Morphometric Analysis of Temporal Bone Radiology for Cochlear Implant Candidacy

Abstract

Cochlear Implantation (CI) is a well-accepted treatment for severe-to-profound sensorineural hearing loss, refractory to conventional hearing amplification. Pre-operative Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) play pivotal roles in patient selection to rule out findings that preclude surgery or identify conditions that may impact the surgical procedure. A prospective study was carried out in a tertiary care center over three years, from January 2020 to January 2023. One hundred and ninety (380 ears) patients' High-Resolution Computed Tomography (HRCT) studies of the temporal bone and MRI scans of the auditory pathways were analyzed. A reporting format was followed which was devised by a team of senior implant surgeons and senior neuro-radiologists. Our study aims to provide a comprehensive radiologic protocol for CI candidacy including normative data for the essential morphometrics in the Indian setting.

Keywords: Cochlear implant; Morphometrics; Radiologic protocol.

Fig. 1

A Axial section of the temporal bone. B Sagittal section of the temporal bone. The coronal projection (Pink line in A and B) is aligned along the basal turn of right cochlea on the axial A and sagittal B images bringing the entire cochlea into focus on oblique coronal plane C. Pink line – Coronal projection; Blue line – Sagittal projection; Yellow line: Axial Projection

Fig. 2

Cochlear Base Width. In the coronal image of cochlea the distance from the midpoint of round window through the center of the cochlea to the other end gives the Cochlear Base Width

Fig. 3

Cochlear height is measured in the oblique sagittal image as distance from the apical turn to the cochlear aperture (at the midpoint of basal turn)

Fig. 4

Width of basal, middle and apical turns of cochlea are taken separately from inner-to-inner margin in the oblique coronal projection

Fig. 5

a On the axial images at the level of round window niche the distance of mastoid segment of the facial nerve from the posterior annulus of tympanic membrane is obtained. b The coronal image is aligned based on the axial images at the level of the round window niche A. On the coronal projection the distance of facial nerve from the mastoid cortex is measured B. Pink line – Coronal projection; Blue line – Sagittal projection

Fig. 6

In axial projection, cochlear aperture width is measured

Fig. 7

The coronal projection is aligned in line with the internal auditory canal and the diameter of internal auditory canal is taken closer to the fundus

Fig. 8

The oblique sagittal C image showing the contents of the IAC obtained by aligning the sagittal plane on the axial image A and axial plane on the coronal image B. Pink line – Coronal projection; Blue line – Sagittal projection; Yellow line: Axial Projection

Fig. 9

The angle between cochlear basal turn plane (blue line) and head midsagittal plane (red arrow)

Fig. 10

The angle between cochlear basal turn plane (blue line) and a line drawn along the proposed intra-operative axis of surgery, through the mastoid, facial recess and round window, running as near as possible to the posterior wall of the external auditory canal. (Yellow arrow)

Fig. 11

Round window orientation. On axial HRCT images of the temporal bone at the level of the round window niche the orientation of the axes of the basal turn of the cochlea (dashed red line) and posterior wall of the external auditory canal (dashed yellow line) is determined. Parallel orientation of these axes are associated with easier round window accessibility and intersecting axes of these structures are usually associated with difficult access

Fig. 12

Lateralization of the sigmoid sinus. On axial HRCT images of the temporal bone types of sigmoid sinus is determined based on the extension of the sigmoid plate beyond the axis of the posterior SCC (red line), the tympanic segment of the facial nerve (yellow line) and the malleal-incudal axis (blue line). Type 1: located medial to the long axis of the posterior SCC; type 2: located between the long axis of the posterior SCC and the tympanic segment of the facial nerve; type 3: located between the long axis of the tympanic segment of the facial nerve and the malleal-incudal axis and type 4: located beyond the malleal-incudal axis