode designs for long-term DBS.
Electrical Impedance Properties of Deep Brain Stimulation Electrodes during Long-Term In-Vivo Stimulation in the Parkinson Model of the Rat
Deep brain stimulation (DBS) is an invasive therapeutic option for patients with Parkinson’s disease (PD) but the mechanisms behind it are not yet fully understood. Animal models are essential for basic DBS research, because cell based in-vitro techniques are not complex enough. However, the geometry difference between rodents and humans implicates transfer problems of the stimulation conditions. For rodents, the development of miniaturized mobile stimulators and adapted electrodes are desirable. We implanted uni- and bipolar platinum/iridium electrodes in rats and were able to establish chronical instrumentation of freely moving rats (3 weeks). We measured the impedance of unipolar electrodes in-vivo to characterize the influence of electrochemical processes at the electrode-tissue interface. During the encapsulation process, the real part of the electrode impedance at 10 kHz doubled after 12 days and increased almost 10 times after 22 days. An outlook is given on the quantification of the DBS effect by sensorimotor behavioral tests
We have developed equipment suitable for electrical bioimpedance measurement in human hearts, and we have developed a method of detecting capture for CRT devices. We have used bioimpedance to improve treatment of cardiac disease, but not to detect cardiac disease. This means that the aims we set are partially reached. The equipment is in use today in other CRT studies, so we expect to reach the last aim as well. The objectives are considered individually:
• Evaluate feasibility of getting useful results from measurements in a human heart using pacemaker leads during the acute phase of the implantation by analysing finite element models.: The work on finite element models have shown that it is probable to get useful results form four electrode measurements, and to some degree that it is feasible to get results from three electrode measurements. If we, connect the work on three electrode models with knowledge of variable tissue properties in myocardium, we can conclude that also three electrode measurement will give useful results. To be able to quantify the feasibility, we developednew measures of quality that are useful for evaluating measurement set-ups in in
silico experiments. We therefore conclude that the objective is fulfilled.
• Design, build, and test a measurement system suitable for use on human hearts:
We did build a measurement system that successfully has been used for measurements on human subjects. We have demonstrated that the system is capturing 190 spectrums per second with five frequencies form 20 kHz to 750 kHz. The results are presented in real-time, and are saved to file simultaneously for post-processing. We therefore conclude that the objective is fulfilled.
• Start building a knowledge base: We have done a series of measurements withvarying set-ups, and we have collected valuable knowledge of possibilities, limitations, and expected results for a number of set-ups. We are still learning and adjusting the information we have, but we have started. We therefore conclude that the objective is fulfilled.
• Demonstrate practical use of electrical bioimpedance in cardiology: We developed a novel method to determine loss of capture in pace electrodes used for delivering CRT therapy based on electrical bioimpedance. The method is based on bioimpedance measurements within the heart, and has been tried in a clinical study. We therefore conclude that the objective is fulfilled.
The work presented here has enabled our group to continue exploiting electrical bioimpedance as a tool in cardiology.
Reported studies pertaining to needle guidance suggest that tissue impedance available from neuromonitoring systems can be used to discriminate nerve tissue proximity. In this pilot study, the existence of a relationship between intraoperative electrical impedance and tissue density, estimated from computer tomography (CT) images, is evaluated in the mastoid bone of in vivo sheep. In five subjects, nine trajectories were drilled using an image-guided surgical robot. Per trajectory, five measurement points near the facial nerve were accessed and electrical impedance was measured (≤1 KHz) using a multipolar electrode probe. Micro-CT was used postoperatively to measure the distances from the drilled trajectories to the facial nerve. Tissue density was determined from coregistered preoperative CT images and, following sensitivity field modeling of the measuring tip, tissue resistivity was calculated. The relationship between impedance and density was determined for 29 trajectories passing or intersecting the facial nerve. A monotonic decrease in impedance magnitude was observed in all trajectories with a drill axis intersecting the facial nerve. Mean tissue densities intersecting with the facial nerve (971-1161 HU) were different (p <;0.01) from those along safe trajectories passing the nerve (1194-1449 HU). However, mean resistivity values of trajectories intersecting the facial nerve (14-24 Ωm) were similar to those of safe passing trajectories (17-23 Ωm). The determined relationship between tissue density and electrical impedance during neuromonitoring of the facial nerve suggests that impedance spectroscopy may be used to increase the accuracy of tissue discrimination, and ultimately improve nerve safety distance assessment in the future.