Image Shift Principle

• Periapical and bitewing radiographs provide 2 dimensional views
• Mesial-distal and occlusal-apical
• Can not see depth: the buccal-lingual dimension
• Occlusal radiograph can help localize objects in the buccal-lingual dimension. But there is another technique that can be used with periapical radiographs: the image shift principle.
• The images of objects at different distances from the film (different positions in the buccal-lingual dimension) will move relative to each other when two radiographs are exposed at different angulations.
  
  
  • If you increase the horizontal angulation (move tube head more distally), images shoft to the mesial
  • If you decrease the horizontal angulation (move tube head more mesially), images shift distal
  • If you increase the positive vertical angulation or decrease the negative vertical angulation (move tube head further up), images shift down
  • If you increase the negative vertical angulation or decrease the positive vertical angulation (move the tube head farther down), images shift up
  • To summarize: the images will ALL shift in the opposite direction of the movement of the x-ray tube head
  • The farther an object is from the film, the greater its image shifts when angulation is changed
  • Image of buccal object (farther from the intraoral film) shifts more in the opposite direction of the tube head than the image of lingual objects (closer to the intraoral film)
  
  How to localize objects in the buccal-lingual dimension:
• Make a radiograph of area
• Identify relation of unknown object to known structure such as pulp space
• Make a second radiograph at a different horizontal or vertical angle
• On the second radiogrpah which image has shifted more in the opposite direction of the tube head? This is the object that is farther from the film!--i.e. located buccal. All images shift in the opposite direction, but the images of buccal objects shift more the opposite way than lingual objects.
  
  The "SLOB" Rule
• S ame
• L ingual
• O pposite
• B uccal