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Intensifying screens

Cerwin-Vega Subwoofers SL-36 USA made
STUDY
PLAY
film cassette
light-tight container which houses and protects the screens and film during transfer from darkroom to bucky tray and back to dark room
intensifying screens
rectangular, flat, shiny, white plates glued into the inside of both sides of a film cassette
intensifying screens holds the film tightly
between when the cassette door is closed
intensifying screesn function to
convert x-ray photon energy into visible light energy to intensify the film

(darkening effect of the x-ray beam)
50 x-ray photons can make
50 latent image centers on the film directly or make enough visible light to cause the formation of 1700 latent image centers with par speed screens
what is an advantage of x-ray intensifying screens
decrease heat damage to x-ray tube
intensifying screens used because
they decrease greatly the number of x-rays necessary to make an exposure (decrease patient dose) and decrease the tube loading (decrease mAs leads to decrease HU)
Michael Pupin first used
fluorescent screens with glass photographic plates in 1896
typical intensifying screen
base, reflecting coat, phospho layer, protective layer
what is the disadvantage of x-ray intensifying screens
decreases image sharpness and resolution
what function on x-ray intensifying screens
convert x-ray energy into light energy
light emission
luminescence

fluorescence

phosphorescence
luminescence
the emission of light from a substance following stimulation by chemicals, light, ionizing radiation
fluorescence
the instantaneous (within 10^-8 sec) emission of light from substances
in radiology, the emission of light from
special inorganic crystals (phosphors) after stimulation by x-rays
phosphorescence
the emission of light after stimulation is delayed beyond 10 ^ -8 sec

(undesirable in radiographic screens)
active ingredient in an intensifying screen
the phosphor layer, which contains the crystals that convert x-ray energy into light energy
calcium tungstate
first commercial phosphor
introduced around 1898 by Thomas Edisons group
rare earth screens
first manufactured in 1973

increased x-ray absorption and conversion efficiency
terbium-activated gadolinium oxysulfide
green emitter
thulium-activated lanthanum oxybromide
blue emitter
the radiologic health branch (RHB) recommends minimum system speed of
400
about what percentage of radiographic film density (darkness) is cause by direct x-ray to film exposure
less than 5% (insignificant)
what percentage of radiographic film density (darkness) is caused by x-ray to light screens to film exposure
greater than 95% (effectively all film darkening)
advantages of intensifying screens
patient ionizing radiation exposures utilizing modern rare earth screens are less than 1/100 the exposure the would be needed for direct x-ray to film exposure
advantages of intensifying screes
decrease tube loading (decrease mAs causes decrease HU)
disadvantage of intensifying screens
image resolution drops from 100 lines/mm to 10 lines/mm
loss of resolution cause (disadvantage of intensifying screens)
primarily by light diffusion within the phosphor crystal layer
within the same family of phosphor crystals image resolution (disadvantage of intensifying screens)
decreases as screen speed increases
phosphor crystal size (image quality with intensifying screes)
the larger the crystal size, the poorer the resultant image (increase light diffusion)
thickness of phosphor crystal layer
the thicker the phosphor layer, the poorer the resultant image (increases light diffusion)
presence of light-absorbing dyes placed in the phosphor layer (image quality with intensifying screes)
light-absorbing dyes increase image quality by decrease light diffusion
presence of TiO2 reflecting layers (image quality with intensifying screes)
reflecting layer decrease image quality by increase light diffusion
film screen contact (image quality with intensifying screes)
anything that interferes with tight film screen contact will decrease image quality (increases light diffusion)
T/F all images produced by intensifying screens have less sharpness and resolution than direct x-ray film exposure
T
which modifications/variables keep this image degradation to a minimum
small phosphor crystal size

thinnest screen phosphor layer

light absorbing dye in phosphor layer

no reflective layer between base and phosphor layer

use of single screen, single emulsion film systems
all modifications decrease
film/screen system speed
screen speed is inversely related to
image quality
screen speed is influenced by
type of phosphor (absorption/conversion ratio)

phosphor crystal size

thickness of phosphor layer

presence of reflecting layer

presence of light absorbing dye

kVp range (K-edge of phosphor)
what was the fisrt phosphors
barium platinocyanide was the first x-ray phosphor used
what is the x-ray absorbing ability of rare earth phosphors as compared to calcium
rare earth phosphors absorb about 50% more x-rays than an equal thickness of calcium tungstate at commonly-used kVps

