5Normal temporal-bone & inner-ear anatomy
You cannot recognise an abnormal cochlea until you can read a normal one, and the temporal bone packs a great deal into a few millimetres. On CT the bone window draws the otic capsule, the two-and-a-half turns of the cochlea around its central modiolus, the vestibule and three perpendicular canals, the oval and round windows and the facial canal; on heavily-T2 MRI the same structures appear inverted, as bright fluid against dark bone. The internal auditory canal has a normal size and orientation worth committing to memory, because deviations from it flag disease. And all of this anatomy is best learned through the embryology that produced it — because the malformations of the next modules are simply development stopped at successive stages. This module is the normal template.
Imaging note
Representative CT and MRI images for this chapter are being added soon. The interactive figures here are original schematic teaching diagrams; to respect copyright we do not reproduce third-party radiographs.
TAnatomy on CT
HRCT bone window depicts the otic capsule, the modiolus and cochlear aperture, the vestibule and three semicircular canals, the vestibular and cochlear aqueducts, the oval and round windows, and the fallopian (facial) canal — alongside the carotid canal and the air spaces of the mastoid. These are the landmarks read on every slice.[2022]
CAnatomy on MRI
On T2/CISS the normal cochlea shows bright scala tympani and scala vestibuliacross the basal, middle and apical turns, and the modiolus, lamina cribrosa, Rosenthal's canal and interscalar septa are identifiable. The cochlear long axis lies about 45–50° to the midsagittal plane — the geometry behind the angled projections used to confirm electrode position later.
CNormal IAC dimensions
Worth memorising: the normal internal auditory canal is about 2–8 mm wide, up to ~8 mm high, ~10 mm long, oriented ~60–65°. A canal <2 mm (narrow) or >8 mm (widened) is a flag — though, as Module 8 stresses, canal size alone never proves the nerve.
TThe embryology that grounds it
Inner-ear development grounds the reading of every malformation. The otic placode appears at week 3, the otocyst by weeks 4–5, the cochlear duct coils to 2.5 turns by weeks 8–10, the organ of Corti completes ~week 25, and the otic capsule ossifies from 14 centres over weeks 15–23. The classifications of the next modules read each deformity as an arrest at one of these stages.
What grounds the reading of malformations?
By when has the cochlear duct coiled to its 2.5 turns?
Why learn malformations through embryology?