The use of cone-beam computed tomography to determine cochlear implant electrode position in human temporal bones

Cone beam computed tomography and histological evaluations of a straight electrode array positioning in temporal bones

CONCLUSION

CBCT allows a precise evaluation of the dimensions and of the shape of the cochlear duct that is of primary importance for the choice of the length and the design of the electrode array. This radio-histological study confirms that CBCT is a reliable tool to be used in clinical studies to evaluate the position of straight electrode arrays within the cochlear scala.

OBJECTIVE

To validate the reliability of cone beam computed tomography (CBCT) in the evaluation of cochlear anatomy and positioning of a straight electrode array, by comparing radiological images to histological analysis of cochlear implanted temporal bones.

METHODS

Eight temporal bones for four subjects were analysed before and after cochlear implantation with a straight electrode array. The size of the cochlea, the dimensions, and shape of the cochlear duct at 180° and 360° were evaluated on CBCT performed before implantation. Temporal bones then underwent histological analysis to determine the position of the electrode array. Scalar localization of the electrode array was assessed on CBCT performed after implantation, and compared with histological study.

RESULTS

CBCT is a reliable radiological technique to differentiate variability not only of cochlear dimensions, but also of size and shape of the cochlea. Histological analysis confirmed the scalar position of the electrode array assessed by the CBCT in 7/8 temporal bones, but soft tissue trauma such as dissection of the spiral ligament was not identified by the CBCT.

Cone-beam computed tomography in children with cochlear implants: The effect of electrode array position on ECAP

OBJECTIVES

To assess the feasibility of using cone-beam computed tomography (CBCT) in young children with cochlear implants (CIs) and study the effect of intracochlear position on electrophysiological and behavioral measurements.

METHODS

A total of 40 children with either unilateral or bilateral cochlear implants were prospectively included in the study. Electrode placement and insertion angles were studied in 55 Cochlear^® implants (16 straight arrays and 39 perimodiolar arrays), using either CBCT or X-ray imaging. CBCT or X-ray imaging were scheduled when the children were leaving the recovery room. We recorded intraoperative and postoperative neural response telemetry threshold (T-NRT) values, intraoperative and postoperative electrode impedance values, as well as behavioral T (threshold) and C (comfort) levels on electrodes 1, 5, 10, 15 and 20.

RESULTS

CBCT imaging was feasible without any sedation in 24 children (60%). Accidental scala vestibuli insertion was observed in 3 out of 24 implants as assessed by CBCT. The mean insertion angle was 339.7°±35.8°. The use of a perimodiolar array led to higher angles of insertion, lower postoperative T-NRT, as well as decreased behavioral T and C levels. We found no significant effect of either electrode array position or angle of insertion on electrophysiological data.

CONCLUSION

CBCT appears to be a reliable tool for anatomical assessment of young children with CIs. Intracochlear position had no significant effect on the electrically evoked compound action potential (ECAP) threshold. Our CBCT protocol must be improved to increase the rate of successful investigations.

The importance of electrode location in cochlear implantation

Objectives

As indications for cochlear implantation have expanded to include patients with more residual hearing, increasing emphasis has been placed on minimally traumatic electrode insertion. Histopathologic evaluation remains the gold standard for evaluation of cochlear trauma, but advances in imaging techniques have allowed clinicians to determine scalar electrode location in vivo. This review will examine the relationship between scalar location of electrode arrays and audiologic outcomes. In addition, the impact that surgical approach, electrode design, and insertion depth have on scalar location will be evaluated.

Data Sources: PubMed literature review

Review Methods: A review of the current literature was conducted to analyze the relationship between scalar location of cochlear implant electrode arrays and speech perception outcomes. Further, data were reviewed to determine the impact that surgical variables have on scalar electrode location.

Results

Electrode insertions into the scala tympani are associated with superior speech perception and higher rates of hearing preservation. Lateral wall electrodes, and round window/extended round window approaches appear to maximize the likelihood of a scala tympani insertion. It does not appear that deeper insertions are associated with higher rates of scalar translocation.

Conclusion

Superior audiologic outcomes are observed for electrode arrays inserted entirely within the scala tympani. The majority of clinical data demonstrate that lateral wall design and a round window approach increase the likelihood of a scala tympani insertion.

Level of Evidence

N/A.

Intraoperative Electrophysiologic Variations Caused by the Scalar Position of Cochlear Implant Electrodes

OBJECTIVE

The position of cochlear implant (CI) electrodes in the cochlea is fundamental for the interaction between the implant and the neurons of the spiral ganglion. The scalar position of the electrode in the cochlea is assumed to be an important parameter for the clinical outcome. In our study, the intraoperative electrophysiologic characteristics in dependence of the position of CI electrodes in the scala tympani or in the scala vestibuli after scalar change should be determined.

