Computed tomography is the primary imaging modality for the evaluation of paranasal sinus disease and temporal bone pathology, serving as an indispensable tool for otolaryngologists (ENT surgeons) in pre-operative planning for functional endoscopic sinus surgery (FESS) and ear surgery. Unlike most CT applications where pathology is the primary focus, CT of the sinuses and temporal bone demands an equally rigorous understanding of normal anatomy — because the primary value of these studies lies in mapping the precise three-dimensional anatomy of surgically critical structures to guide safe operative planning.

CT of the Paranasal Sinuses: Anatomy and Chronic Rhinosinusitis

The paranasal sinuses include four bilaterally paired groups: the maxillary sinuses (largest, in the cheeks), ethmoid sinuses (multiple small air cells between the eye orbits), frontal sinuses (in the forehead), and sphenoid sinuses (deep, adjacent to the sella turcica and optic nerves). CT of the sinuses is performed without IV contrast, in thin-slice (≤1 mm) coronal, axial, and sagittal reformats, using a bone algorithm to maximize mucosal and bony detail.

The ostiomeatal complex (OMC) — the functional drainage pathway of the maxillary, anterior ethmoid, and frontal sinuses — is the most critical anatomical region to evaluate. It includes the maxillary ostium, infundibulum, hiatus semilunaris, ethmoid bulla, middle meatus, and frontal recess. Obstruction at any point within the OMC from mucosal inflammation, anatomical variants (concha bullosa — pneumatization of the middle turbinate — is the most common), or polyposis leads to retained secretions and chronic rhinosinusitis (CRS) in the dependent sinuses. The Lund-Mackay scoring system is the standard CT staging tool for CRS, assigning scores (0–2) to each sinus group and the OMC bilaterally, with a maximum total score of 24.

Critical Anatomical Variants Relevant to FESS Safety

Pre-operative CT sinus review must systematically evaluate several anatomical variants that significantly impact surgical risk:

  • Agger nasi cells: The most anterior ethmoid cells, present in 98% of patients — their size determines the approach to the frontal recess and frontal sinusotomy
  • Onodi cell (Sphenoethmoid cell): A posterior ethmoid cell that pneumatizes superolateral to the sphenoid sinus, bringing the optic nerve into close proximity with the posterior ethmoid dissection plane — a critical safety variant to identify before surgery
  • Dehiscent lamina papyracea: Absence of the thin bony plate separating the ethmoid from the orbit — risk of orbital penetration during ethmoidectomy
  • Low-lying skull base (cribriform plate): A descended ethmoid roof increases the risk of intracranial penetration during superior dissection
  • Deviated nasal septum: Must be documented with direction and degree, as it directly impacts surgical access and may require concurrent septoplasty

CT of the Temporal Bone: Protocol and Key Structures

CT of the temporal bone is performed with submillimeter slice thickness (0.5–0.625 mm) using a high-resolution bone kernel algorithm, with multiplanar reformats in the axial and coronal planes as minimum. The primary structures evaluated include: the external auditory canal (EAC), tympanic membrane (inferred), middle ear cavity and ossicular chain (malleus, incus, stapes — ossicular erosion is key in cholesteatoma), mastoid air cells, inner ear structures (cochlea, semicircular canals, vestibule), facial nerve canal (entire course from fundus to stylomastoid foramen), internal auditory canal (IAC), and jugular bulb (high-riding jugular bulb is a surgical hazard in posterior fossa and cochlear implant surgery).

Cholesteatoma: The Most Important Middle Ear CT Diagnosis

Cholesteatoma is an epidermal inclusion cyst of keratinizing squamous epithelium within the middle ear or mastoid, capable of progressive bony erosion and serious complications including facial nerve palsy, labyrinthine fistula, sigmoid sinus thrombosis, meningitis, and intracranial abscess. CT is the key pre-operative imaging modality. Findings include: a soft tissue mass in Prussak's space (the lateral epitympanic recess — classic location for pars flaccida cholesteatoma), lateral epitympanic wall (scutum) erosion, ossicular erosion (particularly the long process of the incus and stapes superstructure), mastoid opacification, and — in advanced cases — lateral semicircular canal fistula, tegmen erosion, or sinus plate erosion.

Otosclerosis: Spongiosis of the Otic Capsule

Otosclerosis is a primary disorder of otic capsule bone remodelling causing progressive conductive and sensorineural hearing loss. CT demonstrates characteristic hypodense halos around the cochlea — the "halo sign" or "fourth turn sign" — representing spongiotic (low-density) bone replacing the normally dense otic capsule. The most common and earliest location is the fissula ante fenestram — a small cleft anterosuperior to the oval window — whose involvement causes stapedial fixation and the characteristic progressive conductive hearing loss of fenestral otosclerosis. CT grading of activity and extent guides surgical planning for stapedectomy or cochlear implant candidacy.