Facial Bones, Orbits & High-Resolution Temporal Bones

Acquisition settings and contrast timings below are illustrative adult protocol examples. Select the authorized protocol for the indication, scanner, body size, access device, and clinical condition. Treatment decisions belong to the responsible clinical team.

Maxillofacial & Facial Bone CT Protocols

Maxillofacial computed tomography is the gold standard imaging modality for evaluating complex facial trauma, craniofacial deformities, pre-operative surgical reconstructive planning, and suspected osteomyelitis or neoplasms of the splanchnocranium. Because the facial skeleton is composed of delicate, paper-thin bony struts, complex air-filled paranasal cavities, and intricate dental and muscular structures, accurate diagnosis requires rigorous scanning protocols optimized for spatial resolution.

Technical Acquisition Parameters

• Helical Volumetric Acquisition: Maxillofacial CT is acquired in a single volumetric helical sweep from the superior aspect of the frontal sinuses through the inferior border of the mental symphysis of the mandible.
• Thin Collimation: An illustrative fine-detail protocol uses detector collimation of $0.5\text{ to }1.0\text{ mm}$, selected for the scanner and clinical task. Fine longitudinal sampling reduces staircase artifacts and enables seamless, high-fidelity multiplanar reformations.
• Exposure Factors: Standard adult parameters utilize $120\text{ kVp}$ with tube current-time products tailored via automatic tube current modulation ($150\text{ to }250\text{ mAs}$).
• Reconstruction Filters (Kernels): Every raw helical data dataset is routinely reconstructed into two distinct series:
  1. Sharp High-Frequency Bone Kernel: Accentuates high spatial frequency edges to delineate micro-fractures, cortical margins, and suture lines.
  2. Standard Smooth / Soft Tissue Kernel: Suppresses quantum mottle to evaluate soft tissue hematomas, herniated orbital fat, facial muscles, and salivary glands.
• Multiplanar Reformations (MPR) & 3D Volume Rendering (VRT): Direct axial images alone are grossly inadequate for surgical management. Isotropic submillimeter data must be reformatted into true coronal planes (perpendicular to the hard palate) and sagittal planes (parallel to the nasal septum) at contiguous $1.0\text{ mm}$ increments. Furthermore, 3D Volume-Rendered Technique (VRT) reconstructions provide surgeons with an intuitive global topographic map of facial buttress displacements and rotational deformities.

Maxillofacial Trauma Patterns & Classifications

The facial skeleton relies on interconnected vertical and horizontal structural pillars (buttresses) designed to absorb and distribute masticatory forces and protect the intracranial vault:

1. Le Fort Fractures (Midfacial Dysjunction): Classic classification established by René Le Fort describing complex trans-facial fractures that detach the midface from the cranium. Pterygoid plate disruption is an important component of Le Fort patterns, but a pterygoid fracture alone is insufficient. Real injuries can be unilateral, asymmetric or combine patterns; inspect the complete fracture course.
   • Le Fort I (Horizontal Maxillary Fracture / 'Floating Palate'): A horizontal fracture line passing above the maxillary alveolar ridge, traversing the lower nasal septum, lower pyriform aperture, anterolateral walls of the maxillary sinuses, and inferior third of the pterygoid plates. Detaches the tooth-bearing maxillary alveolar process and hard palate from the upper facial skeleton.
   • Le Fort II (Pyramidal Fracture): A pyramidal-shaped fracture configuration extending superiorly through the nasal bones and nasofrontal suture, across the frontal process of the maxilla, lacrimal bones, medial orbital floor, through the infraorbital rim, down through the zygomaticomaxillary buttress, and across the mid-pterygoid plates. The entire central triangular midface (nose and maxilla) is mobilized.
   • Le Fort III (Craniofacial Dysjunction): The most substantial midface fracture. The fracture line passes horizontally through the nasofrontal suture, ethmoid bones, along the medial orbital wall, superior orbital fissure, across the lateral orbital wall (frontozygomatic suture), through the zygomatic arch, and through the high sphenoid pterygoid plates. This completely severs the entire facial skeleton from the cranial base, creating a totally detached midface.
2. Zygomaticomaxillary Complex (ZMC / Tripod / Quadripod) Fractures: The most common midface fracture following nasal bone fractures. Caused by a direct impact to the malar eminence. Disruption involves four distinct bony attachments:
   • Lateral orbital rim / wall (frontozygomatic suture)
   • Zygomatic arch
   • Inferior orbital rim and orbital floor (maxillary sinus roof)
   • Zygomaticomaxillary buttress (lateral wall of the maxillary sinus)
   • Clinical Sequelae: Trismus (inability to open the mouth due to impingement of the depressed zygomatic arch against the coronoid process or temporalis muscle), cheek flattening, infraorbital nerve ($V_2$) anesthesia, and diplopia.
3. Naso-Orbito-Ethmoid (NOE) Fractures: Direct impact to the central upper midface fracturing the nasal bones, ethmoid labyrinth, and lacrimal bones. Carries high risk of tearing the medial canthal tendon (producing telecanthus / widened intercanthal distance) and comminuting the cribriform plate, resulting in dural tears, tension pneumocephalus, and persistent CSF rhinorrhea.
4. Temporomandibular Joint (TMJ) Fractures: Mandibular fractures frequently involve the condylar process and subcondylar neck. CT protocols evaluate condylar displacement, glenoid fossa fractures, and bony articular relationships; MRI is better suited to evaluating the articular disc.

