4MRI sequences & the oblique-sagittal nerve view
If CT is the map of the bone, MRI is the map of what lives inside it. With the right sequence the fluid-filled labyrinth lights up like a luminous cast, the cochlear and vestibular nerves stand out as dark threads against bright cerebrospinal fluid, and the earliest fibrosis that will one day become bone shows up as a quiet loss of signal long before CT sees anything. The single most important view is a thin oblique-sagittal slice cut across the internal auditory canal, which lays the four nerves out face-on so the cochlear nerve can be measured against the facial — the check that decides whether there is a nerve for the implant to stimulate at all. This module covers the sequences and planes that make the soft tissue visible.
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.
TWhat only MRI shows
MRI is the ideal way to see what CT cannot: the membranous labyrinth, the cochlear and vestibular nerves, the IAC contents, and pre-ossific fibrosis — all without ionising radiation.[2022]This is why MRI is the screening modality of the Rational Checklist (Module 1): its fishbone read works from the brain inward to the cochlear aperture and the nerve, settling most of candidacy before HRCT is ever considered.[2026]
CHeavily-T2 sequences
The workhorses are heavily-T2-weighted 3D gradient-echosequences — CISS (Siemens) and FIESTA-C (GE) — which make CSF, perilymph and endolymph uniformly bright against dark nerves, displaying all three cochlear turns, the vestibule and the canals; thin-section fast-spin-echo T2 is the older alternative. Because cortical bone and air are signal-void, the labyrinth appears as a bright cast that can be 3D-reconstructed.
CThe oblique-sagittal nerve view
Beyond routine axial and coronal planes, the key plane is an oblique- sagittal slice perpendicular to the IAC nerves, giving an en-face cross-section of all four: facial anterosuperior, cochlear anteroinferior, and the superior and inferior vestibularnerves posteriorly. This is the view used to compare the cochlear nerve's calibre with the facial nerve and the other side — the basis of the nerve assessment in Module 8.[2006]
CField strength & gadolinium
Both 1.5 T and 3 T are used; 3 T gives higher signal-to-noise and thinner sections, aiding fine cochlear-nerve assessment. Gadolinium is rarely needed for the routine work-up but identifies labyrinthitis (enhancement) and screens for a retrocochlear schwannoma or meningioma, which enhance strongly — the retrocochlear question CT cannot answer.
Which imaging best answers this?
What do heavily-T2 sequences (CISS/FIESTA) show?
What is the oblique-sagittal nerve view for?