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

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.

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.

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.

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.

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.