3HRCT technique & temporal-bone planes
A temporal-bone CT looks deceptively like any other scan, but the way it is acquired and displayed is specific to the tiny, dense structures it must resolve. Thin sections and a bone reconstruction algorithm sharpen the otic capsule and ossicles; near-isotropic voxels mean a single axial acquisition can be reformatted, distortion-free, into coronal, sagittal, oblique and 3D views. The 'axial' plane itself is not the true horizontal — it is tilted along the lateral semicircular canal — and the image is read on a wide bone window that makes dense detail leap out while flattening soft-tissue contrast. Knowing what that contrast choice reveals, and what it hides, defines exactly where MRI must take over. This module is about making and reading the CT.
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.
TThin sections, isotropic voxels
HRCT uses thin sections (0.6–0.7 mm, up to 1 mm) with a high-spatial-frequency bone reconstruction algorithm on multidetector scanners, covering the whole temporal bone in under a minute with near-isotropic ~0.35 mm voxels. Isotropy is the key gift: one axial scan reformats distortion-free into coronal, sagittal, oblique and 3D-rendered planes — no re-scan needed.[2022]
CThe tilted temporal-bone plane
The temporal-bone “axial” plane is not true horizontal: it lies along Reid's baseline, tilted ~30°, in the plane of the lateral semicircular canal; the coronal plane is perpendicular to it. Reading from the correct plane is what makes the landmarks recognisable.
CThe bone window
The same data display very differently depending on the window. The temporal bone is read on a wide bone window (level ~+300 HU, width ~2000) so the otic capsule, ossicles and facial canal stand out. Reference values anchor interpretation: air −1000, fat ~−75, water/CSF 0, grey matter ~40, bone ~+1000.
CWhat CT cannot show
CT's strengths define its blind spots. It cannot characterise a soft-tissue density — cholesteatoma, tumour, granulation and fluid all look alike — it cannot reliably show early pre-ossific fibrosis, it cannot confirm an absent cochlear nerve, and it cannot assess retrocochlearpathology. Each of those is exactly what the next module's MRI is for. Radiation is kept ALARA — the lens is the sensitive target (Module 13). It is precisely because CT cannot show the nerve, soft tissue or retrocochlear disease that the Rational Checklist (Module 1) makes MRI the screen and CT the selective tool, read inside-out from the cochlear lumen when a bony question demands it.[2026]
What is the correct temporal-bone axial plane?
Why does isotropic-voxel HRCT only need one acquisition?
Which plane is the temporal-bone 'axial' plane?