Periapical Radiography: Bisecting Angle Technique

• Alternative to paralleling technique
• Not as accurate:
• Prone to more distortion than paralleling technique
• Must use BSA technique if anatomically difficult to position film in parallel alignment:
  
  • Shallow palate 
  • Shallow floor of mouth
    
    Two differences between BSA and paralleling techniques:
    
  • Film placed as close as possible to crown of tooth, so film is NOT parallel to long axis – tilts at an angle from tooth.
  • Vertical angle of central ray perpendicular to line that bisects angle between long axis of tooth and film.
    
    

    

    If vertical angle is aligned perpendicular to bisecting
    line, the tooth is projected on the film with minimal
    distortion, although it is usually more distorted than
    it would appear on a paralleling periapical radiograph

    
    

    
    

    
    All other aspects of BSA technique are the same as paralleling:

    • Horizontal angulation
    • Film positioning in arch
    • Alignment of central ray to cover film