diagnostic quality Flashcards
diagnostic quality achieved with
- minimal magnification, distortion, penumbra
- anatomical accuracy
- adequate coverage
- density and contrast
qualities of a diagnostic radiograph
- sharp
- shape and size same as object
magnification
caused by divergent path of beam
-x rays travel in diverging straight lines
2 rules to keep mag at a minimum
- keep object as close to image receptor as possible
2. keep the focal spot to image receptor distance as large as possible
distortion
unequal mag of an object
paralleling technique
minimizes distortion and more anatomically accurate
bisecting angle tech
inherent distortion
what leads to foreshortening?
improper use of paralleling tech
for the maxillary arch in paralleling tech, the image receptor must be where?
in center of oral cavity
-teeth biting on 1st/2nd groove
where do teeth bite for mandible?
middle of block
mag —- the farther away the image rec is from object
increases
long cone paralleling tech
- better definition
- less mag
- less distortion
- better anatomic accuracy
indications of bisecting angle tech
- narrow palate
- small mouth
- shallow floor of mouth (ankyloglossia)
- difficult patient/lack of cooperation
penumbra
unsharpness
- bc focal spot has finite dimensions
- edge of an object is imaged many times
how to minimize penumbra
- use as small a focal spot as possible (line focus principle)
- use a long focal spot to object distance
- minimize object to image receptor distance
difference between long and short cone
- long cone is sharper
- magnification
line focus principle
- focal spot large enough to dissipate heat
- small enough to have a sharp image
rules for dimensional accuracy
- small focal spot
- long source-object distance
- short object-image receptor distance
- object parallel to image receptor
- central ray at right angle to teeth and image receptor
how is anatomical accuracy obtained?
when the image receptor is placed parallel to the long axis of the teeth and beam is directed perpendicular to both the image receptor and teeth
anatomical accuracy
- labial and lingual CEJs of ant teeth superimposed
- buccal and lingual cusps of molars superimposed
- contact open
- no superimposition of zygo bone
- buccal portion of alv crest is superimposed over lingual portion of alv crest
adequate coverage
area of interest well covered in radiograph
density
degree of image blackening
contrast
difference in densities
4 most important factors that control density
- milliampere
- exposure time
- kilovoltage
- source to image receptor distance
what influences quantity ?
exposure time
mA
kV
BID
what influences quantity and quality?
kV
the density of the radiograph varies directly and proportionally with —- and —-
mA and exposure time
kVp controls?
- penetrating power of the beam
- increase–> increase power
- the more penetrating beam, the more radiation strikes the image receptor
increasing vs decreasing kVp
increasing kV darkens image
decreasing kV lightens image
intensity of x ray beam is —– to the square of the distance from the source (focal spot)
inversely proportional
doubling the distance does what?
produces 1/4 the density
decreasing the distance by one half does what?
produces 4 times the density
few x rays reach the film thru areas of what?
enamel and cortical bone
many x rays are transmitted through what?
pulp, soft tissue, air around patient
short scale contrast
few shades of gray
high contrast
low Kvp
-good for looking at caries
long scale contrast
many shades of gray
high kvp
low contrast
-good for interpreting fine details–perio disease, periapical disease, bony lesions
contrast depends on
subject contrast
image receptor contrast
scatter radiation
computer display
subject contrast depends on
- make up of patient
- energy of beam
what is the main controller of subject contrast
kVp
what reduces radiographic contrast?
scatter radiation
image receptor contrast depends on
- type of receptor
- film–also depends on chemistry
- extraoral film depends on type of screen phosphor
characteristic curve
-describes contrast, latitude (exposure range), and speed of film and digital receptors
digital imaging can help what?
reduce patient dose
—- and —- have similar contrast and latitude
film and hard sensors
PSP plates
lower contrast but greater latitude
most desirable image
faint outline of soft tissue in edentulous spaces and area distal to molar teeth
ideal viewing conditions
human eye = 60 gray levels
<30 in operator
bit depth
digital contrast
- current–12 or 16 bits
- conventional computer can only see 8 bits =256 gray levels
