In vivo measurements of the insertion depth of cochlear implant arrays using flat-panel volume computed tomography.

OBJECTIVE: The development of a procedure for the measurement of insertion depth angles of cochlear implant electrode arrays based on flat-panel computed tomography (FPCT) and the application of this technique to in vivo postoperative images.
BACKGROUND: The knowledge of the insertion depth angle of electrode arrays is relevant for the preservation of low-frequency residual hearing and for optimizing speech coding strategies. Until now, the angular position of electrodes was derived from 2-dimensional radiographs.
METHOD: In the present study, 3-dimensional (3D) radiographs provided by FPCT were used to determine the insertion depth according to angular electrode positions with higher accuracy. For this purpose, a new evaluation procedure was designed and applied to radiographs of 15 cochlear implant patients.
RESULTS: In contrast to 2-dimensional radiographs, the obtained 3D images show all 3 semicircular channels and therefore allow the determination of a clear reference, which is required for precise insertion angle measurements. Furthermore, the presented FPCT radiographs visualize distinct electrodes. Despite the constant length of the implanted electrode arrays, we have found a considerable variation of measured insertion depth angles, which is consistent with published observations on the variability and the gender dependence of the size of human cochleae.
CONCLUSION: FPCT provides 3D high-resolution radiographic data that enable the determination of the insertion depth angle with high accuracy and, potentially, an angle determination of individual electrodes. Therefore, this low-dose technique is especially appropriate for postoperative investigations after cochlea implantation.