High resolution evaluation of various cochlear implant electrodes’ scalar position by 3T MRI

Objective: The estimation of scalar electrode position is a central point of quality control during the cochlear implant procedure. Ionic radiation is a disadvantage of commonly used radiologic estimation of electrode position. Recent developments in the eld of cochlear implant magnets, implant receiver magnet position, and MRI sequence usage allow the postoperative evaluation of inner ear changes after cochlear implantation. The aim of the present study was to evaluate the position of lateral wall and modiolar cochlear implant electrodes using 3T MRI scanning. Material and Methods: In a prospective study, we evaluated 20 patients (10x Med-El Flex 28; 5x HFMS AB and 5x SlimJ AB) with a 3T MRI and a T2 2D Drive MS sequence (voxel size: 0.3 x 0.3 x 0.9 mm) for the estimation of the intracochlear position of the cochlear implant electrode. Results: In all cases, MRI allowed a determination of the electrode position in relation to the basilar membrane. This observation made the estimation of 19 scala tympani electrode positions and a single case of electrode translocation possible. Conclusion: 3T MRI scanning allows the estimation of lateral wall and modiolar electrode intracochlear scalar positions.


Introduction
Quality control in modern medicine plays an increasingly important role and follows structured evaluation pathways. 1 In otology, multiple methods and stages of quality control have been discussed and implemented. 2 A central point in cochlear implantation quality control is based on the estimation of electrode inside the cochlea, originally performed with a 2D X-ray. 3 [5][6][7] Based on these ndings, quality control was not only a tool for the evaluation of surgeons' abilities in the theater, 8 but a tool for the evaluation of different electrodes as well. 9 The radiological in vivo results were con rmed by temporal bone ndings 10 and histological comparisons. 11 The disadvantages of ionic radiation led to the evaluation of electrophysiological tools for the estimation of electrode translocations performed by a neural response telemetry (NRT) ratio 12 with limitations of electrode design and manufacturer brand.
The technique of an overlay of the postoperative digital volume tomography/ computed tomography (DVT/CT) and the preoperative magnet resonance imaging (MRI) integrated MRI for the rst time into the evaluation of electrode position, 13  The aim of the present study is to show that an estimation of electrode translocation can be performed with different electrode designs using a 3T MRI.

Materials And Methods
In this prospective study, 20 patients received cochlear implantation between January and December 2020. In all cases, the cochlear implant receiver magnet was positioned 8 cm or more behind the external auditory canal. 15 In addition to regular postoperative DVT for the estimation of electrode position, a 3T MRI scanning was performed.

Results
Evaluation of cochlear implant electrode by 3T MRI showed 2 visual indicators of electrode position in the axial plain. For the basal turn, a diminishing signal indicates the electrode localisation. At the lateral portion of the cochlea, each electrode was positioned depending on its design: either laterally, without any further lateral T2 signal ( Fig. 1a and 1b), or more medially positioned with a T2 signal (Fig. 3). Lateral diminishing signal varied depending on electrode design: Flex 28 was round, and SlimJ laterally atter. As a second indicator, a basilar membrane was visible. This basilar membrane diminishing signal allowed de nitive estimation of the electrode location in the scala tympani or scala vestibuli.
At the rst turn, a design-speci c difference of the diminishing signal of the electrodes was obvious. We were able to differentiate between a round signal diminishing and a atter signal diminishing ( Fig. 2a and Fig. 2b). The basilar membrane signal ( Fig. 2a and 2 b) and the localization of the uid signal diminishing indicate scala tympani positions at the rst turn for these two types of lateral wall electrodes.
Modiolar electrode characteristics allowed us to visualize a lateral T2 signal from the electrode at the basal turn and a basilar membrane pattern ( Fig. 3 and 4). This basilar membrane pattern allowed us to locate the electrode in the scala tympani de nitely.
At the rst turn, the estimation of the electrode position depends on the estimation of the basilar membrane and the diminishing localisation of the T2 signal by the electrode. In Fig. 5, diminishing occurred above the T2 signal and the basilar membrane signal, indicating uid. This indicated the pattern of an electrode translocated into the scala vestibuli. Fig.7 of the same patient (No.10) con rms the irregular high position of the electrode in the rst turn. In Fig. 3 and 4, the signal diminished above the electrode and the basilar membrane, indicating an electrode position in the scala tympani. Fig.6a and b are exemplary DVT of regular electrode positions at the oor of the basal and the rst turn.
Independent evaluation of electrode positions by one surgeon and a neuroradiologist using DVT/ CT and MRI con rmed the estimated MRI positions in all and in DVT/CT cases in almost all cases (Test: Quadratic-Weighted Cohen's κ with 95% con dence intervals; MRI: Kappa= 1.000; SE of kappa = 0.000, 95% con dence interval: From 0.704 to 1.00 ; DVT: Kappa= 0.898;SE of kappa = 0.099, 95% con dence interval: From 0.704 to 1.00).
Limitations of the study persist in terms of MRI resolution. Although voxel size of 0.3 mm could be used, a free reconstruction related to a non 3D sequence (building of iso voxel) was not possible. A 3 D MRI sequence commonly generates artifacts, which do not allow a visualization of the IAC and cochlea.

Discussion
Quality control plays an important role in the structuration of medical pathways. 1 Even in otology, quality control is well known. 2 In the surgical portion of a cochlear implant procedure, electrode placement is of high importance for the audiological outcome. 7 The value of different radiological techniques for estimation of electrode positions lies in the evaluation of the surgeon ability of positioning and the properties of the electrode design to ensure the desired target position is met. Different radiological techniques have been shown to determine electrode position in 2D 3 or 3D 4 patterns.
The combination of postoperative DVT with an overlapping preoperative MRI allows electrode positions to be estimated relative to the MRI. 13 The disadvantage of all these techniques is the occurrence of ionic radiation. Electrophysiological techniques such as the NRT ratio 12  We would like to thank S. Trees for her excellent radiological support.

Statement of ethics:
The data used to support this study's ndings are available from the corresponding author upon request.
The study was approved by the institutional review board of the Klinikum Bielefeld, Germany (IRB-klibi-HNO-2020/001) and the ethical board of the University Münster (2019-135-f-S). Patients gave written informed consent for the use of their clinical records in this study. The study was conducted in accordance with the World Medical Association Declaration of Helsinki.
Disclosure Statement: The corresponding author received grants and travel support in the last ve years from Advanced Bionics, MED-El and Cochlear.
Data availability statement: The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.