according to ORAL RADIOLOGY PRINCIPLES AND INTERPRETATION,SEVENTH EDITION white & pharroah Chapter 6

1. CHAPTER 6
Projection Geometry

2. IMAGE SHARPNESS AND RESOLUTION
 Sharpness: how well a boundary between two areas of
differing radiodensity is revealed.
 Image spatial resolution: how well a radiograph is able to
reveal small objects that are close together.

3.  x rays are produced at the target in an x-ray tube
originate from different points and travel in straight lines,
their projections of a feature of an object do not occur at
exactly the same location on an image receptor.
image of the edge of an object is slightly blurred rather than
sharp

5. There are three means to maximize image
sharpness:

6. 1. Use as small an effective focal spot as practical.

7. 2. Increase the distance between the focal spot and
the object by using a long, open-ended cylinder.

8. 3. Minimize the distance between the object and the
image receptor.

9. IMAGE SIZE DISTORTION
 Increasing the focal spot-to-image receptor distance
and decreasing the object-to-image receptor distance
minimizes image magnification.
 The use of a long, open-ended cylinder as an aiming
device on an x-ray machine thus reduces the
magnification of images on a periapical view.

10. IMAGE SHAPE DISTORTION
 This situation arises when not all the parts of an
object are at the same focal spot-to-object distance.
 Such a phenomenon is seen by the differences in
appearance of the image on a radiograph compared
with the true shape.

11. There are two means to minimize shape
distortion:

12. 1. Position the image receptor parallel to the long
axis of the object.
 A) the central ray of the x-ray beam is perpendicular to the
image receptor, but the object is not parallel to the image
receptor. This type of shape distortion is
 called fore-shortening

13.  B) the x-ray beam is oriented at right angles to the
object but not to the image
 receptor; this results in elongation

14. 2. Orient the central ray perpendicular to the object
and image receptor.
 Image shape distortion occurs if the object and image
receptor are parallel, but the central ray is not
directed at right angles to each. This distortion is most
evident on maxillary
 molar views

15. BISECTING-ANGLE TECHNIQUES

16. PARALLELING-ANGLE TECHNIQUES

17. OBJECT LOCALIZATION
 the dentist may wish to use radiographs to determine the
location of a foreign object or an impacted tooth within the
jaw.
 Three methods are frequently used to obtain such three-
dimensional information.

18. The first is to examine two images projected at right
angles to each other.

19. The second method is to use the tube-shift
technique employing conventional periapical views.

20. Third, in recent years, the advent of cone-beam
imaging has provided a new tool for obtaining three-
dimensional information.

21. The position of the maxillary zygomatic process in
relation to the roots of the molars can help in
identifying the orientation of views.

22. EGGSHELL EFFECT
 The top photon has a tangential path through the apex of
the egg and a much longer path through the shell of the
egg
 lower photon, which strikes the egg at right angles to the
surface and travels through two thicknesses of the shell.

23. expanded cortex
 The periphery of the expanded cortex is more opaque
than the region inside the expanded border.
 The cortical bone is not thicker on the cortex than over
the rest of the lesion, but rather the x-ray beam is more
attenuated in this region because of the longer path
length of photons through the bony cortex on the
periphery.

24. expanded cortex