10Reading the Malformed Ear: CT and MRI
CT and MRI are not rivals but partners. CT draws the bone - the cochlear turns, the modiolus, the width of the canal that carries the nerve. MRI fills the labyrinth with bright fluid and, on the right oblique slice, shows whether a cochlear nerve is actually there. Neither alone defines candidacy in a malformed ear; together they do.
FTwo modalities, two questions
High-resolution CT answers the bony question: how many cochlear turns, is the modiolus present, are the interscalar septa intact, how wide is the vestibular aqueduct, how wide is the bony cochlear nerve canal, where does the facial canal run. MRI answers the soft-tissue question: is the membranous labyrinth filled with normal fluid signal (bright on heavily T2-weighted CISS/FIESTA), and - decisively - is a cochlear nerve present in the internal auditory canal. Inner-ear malformations are found in roughly 20 percent of congenitally deaf children imaged with HRCT, so a structured protocol matters; many anomalies are subtle and missed on a quick read.[2012][2017]
TWhat CT measures
Cochlear turns and partition: a normal cochlea has 2.5-2.75 turns with a coiled modiolus and interscalar septa; loss of the apical septum with a confluent apex is incomplete partition type II (Mondini), a near-empty cochlea with an absent modiolus is IP-III, and a featureless sphere is a common cavity. Vestibular aqueduct: enlarged by Valvassori-Clemis criteria when wider than 1.5 mm at the midpoint (or by Cincinnati criteria, midpoint > 0.9 mm / operculum > 1.9 mm); EVA is the single commonest bony anomaly. Bony cochlear nerve canal (BCNC): the channel from the IAC fundus into the modiolus; a width below about 1.4 mm flags possible cochlear-nerve deficiency and prompts close MRI scrutiny. Facial-nerve canal and ossicles must also be tracked on CT - an anomalous facial course and ossicular anomalies travel with severe dysplasias and CHARGE.[2012][2017]
TWhat MRI sees that CT cannot
Heavily T2-weighted thin-section sequences (CISS / FIESTA / DRIVE) make perilymph and endolymph bright, outlining the membranous labyrinth and revealing fibrosis or ossification that CT can miss as a normal-looking but fluid-empty turn. The parasagittal-oblique reconstruction perpendicular to the IAC is the key view: it splits the IAC into four nerves - facial (antero-superior), cochlear (antero-inferior), superior and inferior vestibular - so the surgeon can confirm a cochlear nerve is present and judge its calibre relative to the facial nerve. A normal cochlear nerve at the fundus is roughly 1.0-1.2 mm and should be at least as thick as the facial nerve; thinner-than-facial is hypoplasia, absent is aplasia. MRI also screens the brain for the central anomalies (white-matter, cortical, vestibulocochlear-pathway) that affect rehabilitation expectations.[2012][2011]
CFrom images to a surgical plan
A reporting checklist should assign a Sennaroglu type, state modiolus status, BCNC width, VA width, cochlear-nerve presence and calibre, facial-nerve course and any cochleostomy/round-window accessibility. Imaging flags the gusher risk before incision: an absent modiolus or an IAC-cochlea communication (IP-III, IP-I) predicts a perilymph/CSF gusher, and these are read off the CT/MRI, not discovered at surgery. A discrepancy between CT and MRI is itself information: a CT-normal-looking cochlea with no nerve on MRI is a non-candidate for a standard cochlear implant and a possible auditory-brainstem-implant case - the two modalities together, not either alone, define candidacy.[2017][2004][2012]
What is the single most important imaging step still required before finalising candidacy?
Which structure is best assessed on a parasagittal-oblique MRI reconstruction through the internal auditory canal?
A vestibular aqueduct is considered enlarged by the Valvassori-Clemis criterion when its midpoint width exceeds:
On CT, an absent modiolus with a wide communication between the internal auditory canal and the cochlear basal turn should prompt the surgeon to anticipate: