EX2; Geometry of Image Formation Flashcards
This is how well defined is a boundary between two areas of differing radiodensities
image sharpness
This is how well an image is able to reveal very small object that are very close together
image resolution
What are four desired characteristics of image formation
minimum magnification
distortion
superimposition
maximun detail
Does the x-ray beam diverge or converge
diverge
What three things does magnification depend upon
object-film distance
source-object distance
source-film distance
Does the magnification increase or decrease if there is a constant SOD and an increased OFD
increase
An increase in magnification affects the OFD how
increases
Does the magnification increase or decrease if there is a constant SFD and an increased OFD
increases
OFD should be small or large
small
Does the magnification increase or decrease if there in a constant OFD and a decreased SFD
increases
An increase in magnification affects SFD how
decreases
Does the magnification increase or decrease if there is a constant ODF and an increased SFD
decreased
SFD should be long or short
long
What are the dimensions of a short and long cone
short = 8in or less long = 12"-16"
This is variation seen in an image from the true shape or proportions of the object
distortion
What are the two types of distortion
size
shape
Distortion seen in an image is due to what
disproportionate (non-uniform_ magnification
some parts are more magnified than others
This occurs when object and sensor are parallel and the central ray is perpendicular to both the object and the film
minimun distortion
If the relationship between the object and film is angular (not parallel) what occurs
more acute distortion
If the central ray is directed at right angle to the object, what occurs
elongation
If the central ray is directed at right angle to the sensor, what occurs
foreshortening
If vertical angulation of the bisecting angle is too great, what occus
image foreshortening
image will shift occlusally or incisally
If vertical angulation of the bisecting angle is too small, what occurs
image elongation
image will shift apically
If vertical angle is too great of the parallel technique, what occurs
no foreshortening
the image will shift occlusally and may project off the receptor
If vertical angle is too small of the parallel technique, what occurs
no additional elongation
image may shift apically; root apices may project off the receptor
If a mistake is made using parallel technique, what occurs compared to bisecting angle
parallel technique mistake; magnification
BA technique mistake; elongation or foreshortening
This is mandatory for paralleling technique due to a need for an increased SFD and not required for BA technique
long cone
This refers to the recorded sharpness of boundaries of structures
radiographic definition (sharpness)
What are four variables of radiographic definition
size of the source (focal spot)
OFD, SFD, SOD
movement
nature of image receptor
The size of the source and definition are related how
inversely related
These are factors that increase magnification and decrease definition
OFD
SFD
SOD
This of the source of radiation; not vey significant; termed geometric unsharpness
movement
This of the patients and/or image receptor; very significant; termed motion unsharpness
movement
The source of radiation is superior to point of entry of CR (PRD pointing downwards) with what vertical angulation involving maxillary teeth
positive VA
An increased or positive VA (maxillary) can cause the image to shift how
down; the occlusal/incisal edges may be projected off
A decreased or negative VA (maxillary) can cause the image to shift how
up; the apices may project off the receptor
The source of radiation is inferior to point of entry of CR (PRD pointing upwards) with what vertical angulation involving mandibular teeth
negative VA
An increased VA (mandibular) can cause the image to shift how
up; the occlusal/incisal edges may be projected off
A decreased VA (mandibular) can cause the image to shift how
down; the apices may project off the receptor
The VA for all maxillary periodicals, bitewings, and occlusal radiographs is always what
positive; greater for PA than bitewings
What is the angle of VA
0 degrees; CR parallel to occlusal plane
What is the 0 degree of HA
CR is parallel to sagittal plane and directed anterioposteriorly
What is the 90 degree of HA
CR is perpendicular to the sagittal plane
What is the 180 degree of HA
CR is parallel to the sagittal plane and directed posteroanteriorly
Improper HA can cause what
interproximal overlap
Increasing HA causes images to what
shift anteriorly and results in overlapping
Decreasing HA causes images to what
shift posteriorly and results in overlapping
HA increases as the source of radiation moves where
posteriorly
HA varies from what degrees in a PA
0 (incisor) to 90 (molar)
