10.2 Temporal Bone, Orbit & Sinus CT

Why This Topic Matters on the ARRT CT Exam

This section covers three more of ARRT's eight named Head leaf items: temporal bones/internal auditory canal (IAC), orbits, and sinuses. All three share a common technical thread — thin-slice, bone-algorithm imaging of small, anatomically dense structures — but each has its own protocol logic, indications, and hallmark pathology. These three regions generate some of the densest anatomy-and-pathology questions on the exam because ARRT's "Focus of Questions" themes (cross-sectional anatomy, landmarks, protocol factors, and contrast decisions) are all directly testable on hearing-loss, trauma, and sinusitis vignettes.

Core Terms and Technique

Temporal Bone (High-Resolution CT)

High-resolution CT (HRCT) of the temporal bones uses the thinnest sections routinely acquired anywhere in CT — typically submillimeter (0.5–0.6 mm) — reconstructed with a sharp/bone algorithm at a small, targeted display field of view. Rather than performing two separate direct-axial and direct-coronal scans (which doubles dose and risks motion between acquisitions), most departments acquire a single helical axial volume and generate coronal reformats from the same raw data, with the reformatting plane chosen perpendicular to the axial plane (which itself is set parallel to the lateral semicircular canal). Routine temporal bone CT is noncontrast; contrast is reserved for suspected tumor or infectious extension beyond the bone.

Key structures a CT technologist must recognize:

• Ossicular chain — malleus, incus, and stapes; the malleus-incus articulation resembles an "ice cream cone" on axial images
• Oval window and round window — inner ear entry points
• Scutum — a sharp bony spur; blunting suggests chronic middle ear disease
• Tegmen tympani — the thin bony roof separating the middle ear/mastoid from the middle cranial fossa; dehiscence risks CSF leak or encephalocele
• Facial nerve canal — labyrinthine, tympanic, and mastoid segments
• Internal auditory canal (IAC) — CT evaluates bony canal widening/asymmetry (a clue to vestibular schwannoma), though MRI remains the primary modality for characterizing the soft-tissue mass itself
• Cholesteatoma — a chronic, erosive middle-ear soft-tissue mass from chronic otitis media, classically eroding the scutum and ossicles; CT is used for surgical planning, not primary diagnosis

Orbit CT

Orbital CT uses thin-section (roughly 1–2 mm, or submillimeter with reformats) axial acquisition with coronal (and sometimes sagittal) reformats, reviewed in both bone and soft-tissue algorithms/windows. Indications include trauma, proptosis, suspected radiopaque foreign body, and orbital cellulitis/abscess (which requires IV contrast). CT — not MRI — is the modality of choice whenever a metallic intraorbital foreign body is suspected, since MRI's magnetic field can move a ferromagnetic fragment and cause further injury.

• Blowout fracture: the orbital floor (thinnest wall, forming the maxillary sinus roof) is the most common fracture site. CT shows the "teardrop sign" — herniated orbital fat and/or inferior rectus muscle prolapsing through the fracture defect into the maxillary sinus — often with an air-fluid level in the sinus from blood. Entrapment of the inferior rectus (causing diplopia on upward gaze) is a clinical, not radiologic, diagnosis, though a "trapdoor" fracture pattern on CT raises suspicion.
• Globe rupture: a surgical emergency. CT shows the "flat tire" sign — collapse/deformity of the normally spherical globe — sometimes with intraocular air or hemorrhage. Recognizing this finding and escalating immediately is a technologist responsibility, not just a radiologist one.

Sinus CT

Routine paranasal sinus CT for chronic sinusitis is a thin-section, noncontrast, bone-algorithm study, classically reviewed in the coronal plane (either acquired directly coronal or, more commonly today, acquired axially and reformatted coronal) because the coronal plane best displays the osteomeatal complex (OMC) — the shared drainage pathway (infundibulum, hiatus semilunaris, maxillary ostium, and ethmoid bulla) for the frontal, maxillary, and anterior ethmoid sinuses. Obstruction here is the anatomic bottleneck behind most recurrent sinus infections.

Pre-operative sinus CT — ordered before Functional Endoscopic Sinus Surgery (FESS) — uses thin sections and a sharp bone kernel specifically to map anatomic variants that raise surgical risk, such as a concha bullosa (aerated middle turbinate), Haller cells (infraorbital ethmoid air cells), and Onodi cells (posterior ethmoid cells abutting the optic nerve). Contrast is added only when an infectious complication (orbital or intracranial extension) or tumor is suspected — it is not part of the routine anatomic protocol. Disease burden can be quantified with the Lund-Mackay score (0–2 points per sinus group per side, plus the OMC, for a maximum of 24), though ARRT's blueprint emphasizes protocol logic and anatomy over memorizing staging systems.

Protocol Comparison Table

Exam Scenario Walkthrough

A 40-year-old with progressive conductive hearing loss and chronic otitis media undergoes HRCT temporal bones, which shows soft tissue in Prussak's space eroding the scutum and ossicular chain — classic cholesteatoma, mapped here for surgical planning. A separate patient, assaulted and struck in the eye, shows a collapsed globe on orbital CT — the flat tire sign — requiring urgent ophthalmology notification. A third patient with years of recurrent sinusitis is scheduled for FESS; the pre-operative sinus CT is read in the coronal plane specifically to evaluate the osteomeatal complex and flag a concha bullosa before the surgeon operates.

Takeaways

• Temporal bone HRCT uses submillimeter sections and a sharp bone algorithm, typically acquired as a single axial helical volume with coronal reformats rather than two direct scans.
• The scutum, tegmen tympani, and ossicular chain are the highest-yield temporal bone landmarks; cholesteatoma classically erodes the scutum and ossicles.
• CT, not MRI, is the modality of choice when a metallic orbital foreign body is suspected; orbital floor blowout fractures show the "teardrop sign," while globe rupture shows the "flat tire sign."
• Routine sinus CT is reviewed in the coronal plane to evaluate the osteomeatal complex, the shared drainage pathway behind most sinus disease.
• Contrast is added to temporal bone, orbit, and sinus protocols only for suspected tumor or infectious complication — never as part of the routine anatomic study.