Geometric Characteristics of Imaging

All radiographic projections are plagued by three problems:

• Unsharpness (penumbra)
• Magnification
• Distortion

X-rays originate from a small plane (focal spot) as numerous point sources. Each point source forms its own image. Edges of images will not be at the same point on the film or sensor, hence images are always unsharp (blurred).
X-rays travel in diverging lines, which projects edges of objects farther away from each other. This results in magnification.
Structures have 3 dimensions, but are projected onto a 2 dimensional film/sensor. Different parts of the structure are varying distances, hence magnification is different at each distance distorting image.

Unsharpness (Penumbra)

Unsharpness is minimized by:

• Long source-object distance --longer PID
• Short object-flim distance --place film closer to tooth surface
• Small focal spot --done by manufacturer
• Limiting patient motion

Magnification

Equal Enlargement of all parts of the image.
Magnification is reduced by:

• Long source-object distance --longer PID
  
• Short object-flim distance --place film closer to tooth surface
  
• Small focal spot

Distortion

Unequal enlargement of parts of the image. Normal crown to root ratio is 1:2. Distortion appears across all teeth. If patient has naturally long roots then it will appear in only a few teeth.
Reduced by:

• Parallel alignment of teeth and film --film parallel to long axis of tooth
• Perpendicular alignment of beam and teeth/ x-ray is at right angle to film

Summary: Accurate Image Formation

• Long source-object distance --longer PID
  
• Short object-film distance --place film closer to tooth surface
  
• Small focal spot
• Perpendicular alignment of beam and teeth/ x-ray is at right angle to film

Intra oral Radiographic Landmarks

Teeth are composed primarily of dentin, with an enamel cap over the coronal portion and a thin layer of cementum over the root surface.

Enamel: because it is 90% mineralized it is the most radiopaque tissue in the body
Dentin: is about 75% mineralized, and is less radiopaque
Pulp chamber: radiolucent
Root Canals: extension of pulp into roots
Cementum: indistinguishable from dentin

Periodontal Ligament (PDL) Space

• Radiolucent line outside roots
• Varies in thickness: usually thinner in middle root, wider near apices and alveolar crest
• PDL is thin around unerupted and unopposed teeth
• If root has two convexities a "double PDL" can be seen. Common in upper posterior teeth and the mesial roots of lower molars.

Lamina Dura

• Radiopaque line around PDL, representing bone into which ligaments attach into alveolus
• Appearance can vary due to different horizontal angulations

Alveolar Crest

• Gingival margin of alveolar process extending between teeth
• Appears as radiopacity at superior aspect of alveolar process
• Pointed in anterior teeth, flat in posterior teeth
  Note: 0.5-2.0mm is the normal distance from CEJ to alveolar crest. More than this can indicate bone loss (periodontal disease).

Trabecular bone (Cancellous bone)

• Thin interlacing radiopaque plates and rods (trabeculae) interspersed between cortical plates
• Maxilla: porous & numerous, producing a granular pattern
• Mandible: fewer trabeculae, often oriented horizontally in step-ladder pattern
• Posterior mandible: trabeculae often form step-ladder pattern
• Anterior mandible: trabeculae more sparse
• Where cortical plates are thin (maxilla and anterior mandible), trabeculae may be more numerous to bolster the jaw
• Where cortical plates are thick (posterior mandible), not as many trabeculae are needed for strength
  
  

  

  "Step-ladder" trabecular bone in mandible

  
  

Dental Follicle (Dental sac)

• Connective tissue surrounding developing tooth
• Usually appears as thin, uniform radiolucency around unerupted tooth
• May be thicker in some areas (e.g., maxillary canine)
  
  
  
  
  

Dental Papilla

• Apical portion of dental follicle
• Delicate connective tissue that forms the dental pulp
• Appears as radiolucency 

Nutrient Canals

• Carry neurovascular tissue to bone and teeth
• Appear as thick, uniform vertical radiolucent lines
• Most prominant in anterior mandible

Arrows pointing to nutrient canals

Incisive Foramen/Nasopalatine Foramen

• Opening of nasopalatine canals
• Carries nasopalatine vessels and nerves
• Midline of anterior maxilla behind central incisors at junction of median palatal and incisive sutures
• Solitary radiolucency between centrals
• Indistinct border/ well-defined
• Heart-shaped
• Differentiate from periapical inflammation (if lamina is breached then it is an infection)

