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Image Production

Chap 3 Screens

-in a cassette converts a small amt of x-ray energy to a much greater amt of visible light energy intensifying screens
What % of the x-ray that strike a radiographic intensifying screen interact with the screen? 30%
-visible light emitted luminescence
-any material that emits light in response to some outside stimulation luminescent material or a phosphor
Explain what happens to cause luminescence outer-shell electrons are raised to excited levels, the hole left is filled when the excited electron returns to its normal state, and light is emitted
-emission of visible light from screens after x-rays interact with the atoms of the screen's phosphor crystals luminescence
What are the two types of luminescence? fluorescence & phosphorescence
-yields a more predictable density on the film bc light emission ceases after exposure fluorescence
-screen lag, after glow phosphorescence
What are the 4 layers of a screen? base, reflective layer, phosphor, protective coating
farthest layer from the film base
What is the purpose of the base layer of the screen? provides mechanical support for the phosphor layer
Where is the reflective layer located? btwn the phosphor layer and the base
What is the purpose of the reflective layer? to redirect light to the film
What does isotropically mean? equally in all directions
-the amt of light emitted and the efficiency or ease with which a screen converts x-ray energy to light energy conversion efficiency
WHich layer is known as the active layer? phosphor layer
What is the purpose of the phosphor layer? to convert the energy of the xray beam into visible light
Which layer of the screen is closest to the film? protective coating
What is the purpose of the protective coating layer? resist abrasion, eliminate static buildup, allows for routine cleaning
Why is the back lid of the cassette have lead foil? to reduce backscatter
What are the 4 characteristics that crystals must posses? high atomic #, high conversion efficiency, spectrally matched, no afterglow
Why is it important for the crystals to have a high atomic number? so that the probability of xray interaction is high, called quantum detection efficiency
What color does calcium tungstate emit? rare earth? blue/violet blue/green
Who developed calcium tungstate? Thomas Edison
What is the primary advantage of rare earth screens? reduce pt dose
What are 6 intrinsic factors affecting screen speed? phosphor composition, active layer thickness,reflective layer, dye, crystal size, crystal concentration
The thicker the active layer, the ____ is the relative number of xrays converted into light. higher
High speed screens have _____ active layers; fine detailed screens have ____ active layers. thick thin
HOw can the speed of a film/screen combination be increased? by larger crystals, thinner crystal layer, and using a reflective backing
The presence of the reflective layer does what to screen speed but has what effect on resolution (detail)? increases screen speed, reduces resolution
Why are light-absorbing dyes added to phosphors? to control the spread of light
What effect does dye have to resolution and screen speed? increases resolution and reduces screen speed
The higher the crystal concentration, the _____ the screen speed but recorded detail will ____. higher decrease
What are two extrinsic factors that affect screen speed? kVp temperature
As kVp increases, what happens to the screen speed? increases
As temp is lowered, what happens to screen speed? increases
What are some qualities for a base material of high quality? rugged and moisture resistant not suffer radiation damage/age flexible
-the precentage of x-rays absorbed by the screen detective quantum efficiency
What kind of cassette will result in the highest intensification factor? high speed cassette
-the screen's ability to produce an accurate and clear image resolution
-device to test the resolution of the screen line bar test tool
-the ability of the screen to image similar tissues such as liver and pancreas or gray matter and white matter contrast resolution
what system speeds do rare earth screens use? 200 or 400
What is an advantage of direct exposure (not using screens) increased resolution
What is another word for radiographic noise? quantum mottle
What is the only way to reduce quantum mottle? To use a slower screen or to increase the mAs
HIger speed = ____ mottle more
How often should busy depts clean the screens? once a mth
What will be the result of poor film-screen contact? hazy area
Screen speed ____ as the screen ages? decreases
What are 7 causes of poor screen-film contact? misues, dropping the cassette, warped front panel, warped screen due to excessive moisture, worn screen backing, foreign particulate btwn the screen and film, trapped airc
Created by: erikasmith28