9/10: Projection Geometry Flashcards
What are the geometric characteristics?
Image sharpness
Image magnification
Image shape distortion
This is the fuzzy, unsharp margin of radiographic image
Unsharpness
What are examples of unsharpness margins?
Penumbra
Edge gradient
What are the 3 rules to maximize sharpness?
- Radiation source should be be as small as possible
- Source-to-object distance should be as long as possible
- Object-to-receptor distance should be as short as possible
This is the shadow of the image we receive
Umbra
This is the unsharp number. Area around the margins of the object
Penumbra
The closer the image, therefore the larger the focal spot, the
Larger the penumbra
The further the image, therefore the smaller the focal spot, the
Smaller the penumbra
How can you control source-to-object distance?
Length of cone → larger = less divergence → more sharpness
This is the equal enlargement of radiographic image, compared to the actual size of the object
Magnification
What are the 2 rules to minimize magnification?
- Source-to-object distance should be as long as possible
- Object-to-receptor distance should be as short as possible
When you magnify, the image shows
True shape of an object
This is the “unequal enlargement”. Variation from true shape of object, unequal magnification of part of object. Improper alignment of receptor, object, beam
Distortion
What are 2 rules to minimize shape distortion?
- Object and receptor should be parallel
- Beam should be perpendicular to both object and receptor
What are the types of distortion?
Foreshortening
Elongation
This is when the tooth is not parallel to receptor, and beam directed perpendicular to receptor
Foreshortening
This when tooth is not parallel to receptor, and beam directed perpendicular to tooth
Elongation
What are the 5 rules for accurate image formation?
- Focal spot as small as possible
- Source-object distance as long as possible
- Object-receptor distance as short possible
- Object parallel to receptor
- Beam perpendicular to object and receptor
When doing PAs, what should you use for the paralleling technique?
Long cone
When doing PAs, what should you use for the bisecting angle technique?
Long or short cone
why do you want the receptor paralell to the tooth
increaes S-R distance
increases O-R distance
Of the PA techniques, what is the preferred method?
Paralleling technique
If the paralleling technique cannot be used, what technique should be used?
Bisecting angle technique
The bisecting angle technique is based on what?
Rule of isometry
