Projection Geometry-Day 2 Flashcards
3 Problems in projection geometry
Unsharpness
magnification
distortion
3 Projection problems always present because
The focal spot is an area, never a point
The X-rays diverge from their source
Images are 2-D representations of 3-D structures
Spatial resolution
The degree of how well the image can display two objects that are close together
Measured in line pairs per mm
Higher spatial resolution is desirable
Sharpness
The degree of how well the image can display the boundary between two differing radiodensities
Sharp boundaries are desirable
Unsharpness at the boundary of an object is due to
penumbra and parallax
Penumbra
The space of partial illumination between the perfect shadow and full light
How to reduce penumbra
1) Use as small an effective focal spot as practical
- 0.4mm
2) Increase the distance between the focal spot and the object
3) Minimize the distance between the object and the image receptor
Parallax
Apparent change in position or size of a subject when viewed from different perspectives
Dental film – double emulsion
Movement due to
Just like a camera, create blurry lines:
Source movement (tube head movement) Object movement (patient movement) Receptor movement (patient movement of sensor/film in mouth)
Image size distortion due to
divergent paths of photons in x-ray beam
Image shape distortion due to
Unequal magnification in different parts of the same object
What is present in all radiographs
distortion
To decrease size distortion must
Increase in source to object