geometry of image formation Flashcards
image sharpness
how well defined is a boundary between 2 areas of differing radiodensities (i.e. DEJ)
image resolution
how well an image is able to reveal very small objects that are very close together
what are the 4 desired characteristics of image formation?
minimum magnification
minimum distortion
minimum superimposition
MAXIMUM detail
magnification depends on what 3 distances?
- object-film distance (OFD)
- source-object distance (SOD)
- source-film distance (SFD)
We want OFD to be ________ for clear and sharp images.
SHORT
we want SFD to be ________ for defined images
LONG
if you increase OFD, the magnification _______
increases
constant OFD + decreased SFD ____________ magnification
increases
constant OFD + increased SFD ___________ magnification
decreases
constant SOD + increased OFD (film moves away from object) __________ magnification
increases
constant SFD + increased OFD (object moves toward source) ____________ magnification
increases
decreased SFD __________ magnification
increases
4 names for source of radiation
source
target
focal spot
anode
distortion
variation seen in image from true shape or proportions of object
distortion in an image is due to what?
disproportionate magnification
distortion depends on ___________; magnification depends on __________
angulation; distance
minimum distortion occurs when object and sensor are ________ and central ray is ______________ to both __________ and _______
parallel
perpendicular
object
film
what happens when central ray is directed at right angle to object?
elongation
what happens when central ray is directed at right angle to sensor?
foreshortening
parallel technique
- image shifts __________ if vertical angle is too great and shifts _____________ if vertical angle is too small
occlusally
apically
bisecting angle technique
– image shifts __________/_________ if vertical angle is too great and shifts _____________ if vertical angle is too small
occlusally/incisally
apically
paralleling technique (3 characteristics)
- placement of sensor parallel to teeth is easy
- if mistake is made no foreshortening or magnification
- long cone is mandatory due to need for increased SFD
bisecting angle technique (3 characteristics)
- direct invisible central ray of invisible x-ray beam at right angle to imaginary bisector
- if mistake is made it will result in foreshortening or elongation
- long cone is not necessary
radiographic definition
recorded sharpness of boundaries of structures
umbra
innermost and darkest part of a shadow where light source is completely blocked by occluding body
the _________ the umbra, the sharper the image
smaller
penumbra
space in periphery of main target of radiation therapy
size of source is _________ related to definition
size of source is _________ related to unsharpness
inversely
directly
all factors that ________ magnification will decrease definition
increase
movement of source of radiation =
geometric unsharpness
movement of patient or image receptor =
unsharpness
zero degree VA
central ray is parallel to occlusal plane
increased/positive VA: source of radiation is __________ to point of entry of central ray (PID is pointing _________) and the image shifts _________ (_______/_________ edges may be projected off receptor)
superior
downwards
down
incisal/occlusal
are all maxillary radiographs positive?
yes
decreased/negative VA: image shifts ____ (_______ may be projected off receptor)
up
apices
for mandibular teeth a the source of radiation for negative VA is _________ to point of entry of central ray. PID is pointing _________
inferior
upwards
are all mandibular radiographs negative?
yes
increasing VA in mandibular teeth causes image to shift ______ where what edges may be projected off the receptor?
up
occlusal/incisal
decreasing VA in mandibular teeth causes image to shift _____ where ______ may be projected off the receptor
down
apices
VA for maxillary periapicals is ____________ than bitewings
greater
VA for maxillary anterior periapicals is __________ than VA for posterior periapicals
greater
Horizontal angulation
- 0 degrees: central ray is _________ to sagittal plane and directed _____________
parallel
anteroposteriorly
HA
- 90 degrees: central ray is __________ to sagittal plane
perpendicular
HA
- 180 degrees: central ray is __________ to sagittal plane and directed _____________
parallel
posteroanteriorly
improper HA causes _________
interproximal overlapping
increasing HA causes images to shift ____________
decreasing HA causes images to shift ___________
anteriorly
posteriorly
source of radiation moves ____________
posteriorly
in periapical radiography, HA varies from ____ degrees (incisor PA) to _____ degrees (molar PA)
0-90
