14.2 Intraoral Radiographic Techniques, Positioning & Geometry

14.2 Intraoral Radiographic Techniques, Positioning & Geometry

Achieving high-diagnostic-yield intraoral radiographs requires a precise understanding of projection geometry, anatomic landmarks, and positioning techniques. Radiographs serve as two-dimensional representations of three-dimensional anatomical structures; thus, any deviation from optimal geometric alignment introduces distortion, magnification, or anatomical superimposition. In Australian dental practice, standardized technique execution minimizes technical retakes, aligning directly with ALARA safety principles. Candidates preparing for the ADC Written Examination must master the geometric principles of shadow casting, contrast the paralleling and bisecting-angle periapical techniques, execute bitewing protocols, and apply tube-shift localization principles.

---

1. Principles of Ideal Projection Geometry (Rules of Shadow Casting)

To produce a radiograph that accurately depicts the true size, shape, and spatial relationships of teeth and supporting periodontium, five fundamental geometric rules must be satisfied:

1. Small Focal Spot Size: The X-ray source (focal spot on the tungsten anode) should be as small as possible to minimize penumbra (partial shadow at image borders) and maximize image sharpness.
2. Long Source-to-Object Distance: The distance between the focal spot and the target tooth should be maximized. A longer distance (e.g., using a $20\text{ cm}$ or $40\text{ cm}$ PID) produces a more parallel, less divergent central beam, reducing geometric magnification.
3. Short Object-to-Receptor Distance: The distance between the tooth and the digital sensor/film should be as small as possible to prevent image magnification and penumbral blurring.
4. Parallelism (Object Parallel to Receptor): The long axis of the tooth and the plane of the image receptor must be placed parallel to each other to prevent shape distortion (foreshortening or elongation).
5. Perpendicularity (Beam Perpendicular to Object & Receptor): The central X-ray beam must intersect both the long axis of the tooth and the receptor plane at a $90^\circ$ right angle to avoid dimensional distortion.

---

2. Paralleling Technique vs. Bisecting-Angle Technique

PARALLELING TECHNIQUE (Gold Standard)             BISECTING-ANGLE TECHNIQUE (Cieszynski)

        Tooth        Receptor                         Tooth    Imaginary    Receptor
       Axis (│)      Plane (│)                       Axis (╲)   Bisector (┆)  Plane (│)
          │             │                               ╲         ┆          │
          │             │                                ╲        ┆          │
 Central  │             │                      Central    ╲       ┆          │
 Beam ───►├─────────────┤                      Beam ───────┼──────►┆──────────┤
 (90°)    │   Parallel  │                      (90° to     ╲  90°  ┆          │
          │             │                       Bisector)   ╲      ┆          │

A. Paralleling Technique (Extension Cone Paralleling - ECP)

• Mechanism: The image receptor is supported in a rigid sensor holder (e.g., Rinn XCP aiming ring system) and positioned parallel to the long axis of the tooth. Because anatomical structures (e.g., palatal vault, floor of mouth) may prevent placing the sensor flush against the tooth while maintaining parallelism, the sensor is positioned deeper within the oral cavity. The PID is aligned perpendicular to both the tooth and sensor.
• Advantages:
  • Produces an accurate, anatomically true image with minimal geometric distortion.
  • Highly reproducible across longitudinal appointments, essential for evaluating periodontal bone changes or endodontic healing.
  • Aiming rings simplify alignment and eliminate cone-cut errors.
• Disadvantages & Clinical Workarounds:
  • Difficult in patients with shallow palatal vaults, prominent tori, low floor of mouth, or severe gag reflex. Utilizing cotton rolls or modified sensor holders aids placement.

B. Bisecting-Angle Technique

• Mechanism: Based on Cieszynski's Rule of Isometry (which states that two triangles are equal if they share a common side and have two equal angles). The receptor is placed directly against the lingual/palatal tissue of the tooth, forming an angle with the long axis of the tooth. The clinician visualizes an imaginary bisector line that divides this angle in half, and directs the central X-ray beam perpendicular to this imaginary bisector.
• Geometric Errors in Bisecting-Angle Radiography:
  • Foreshortening: Caused by excessive vertical angulation of the PID (central beam directed perpendicular to the receptor instead of the bisector). The resulting image depicts teeth that appear unnaturally short and squat.
  • Elongation: Caused by insufficient vertical angulation of the PID (central beam directed perpendicular to the tooth axis instead of the bisector). The resulting image depicts teeth that appear stretched out and elongated.

---

3. Bitewing Radiography Protocols & Alignment

Bitewing radiographs provide the primary diagnostic screening tool for interproximal caries detection and alveolar bone height evaluation.

A. Horizontal Bitewings

• Indications: Detection of proximal enamel and dentinal caries, evaluation of existing restoration margins, and assessment of early-to-moderate crestal bone loss.
• Horizontal Angulation: The central X-ray beam must be directed parallel to the interproximal contact spaces of the teeth being imaged. Incorrect horizontal angulation results in overlapping of adjacent proximal enamel surfaces, obscuring incipient lesions.
• Vertical Angulation: Set at $+5^\circ\text{ to }+8^\circ$ to compensate for the slight curve of Spee and slight palatal tilt of maxillary crowns.
• Coverage Standards:
  • Premolar Bitewing: Must capture the distal half of the mandibular canine, premolars, and interproximal contacts.
  • Molar Bitewing: Must capture the distal surface of the most posterior erupted molar.

B. Vertical Bitewings

• Indications: Mandatory in patients presenting with moderate-to-severe periodontitis (alveolar bone loss $>4-5\text{ mm}$). Standard horizontal bitewings fail to capture bone crests that have migrated apically, placing them outside the sensor field of view. Vertical bitewings rotate the long axis of the receptor $90^\circ$ to capture both maxillary and mandibular root shafts and deep bony defects.

---

4. Occlusal Radiography & The SLOB Rule (Clark's Tube-Shift Technique)

A. Occlusal Radiographic Projections

Occlusal radiographs utilize a Size 4 sensor or phosphor plate held between the occlusal surfaces of teeth:

• Maxillary Topographic Projection: Visualizes palate, anterior maxilla, impacted mesiodens, or palatal clefts ($+65^\circ$ vertical angle).
• Mandibular Cross-Sectional Projection: Central beam directed at $90^\circ$ to sensor plane under the chin. Excellent for locating submandibular gland sialoliths in Wharton's duct or detecting buccal/lingual cortical plate expansion.

B. Spatial Localization: The SLOB Rule (Clark's Technique)

Determining whether an un-erupted tooth (e.g., impacted maxillary canine), supernumerary tooth, foreign body, or extra root canal (e.g., MB2 canal in maxillary first molars) is positioned buccally or lingually requires a two-dimensional tube-shift technique.

                  SLOB RULE (Same Lingual, Opposite Buccal)

         Initial Projection                       Tube Shifted Mesially

       Buccal Object (○)                       Buccal Object (○) -> Moves Distal (Opposite)
       Lingual Object (●)                      Lingual Object (●) -> Moves Mesial (Same)

1. Protocol: Take an initial standard radiograph. For the second radiograph, shift the X-ray tubehead horizontally (mesially or distally) or vertically (superiorly or inferiorly) while keeping the receptor stable.
2. Diagnostic Mnemonic — SLOB:
   • Same Lingual: If the object of interest moves in the same direction as the movement of the X-ray tubehead, the object is located lingual (or palatal) to the reference tooth.
   • Opposite Buccal: If the object moves in the opposite direction relative to the movement of the X-ray tubehead, the object is located buccal to the reference tooth.