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DQE
detection quantum efficiency
Percent of x-rays absorbed by screen
CE
conversion efficiency
Light emitted for each x-ray absorbed
screen layers
protective layer
phosphor - active layer (the thicker the more converted to light)
reflective layer
base
Stimulus
outer shell electrons absorb some energy and raised to higher energy level
and creates a hole in the outer shell
Phosphorescence
visible light emitted after stimulation stops
elec returns to normal after more than one revolution
fluorescence
Visible light emitted only while the phosphor is stimulated
elec returns to normal in one revolution
intensifying factor equation
IF = exposure required without screens/ exposure required with screens
speed is determined by
–Number of x-rays interacting with the phosphor
–Efficiency of conversion of x-ray energy into light
dye is added to screens to:
–Added to control spread of light
–Improve spatial resolution, decrease speed
noise
Appears on radiograph as speckled background
Occurs more with fast screens, high kvp
Noise reduces image contrast
spatial resolution
ability to produce ACCURATE image
How small an object can be imaged
contrast resolution
ability to distinguish between similar tissues
image detail
determined by spatial resolution and contrast resolution
faster screens
larger crystals
low spatial resolution
front of screen
low atomic number so xr aren't attenuated
conversion efficiency
Amount of visible light emitted to xray energy absorbed
calcium tungstate about 5% of absorbed converted (amber safe light)
Rare earth absorbs about 15 –20% converted (red safe light)
screen
speed technique equation
relative speed from/ relative speed to x mAs old = mAsnew
grid ratio
The higher the strip and the smaller the interspace---the straighter the beam must be to get thru
The shorter the strip, or the wider the interspace, the more angled beam can get thru
grid frequency
number of lead strips per inch
.. high freq requires higher tech and results in high pt dose
disadvantages of air gap tech
-image magnification
-Loss of recorded detail
* so you have to use a longer SID
bucky factor values from nongrid to 16:1
ng=1
5=2
6=3
8=4
10&12=5
16=6
wetting the image:
•Allows chemicals to reach the emulsion
•swells emulsion and causes it to expand
developing agents
–hydroquinone acts slow and gives blacks
–phenidone acts fast and gives shades of gray
preservative
controls oxidation in the developer
helps maintain proper development rate and keeps chemistry lasting longer
3 factors affecting developing
•Time
•temperature
•concentration of chemicals
all lead to inc development of crystals giving more density
fixer:
•Stops action of developer
•neutralizes the alkaline
•acetic acid
buffer
helps maintain the ph
6 principle components of processing
•Transport
•temperature control
•circulation
•replenishment
•dryer
•electrical
3 types of rollers
transport rollers–1 inch
Master rollers–3 inch
planetary rollers
Turnaround assembly
–master roller
–planetary roller
–guide shoe
film layers
• supercoating
• emulsion
• adhesive layer
• Base
2 parts of the emulsion
–gelatin
–silver halide crystal
speed of films depends on
the thickness of emulsion=greater speed b/c more sensitive to light
reciprocity law
Exposure = intensity x time
fails at very long and short times
storage of film
68 F up to one year
40-60% humidity
no light
jeep away from rad
factors affecting scatter
kvp
field size
pt thickness
grid ratio formula
height of grid/distance of interspace
grid frequency
10,000/ width of strip + width of interspace
CIF
contrast with grid/ contrast w.o grid
Bucky Factor
hit top/ hit bottom
tech change for grids
ratio of BF to/ ratio of BF from *mAs
selectivity
primary transmitted thru/scatter transmitted thru
cut off
SID/ grid ratio
magnification factor
image size/ object size
or
SID/SOD
noise components
film graininess
structure mottle
quantum mottle
scatter radiation
shape distortion depends on:
object thickness
object position
object shape
prevent shape distortion by:
part parallel to grid
CR perpendicular to part
CR perpendicular to IR
if you angle the part what happens
foreshortened
angle the IR what happens
elongation
prevent size distortion by:
part close to IR
straight beam.. can make straighter by inc SID
reduce magnification by using
high SID and low OID
Author
nethers-1944
ID
10605
Card Set
exposure
Description
grids, screens etc
Updated
2010-03-15T16:49:47Z
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