Evaluation of CI electrode position from imaging: comparison of an automated technique with the established manual method

Abstract

Background: A manual evaluation of the CI electrode position from CT and DVT scans may be affected by diagnostic errors due to cognitive biases. The aim of this study was to compare the CI electrode localization using an automated method (image-guided cochlear implant programming, IGCIP) with the clinically established manual method.

Methods: This prospective experimental study was conducted on a dataset comprising N=50 subjects undergoing cochlear implantation with a Nucleus® CI532 or CI632 Slim Modiolar electrode. Scalar localization, electrode-to-modiolar axis distances (EMD) and angular insertion depth (aDOI) were compared between the automated IGCIP tool and the manual method. Two raters made the manual measurements, and the interrater reliability (±1.96·SD) was determined as the reference for the method comparison. The method comparison was performed using a correlation analysis and a Bland-Altman analysis.

Results: Concerning the scalar localization, all electrodes were localized both manually and automatically in the scala tympani. The interrater differences ranged between ±0.2 mm (EMD) and ±10° (aDOI). There was a bias between the automatic and manual method in measuring both localization parameters, which on the one hand was smaller than the interrater variations. On the other hand, this bias depended on the magnitude of the EMD respectively aDOI. A post-hoc analysis revealed that the deviations between the methods were likely due to a different selection of mid-modiolar axis.

Conclusions: The IGCIP is a promising tool for automated processing of CT and DVT scans and has useful functionality such as being able to segment the cochlear using post-operative scans. When measuring EMD, the IGCIP tool is superior to the manual method because the smallest possible distance to the axis is determined depending on the cochlear turn, whereas the manual method selects the helicotrema as the reference point rigidly. Functionality to deal with motion artifacts and measurements of aDOI according to the consensus approach are necessary, otherwise the IGCIP is not unrestrictedly ready for clinical use.

Keywords: Angular depth of insertion; Cochlear implant; EMD; Electrode localization; Electrode-to-modiolus-distance; IGCIP; aDOI; imaging.

Fig. 1

Schematic illustration of spatial parameters to be measured: aDOI, angle of insertion depth, relative to the chord produced between the mid-modiolar axis “MOD” (helicotrema) and the center of the round window “RW” (0°); EMD, distance from the center of an electrode “E” to MOD

Fig. 2

(left) Scatter diagram of electrode-to-modiolar axis distances (EMD) measured manually and with the automatic tool; (right) Bland-Altman plot of EMD with mean (bias) and ±1.96 standard deviation (limits of agreement, LoA) differences between the automatic and manual method. EMD was analyzed from N=50 electrode arrays at each even-numbered electrode contact from E2 to E22, as well as at E1

Fig. 3

(left) Scatter diagram of insertion depth angles (aDOI) measured manually and with the automatic tool; (right) Bland-Altman plot of aDOI with mean (bias) and ±1.96 standard deviation (limits of agreement, LoA) differences between the automatic and manual method. aDOI was analyzed from N=50 electrode arrays at each even-numbered electrode contact from E2 to E22, as well as at E1

Fig. 4

Polar plot, illustrating the mean value of EMD (radius) and aDOI (angle) with the automatic and manual method for each of the electrodes analyzed (E1, E2, E4, E6, E8, E10, E12, E14, E16, E18, E20, E22). The center of the polar plot represents the mid-modiolar axis (helicotrema) of the cochlea

Fig. 5

Difference pattern effect from selecting different position of the mid-modiolar axis in a hypothetical CI electrode insertion

Fig. 6

Differences between the automatic and manual method after eliminating the systematic differences may be addressed by selecting a different location of the mid-modiolar axis. Bland-Altman plots of EMD (left) and aDOI (right) with mean (bias) and ±1.96 standard deviation (limits of agreement, LoA) differences between the automatic and manual method. EMD and aDOI were analyzed from N=50 electrode arrays at each even-numbered electrode contact from E2 to E22, as well as at E1