15Post-operative electrode position
The operation ends with a question imaging must answer: where exactly did the electrode end up? A good array reads as a smooth, evenly-spaced spiral sitting in scala tympani, but two complications hide in the detail — the electrode slipping from scala tympani into scala vestibuli, and the tip doubling back on itself — both judged against a single bony landmark, the lamina spiralis ossea. A simple plain film confirms it if the projection is aligned to the cochlea's tilted axis; counting the rings then needs to know which device was used. Where finer detail is wanted, cone-beam and high-speed flat-detector CT now give high-resolution, low-artefact, low-dose pictures of scalar position. This module reads the array once it is in.
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.
TA good placement
A well-placed array sits in scala tympani / the initial middle turn as a smooth wide-arc spiral with evenly spaced electrodes and no kinks; the extra non-active electrodes medial to the round window are countable on film. Insertion depth is quantified as the angular depth (~420–600°).
CDislocation & tip fold-over
The two complications sought are scalar dislocation (the array crossing from scala tympani into scala vestibuli, linked to poorer outcomes) and tip fold-over — both referenced to the lamina spiralis ossea, which is why visualising that landmark is a graded image-quality target. Imaging also detects partial/over-insertion, kinking, and misplacement into the vestibule, canal, carotid canal or eustachian tube.
CPlain-film projections
Plain radiography is the simple first-line confirmation when the projection aligns the central ray with the modiolar axis (~45–50° to the midline). The modified Stenvers view is the commonest; off-axis the spiral foreshortens and the rings cannot be reliably counted. Counts differ by device — Advanced Bionics 16, Cochlear 22, Med-El 12 — so knowing the implant aids the count (Devices chapter).
CCBCT & flat-detector CT
Cone-beam CT is the preferred low-dose, high-resolution, low-metal-artefact option for confirming scalar position (~75–300 µm voxels, ~40 s, large dose reduction vs MSCT). Flat-detector CT on angiography systems is the widely-available standard for post-op control, and high- speed FD-CT (7/9/14 s protocols) cuts scan time to beat motion artefact — though dose rises steeply with image quality.[2022]
What complication is this, and why does it matter?
Which two complications does post-op imaging chiefly seek?
Why does plain-film projection geometry matter?