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Temporomandibular joint CT

The mandibular condyle articulates with the temporal bone's mandibular fossa and articular eminence. A targeted CT uses thin bone reconstructions and oblique sagittal and coronal planes relative to the condylar axis. Assess cortical erosion, osteophytes, sclerosis, fracture and the condyle's position. Closed-mouth and, when specifically ordered and safely tolerated, open-mouth views answer different positional questions; do not force opening after trauma. Include both joints when prescribed for comparison. CT is useful for the osseous components, while MRI generally provides the better assessment of disc position and other internal soft-tissue derangement. A normal bone CT cannot exclude disc displacement.

CT Protocols of the Orbits

Computed tomography of the orbits requires specialized technique to resolve fine extraocular muscle bellies, the optic nerve, intraorbital fat, and the delicate bony walls forming the orbital pyramid.

Scan Technique & Lens Dose Optimization

• Positioning: The patient is placed supine with the head positioned symmetrically. The scan plane is angled parallel to the infraorbitomeatal line (IOML). Angling parallel to the IOML aligns the scan plane along the long axis of the optic nerve and orbital floor while directing the direct primary beam below the vertex.

• Lens dose: an orbit examination necessarily covers the required orbital structures. Supported organ-based modulation can alter anterior tube output, but the angular range, reduction and dose redistribution are scanner-specific. Optimize the approved technique without excluding the diagnostic anatomy.

• Intravenous Contrast Administration: Indicated when evaluating orbital cellulitis, subperiosteal abscess, idiopathic orbital pseudotumor, Graves' ophthalmopathy with optic nerve compression, or lacrimal gland neoplasms. Non-contrast imaging is standard for trauma and pre-MRI metallic foreign body screening.

Dedicated Orbital Display Windows

• Orbital Soft Tissue Window (WW 350 to 400, WL 40): The orbit contains abundant low-density retrobulbar fat ($-100\text{ to }-120\text{ HU}$), which acts as a pristine natural contrast medium outlining the water-density extraocular rectus muscles ($+30\text{ to }+50\text{ HU}$), the optic nerve sheath complex, and the superior ophthalmic vein.
• Orbital Bone Window (WW 2500, WL 500): Visualizes the delicate bony boundaries: the lamina papyracea medially ($0.2\text{--}0.4\text{ mm}$ thick) and the paper-thin maxillary orbital floor.