 

Median Palatal Suture (Intermaxillary Suture)

• Union of palatal shelves
• Appears as thin, uniform radiolucent line vertically positioned between central incisors
• May be widened at alveolar crest (normal variant)
  

Nasopalatine Canals

• Extended from floor of nasal cavities to palate
• Transmit nasopalatine vessels
• Radiographic appearance: paired radiolucencies, vertical (nose to palate), thick and uniform

 

Nasal Fossae

• Paired air cavities superior to coral cavity
• Appear as radiolucencies overlying anterior teeth
• Associated with: anterior wall of nasal fossa ("floor of nasal fossa"), nasal septum, and inferior turbinate one (concha)

Anterior Wall ("Floor") of Nasal Fossa

• Inferior aspect of nasal fossa
• Projected on a tangent in anterior periapical radiographs
• Appears as bilateral, densely radiopaque, uniformly thick lines extending laterally from midline subadjacent to nasal fossae
• Not truly the "floor" of nasal cavity

Nasal Septum

• Consists of vomer and cartilage
• Appears as vertical, irregular radiopacity between nasal fossae
• Mucoperiosteum produces faintly radiopaque image around septum
• Often deviated

Inferior Turbinate Bone

• Attached to lateral wall of nasal fossa
• Delicately radiopaque, curved structures

Anterior Nasal Spine

• Protuberance of bone subadjacent to midline of nasal fossae at inferior junction of nasal septum
• 1.5 to 2.0cm superior to alveolar process

Soft Tissue of Nose

• Seen over maxillary anterior teeth and nasal fossae
• Faintly radiopaque, rounded line of contrast

Lateral Fossa

• Depression--buccal aspect of lateral incisor root
• Appears as diffuse, faintly radiolucent image overlying lateral root

 

Maxillary Sinus

• Air filled cavity lined by mucoperiosteum in maxilla
• Usually extends from canine to molars
• Radiographic appearance: round or oval radiolucency surrounded by thin, uniform radiopaque wall
• Wall of nasal fossa may be imposed, producing the "inverted Y" appearance on canine periapical radiographs
• When maxillary teeth are missing, the sinus can expand into the edentulous space in a process called "pneumatization"
• Some sinuses have vertical walls (septae) in the sinus cavity. Septae run vertically from the inferior aspect of the sinus to the superior aspect, which is the floor of the orbit

Nasolabial Fold

• Line of contrast depicting the line extending from the corner of the nose to the corner of the lip

Zygomatic process of Maxilla

• Thickened extension on buccal surface of maxilla
• Articulation of the zygoma with the maxilla
• Buttress that prevents maxilla from separating from base of skull
• Arises between 1st and 2nd molars on buccal surface of maxilla
• Thick, densely radiopaque "U" shaped structure superimposed over maxillary sinus
• Zygoma attaches to process and extends posteriorly
  
  

Zygoma

• Articulates anteriorly with zygomatic process of maxilla
• Appears inferiorly and posteriorly to zygomatic process
• Uniformly radiopaque, rounded structure superimposed on maxillary sinus

Maxillary tuberosity

• Posterior aspect of maxilla

Pterygoid Plates and Hamulus Process

• Pterygoid plates--appear as single rectangular radiopacity
• Hamular process extends inferiorly from medial pterygoid plate
• Appears as "punching bag" radiopacity

Nasolacrimal Canal

• Extends from medial aspect of inferior orbit to nasal cavity below inferior turbinate

Coronoid Process of Mandible

• Beak-shaped process at superior aspect of mandibular amus
• Often appears on maxillary molar periapical radiographs
• Well defined, uniformly radiopaque structure arising at posterior and inferior aspect of maxillary molar PA

Genial Tubercles

• Bony extensions on lingual surface of anterior mandible
• Attachments for genioglossus and geniohyoid muscle
• Appear as linear, uniformly radiopaque structures at midline

Lingual Foramen

• Opening in the lingual surface of the mandible
• Terminal branches of incisive branch of mandibular canal exit through foramen
• Appears as a small radiolucency surrounded by radiopaque cortical wall of canal