MATERIALS AND METHODS

The intraoperative impedances and neural response telemetry (NRT) data of 23 patients implanted with a Nucleus Advance Contour (Cochlear Pty, Sydney, Australia) electrode were recorded. One CI surgeon and two radiologists evaluated the electrode array's position independently radiologically by flat-panel tomography. Results from 17 patients with the electrode positioned in the scala tympani and six patients with the electrode changing intraoperatively from the tympanic into the vestibular scala were retrospectively analyzed.

RESULTS

We found a statistically significant difference with an NRT threshold-based ratio for the groups. An estimation of the (radiologically confirmed) scalar position based on the NRT ratio was possible retrospectively.

CONCLUSION

The evaluation of specific intraoperative electrophysiologic data allowed separating between a regular and an irregular (i.e., scalar changing) position of CI electrodes. This noninvasive methodology can support the postoperative radiologic evaluation of the CI electrode array position.

Radiation dose reduction in postoperative computed position control of cochlear implant electrodes in lambs - An experimental study

OBJECTIVE

Cochlear implants (CI) are standard treatment for prelingually deafened children and postlingually deafened adults. Computed tomography (CT) is the standard method for postoperative imaging of the electrode position. CT scans accurately reflect electrode depth and position, which is essential prior to use. However, routine CT examinations expose patients to radiation, which is especially problematic in children. We examined whether new CT protocols could reduce radiation doses while preserving diagnostic accuracy.

METHODS

To investigate whether electrode position can be assessed by low-dose CT protocols, a cadaveric lamb model was used because the inner ear morphology is similar to humans. The scans were performed at various volumetric CT dose-indexes CTDIvol)/kV combinations. For each constant CTDIvol the tube voltage was varied (i.e., 80, 100, 120 and 140kV). This procedure was repeated at different CTDIvol values (21mGy, 11mGy, 5.5mGy, 2.8mGy and 1.8mGy). To keep the CTDIvol constant at different tube voltages, the tube current values were adjusted. Independent evaluations of the images were performed by two experienced and blinded neuroradiologists. The criteria diagnostic usefulness, image quality and artifacts (scaled 1-4) were assessed in 14 cochlear-implanted cadaveric lamb heads with variable tube voltages.

RESULTS

Results showed that the standard CT dose could be substantially reduced without sacrificing diagnostic accuracy of electrode position. The assessment of the CI electrode position was feasible in almost all cases up to a CTDIvol of 2-3mGy. The number of artifacts did not increase for images within this dose range as compared to higher dosages. The extent of the artifacts caused by the implanted metal-containing CI electrode does not depend on the radiation dose and is not perceptibly influenced by changes in the tube voltage. Summarizing the evaluation of the CI electrode position is possible even at a very low radiation dose.

CONCLUSIONS

CT imaging of the temporal bone for postoperative electrode position control of the CI is possible with a very low and significantly radiation dose. The tube current-time product and voltage can be reduced by 50% without increasing artifacts. Low-dose postoperative CT scans are sufficient for localizing the CI electrode.

Scalar localization by cone-beam computed tomography of cochlear implant carriers: a comparative study between straight and periomodiolar precurved electrode arrays

OBJECTIVE

To compare the incidence of dislocation of precurved versus straight flexible cochlear implant electrode arrays using cone-beam computed tomography (CBCT) image analyses.

STUDY DESIGN

Consecutive nonrandomized case-comparison study.

SETTINGS

Tertiary referral center.

PATIENTS

Analyses of patients' CBCT images after cochlear implant surgery.

INTERVENTION(S)

Precurved and straight flexible electrode arrays from two different manufacturers were implanted. A round window insertion was performed in most cases. Two cases necessitated a cochleostomy. The patients' CBCT images were reconstructed in the coronal oblique, sagittal oblique, and axial oblique section.

MAIN OUTCOME MEASURES

The insertion depth angle and the incidence of dislocation from the scala tympani to the scala vestibuli were determined.

RESULTS

The CBCT images and the incidence of dislocation were analyzed in 54 patients (61 electrode arrays). Thirty-one patients were implanted with a precurved perimodiolar electrode array and 30 patients with a straight flexible electrode array. A total of nine (15%) scalar dislocations were observed in both groups. Eight (26%) scalar dislocations were observed in the precurved array group and one (3%) in the straight array group. Dislocation occurred at an insertion depth angle between 170 and 190 degrees in the precurved array group and at approximately 370 degrees in the straight array group.

CONCLUSION

With precurved arrays, dislocation usually occurs in the ascending part of the basal turn of the cochlea. With straight flexible electrode arrays, the incidence of dislocation was lower, and it seems that straight flexible arrays have a higher chance of a confined position within the scala tympani than perimodiolar precurved arrays.