35.4 Lateral Cephalometry and Cone Beam CT

6. Lateral Cephalometry & Cone Beam Computed Tomography (CBCT)

Lateral Cephalometric Radiography

• Standardized Geometry: Standardized skull projection obtained with a specialized cephalostat maintaining a fixed distance of 1.5 metres between the X-ray focal spot and the patient's midsagittal plane, with a receptor distance of $15\text{ cm}$.
• Natural Head Position (NHP): The patient stands in the natural head posture, looking into their own eyes in a wall mirror on the opposite wall, with ear rods gently seated in the external acoustic meatus and an anterior nasal positioner.
• Soft Tissue Filter (Aluminium Wedge): An adjustable aluminium attenuating wedge is positioned anteriorly to absorb a portion of the primary beam passing through the soft tissues of the facial profile. This enables simultaneous, high-contrast visualization of both bony cephalometric landmarks (Sella, Nasion, Points A and B) and the soft tissue profile outline (nose, lips, chin contour).
• Primary UK Indications: Pre-treatment and post-treatment skeletal assessment in orthodontics; orthognathic surgical planning and surgical splint manufacturing.

Cone Beam Computed Tomography (CBCT)

CBCT produces three-dimensional volumetric images of the maxillofacial skeleton by rotating a divergent, cone-shaped or pyramidal X-ray beam coupled to a two-dimensional flat panel detector (FPD) around the patient's head in a single 180° to 360° sweep.

Medical Fan-Beam CT vs Dental Cone-Beam CT (CBCT)

     A. Conventional Medical CT (Fan-Beam)         B. Dental CBCT (Cone-Beam)

          [X-Ray Source]                               [X-Ray Source]
                │                                           ╱│╲
                │ Fan-Beam                                 ╱ │ ╲ Cone-Beam
                ▼                                         ▼  ▼  ▼
          ┌────────────┐                               ┌─────────────┐
          │ Axial Slice│                               │ Volumetric  │
          └────────────┘                               │ 3D Cylinder │
                │ Multiple rotations                   └─────────────┘
                ▼ (Helical)                                  │ Single rotation
          Anisotropic Voxels                                 ▼
          (Unequal: slice thickness > pixels)          Isotropic Voxels
          Dose: Very High (e.g. 1000–2000 µSv)         (Equal: x = y = z; 0.075–0.4 mm)
                                                       Dose: Low-to-Moderate (20–300 µSv)

Isotropic Voxels vs Anisotropic Voxels

• Isotropic Voxels (CBCT): The smallest volumetric building block (voxel) is a perfect cube with identical dimensions in all three spatial planes ($x = y = z$, typically 0.075 to 0.4 mm).
• Diagnostic Advantage: Permits true 1:1 measurements in any reconstructed plane (axial, sagittal, coronal, or oblique) without distortion or partial volume averaging artifacts.
• In contrast, medical CT often acquires slices where pixel size is smaller than slice thickness (anisotropic voxels), degrading secondary reconstructions.

Field of View (FOV) Selection Criteria

Under European SEDENTEXCT and UK FGDP/CGDent CBCT guidelines, selecting the Field of View must be strictly driven by the specific clinical diagnostic task:

1. Small / Localized FOV ($\le 5 \times 5\text{ cm}$):
   • Dose: Lowest effective radiation dose ($20\text{ to } 50\text{ }\mu\text{Sv}$).
   • Clinical Indications: Endodontic assessment (detecting missed canals such as MB2 in maxillary molars, persistent periapical periodontitis, root resorptions, vertical root fractures); localized impacted teeth (assessing intimate 3D relationship between an impacted canine and incisor roots); pre-surgical implant assessment for a single site.
2. Medium FOV ($6 \times 6\text{ cm to } 8 \times 8\text{ cm}$):
   • Clinical Indications: Full arch assessment; multiple adjacent implant sites; bilateral impacted third molars with intimate inferior alveolar nerve relationships.
3. Large FOV ($> 10 \times 10\text{ cm to } 15 \times 15\text{ cm}$):
   • Dose: Substantially higher effective dose ($100\text{ to } 300+\text{ }\mu\text{Sv}$).
   • Clinical Indications: Orthognathic surgical planning; complex craniofacial trauma; extensive maxillofacial skeletal deformities; cleft palate reconstruction.