Orbital Blowout Fractures: Traumatic Pathology

• Mechanism of Injury: Blunt impact to the globe by an object larger than the orbital aperture (e.g., fist, tennis ball, baseball). The kinetic energy compresses the globe, transmitting sudden hydraulic pressure throughout the closed orbital cavity. The orbital rim remains intact, but the thinnest structural boundaries rupture:
  1. Orbital Floor (Maxillary Sinus Roof): The most frequent blowout site (frequency varies by population).
  2. Medial Orbital Wall (Lamina Papyracea): The second most common site, communicating with the ethmoid air cells.
• Imaging Hallmarks:
  • Orbital Emphysema: Free air bubbles within the retrobulbar or preseptal fat originating from ruptured ethmoid or maxillary sinus cavities.
  • Tear-Drop / Trapdoor Sign: Soft-tissue mass protruding inferiorly through the fractured orbital floor into the superior aspect of the maxillary sinus, consisting of herniated retrobulbar fat and blood.
  • Inferior Rectus Muscle Entrapment: The inferior rectus muscle belly or adjacent fascial connective tissue slips become incarcerated within the hinged bony fracture margins of the floor. This constitutes an acute ophthalmic surgical emergency. Incarceration leads to muscle ischemia, irreversible necrosis, and severe vertical diplopia with mechanical restriction of upward gaze.
  • Enophthalmos: Posterior and inferior sinking of the ocular globe due to expansion of the orbital volume and fat atrophy.

Differential Diagnosis: Graves' Ophthalmopathy vs. Orbital Myositis

Extraocular muscle enlargement is a common diagnostic dilemma encountered on orbital CT:

1. Graves' Ophthalmopathy (Thyroid-Associated Orbitopathy):
   • Etiology: Autoimmune deposition of glycosaminoglycans and hydrophilic mucopolysaccharides within orbital fibroblasts, causing muscle swelling and fat proliferation.
   • Morphology: Often bilateral fusiform enlargement of extraocular muscle bellies with typical relative sparing of the anterior tendinous insertions at the globe (creating a classic "coca-cola bottle" or spindle appearance).
   • Muscle Involvement Sequence (Mnemonic: I'M SLOW):
     1. I: Inferior rectus (commonly involved)
     2. M: Medial rectus
     3. S: Superior rectus
     4. L: Lateral rectus
     5. O: Oblique muscles (least frequently involved)
   • Clinical Sequelae: Painless progressive exophthalmos (proptosis); apical muscle crowding can compress the optic nerve at the orbital apex, causing blinding compressive optic neuropathy.
2. Orbital Pseudotumor / Idiopathic Orbital Myositis:
   • Etiology: Non-granulomatous, non-infectious inflammatory phlegmon.
   • Morphology: Typically unilateral, presenting with acute, agonizing retrobulbar pain, chemosis, and diplopia.
   • Diagnostic Feature: Diffuse muscle belly enlargement that DIRECTLY INVOLVES the anterior tendinous insertion, extending seamlessly to the scleral attachment. The clinical team distinguishes inflammatory disease from infection and other mimics before treatment.

Orbital metal assessment before MRI

A history of potentially penetrating ocular metallic injury warrants the MR safety team's assessment. Employment as a metalworker alone does not automatically require CT for every patient. Appropriate orbital radiographs or a radiologist's review of an adequate prior CT may satisfy the assessment under the MR safety protocol. CT cannot promise reliable detection of every 0.1 mm fragment. A suspected foreign body must be evaluated before MRI clearance; the technologist should not clear the patient from occupation history or a casual image glance.

Reference: ACR MR safety resources.

High-Resolution Temporal Bone CT (HRCT)

High-Resolution Computed Tomography (HRCT) of the petrous temporal bones represents the high-resolution application of spatial resolution in diagnostic medical imaging. The petrous pyramid houses the delicate microscopic structures of the middle ear ossicular chain, the fluid-filled cochlea, semicircular canals, and the intricate micro-canals conveying cranial nerves.

Targeted temporal-bone acquisition and reconstruction

Use the scanner's approved thin-section acquisition and a high-resolution bone reconstruction for the osseous task. Reconstruct each temporal bone with a small field of view. A 100 mm DFOV and 512 matrix give 100/512 = 0.195 mm pixels; a 120 mm DFOV gives 0.234 mm pixels. Pixel size is sampling, not a guarantee that an equally small structure is resolved. Slice sensitivity, focal spot and reconstruction also limit detail.

Axial and coronal images depict the ossicles, cochlea, vestibule, semicircular canals and facial nerve canal. Pöschl reformations are parallel to the superior semicircular canal plane; Stenvers views are orthogonal to that plane. Use the prescribed oblique planes when evaluating suspected canal dehiscence. Bone windows reveal cortical boundaries, while a separate appropriate soft-tissue reconstruction can be useful for selected inflammatory or adjacent soft-tissue questions. A sharp bone image alone is not a complete soft-tissue examination.

