Projection Geometry Flashcards
What are three problems that will always be present in radiographs
unsharpness, magnification, distortion
Why are there always projection problems
- the focal spot is an area, never a point
- The X-rays diverge from their source
- Images are 2-d representation of 3 d structures
What is sharpness
the degree of how well the image can display the boundary between two differing radio densities
the unsharpness at the boundary of an object
penumbra
Spatial resolution
the degree of how well the image can display two objects that are close together
How is spatial resolution measured
line pairs/ mm
lp/mm
_______ spatial resolution is desirable
higher
Spatial resolution and sharpness are both influenced by
- focal spot size
- receptor composition
- movement
____ focal spot size has higher sharpness and spatial composition
small
Order the following receptors based on there sharpness and spatial resolution
PSP, Film, CCD, CMOS
Film> CCD> CMOS > PSP
Movement of the tube head is
source movement
movement of the patient is
object movement
patient movement of sensor/film in mouth is
receptor movement
Large focal spot will cause sharpness to
decrease
long source to object distance will cause sharpness to
improve
long object to receptor distance will cause sharpness to
decrease
What angulation is the dental x-ray set at
20 degrees
What is the size of the focal spot in dental X-rays
1mmx3mm
Distortion can cause
foreshortening or elongation
foreshortening is due to
the tooth not parallel to the receptor, beam perpendicular to the receptor
elongation is due to
the receptor is not parallel to the tooth and it is not perpendicular to the beam, but is perpendicular to the beam
this technique increased sharpness, and decrease distortions
parallel technique
In parallel technique the tooth is parallel to the
receptor
This technique is used due to its accuracy in length
bisecting angle
