35.3 Dental Panoramic Tomography: Principles, Positioning and Errors

4. Dental Panoramic Tomography (DPT / OPG): Principles & Positioning

Dental Panoramic Tomography (also termed Orthopantomography or OPG) produces a single, continuous tomographic image of the entire maxillofacial complex, including both dental arches, the maxillary antra, the ramus, and the temporomandibular joints (TMJs).

Rotational Geometry of Panoramic Radiography

              Image Receptor
                 ┌──────┐               Direction of Rotation
                 │      │                ╭──────────────╮
                 └──────┘                │              │
                    ▲                    ▼              │
                    │                   ╭────╮          │
                    │        Focal      │Head│          │
     Narrow Slit ───┼──────> Trough ───>│    │          │
     X-Ray Beam     │      (Sharp Zone) ╰────╯          │
                    │                    ▲              │
                 ┌──────┐                │              │
                 │      │────────────────╯              │
                 └──────┘                               ▼
               X-Ray Tube

Physical Principles of Panoramic Tomography

1. Rotational Narrow-Beam Tomography: The X-ray source and the image receptor rotate continuously in opposite directions around the patient's stationary head during a 12- to 15-second exposure cycle. The X-ray beam emerges through a narrow vertical slit collimator, exposing the moving receptor through a secondary slit.
2. The Focal Trough (Image Layer):
   • A three-dimensional, horseshoe-shaped curved zone of sharpness corresponding to the anatomical shape of human dental arches.
   • Anatomical structures positioned within the focal trough appear sharply defined and clearly focused.
   • Structures positioned outside the focal trough (anteriorly, posteriorly, or laterally) are blurred out of recognition, magnified, or minified, fading into the background.
   • Geometry: The focal trough is narrow in the anterior incisor region ($< 10\text{ mm}$ width) and significantly wider in the posterior molar region ($> 20\text{ mm}$ width). Thus, anterior positioning requires millimeter-level precision.

Standard Patient Positioning Protocol

To align the patient's dentition precisely within the narrow focal trough, five positioning criteria must be fulfilled:

1. Anterior Bite Block (Edge-to-Edge Incisors): The patient must bite into the upper and lower locating grooves of the plastic bite block with incisal edges in an edge-to-edge relationship. This positions the maxillary and mandibular anterior roots precisely within the narrowest section of the focal trough.
2. Midsagittal Plane (Transverse Symmetry): The midsagittal light beam must bisect the patient's face vertically through the midline (philtrum and glabella). Head rotation or lateral tilt causes asymmetric horizontal magnification.
3. Frankfort Horizontal Plane (Vertical Tilt): The Frankfort plane—running from the superior border of the external acoustic meatus (porion) to the inferior rim of the bony orbit (orbitale)—must be aligned strictly parallel to the floor. Clinically, this is verified using the horizontal alignment light beam.
4. Canine Vertical Alignment Light: The vertical reference beam must be aligned with the contact point between the maxillary canine and lateral incisor (or canine cusp tip), ensuring correct anteroposterior carriage positioning.
5. Tongue Posture & Cervical Spine:
   • The patient must swallow and press the entire dorsum of the tongue firmly flat against the hard palate throughout the entire exposure.
   • The patient must stand or sit upright with shoulders relaxed and the neck extended ('step forward into the machine'), eliminating spinal slumping.

5. Panoramic Positioning Errors, Artifacts & Ghost Images

Positioning errors are the single greatest cause of non-diagnostic panoramic radiographs requiring re-exposure in the UK.

Diagnostic Impact of Vertical Head Tilt in Panoramic Radiography

   A. Chin Tilted Too High (Frankfort Plane Angled Upwards)
      • Occlusal plane appears flat or inverted (downward 'frown')
      • Hard palate radiopaque line superimposed over maxillary tooth apices
      • Mandibular condyles projected off the lateral edges of the image

   B. Chin Tilted Too Low (Frankfort Plane Angled Downwards)
      • Occlusal plane exaggerated upwards (extreme 'joker smile' / V-shaped)
      • Severe foreshortening and compression of mandibular incisors
      • Mandibular condyles projected very high near upper margin of image
      • Symphysis / lower cortex cut off at bottom of image

Comprehensive Breakdown of Panoramic Positioning Faults

Ghost Images (Secondary Images)

Ghost images arise when an anatomically dense object is located between the X-ray tubehead and the center of rotation on one side, projecting across to the contralateral image receptor.

[!IMPORTANT] The Four Cardinal Rules of Ghost Images:

1. Contralateral Location: The ghost image always appears on the opposite side of the true object.
2. Superior Position: The ghost image is always projected higher up than the true anatomical object (due to the upward $+5^\circ$ to $+8^\circ$ angulation of the panoramic X-ray beam).
3. Blurred & Magnified: The ghost image is blurred, horizontally stretched, and magnified because it was positioned outside the focal trough close to the tubehead.
4. Identical Morphology: It retains the generalized shape of the primary object (e.g., metallic earrings, dense mandibular angles, cervical vertebrae).

Choosing a Panoramic Radiograph Appropriately

The examinable limitation of panoramic radiography is that it is a survey examination with relatively poor resolution, magnification that varies across the image, and a focal trough outside which structures are blurred. It is therefore justified for assessing third molars, for gross pathology of the jaws, for trauma, for orthodontic assessment of the developing dentition, for the edentulous or pre-prosthetic patient, and where intraoral views cannot be tolerated. It is not an appropriate examination for approximal caries, for periapical assessment of an individual tooth, or as a routine screening film for a new patient — the standard of care is selection criteria applied to the individual, and taking a panoramic radiograph on every new patient is a breach of IR(ME)R justification.

Candidates should also recognise the normal anatomical structures and the artefacts that a panoramic image reliably produces, because an SBA may present a finding and ask whether it is pathological. The soft palate, the dorsum of the tongue, the nasal septum, the inferior border of the mandible, the hyoid bone and the cervical spine all superimpose in predictable places. The air space between the tongue and the palate produces a radiolucent band across the maxillary apices that is regularly mistaken for pathology; the cervical spine produces a central radiopaque column; and earrings, spectacles, dentures and other metallic objects produce ghost images on the opposite side, higher and more blurred than the original.