DEN 130 1 H - Dental Radiology- 42

Extraoral Imaging

Chapter 42
Learning Objectives Lesson 42.1: Extraoral Imaging (Slide 1 of 2)

  • Key Terms: Pronounce, define, and spell the key terms related to extraoral imaging.

  • Panoramic Imaging:

    • Purpose and Uses: Understand the purpose and applications of panoramic imaging.

    • Equipment Used: Identify the tools and machines utilized in panoramic imaging.

    • Patient Preparation and Positioning: Know the steps for adequately preparing and positioning a patient for panoramic imaging.

    • Potential Errors: Be aware of the errors that can occur during patient preparation and positioning.

Learning Objectives Lesson 42.1: Extraoral Imaging (Slide 2 of 2)

  • Three-Dimensional Imaging:

    • CT vs. CBCT: Distinguish between computed tomography (CT) scans and cone beam computed tomography (CBCT).

    • Uses of 3D Imaging: Discuss the applications for three-dimensional imaging in dental practices.

    • Advantages and Disadvantages of CBCT: Analyze the benefits and drawbacks of using CBCT technology.

    • Extraoral Film Projections: Identify the specific purposes of various types of extraoral film projections.

Introduction

  • Extraoral Images Definition: Extraoral images are radiographs taken outside the mouth, essential for evaluating large areas of the skull or jaw.

  • When Used: Extraoral radiographs are particularly crucial when patients cannot open their mouths for film placement.

  • Subtle Changes Detection: These radiographs are not ideal for detecting subtle changes; they are less effective in diagnosing caries or early periodontal changes.

  • Technological Advances: Highlight the advancements in digital panoramic units and cone beam computed tomography (CBCT).

  • Image Quality: Digital extraoral images generally provide better resolution compared to film-based images.

Panoramic Imaging (Slide 1 of 2)

  • Overview: Panoramic imaging enables viewing the entire dentition and associated structures in a single image.

  • Uses: Utilized for locating impacted teeth, detecting jaw lesions, and observing eruption patterns.

  • Historical Context: Previously, panoramic images were not recommended for diagnosing dental caries due to overlapping posterior contact areas; bitewing images were necessary for supplementing panoramic views.

Panoramic Imaging (Slide 2 of 2)

  • Technological Improvement: The advent of advanced digital panoramic units equipped with a special C-arm now allows for the detection of small interproximal carious lesions.

  • Image Clarity: These machines can “open up” contacts in the premolar areas, previously obscured in traditional imaging.

Types of Panoramic Units

  • Classification: There are two main types of panoramic machines:

    • Film-Based Imaging: Traditional method using film.

    • Direct Digital Imaging: Utilizes sensors instead of film, producing immediate results displayed on a computer monitor.

  • Core Difference: The key distinction lies in the image receptor; digital units capture images electronically.

Basic Concepts

  • Mechanics of Panoramic Imaging: Both the film/sensor and tubehead rotate around the patient, creating a sequence of individual images that are combined to produce a comprehensive view of the maxilla and mandible.

Focal Trough (Slide 1 of 2)

  • Definition: Focal trough is an imaginary three-dimensional curved zone where structures appear distinctly on a panoramic radiograph.

  • Positioning Importance: Accurate positioning of patient jaws within this zone results in clearer radiographs; outside this zone, images become blurred or indistinct.

Focal Trough (Slide 2 of 2)

  • Variability: The size and shape of the focal trough differs by panoramic unit manufacturer.

  • Performance Factors: Radiograph quality is contingent on the patient's jaw positioning within the trough and how closely it conforms to the focal trough specifications.

Equipment

  1. Tubehead

    • Comparable to that of intraoral x-ray tubeheads, includes a filament for electron production and a target for radiograph generation.

    • Collimator: Employs a lead plate shaped as a narrow vertical slit, differing from intraoral tubeheads, as vertical angulation is non-adjustable in panoramic tubeheads.

    • Rotation Mechanism: The tubehead rotates behind the patient as the film rotates in front.

  2. Head Positioner

    • Designed to align the patient's teeth for maximum accuracy.

    • Consists of a chin rest, notched bite-block, forehead rest, and lateral guides.

    • Each unit's specifics vary, requiring adherence to manufacturer guidelines for correct patient positioning.

  3. Exposure Controls

    • Settings allow for adjustments in milliamperage and kilovoltage to cater to patients of varying sizes.

    • Note: Exposure time is fixed and cannot be altered.

  4. Film and Intensifying Screens

    • Film-based panoramic imaging employs extraoral screen film housed in a cassette sensitive to light emitted from intensifying screens.

Common Errors

  • Diagnostic Image Quality: Ensuring diagnostic quality panoramic images while minimizing patient exposure requires recognition and prevention of common errors during patient preparation and positioning.

Patient Preparation Errors (Slide 1 of 2)

  1. Ghost Images:

    • Occurs if metallic/radiodense objects are not removed before exposure, creating a “ghost” image opposite the real object, which looks blurred and larger.