Detailed Micro-Anatomy of the Temporal Bone

1. External Auditory Canal (EAC) & Tympanic Cavity:
   • Tympanic Membrane & Scutum: The scutum is the sharp bony spur formed by the superior lateral wall of the epitympanic recess. The scutum is adjacent to the pars flaccida and Prussak’s space; do not equate it with the entire tympanic membrane attachment.
   • Prussak's Space (Lateral Epitympanic Recess): The microscopic space bounded laterally by the scutum and pars flaccida, and medially by the neck of the malleus. Clinical Significance: Prussak's space is the primary origin site for acquired cholesteatomas—destructive, expansile keratinizing squamous epithelial cysts that scallop and destroy the scutum, disrupt the ossicular chain, and erode the adjacent facial nerve canal and tegmen tympani.
2. The Middle Ear Ossicular Chain:
   • Malleus (Hammer): Head located in the epitympanic recess (attic); handle (manubrium) embedded in the tympanic membrane.
   • Incus (Anvil): Articulates with the malleus head via the malleoincudal joint. On axial CT, the malleoincudal complex resembles an "ice cream cone" (the rounded head of the malleus represents the scoop of ice cream, while the body and short process of the incus form the triangular cone). Disruption of the ice cream cone configuration is characteristic for traumatic ossicular dislocation.
   • Stapes (Stirrup): The long process of the incus connects via the microscopic lenticular process to the stapes head (incudostapedial joint, which can be disrupted in head trauma). The anterior and posterior stapes crura extend to the oval footplate, which sits directly within the oval window (fenestra vestibuli).
3. The Inner Ear (Bony Labyrinth):
   • Cochlea: The sensory organ of hearing, making approximately 2.5 helical turns around a central spongy bony axis called the modiolus. The basal turn abuts the round window (fenestra cochleae).
   • Vestibule & Semicircular Canals: Three mutually orthogonal fluid-filled canals (lateral / horizontal, superior / anterior, and posterior) that detect rotational acceleration. Thinning or absence of bone over the superior canal roof constitutes Superior Semicircular Canal Dehiscence (SSCD), producing the Tullio phenomenon (dizziness and vertigo induced by loud sounds or barometric pressure shifts).
   • Internal Auditory Canal (IAC): Transmits Cranial Nerve VII (facial nerve), Cranial Nerve VIII (cochlear, superior vestibular, inferior vestibular nerves), and the labyrinthine artery from the cerebellopontine angle cistern.
4. Neurovascular Canals:
   • Facial Nerve Canal ($CN\ VII$ / Fallopian Canal): Intricate, z-shaped canal with three segments:
     1. Labyrinthine Segment: From IAC to the geniculate ganglion (the narrowest portion, $<0.7\text{ mm}$). The greater petrosal nerve branches at the geniculate ganglion.
     2. Tympanic (Horizontal) Segment: Runs along the medial wall of the middle ear immediately inferior to the lateral semicircular canal and superior to the oval window.
     3. Mastoid (Vertical) Segment: Descends through the mastoid process to exit the skull base via the stylomastoid foramen.
   • Carotid Canal: Carries the petrous segment of the internal carotid artery through the anteromedial temporal bone, immediately anterior to the cochlea and middle ear.
   • Jugular Foramen: Situated posteromedial to the carotid canal, transmitting the internal jugular vein and Cranial Nerves IX (glossopharyngeal), X (vagus), and XI (accessory).

Temporal-bone trauma

Historical longitudinal and transverse labels describe fracture direction relative to the petrous ridge. Modern assessment emphasizes whether the fracture violates the otic capsule, and its relationship to the ossicles, facial nerve canal, carotid canal and tegmen. Otic-capsule involvement raises concern for sensorineural hearing loss and other complications. Ossicular disruption can produce conductive loss. Facial weakness may reflect different injury mechanisms; a fracture label does not by itself prove complete nerve transection or mandate one operation.

Communicate a fracture crossing a critical canal, CSF-leak concern or clinically important orbital injury promptly. Thin source images and correctly oriented reformations help the specialist assess anatomy; percentage-based predictions do not replace that assessment.

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