Module 6: X Rays

Dental Imaging Examinations:

1. Intraoral projections (inside mouth)

   • examine teeth + surrounding structures

   • periapical, interproximal, occlusal examinations

2. Extra-oral projections

   • placed outside the mouth

   • to examine large areas of skull + jaw

Projection Geometry Principles

1. the source of radiation should be as small as possible

   • small focal spot

   • sharper images

2. The source-to-object distance should be as long as possible

   • When you decrease the source object distance, you make the shadow larger + increase width of the penumbra (shadow around shadow)

3. The object-to-receptor distance should be as short as possible

   • results in sharper images

4. The object should be parallel to the receptor

   • prevents forshortening or elongation

5. The beam shoudld be perpendicular to the object and receptor

   • prevents forshortening or elongation

X-Ray Techniques

1. Paralleling technique

   • receptor is parallel to the long axis of the tooth, PID is perpendicular

   • may not be possible because of anatomy, small mouths, gag reflex, etc.

2. Bisecting technique

   • rule of isometry

   • two triangles are equal if the triangles have two equal angles and share a common side

Topic 1: Bisecting Angle

Bisecting Angle Technique (BAT)

• Also called:

  • Bisecting angle method

  • Short cone technique

• Type: Intra-oral radiographic technique

• Use: Producing periapical radiographs

• Indication: When paralleling technique is not possible

  • Small mouths

  • Limited mouth opening

  • Anatomical constraints

Purpose

• To obtain detailed images of:

  • Tooth

  • Surrounding structures

Key Concept

• Based on bisecting the angle between:

  • Long axis of the tooth

  • Plane of the receptor (film)

• X-ray beam is directed perpendicular (90°) to this bisector

Five Main Principles

1. Bisecting Angle Concept

• Imaginary line bisects angle between tooth and receptor

• Central ray directed through this bisector

2. Film Placement

• Placed on lingual/palatal surface

• Positioned:

  • As close as possible to tooth

  • Covers area of interest

  • Extends beyond tooth margins

3. Central Ray Alignment

• Directed perpendicular to bisecting angle

• Passes through intersection of:

  • Tooth long axis

  • Film plane

4. Vertical Angulation

• Adjusted to align with bisector

• Ensures correct image length

• Errors → distortion

5. Horizontal Angulation

• Directed through contact areas

• Ensures:

  • Open contacts

  • Symmetrical image

Geometric Principle (Rule of Isometry)

• Two triangles are equal if:

  • Two angles are equal

  • They share a common side

Application in BAT:

• Creates two equal imaginary triangles

• Results:

  • Accurate tooth length

  • True representation

Technique Steps

1. Place receptor along lingual surface

2. Identify angle between tooth and receptor

3. Visualise imaginary bisector

4. Direct central ray 90° to bisector

5. Produce two congruent right triangles

Position-Indicating Device (PID) Angulation

Horizontal Angulation

• Direction: Side-to-side

• Goal:

  • Beam passes through contact points

  • Produces open contacts

• Error:

  • Overlapping contacts → non-diagnostic image

Vertical Angulation

• Direction: Up-and-down

• Measured in degrees

• Determines image length

Errors:

• Excessive angulation → Foreshortening

  • Tooth appears shorter

• Insufficient angulation → Elongation

  • Tooth appears longer

Advantages

• Useful when paralleling technique not possible

• No need for beam alignment device in some cases

• Short PID (8-inch):

  • Reduced exposure time

Disadvantages

1. Image Distortion

• Caused by:

  • Short PID → increased divergence → magnification

  • 3D object projected onto 2D receptor

2. Angulation Errors

• Difficult to visualise bisector

• No aiming ring → reduced accuracy

• Errors lead to:

  • Elongation

  • Foreshortening

Limitations

• Requires high operator skill

• Greater chance of anatomical distortion

Topic 2: Occlusal Technique

Occlusal Radiographs

Definition

• Used to examine large areas of:

  • Maxilla

  • Mandible

• Less common than:

  • Bite-wings

  • Periapical radiographs

• Often used in conjunction with other radiographs

Indications (Uses)

• Localisation of:

  • Impacted teeth

  • Retained roots

  • Supernumerary teeth

• Detection of:

  • Foreign bodies

  • Salivary stones (e.g. submandibular gland)

  • Cysts

  • Benign tumours

  • Malignancies

• Assessment of:

  • Cleft palate

  • Size and shape of maxilla/mandible

Relevance

• Previously more common before CBCT (3D imaging)

• Still important and clinically relevant today

General Principles

Receptor Placement

• Positioned between maxillary and mandibular teeth

• Provides broad view of dental arch

Receptor Orientation

• Tube side faces arch being examined

  • Film:

    • White side → faces exposed arch

  • Digital sensor:

    • Flat/non-wired side → faces arch

Stabilisation

• Patient gently bites on receptor

• Important:

  • Bite lightly

  • Avoid damaging sensor or causing bite marks

Maxillary Occlusal Projections

1. Topographic Projection

• Examines:

  • Palate

  • Anterior maxillary teeth

2. Lateral Projection (Right/Left)

• Examines:

  • Palatal roots of molars

• Also used for:

  • Foreign bodies

  • Posterior maxillary lesions

3. Pediatric Projection

• Examines:

  • Anterior maxillary teeth

• Recommended for:

  • Children ≤ 5 years

Mandibular Occlusal Projections

1. Topographic Projection

• Examines:

  • Anterior mandibular teeth

2. Cross-Sectional Projection

• Examines:

  • Buccal and lingual aspects

• Used for:

  • Foreign bodies

  • Salivary stones (floor of mouth)

3. Pediatric Projection

• Examines:

  • Anterior mandibular teeth

• Recommended for:

  • Children ≤ 5 years

Vertical Angulations

Key Points to Remember

• Provides wide coverage of jaws

• Useful for localisation and pathology detection

• Correct receptor placement + angulation is critical

• Still valuable despite advances like CBCT

Here’s a side-by-side comparison sheet designed for quick exam revision:

Bisecting Angle Technique vs Occlusal Radiographs

Technique & Angulation

Projections / Applications

Advantages vs Disadvantages

Key Concepts to Memorise

Bisecting Angle Technique

• Rule of isometry

• Central ray ⟂ bisector

• Common errors:

  • Foreshortening = too steep

  • Elongation = too flat

Occlusal Radiographs

• Bite to stabilise receptor

• Used for large area + localisation

• Know vertical angulations:

• Maxillary:

  • +65° (topographic)

  • +60° (lateral/pediatric)

• Mandibular:

  • -55° (topographic/pediatric)

  • 90° (cross-sectional)

Quick Exam Tip

• BAT = Accuracy of tooth length (geometry-based)

• Occlusal = Coverage + localisation (big picture)

Topic 3: Localization Technique

Here are your notes, organised clearly for exam revision:

Localization Techniques in Dental Radiography

Definition

• Methods used to determine the exact 3D position of a tooth or object in the jaws

• Needed because radiographs are 2D images of 3D structures

Limitations of Radiographs

• Show:

  • Superior–inferior

  • Anterior–posterior

• Do NOT show:

  • Buccal–lingual (depth) position

➡ Therefore, localization techniques are required

Clinical Importance

• Provides 3D information for:

  • Diagnosis

  • Treatment planning

  • Surgical procedures

Common Uses

• Impacted/unerupted teeth

• Retained roots

• Root position

• Foreign bodies

• Salivary stones

• Jaw fractures

• Broken instruments/needles

Types of Localization Techniques

1. Buccal Object Rule (SLOB rule)

2. Right-Angle Technique

Principle of Parallax (Tube Shift)

• Definition: Apparent movement of an object when X-ray beam angle changes

• Compared to a reference object (usually adjacent tooth)

Key Concept

• Object farther from tube → moves same direction as tube

• Object closer to tube → moves opposite direction

Tube Shift Directions

• Horizontal

• Vertical

Horizontal Tube Shift Technique (Clark’s Technique)

• Based on parallax principle

• Also called:

  • Clark’s technique (1910)

  • Uses SLOB rule

Method

• Originally: 3 radiographs (mesial, distal, central)

• Now: 2 radiographs sufficient

Purpose

• Determine buccal vs lingual position

Vertical Tube Shift Technique (VTS)

• Described by Keur (1986)

Radiographs Used

• Panoramic radiograph (PR)

• Occlusal radiograph (OR)

Angulations

• PR: ~ +7° (effective)

• OR: +60° to +65°

• Difference: ~53°–58°

Key Point

• Requires only one additional exposure if PR already taken

Buccal Object Rule (SLOB Rule)

Definition

• Technique to determine if object is buccal or lingual

Method

1. Take first radiograph

2. Shift tube (horizontal or vertical)

3. Take second radiograph

4. Compare object movement

SLOB Rule

• Same direction = Lingual

• Opposite direction = Buccal

Applications

• Impacted teeth

• Root canals

• Foreign bodies

• Retained roots

Right-Angle Technique

Definition

• Uses two radiographs at 90° to each other

Method

1. Periapical radiograph

   • Shows:

     • Superior–inferior

     • Anterior–posterior

2. Occlusal radiograph

   • Beam directed perpendicular (90°)

   • Shows:

     • Buccal–lingual

     • Anterior–posterior

Outcome

• Combined images give full 3D localisation

Common Use

• Often used for mandibular objects

Key Comparisons

Key Exam Points

• Radiographs = 2D → need localization for 3D

• Parallax = apparent movement

• SLOB rule: Same Lingual, Opposite Buccal

• Right-angle technique uses:

  • Periapical + Occlusal

• Always compare two images

MY NOTES:

• Incorrect horizontal angulation results in overlapped contact areas

• Foreshortening of images from excessive vertical angulation

• Elongation of images results from insufficient vertical angulation

Advantages 

1. Can be used without an alignment device when the patient's anatomy (shallow palate, bony growths, sensitive mandibular premolar areas) precludes its use.

2. A decreased exposure time. When a short (8-inch) PID is used with the bisecting technique, a shorter exposure time is recommended. 

Disadvantages: The primary disadvantage is the image distortion. It can be summarised as follows: 

• Image distortion: Distortion occurs when a short PID is used; a short PID increases X-ray divergence, resulting in image magnification. Distortion also occurs when a tooth (3D structure) is projected onto a receptor (2D structure); structures that are farther away from the receptor appear more elongated than those closer to the receptor. 

• Angulation problems: Without a beam alignment device and aiming ring, it is difficult for the dental radiographer to visualise the imaginary bisector and determine the vertical angulation. Any error in vertical angulation will result in image distortion (elongation or foreshortening).