    • Solution: Instruct patients to remove all radiodense objects from head and neck prior to positioning.

Visual Example

  • Ghost Image: A photographic example displaying the effect of a ghost image.

Patient Preparation Errors (Slide 2 of 2)

  1. Lead Apron Artifact:

    • Resulting from incorrectly placed lead aprons or using aprons with thyroid collars during exposure, producing a cone-shaped radiopaque artifact.

    • Impact: This artifact disrupts diagnostic clarity.

    • Solution: Use a lead apron without a thyroid collar, placed low around the neck to avoid blocking the x-ray beam.

Patient Positioning Errors: Lips and Tongue

  • Correction: Patient lips must securely close on the bite-block; leaving them open results in a dark radiolucent shadow obscuring anterior teeth. The tongue must maintain contact with the palate; if not, shadows could obstruct maxillary tooth apices.

  • Solution: Instruct by stating, "Close your lips around the bite-block, swallow, then raise your tongue to the palate."

Patient Positioning Errors: Chin Too High

  • Consequence: If the Frankfort plane is positioned incorrectly with the chin too high, superimpositions occur (hard palate/floor of nasal cavity over maxillary roots), leading to blurred and magnified maxillary incisors and a “reverse smile line.”

  • Solution: Position the patient so that the Frankfort plane is parallel to the floor.

Patient Positioning Errors: Chin Too Low

  • Consequence: If the chin is too low, mandibular incisors may appear blurred, anterior apical detail is lost, condyles are not visible, creating an “exaggerated smile line.”

  • Solution: Ensure the patient's Frankfort plane is parallel to the floor.

Patient Positioning Errors: Posterior to Focal Trough

  • Consequence: When anterior teeth are too far back, they appear distorted (“fat” and out of focus).

  • Solution: Position the patient to ensure anterior teeth are aligned end-to-end in the bite-block groove.

Patient Positioning Errors: Anterior to Focal Trough

  • Consequence: If anterior teeth are misplaced forward, they appear “skinny” and out of focus.

  • Solution: Align the anterior teeth end-to-end within the bite-block groove.

Patient Positioning Errors: Spine Not Straight

  • Consequence: An improperly aligned spine leads to cervical spine radiopacity obscuring essential diagnostic information.

  • Solution: Instruct the patient to stand or sit tall with a straight back.

Cone Beam Computed Tomography (CBCT)

  • Functionality: During CBCT exams, the arm rotates around the patient's head, executing a full 360-degree rotation, capturing between 200-600 2D images.

  • 3D Image Formation: These images are digitally combined to create formats that reveal significant dental and surgical information.

Advantages of CBCT

  • 3D Views: Provides multidimensional perspectives of the mouth, face, and jaw.

  • Software Aid: Advanced software allows visualization of all anatomical structures including soft tissue, with features to overlay facial images onto radiographs.

  • Image Manipulation: Digitally captured images are easily adjustable, colorized, and shareable online for further consultation.

Additional Advantages

  • Enhanced Diagnostics: CBCT significantly improves diagnostic capabilities for:

    • Implant Placement: Facilitates accurate positioning of implants.

    • Surgical Extractions: Aids in the extraction of impacted teeth.

    • Mandibular Nerve Localization: Identifies the mandibular nerve location prior to surgical interventions.

Common Uses of CBCT

  • Adoption: Increasingly utilized by dentists and dental specialists in their practices.

  • Training: Requires training to effectively operate CBCT devices and interpret 3D data formats.

Specialized Extraoral Imaging

  • Equipment: Extraoral images can be captured using either film-based or digital systems, providing larger area views, including the skull and jaws.

  • Standard Equipment: Standard intraoral x-ray machines may accommodate extraoral radiographs with additional head positioning and beam alignment aids; panoramic units can utilize a cephalostat for improved patient positioning.

Skull Radiography (Slide 1 of 2)

  • Application: Commonly utilized in oral surgery and orthodontics, while standard intraoral radiographs are sometimes sufficient for skull films, most require extraoral units with cephalostats.

Skull Radiography (Slide 2 of 2)

  • Interpretation Difficulty: Skull radiographs can be challenging to interpret due to overlapping anatomical structures.

  • Common Projections:

    • Lateral Cephalometric Projection

    • Posteroanterior Projection

    • Temporomandibular Joint Projection

Lateral Cephalometric Projection

  • Evaluation Uses: Assesses facial growth, trauma, disease, and developmental abnormalities; displays facial bones and soft tissue profile.

Posteroanterior Projection

  • Evaluation Uses: Used similarly to Lateral Cephalometric for assessing growth, trauma, and abnormalities; shows frontal and ethmoid sinuses, orbits, and nasal cavities.

Temporomandibular Joint (TMJ) Radiography

  • Examination Challenges: TMJ radiographs are complex due to adjacent bony structures; soft tissues like the articular disc are not visible via standard radiography.

  • Special Techniques Required: Techniques such as arthrography and magnetic resonance imaging (MRI) are used for thorough evaluation.

  • Projection Uses: View TMJ bone and assess joint relationships.