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Geometry of Image Formation

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									Geometry of Image Formation

              Lecture 5
  Prof. Janet Gruber, R.D.H., M.S.
Principles of Shadow Casting
 The goal of taking dental radiographs is to
 record the image of the teeth and
 surrounding structures so that they are:

    1. Accurate in size.
    2. Free from distortion.
    3. Correctly proportioned.
Two Basic Techniques for
Exposing Periapical Films
 Bisecting Angle Technique - AKA- Short
  Cone Technique
 Paralleling Technique - AKA - Long Cone
    Objective of both is to direct the central ray at
    the most favorable horizontal and vertical
    angle, through the patient’s tissue and onto the
    film, so that the least distortion and most
    accurate representation is obtained.
Five Rules for Image Formation
1. The source of radiation (focal spot) should
  be as small as possible.
2. The distance between the object (tooth) and
  the film should be as small as possible.
3. The object and film should be parallel to
  each other.
4. The source distance should be as long as
  possible. (BID)
5. The radiation must strike the tooth, object
  and film at right angles.
Technique VS. Rules
 Bisecting angle technique fails to meet rules
  3,4 & 5.
 Paralleling technique fails to meet rule 2.
Bisecting Angle Technique
 AKA short cone technique
 film as close to the tooth as possible
 x-ray beam directed perpendicular to the
  imaginary line that bisects the angle formed
  by the recording plane and the long axis of
  the tooth
 an 8”, 12” or 16” BID may be used
Disadvantages of Bisecting
 difficult to estimate bisector
 short cone = more divergent rays
     magnification
     penumbra
        size of focal spot
        target film distance

        object film distance

 dimensional distortion
 more superimposition
 CR directed more toward gonads
 set head position required
Advantages of Bisecting
 shorter exposure time
 don’t need film holders (we prefer them)
 can use in all situations…gagger, small
  mouth, child etc.
 short BID can be used if operatory has
  limited space
Paralleling Technique
 AKA long cone technique or right angle
  technique (1920 McCormick)
 obeys more rules of image formation than
 uses film holders to place the film as
  parallel to the tooth’s long axis as possible
 CR perp. to film and tooth
 must use a long BID
Disadvantages of Paralleling
 anatomical limitations
 film holders take more time to assemble
 need more operatory space for a long BID
 longer exposure time, but the patient
  receives the same amount of radiation
Advantages of Paralleling
 minimal distortion and magnification
 easier to determine vertical and horizontal
  angulation compared to bisecting technique
 CR never directed toward gonads
 standard reproducible technique compared
  to bisecting
 set head position not required
 better radiograph obtained than with
Distance Terms
 Focal Film Distance = source to film
 Focal Object Distance = source to tooth
 Object film Distance = tooth to film
X-Ray Film Quality
Quality is controlled by:
 density
 contrast
 geometric unsharpness
 magnification
 distortion

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