UK Statutory & European Radiation Protection Guidelines (SEDENTEXCT & CGDent)

• Never a Screening Tool: CBCT must never be used routinely or as a screening modality.
• Pre-requisite 2D Imaging: A CBCT scan is justified only when clinical examination combined with conventional 2D radiographs (periapicals or DPT) cannot provide the necessary diagnostic information, and where the 3D information will directly alter patient management.
• ALARP / ALADA Principles: Radiation exposure must be kept As Low As Reasonably Practicable (ALARP) or As Low As Diagnostically Acceptable (ALADA). Clinicians must always choose the smallest FOV, lowest tube current ($mA$), and lowest possible exposure time compatible with diagnostic efficacy.
• Reporting Obligation: Under IR(ME)R17, every CBCT dataset must undergo a formal, documented clinical radiological evaluation of the entire volumetric field of view, not merely the teeth of interest. Clinicians lacking competence to interpret the full volume must commission a specialist dental and maxillofacial radiologist report.

7. Clinical Traps, Pitfalls & Worked Clinical Scenario

[!CAUTION] Clinical Trap: Misinterpreting Palatoglossal Air Shadow as Horizontal Bone Loss: On a panoramic radiograph, if the patient fails to press their tongue firmly against the roof of the mouth, a thick, radiolucent band (the palatoglossal air space) is projected across the maxillary alveolar crest and anterior root apices. Inexperienced clinicians frequently mistake this radiolucency for generalized horizontal alveolar bone loss or apical periodontitis. Always examine the cortical crest in conjunction with bite-wing radiographs before diagnosing periodontal destruction.

[!WARNING] Clinical Trap: Ordering Full Craniofacial CBCT for Endodontic Retreatment: Requesting a large-FOV ($15 \times 15\text{ cm}$) CBCT to investigate a suspected cracked tooth or missed MB2 canal on an upper first molar is a severe regulatory violation under IR(ME)R17 and CGDent guidance. Large FOVs deliver excessive radiation doses, reduce spatial resolution (larger voxel size $\approx 0.3-0.4\text{ mm}$), and introduce scatter from distant metallic restorations. Endodontic assessment strictly mandates a small/focused FOV ($\le 5 \times 5\text{ cm}$) with high-resolution sub-millimetre voxels ($\le 0.1\text{ mm}$).

Worked Clinical SBA Scenario

Scenario: A 42-year-old male presents to a UK dental practice for radiographic assessment prior to restorative treatment. A Dental Panoramic Tomogram (DPT) is taken. On reviewing the processed image, the practitioner observes that the occlusal plane forms a prominent, exaggerated upward curvature resembling a deep 'smile' (joker smile), the mandibular incisor crowns and roots are severely foreshortened and out of focus, the condyles are positioned high up touching the superior border of the image, and the lower border of the mandibular symphysis is truncated. What is the specific patient positioning error that caused these combined artifacts, and how must it be rectified for a diagnostic repeat?

Clinical Reasoning Formulation:

1. Artifact Analysis:
   • Exaggerated upward occlusal curve ('joker smile'): Produced when the mandibular arch is tipped downward into extreme forward flexion.
   • Foreshortened mandibular incisors: When the head is tilted downwards, the mandibular incisors are angled anteriorly and outside the narrow anterior focal trough, resulting in vertical distortion and severe foreshortening.
   • High condyles & truncated symphysis: Downward flexion elevates the posterior condyles towards the top of the rotational beam path while dropping the anterior chin below the sensor exposure window.
2. Identification of Positioning Error: This constellation of features is pathognomonic for chin tilted too low (excessive downward tilt of the Frankfort horizontal plane).
3. Corrective Action: The clinician must re-position the patient with the Frankfort horizontal plane (porion to orbitale) precisely parallel to the floor, verified by aligning the horizontal alignment light beam directly with the infraorbital rim and external acoustic meatus before exposure.