Radiography: intra-oral parallel and bisecting angle

• See dental radiography overview Dental radiography: overview.
• Positioning X-ray film inside mouth minimizes superimposition of irrelevant structures.

• Parallel technique: if angle between tooth and film=<15°, ie mandibular, molars and (most) premolars.
• Target teeth situated parallel to x-ray film with xray beam at 90° to film  .
• Bisecting angle technique: to give an image the same length as the object: if angle between tooth and film = >15° (maxillary molars/premolars, mandibular and maxillary incisors/canines, also mesial/anterior premolars due to mandibular symphysis)  .

Advantages

• Parallel: simple.
• Bisecting angle: minimizes superimposition of other structures. Accurate images despite anatomy preventing parallel film placement.

Disadvantages

• Parallel: limited to mandibular molars and pre-molars.
• Bisecting angle: more technical to obtain; inaccuracies with flawed technique; good at casting 'shadow' due to superimposition if structures occurs.

• Accuracy in obtaining bisecting angle line.
• Head positioning if using standard xray machine.

• Extra-oral parallel technique Radiography: dental extra-oral parallel.
• Intra-oral views Radiography: skull (basic).

Procedure

• 15-30 min depending on skill of radiographer and number of views required.

Criteria for choosing test

• Whether angle between tooth and film = < or >15°.
• Whether one technique will minimize superimpostion best.

Minimum equipment

• See dental radiography: overview Dental radiography: overview.
• X-ray machine.
• Viewer.

Ideal equipment

• Film holders/bite blocks . Tissue or foam squares (sink cleaners!) aid positioning.
• Radiograph magnifier/tube viewer.
• Dental film.
• Dental film processor  .
• Dental film viewer.
• Radiograph marking pen.
• Dental non-screen film:
  • Use dental film sizes:
    • Occlusal (54x70 mm) - for large teeth in dogs.
    • Periapical size 2/adult (30x40 mm) - for most teeth in dogs and cats.
    • Periapical size 0-1/child (20x30 mm) - for cats (especially good for mandibular cheek teeth).
  • Dental film speed of D is optimal for wet processing.
    Practice with cadavers using paper clip to detect distortions (radiograph paper clip on tooth then place paper clip on developed radiograph for size comparison).

Minimum consumables

• Non-screen, dental film.
• Radiographic processing chemicals.
• Dental radiographs; many different types including some self-developing (use intra-envelope developing, eg squeeze developer into film envelope, massage envelope, pull tab on envelope → release film.

• Using dental non-screened film + dental xray unit:
• Film focal distance set by cone (20-30 cm); simply place end of cone onto area of interest.
• Dental xray machines have fixed mA and kV, only the time setting alters.
• Usual fixed settings are around 50-70 kV and 8-15 mA.
• Time settings may be adjusted by a dial or digitally purely by observing the time reading or by use of pictorial aids.
• The pictorial aids are manufacturers suggested settings; machines intended for humans show different sized people and a choice of tooth types/positions while veterinary dental xray machines show different sized dogs (sometimes also cat, if not use small dog setting) and their teeth.
• To use the pictorial settings simply select the size and tooth type/position which best matches your patient! Remember to allow for anticipated skull thicknesses/density!
• Since all methods alter only time setting, the suggested settings below are equally applicable to each.
• Suggested machine settings:
  • Small dog/cat:
    • Maxillary incisors - 0.25-0.32 s. Mandibular incisors - 0.25-0.32 s.
    • Maxillary canines - 0.32-0.40 s. Mandibular canines - 0.32 s.
    • Maxillary premolars and molars - 0.32-0.40 s. Mandibular premolars and molars - 0.20-0.32 s.
  • Medium dog:
    • Maxillary incisors - 0.32-0.40 s. Mandibular incisors - 0.32-0.40 s.
    • Maxillary canines - 0.40-0.63 s. Mandibular canines - 0.32-0.50 s.
    • Maxillary premolars and molars - 0.40-0.63 s. Mandibular premolars and molars - 0.32-0.40 s.
  • Large dog:
    • Maxillary incisors - 0.40-0.63 s. Mandibular incisors - 0.40-0.50 s.
    • Maxillary canines - 0.63-1.00 s. Mandibular canines - 0.40-0.50 s.
    • Maxillary premolars and molars - 0.63-1.00s. Mandibular premolars and molars - 0.40-0.50 s.
• Using veterinary xray machine + non-screen film:
  • Film focal distance = 30-50 cm.
  • Suggested maching settings:
    • Small dog/cat: 65 kV, 20 mAS.
    • Medium dog: 70 kV, 20 mAS.
    • Large dog: 70 kV, 25 mAS.

Ideal consumables

• Dental film.
• Dental (rapid) radiographic processing chemicals.

• Fast animal for 12 hours prior to routine anesthesia to prevent reflux esophagitis.

• See dental radiography overview Dental radiography: overview.

Step 1 - Set up

• Similar to conventional skull radiography Radiography: skull (basic).
• Position animal in lateral recumbency with side to be radiographed uppermost.

Step 2 -

• Parallel:
  • Place dental intra-oral film lingual (medial) to relevant tooth.

Step 3 -

• Push film into the mouth so that its edge is level with ventral border of mandible (to ensure whole tooth root will be radiographed).
  Edge of film can be palpated externally.

Step 4 -

• Bisecting angle:
  • Press film as close to teeth as possible without bending it (any bending will distort image).
  • Hold film in place with, eg cotton wool, swab, foam wedge, crumpled paper towel, rubber mouth prop.
  • Bisecting angle - the X-ray beam is directed perpendicular to an imaginary line that bisects the angle between the long axis of the tooth and the film.

Refereed papers

• Recent references from PubMed and VetMedResource.

Other sources of information

• Smithson A (2005) Oral radiology Part 1. UK Vet 10 (8), 57.
• Smithson A (2005) Oral radiology Part 2. 11 (1),UK Vet 40-44.
• Gorrel C (2004) Veterinary Dentistry for the General Practitioner. Saunders.
• Mulligan, Aller, Williams (1998) Atlas of canine and feline dental radiography. In:Veterinary Learning Systems. Trenton, NJ, USA (Execllent reference for dental radiogaphy).