projection geometry,radiology

1. PROJECTION GEOMETRY

2.  Conventional radiograph is a two-dimensional projection
image of a three-dimensional object
 The principles of projection geometry describe the effect
of focal spot size and position (relative to the object and
the film) on image clarity, magnification, and distortion.
 Clinicians use these principles to maximize image clarity,
minimize distortion, and localize objects in the image field

3. Image Sharpness and Resolution
 Sharpness measures how well a boundary between
two areas of differing radiodensity is revealed.
 Image spatial resolution measures how well a
radiograph is able to reveal small objects that are
close together.

4. 3 methods for minimizing this loss of
image clarity and improving the quality
of radiographs
 1. Use as small an effective focal spot as practical.
 Dental x-ray machines should have a nominal focal spot
size of 1.0 mm or less.
 Some tubes used in extraoral radiography have effective
focal spots measuring 0.3 mm, which greatly adds to
image clarity.

5. Decreasing the angle of the target perpendicular to the long axis of the electron beam
decreases the actual focal spot size and decreases heat dissipation and thereby tube
life.
It also decreases the effective focal spot size, thus increasing the sharpness of the
image.

6. 2. Increase the distance between the focal spot and the object by using
a long, open-ended cylinder
Increasing the distance between the focal spot and the object results in an image with
increased sharpness and less magnification of the object

7. 3. Minimize the distance between the object and the
film.
Decreasing the distance between the object and the film increases the sharpness and
results in less magnification of the object.

8. OBJECT LOCALIZATION
 To determine the location of a foreign object or an
impacted tooth within the jaw.
 Two methods are frequently used to obtain such
three-dimensional information
 RIGHT-ANGLE OR CROSS-SECTION TECHNIQUE
 TUBE SHIFT TECHNIQUE OR BUCCAL OBJECT RULE AND
CLARK ’ S RULE

9. RIGHT-ANGLE OR CROSS-SECTION
TECHNIQUE
two projections taken at right angles to one another localize an
object

10. TUBE SHIFT TECHNIQUE OR BUCCAL
OBJECT RULE OR CLARK ’ S RULE
The relative positions of radiographic images of two separate
objects change when the projection angle at which the images were
made is changed.

11. These relationships can be easily remembered by the
acronym SLOB : S ame L ingual, O pposite B uccal
If it does not move with respect to the reference object, it lies at
the same depth (in the same vertical plane) as the reference
object.