CT and MR Imaging Cochlear Distance Measurements May Predict Cochlear Implant Length Required for a 360° Insertion

Abstract

A preoperative prediction of the 360 degrees point insertion depth would aid the planning of electric acoustic stimulation (EAS) implantation. The purpose of this study was to establish whether the distance between the round window and the opposite cochlear wall on CT or MR imaging may be used to predict the length of a cochlear implant electrode array required to be inserted to the 360 degrees point of the basal turn. CT and MR imaging data were studied in 19 patients undergoing cochlear implantation. Distances were measured between the round window and the opposite outer cochlear wall on an oblique paracoronal reformatted image. Adjusted distance measurements were applied to a spiral function to estimate the length of an electrode array extending between the round window entry point and the 360 degrees point. This was compared with measurements of implant length to this insertion depth on postoperative CT. Intraobserver reproducibility for each of the 2 observers was r = 0.85/0.55 for CT and r = 0.87/0.67 for MR imaging. Interobserver reproducibility was r = 0.68 for CT and r = 0.84 for MR imaging. There was no bias between CT and MR imaging measurements, with a mean difference of less than 0.1 mm. CT and MR imaging estimates markedly correlated with the actual length of the electrode array extending to the 360 degrees insertion depth (SD between the estimated and actual length was 0.84 mm for CT and 0.87 mm for MR imaging). CT and MR imaging measures of cochlear distance (CD) were used to predict insertion depths to 360 degrees , and these were markedly concordant with the actual length of the electrode array required to reach this point. MR imaging measurements were more precise and similar in accuracy to those obtained with CT.