Mental Ridges

• Protuberances on anterior aspect of mandible, sweeping up from the canine area to the midline
• Represent the prominence of the chin
• Appear as symmetrical, curved radiopaque lines

Mental Fossa

• Depression on anterior aspect of mandible above mental ridge
• Appears as a diffuse radiolucency around the roots of incisors

 

Mental Foramen

• Opening in buccal surface of mandible for branch of trigeminal nerve and vessels
• Usually located between 1st and 2nd premolars near apices
• Usually appears as round or oval radiolucency, may have corticated border

Mandibular Canal (Inferior Alveolar Canal)

• Transmits mandibular branch of trigeminal nerve & vessles
• Neurovascular tissue enters mandible at the mandibular foramen
• Appears as thick, linear radiolucency inferior to roots of teeth
• Visualized in molar and premolar area

Internal Oblique Ridge (Mylohyoid Ridge)

• Protuberance of bone (lingual surface of mandible)
• Appears as a linear radiopaque structure running obliquely downward and forward from molars to premolars, at level of root apices
• Attachment of mylohyoid muscle
• Located lower than external obliques ridge

External Oblique Ridge

• Continuation of anterior border of mandibular ramus
• Located superior to internal oblique ridge
• Arises lateral to alveolar process from 3rd molar to 1st molar area
• Thick in psterior areas, thinner anteriorly
• Appears as linear radiopaque structure

Submandibular Gland Fossa

• Depression in the lingual aspect of mandible below mylohyoid ridge in molar region
• Submandibular salivary gland is located there
• Depression produced a diffuse radiolucency with sparse trabecular pattern
• Submandibular gland fossa often looks very dark in contrast to mylohyoid ridge superior to it

Radiology

Types of Intra Oral Radiographs and Their Indications 
Reference:http://www.amazon.com/Oral-Radiology-Interpretation-Stuart-White/dp/0323049834/ref=dp_ob_title_bk

 Periapical Radiograph

• Visualizes crowns, contact points, roots, apices and area around the apices
• Indication: periapical pathosis, lesions in alveolar process, jaws, and proximal caries in anterior teeth
• Method: paralleling technique; the film is placed parallel to the long axis of the tooth, and x-ray should always be 90 degrees to the long axis of the tooth and the film
  Note: x-ray is what comes out of the machine, radiograph is the image

Interproximal (Bitewing) Radiograph

• Has less distortion than periapical
• Used to visualize crowns with contact, periodontal bone level
• Indications: proximal and occlusal caries and periodontal bone loss on posteriors
• Method: x-ray beam is at a 90 degree angle to object and film.
  Note: if x-ray is not 90 degrees you get image distortion. Bitewing radiographs have the least distortion!!
• A full mouth series consists of both periapicals and bitewings.

 

Occlusal Radiographs

A special type of intraoral radiograph made with the film held between the occluded teeth. Used to visualize bucco-lingual aspect.

Maxillary Topographical Occlusal/ Anterior Maxillary Occlusal Projection

• Visualizes large areas of maxilla (palate and alveolus); but teeth are distorted.
• Indications: impacted anterior teeth, large pathologic lesions, and localizing objects in the buccal-lingual dimension.
  Note: Can visualize buccal-lingual aspect with occlusal radiogrpahs but CANNOT with periapical and bitewing, which only show mesio-distal aspect. All radiograps are 2D depictions of a 3D object.
  

  

  The primary field includes the anterior maxilla and it's dentition and the anterior floor of the nasal fossa and teeth from canine to canine.

  
  

Anterior Mandibular Occlusal Projection

• Visualizes anterior mandibular alveolar process
• Indications: developing permanent teeth in children, and large pathologic lesions in anterior mandible.
  

  

  This projection includes the anterior portion of the mandible, the dentition from canine to canine, and the inferior cortical border of the mandible.

Cross-sectional Mandibular Occlusal Projection

• Visulalizes floor of the mouth and body of the mandible; but teeth are distorted.
• Indications: identifying objects in floor of mouth (eg., sialoliths), and localizing objects in the buccal-lingual dimension.
  
  

  

  This projection includes the soft tissue of the floor of the mouth and reveals the lingual and buccal plates of the mandible from second molar to second molar.