Calculation of the Faradaic Impedance of the Electrode-Tissue Interface Improves Prediction of Behavioral T/C Levels in Cochlear Implant Patients.

BACKGROUND: Fitting of cochlear implants is a labor-intensive process, and therefore automated fitting procedures are being sought. The objective of this study was to evaluate if decomposition of the complex impedance of the electrode-tissue interface could provide additional parameters that show improved correlation with the behavioral T/C levels. METHODS: A new method for decomposing the complex impedance of the electrode-tissue interface was developed and tested in 18 patients in a prospective study in a tertiary otologic referral center. RESULTS: The averaged near-field Faradaic resistance (RF) calculated in study subjects shows a very strong correlation (R2=0.80) with the behavioral C levels and can be used for automated fitting in most patients. The standard deviation for the T levels and the C levels calculated for each of the electrode contacts in all study subjects is in the range of 10-15 CL and 15-20 CL, respectively. These higher values of the standard deviations are caused by a few outliers who require that additional parameters have to be added to the metric equation, allowing for the automated prediction of the T/C levels. CONCLUSION: A new method for deriving information from the electrode impedance measurements shows excellent correlation of the Faradaic resistance with the behavioral T/C levels in most patients and can be very useful for fitting cochlear implants based on objective measures. Since some patients still show discrepancies between the predicted T/C levels based on the RF calculation, additional parameters have to be added to the metric equation, allowing for automated prediction of the T/C levels.

Novel Impedance Measures as Biomarker for Intracochlear Fibrosis.

Measurement of the complex electrical impedance of the electrode contacts can provide new insights into the factors playing a role in the preservation of residual hearing with cochlear implants (CIs). However, unraveling the contributions related to the different phenomena from impedance data necessitates more advanced measurement and analysis techniques. The present study explores a new impedance measurement option recently included into the cochlear-implant programming software and aims to contribute to a more solid basis for the clinical use of impedance measures as a biomarker for fibrous tissue formation. Twenty adult CI-recipients were followed from surgery until 1 year after implantation by means of Electrode Voltage Telemetry (EVT), also called Electric Field Imaging or TransImpedance-Matrix measurement, and a 4-point technique for probing the voltage between adjacent electrode contacts. The data were compared to the electrode location derived from computed tomography, and to the device usage log. Using our impedance model for electrical stimulation of the cochlea, the polarization impedance related the electrode-tissue interface was determined, and the bulk impedance (access resistance) was split into a near-field and a far-field component. On average, the polarization impedance increased abruptly after surgery, indicating a strong passivation of the electrode contacts before cochlear-implant initiation. Its initial rise resolved almost completely soon after device switchon (2-4 weeks). The gradual increase of the access resistance mainly happened during the first 40 days on a time scale very similar to that observed in a guinea-pig study correlating impedance changes to fibrous tissue growth. The higher increase towards the round window is consistent with the higher amount of tissue observed in histological animal studies close to the electrode entry point. While the initial changes were due to the near-field resistance, the far-field resistance began to rise only after one month for half of the study group, once the near-field component had reached its critical value. This suggests indeed fibrosis initiating near the electrode contacts and spreading thereafter farther away. The near-field resistance positively correlated to device usage. EVT data allow for a further decomposition of the impedance at a cochlear-implant electrode, yielding a more detailed description of the postoperative intracochlear phenomena, such as fibrosis.

3D-localisation of cochlear implant electrode contacts in relation to anatomical structures from in vivo cone-beam computed tomography.

Positioning of the cochlear implant (CI) electrode in relation to the anatomical structures is a key factor for the hearing outcome and the preservation of residual hearing after cochlear implantation. Determining the exact electrode's location is therefore expected to play an important role in optimisation of the electrode design, the surgical techniques and the post-operative device fitting. The aim of this study is the development and validation of a robust and efficient computerised algorithm for three-dimensional (3D) localisation of the CI-electrode contacts with respect to the relevant cochlear structures, such as the basilar membrane and the modiolus, from modern clinical in vivo cone-beam computed tomography (CBCT). In the presented algorithm, the pre- and post-implantation CBCT are spatially aligned. To localise the anatomical structures, a cochlear microanatomical template derived from lab-based X-ray computed microtomography (µCT) measurements is warped to match the patient-specific cochlear shape acquired from pre-implantation CBCT. The electrode-contact locations, determined from the post-operative CBCT, are superimposed onto the cochlear fine-structure of the microanatomical template to localise the array. The accuracy of this method was validated in a temporal bone study by comparing the distance of the electrode contacts from the modiolar wall, as derived by the algorithm from CBCTs, with the distance determined from synchrotron-radiation (SR) µCT on the same specimens. Due to the achievable spatial resolution, good tissue contrast and limited presence of metallic artifacts, the SRµCT technique is considered to be a golden standard in the proposed approach. In contrast to other approaches, this validation method allowed for the evaluation of the final electrode-to-modiolus distance (EMD) error, and covers the error in co-alignment of the images, in the determination of the electrode contact location and in the localisation of the cochlear structures. The absolute mean error on the EMD parameter was determined at 0.11 mm (max = 0.29 mm, SD = 0.07 mm) across five samples, slightly lower than the voxel size of the CBCT-scans. In a retrospective study, the algorithm was applied to identify scalar translocations of the electrode from clinical in vivo CBCT datasets of 23 CI-recipients, which showed perfect (100%) agreement with the blinded opinion of two experienced neuroradiologists.

Synchrotron radiation X-ray microtomography for the visualization of intra-cochlear anatomy in human temporal bones implanted with a perimodiolar cochlear implant electrode array.

Recently, synchrotron radiation computed microtomography (SRµCT) has emerged as a promising tool for non-destructive, in situ visualization of cochlear implant electrode arrays inserted into a human cochlea. Histological techniques have been the `gold standard' technique for accurate localization of cochlear implant electrodes but are suboptimal for precise three-dimensional measurements. Here, an SRµCT experimental setup is proposed that offers the benefit of a high spatial and contrast resolution (isotropic voxel size = 4.95 µm and propagation-based phase-contrast imaging), while visualizing the soft-tissue structures and electrode array of the cochlear implant simultaneously. In this work, perimodiolar electrode arrays have been tested, which incorporate thick and closely spaced platinum-iridium contacts and wiring. These data can assist cochlear implant and hearing research, can be used to verify electrode segmentation techniques for clinical computed tomography or could be utilized to evaluate cochlear implant electrode array designs.