Untersuchungen zur intracochleären Lage verschiedener Cochleaelektroden mittels DVT

Abstract The development of the cochlear implant (CI) over 50 years ago was considered a milestone in medicine. In order to advance this development the present thesis deals with the radiological analysis of the exact position of different cochlear implants within the cochlea. With regard to the post-operative position monitoring after CI implantation and the further development of the CI electrode array and the operation technique, this topic holds clinical and scientific importance. In the guidelines of the German Society of ENT, post-operative position monitoring after CI implantation is recommended as a standard. This is used to control the final position of the electrodes and to discover possible false insertions. As an alternative to the known methods, such as CT and X-ray, the scientific focus is currently on the digital volume tomography (DVT), also called cone beam computed tomography (CBCT) as an imaging modality. Especially for objects with high contrast differences, such as the bony structures of the temporal bone and the cochlea, the CBCT is an established imaging method. In this context, the thesis examines the CBCT's ability to display the inserted cochlear implant electrodes. Two differently manufactured CI models have been investigated with CBCT imaging. Overall, the CBCT images of 65 patients from the ENT-department of the University Hospital Marburg were examined after CI implantation. Among these, 35 patients were implanted with the CI - model by the company Cochlear® and 30 patients with the CI - model by the company MedEl®. Methodically, the insertion depth and the insertion angle of the CI were measured for each patient. The exact relationship of each electrode within the cochlea was determined by the measurement of their distance to the medial (close to the modiolus) and the lateral wall of the cochlea (distant the modiolus). The results significantly confirm that the CBCT can make an accurate statement about the detailed electrode position within the cochlea. This is evident in the comparison of the image data of the two different implant models. The CBCT depicts that the MedEl® electrodes locate closer to the lateral cochlea wall, far away from the modiolus. The Cochlear® electrodes position closer to the medial wall of the cochlea and are thus close to the modiolus (perimodiolär). Furthermore, it was shown that the electrodes of the Cochlear® model mainly come to lie in the basal turn of the cochlea. The MedEl® electrodes insert within the whole cochlea, reaching from the basal to the apical turn. Using CBCT-imaging, dissimilar positions of CI electrodes are distinctly detectable. All results correctly reflect the characteristics previously published by the CI-manufacturers by showing the potential of the CBCT in visualizing the CI. As one quality criterion for the precision of the CBCT's analysis, the cochlea diameters were measured. In accordance with anatomical studies of the cochlea, the presented results show that the measured cochlear diameters continuously decrease from the basal to the apical turn of the cochlea. This confirms the quality of the measurement method. Furthermore, the diameter of the implant electrodes served as a quality criterion, which mainly corresponded to the manufacturer's information. The error of measurement varies depending on the insertion depth of the electrodes between 1.3 %, regarding the basal electrodes, to 28 % concerning the apical electrodes. Due to its anatomical proximity to the inner ear, the CBCT-imaging of the facial nerve was investigated in this work as well. In all patients, it was possible to track the bony course of the facial nerve within the temporal bone. Moreover, the distance from the facial nerve to the cochlea, or to the CI-electrodes were measured. Furthermore, in 82 % of the cases the CBCT displayed the branching of the chorda tympani from the N. facialis. Considering the complication of iatrogenic injury to nerves as well as the accidental, unwanted stimulation of the facial nerve by a cochlear implant, the relevance of this topic becomes clear. In summary, it can be stated that with the help of CBCT, a precise measurement of the cochlea, its surrounding structures and particularly the intracochlear position of CI electrodes is possible. The CBCT therefore offers a good possibility of post-operative imaging. Based on this, the CBCT based CI imaging may offer a correlating depiction of the patient's auditory impression as far as the CI's anatomical position is concerned. In the future, CBCT imaging in correlation to subjective auditory impressions could be used to optimize the results in the patient's listening experience by developing revised electrode-models as well as operation techniques.