11.3 Panoramic Radiography, Patient Positioning Errors & Extraoral Imaging

Panoramic Radiography, Patient Positioning Errors & Extraoral Imaging

Quick Answer: Panoramic radiography (orthopantomography) is an extraoral technique that captures the entire maxilla, mandible, dentition, temporomandibular joints (TMJs), and maxillary sinuses on a single continuous image. It operates via rotational tomography, projecting a narrow vertical slit beam through a three-dimensional curved image layer termed the focal trough. Patient alignment requires the midsagittal plane perpendicular to the floor and the Frankfort horizontal plane parallel to the floor. Current ADA guidance does not recommend routine patient shielding; applicable regulations still control. If a required shield is used, a thyroid collar must not enter the panoramic beam. If the patient does not press their tongue firmly against the hard palate, a dark palatoglossal air space artifact obscures the maxillary root apices.

Extraoral imaging provides broad anatomical overviews essential for orthodontic evaluation, surgical planning, trauma triage, third molar impaction assessment, and pathology screening. For the DANB NELDA Radiation Health and Safety (RHS) exam, dental assistants must master the physics of rotational tomography, diagnose every patient positioning error, and understand specialized extraoral modalities such as Cone Beam Computed Tomography (CBCT) and Cephalometric projections.

---

1. Principles of Rotational Panoramic Tomography & The Focal Trough

Panoramic imaging is governed by the principles of tomography (imaging a specific anatomical layer while blurring out structures in other planes). In panoramic systems:

1. The x-ray tubehead rotates around the back of the patient's head in one direction.
2. The image receptor (digital sensor or PSP drum) rotates around the front of the patient's face in the opposite direction.
3. The x-ray beam emerges from a narrow vertical slit collimator (producing a beam roughly 1 to 2 mm wide) and is directed slightly upward at a negative vertical angulation of approximately -4° to -7°.

                    ROTATIONAL PANORAMIC FOCAL TROUGH

                             [ X-Ray Tubehead ]
                                     \ (Slit beam)
                                      \
                   +-------------------+-------------------+
                   |                                       |
                   |       FOCAL TROUGH (Image Layer)      |
                   |   +-------------------------------+   |
                   |   |   (Sharp, in-focus anatomy)   |   |
                   |   |        [ DENTITION ]          |   |
                   |   +-------------------------------+   |
                   |                                       |
                   +-------------------+-------------------+
                                      /
                                     / (Opposing rotation)
                             [ Digital Receptor ]

The Focal Trough (Image Layer)

The focal trough is a three-dimensional, horseshoe-shaped curved zone of sharpness. Structures positioned inside the focal trough appear sharp, well-defined, and diagnostically accurate. Structures positioned outside the focal trough (too far forward, backward, or laterally) appear blurred, magnified, or minified and may be obscured by ghost artifacts.

Real Images vs. Double Images vs. Ghost Images

• Real Images: Anatomical structures located between the center of rotation and the receptor (e.g., teeth, alveolar bone, maxillary sinuses).
• Double Images: Structures located in the midline that are intercepted twice by the rotating beam (e.g., hard palate, soft palate, epiglottis, cervical spine, hyoid bone).
• Ghost Images: Radiopaque artifacts created when dense objects (earrings, necklaces, metallic eyeglasses, heavy mandibular angles) located between the x-ray tubehead and the center of rotation are penetrated by the beam. Ghost images appear on the opposite side of the radiograph, higher up than the real object, magnified, and blurred/unsharp.

---

2. Step-by-Step Panoramic Patient Positioning Protocol

To ensure the patient's dentition sits squarely within the focal trough, the dental assistant must execute a strict, standardized preparation sequence:

                PANORAMIC POSITIONING WORKFLOW SEQUENCE

  1. REMOVE DENSE OBJECTS (Jewelry, glasses, dentures, hairpins, piercings)
                         |
                         v
  2. CHECK SHIELDING RULE (If required, keep collar outside beam)
                         |
                         v
  3. BITE IN NOTCH (Incisal edges seated end-to-end in bite-stick grooves)
                         |
                         v
  4. ALIGN PLANES (Midsagittal perpendicular; Frankfort parallel to floor)
                         |
                         v
  5. STAND TALL (Spine erect, neck extended, shoulders relaxed down)
                         |
                         v
  6. TONGUE TO PALATE (Lips closed around stick; tongue pressed to palate)

Detailed Clinical Positioning Checkpoints

1. Object Removal: All metallic, dense, or radiopaque items from the head and neck must be removed: earrings, tongue rings, nose rings, necklaces, eyeglasses, bobby pins, hearing aids, and removable dental prostheses (partial/full dentures, retainers).
2. Shielding Check: Follow applicable law. Routine patient shielding is not recommended by current ADA guidance; if a jurisdiction requires a shield, position it so no thyroid collar or apron edge enters the rotating beam.
3. Bite-Block Alignment: Instruct the patient to bite into the grooves of the plastic bite-stick so that the maxillary and mandibular incisors meet in an end-to-end incisal edge relationship.
4. Midsagittal Plane Alignment: Center the patient's head so that the vertical midsagittal laser beam runs directly down the facial midline (perpendicular to the floor). The head must not be tilted or rotated.
5. Frankfort Horizontal Plane Alignment: Adjust the unit height so that the horizontal laser beam aligns with the Frankfort plane (the line connecting the superior border of the external auditory meatus / porion to the inferior border of the bony orbit / orbitale), running parallel to the floor.
6. Posture Alignment: The patient must step forward slightly and stand tall with an erect spine and relaxed shoulders to prevent the cervical vertebrae from slumping into the beam path.
7. Soft Tissue Seal: Instruct the patient to close their lips tightly around the bite-stick and swallow, holding the tongue flat against the entire roof of the hard palate for the full 15–20 second exposure cycle.