(better k-edge/ave photon E match)
what is the x-ray to light conversion efficiency of rare earth phosphors compared to calcium tungstate
calcium tungstate 5%

rare earths 15-18%
how much better are rare earths than calcium tungstate when it comes to x-ray to light conversion efficiency
3-4 times better
since rare earth phosphors absorb 50% more x-rays than calcium tungstate, and convert 3-4 times as much absorbed x-ray energy into light energy, which type of phosphor is faster (more film density per mAs)
rare earth
considering the film-darkening effect of various speeds of intensifying screens, how much faster is an 800 speed rare earth film/screen system than a 200 speed high plus calcium tungstate film/screen system
4 times
calcium tungstate phosphor emits a continuous spectrum with maximum intensity in the
blue purple region of the EM spectrum
regular halide film is maximally sensitive to
light in the UV and purple blue regions
what is well matched to calcium tungstate phosphor
halide
rare earth phosphors emit a non-continuous (line) spectrum that comes in 2 types
blue light emitting

green light emitting
green light emitting is used with
gadolinium
an example of a green light emitter is
Kodak Lanex
what happens if green emitting screens are used with standard halide (blue-purple sensitive) film
spike of useful green light emitted will not be seen by regular halide film

primary light emission will be wasted

adequate film exposure will not take place

film/screen system becomes slow

patient receives excessive radiation
the solution is special
orthocrhomatic film which is sensitive to green light
T/F
orthochromatic film must be used with any green-emitting system
T/F rare earth screens & film must be carefully matched to ensure compatibility
T
T/F Panchromatic film is not used in radiography
T
regular halide film is not sensitive to (darkroom safe light)
red or green light
what kind of safelight can be safely used with halide film
amber
what film is sensitive to some of the shorter wavelengths emitted by the amber safelight (yellow green) and will be fogged
orthocrhomatic
what safelight can safely be used with green sensitive film
true red safelight
whats the proper safelight color to use with gadolinium based RE screens
red
advantages of screens over non-screen technique
shorter exposure times causes less motion sharpness

smaller focal spots used leading to increase image quality

longer FFDs lead to increase image quality & decrease patient dose

extend x-ray tube life

decrease mAs causes decrease HU

reduce patient/operator exposure to ionizing radiation

screens inherently increase image contrast
advantages of rare earth over calcium tungstate screens
decreased tube loading

reduced patient/operator radiation dose
decreased tube loading (advantages or rare earth over calcium tungstate screens)
shorter exposure times causes less motion unsharpness

smaller focal spots can be used leading to increase image quality

longer FFDs lead to increase image quality & decrease patient dose

extend x-ray tube life
reduced patient/operator radiation dose (advantages or rare earth over calcium tungstate screens)
75% reduction w/ 800 speed rare earth system compared to 200 speed tungstate system
T/F magnification radiography benefits from smaller focal spot sizes
T
disadvantages of rare earth over calcium tungstate screens
cost

mottle
so much speed (light) is produced by so few x-ray photons that mottle (graininess) becomes
a significant problem which degrades image quality
T/F dont use film/screen system speeds greater than 800 (mottle)
T
mottle is a
signal to noise ratio problem
mottle results from
statistical fluctuation in the number of x-ray photons absorbed by the intensifying screens to form the light image recorded on film
if x-ray photons are not evenly distributed in the x-ray beam they
clump randomly producing a pattern on the film
mottle background noise consists of a
random clumping pattern that is only visible on sharp films taken with very few x-ray photons
the fewer the number of photons the
worse the noise
increasing the number of photons
fills in the spaces

covers the clumping pattern
if the film/screen system is so fast that very few x-ray photons (lower mAs) are being used to create the image then the bacground clumping patter/graininess/mottle becomes
prominent and the image (signal) becomes weak and loses clarity
excessive mottle/graininess causes
an unacceptable degradation in image clarity at film/screen system speeds above 800
excessive mottle can only be corrected by using
more x-rays (mAs) which can only be accomplished by switching to a slower film/screen system
gradient/split screens one end is
slower than the other
gradient/split screens can be an
abrupt change (split) or gradual (gradient)
gradient split screens are used primarily in
full spine radiography to compensate for tissue thickness differences in the patient
gradient/split screens slower portion of the screen is placed
over the thinner portion of the patient
gradient/split screens allows the thin portion of the patient to receive
excessive radiation dose but not overexpose the film
gradient/split screens are to be
discouraged, and should be replaced by single-speed screens and compensating filters
film and screens should make tight
contact throughout every part of the film
film screen contact gaps between the screens and films increases
light diffusion which degrades the image
Z screen is
large mesh, Cu, brass or Zn galvanized steel
high Z screen is x-rayed with
an air-gap using the film/screen system in question
processed image is carefully
scrutinized for areas of poor image clarity

(should be uniformly sharp)
T/F never snap shut the door of any cassette unless it is completely dry and loaded with film
T