Chap 3 Screens
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| -in a cassette converts a small amt of x-ray energy to a much greater amt of visible light energy | intensifying screens
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| What % of the x-ray that strike a radiographic intensifying screen interact with the screen? | 30%
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| -visible light emitted | luminescence
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| -any material that emits light in response to some outside stimulation | luminescent material or a phosphor
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| 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
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| -emission of visible light from screens after x-rays interact with the atoms of the screen's phosphor crystals | luminescence
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| What are the two types of luminescence? | fluorescence & phosphorescence
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| -yields a more predictable density on the film bc light emission ceases after exposure | fluorescence
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| -screen lag, after glow | phosphorescence
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| What are the 4 layers of a screen? | base, reflective layer, phosphor, protective coating
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| farthest layer from the film | base
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| What is the purpose of the base layer of the screen? | provides mechanical support for the phosphor layer
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| Where is the reflective layer located? | btwn the phosphor layer and the base
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| What is the purpose of the reflective layer? | to redirect light to the film
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| What does isotropically mean? | equally in all directions
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| -the amt of light emitted and the efficiency or ease with which a screen converts x-ray energy to light energy | conversion efficiency
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| WHich layer is known as the active layer? | phosphor layer
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| What is the purpose of the phosphor layer? | to convert the energy of the xray beam into visible light
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| Which layer of the screen is closest to the film? | protective coating
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| What is the purpose of the protective coating layer? | resist abrasion, eliminate static buildup, allows for routine cleaning
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| Why is the back lid of the cassette have lead foil? | to reduce backscatter
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| What are the 4 characteristics that crystals must posses? | high atomic #, high conversion efficiency, spectrally matched, no afterglow
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| 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
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| What color does calcium tungstate emit? rare earth? | blue/violet
blue/green
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| Who developed calcium tungstate? | Thomas Edison
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| What is the primary advantage of rare earth screens? | reduce pt dose
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| What are 6 intrinsic factors affecting screen speed? | phosphor composition, active layer thickness,reflective layer, dye, crystal size, crystal concentration
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| The thicker the active layer, the ____ is the relative number of xrays converted into light. | higher
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| High speed screens have _____ active layers; fine detailed screens have ____ active layers. | thick
thin
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| HOw can the speed of a film/screen combination be increased? | by larger crystals, thinner crystal layer, and using a reflective backing
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| The presence of the reflective layer does what to screen speed but has what effect on resolution (detail)? | increases screen speed, reduces resolution
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| Why are light-absorbing dyes added to phosphors? | to control the spread of light
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| What effect does dye have to resolution and screen speed? | increases resolution and reduces screen speed
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| The higher the crystal concentration, the _____ the screen speed but recorded detail will ____. | higher
decrease
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| What are two extrinsic factors that affect screen speed? | kVp
temperature
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| As kVp increases, what happens to the screen speed? | increases
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| As temp is lowered, what happens to screen speed? | increases
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| What are some qualities for a base material of high quality? | rugged and moisture resistant
not suffer radiation damage/age
flexible
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| -the precentage of x-rays absorbed by the screen | detective quantum efficiency
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| What kind of cassette will result in the highest intensification factor? | high speed cassette
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| -the screen's ability to produce an accurate and clear image | resolution
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| -device to test the resolution of the screen | line bar test tool
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| -the ability of the screen to image similar tissues such as liver and pancreas or gray matter and white matter | contrast resolution
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| what system speeds do rare earth screens use? | 200 or 400
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| What is an advantage of direct exposure (not using screens) | increased resolution
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| What is another word for radiographic noise? | quantum mottle
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| What is the only way to reduce quantum mottle? | To use a slower screen or to increase the mAs
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| HIger speed = ____ mottle | more
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| How often should busy depts clean the screens? | once a mth
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| What will be the result of poor film-screen contact? | hazy area
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| Screen speed ____ as the screen ages? | decreases
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| 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
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