---

3. Panoramic Positioning Errors: Master Diagnostic Guide

Positioning errors account for over 85% of all non-diagnostic panoramic retakes. Dental assistants must instantly identify each artifact on sight:

                   PANORAMIC CHIN ANGLE DIAGNOSTIC PATTERNS

     [ CHIN TILTED DOWN ]        [ CORRECT POSITION ]        [ CHIN TILTED UP ]
       (Frankfort Down)          (Frankfort Parallel)         (Frankfort Up)
     ====================        ====================        ==================
           \      /                   __________                    ______
            \____/                   (   Normal )                  /      \
                                      \__Curve_/                  /        \
      EXAGGERATED SMILE           GENTLE UPWARD SMILE           FLAT / FROWN
      Condyles cut off top         Ideal Occlusal Plane         Palate over apices

---

4. Extraoral Radiography Modalities: Cephalometrics & CBCT

1. Cephalometric Radiography

Cephalometric projections evaluate facial growth, skeletal relationships, dental malocclusions, and soft tissue facial profiles. A specialized cephalostat equipped with ear rods and an orbital pointer stabilizes the head at a standardized source-to-receptor distance of 5 feet (60 inches).

• Lateral Cephalometric Projection: Beam strikes the side of the head perpendicular to the midsagittal plane. Essential for orthodontic tracing (Downs, Steiner, and Tweed analyses) and orthognathic surgery.
• Posteroanterior (PA) Cephalometric Projection: Beam strikes the back of the head (posterior to anterior). Used to diagnose craniofacial asymmetries, midface fractures, and transverse skeletal discrepancies.

2. Cone Beam Computed Tomography (CBCT)

CBCT is a three-dimensional volumetric imaging technology that has revolutionized dental diagnostics. Unlike conventional medical CT scanners that emit a fan-shaped x-ray beam and capture narrow slices, CBCT utilizes a cone-shaped x-ray beam and a 2D flat panel detector to capture the entire anatomical volume in a single rotation.

                      CBCT MULTIPLANAR RECONSTRUCTION (MPR)

          [ AXIAL PLANE ]            [ CORONAL PLANE ]          [ SAGITTAL PLANE ]
           (Top-to-Bottom)            (Front-to-Back)             (Side-to-Side)
             Horizontal                  Vertical                    Vertical
           Cross-Sections             Anterior/Posterior           Lateral Views

• Multiplanar Reconstruction (MPR): CBCT software reconstructs the 3D data volume into three primary orthogonal planes:
  1. Axial Plane: Transverse (top-to-bottom) horizontal slices.
  2. Coronal Plane: Frontal (anterior-to-posterior) vertical slices.
  3. Sagittal Plane: Longitudinal (side-to-side) vertical slices.
• Isotropic Voxels: CBCT images are composed of three-dimensional cubic volume elements (voxels) that are isotropic (equal dimensions in height, width, and depth, typically 0.075 mm to 0.4 mm), ensuring 1:1 true anatomical measurement without geometric distortion.
• Field of View (FOV): Dental assistants select the appropriate FOV according to the clinical indication to minimize radiation dose:
  • Small FOV (e.g., 5 $\times$ 5 cm): Single tooth or sextant (endodontic root canal morphology, localized root fractures).
  • Medium FOV (e.g., 8 $\times$ 8 cm): Single arch or both dental arches (implant planning, impacted canines).
  • Large FOV (e.g., 15 $\times$ 15 cm): Entire craniofacial complex (orthognathic surgery, TMJ pathology, airway sleep apnea analysis).

3. Specialized Extraoral Skull Views

• Waters Projection (Occipitomental View): Patient's chin touches the receptor while the nose is elevated 0.5 to 1 inch. Primary indication: Evaluation of the maxillary sinuses, orbit floors, and zygomatic arches.
• Submentovertex (SMV) Projection: Central ray directed from under the chin upward through the vertex of the skull. Primary indication: Assessment of the zygomatic arches ("jug-handle" fractures) and base of the skull.
• Reverse Towne Projection: Evaluates fractures of the condylar neck and ramus of the mandible.

---

5. DANB NELDA Clinical Exam Traps & Chairside Pearls

[!CAUTION] DANB Exam Trap #1: Shielding and Panoramic Artifacts Current ADA guidance does not recommend routine patient shielding. Where a jurisdiction still requires shielding, a thyroid collar must remain outside the panoramic beam because it can create a dense radiopaque artifact over the mandible and force a retake.

[!WARNING] DANB Exam Trap #2: Palatoglossal Air Space vs. Pathology An exam question may present a panoramic image showing a horizontal radiolucent (dark) band obscuring the root apices of the maxillary molars and premolars. This is NOT a periapical cyst or maxillary sinus defect—it is the palatoglossal air space, caused when the patient fails to press the dorsal surface of their tongue flat against the hard palate during exposure.

[!NOTE] DANB Exam Trap #3: Ghost Image Characteristics Questions frequently ask how to distinguish a ghost image from a real anatomical object. A ghost image is always: (1) located on the opposite side of the real object, (2) located higher up than the real object, (3) magnified, and (4) blurred / out-of-focus.