Precise evaluation of the postoperative cochlear duct length by flat-panel volume computed tomography - Application of secondary reconstructions

The Photon-Counting CT Enters the Field of Cochlear Implantation: Comparison to Angiography DynaCT and Conventional Multislice CT

INTRODUCTION

Cochlear duct length (CDL) measurement plays a role in the context of individualized cochlear implant (CI) surgery regarding an individualized selection and implantation of the CI electrode carrier and an efficient postoperative anatomy-based fitting process. The level of detail of the preoperative temporal bone CT scan depends on the imaging modality with major impact on CDL measurements and CI electrode contact position determination. The aim of this study was to evaluate the accuracy of perioperative CDL measurements and electrode contact determination in photon-counting CT (PCCT).

METHODS

Ten human fresh-frozen petrous bone specimens were examined with a first-generation PCCT. A clinically applicable radiation dose of 27.1 mGy was used. Scans were acquired before and after CI insertion. Postoperative measurement of the CDL was conducted using an otological planning software and 3D-curved multiplanar reconstruction. Investigation of electrode contact position was performed by two respective observers. Measurements were compared with a conventional multislice CT and to a high-resolution flat-panel volume CT with secondary reconstructions.

RESULTS

Pre- and postoperative CDL measurements in PCCT images showed no significant difference to high-resolution flat-panel volume CT. Postoperative CI electrode contact determination was also as precise as the flat-panel CT-based assessment. PCCT and flat-panel volume CT were equivalent concerning interobserver variability.

CONCLUSION

CDL measurement with PCCT was equivalent to flat-panel volume CT with secondary reconstructions. PCCT enabled highly precise postoperative CI electrode contact determination with substantial advantages over conventional multislice CT scanners.

Accuracy of Preoperative Cochlear Duct Length Estimation and Angular Insertion Depth Prediction

OBJECTIVE

In cochlear implantation with flexible lateral wall electrodes, a cochlear coverage of 70% to 80% is assumed to yield an optimal speech perception. Therefore, fitting the cochlear implant (CI) to the patient's individual anatomy has gained importance in recent years. For these reasons, the optimal angular insertion depth (AID) has to be calculated before cochlear implantation. One CI manufacturer offers a software that allows to visualize the AID of different electrode arrays. Here, it is hypothesized that these preoperative AID models overestimate the postoperatively measured insertion angle. This study aims to investigate the agreement between preoperatively estimated and postoperatively measured AID.

STUDY DESIGN

Retrospective cross-sectional study.

SETTING

Single-center tertiary referral center.

PATIENTS

Patients undergoing cochlear implantation.

INTERVENTION

Preoperative and postoperative high-resolution computed tomography (HRCT).

MAIN OUTCOME MEASURES

The cochlear duct length was estimated by determining cochlear parameters ( A value and B value), and the AID for the chosen electrode was (i) estimated by elliptic circular approximation by the software and (ii) measured manually postoperatively by detecting the electrode contacts after insertion.

RESULTS

A total of 80 HRCT imaging data sets from 69 patients were analyzed. The mean preoperative AID estimation was 662.0° (standard deviation [SD], 61.5°), and the mean postoperatively measured AID was 583.9° (SD, 73.6°). In all cases (100%), preoperative AID estimation significantly overestimated the postoperative determined insertion angle (mean difference, 38.1°). A correcting factor of 5% on preoperative AID estimation dissolves these differences.

CONCLUSIONS

The use of an electrode visualization tool may lead to shorter electrode array choices because of an overestimation of the insertion angle. Applying a correction factor of 0.95 on preoperative AID estimation is recommended.

[Cochlear Implantation: Evaluation of Cochlear Duct Length (CDL)]

Personalized care in the context of cochlear implantation is becoming increasingly important. Choosing the right electrode could improve speech understanding. The measurement of the cochlear length plays an important role: preoperatively, in order to select a suitable electrode length; postoperatively, on the one hand to check the correct electrode position, on the other hand to enable anatomically based fitting of the electrode contacts. Of the various possible localizations of the CDL measurements within the cochlear turns, the one on the organ of Corti (CDL_OC) is the most frequently used and clinically most important. In the CDL measurement, a direct and indirect evaluation can be distinguished. There is also the possibility of reconstructing and measuring the CDL in 3D and calculating it mathematically, e.g. using spiral equations. In this context, measurements based on radiological imaging are gaining increasing importance. Therefore, if there is the possibility of performing higher-resolution imaging, this should be strived preoperatively in order to enable the most precise possible procedure and thus a good outcome. Otological planning software can help to create an interface between new findings regarding CDL measurement and higher-resolution imaging for an individualized cochlear implantation.

[Influence of cochlear parameters on the current practice in cochlear implantation : Development of a concept for personalized medicine. German version]

Since the introduction of cochlear implants into clinical routine, the interest in measuring cochlear parameters, particularly the cochlear duct length (CDL) has increased, since these can have an influence on the correct selection of the electrode. On the one hand, coverage of an optimal frequency band is relevant for a good audiological result, and on the other hand, cochlear trauma due to too deep insertion or displacement of the electrode must be avoided. Cochlear implants stimulate the spiral ganglion cells (SGC). The number of SGC and particularly their distribution can also have an influence on the function of a cochlear implant. In addition, the frequency assignment of each electrode contact can play a decisive role in the postoperative success, since the frequency distribution of the human cochlea with varying CDL shows substantial interindividual differences. The aim of this work is to provide an overview of the methods used to determine the cochlear parameters as well as of relevant studies on the CDL, the number and distribution of SGZ, and the frequency assignment of electrode contacts. Based on this, a concept for individualized cochlear implantation will be presented. In summary, this work should help to promote individualized medicine in the field of cochlear implants in the future, in order to overcome current limitations and optimize audiological outcomes.