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image receptors

chapter 8 fosbinder

QuestionAnswer
What type of radiation produces an image of body parts in direct exposure radiography? transmitted and exit radiation
why does direct exposure radiography increase patient dose? because without screens the total amount of energy exposing film is radiation
What does direct exposure film do to recorded detail? increases it.
define- intensifying screens Device found in cassettes that contain phosphors that convert x-rays in to light which then exposes film
What do the phosphors in intensifying screens do? They are a chemical that emits light when struck by radiation.
what is the purpose of intensifying screens? to decrease patient dose
What percentage of screens can be exposed by light and x-rays? 90-99% light and 10% x-rays
What does an intensifying screen do to recorded detail? decreases recorded detail
What are the 4 layers of the screen? 1.protective layer (closest to film) 2.phosphor layer 3.reflecting or absorbing layer 4.base layer (farthest from film/closest to the cassette)
what is the base layer made of? polyester plastic or cardboard
3 characteristics of the base layer 1.flexible and chemically stable and provides support/stability for phosphor layer. 2.chemically inert 3.radiolucent
Does a screen have both a reflective layer and an absorbing layer? No. one or the other but not both.
What does the reflecting layer do? It's used to reflect light back toward the film.
What does the absorbing layer do? It consists of a light absorbing dye that absorbs low wavelength light photons directed toward the phosphor layer.
how does the reflective layer of the intensifying screen effect the efficiency of the screen? increases efficiency
How does the reflective layer of the intensifying screen effect the overall recorded detail of the image? decreases recorded detail
How does the absorbing layer of the intensifying screen affect the recorded detail of the image? increases recorded detail
What is the phosphor layer known as? The active layer
Why is the phosphor layer the most important screen component? because it contains phosphor materials that absorb transmitted radiation and converts them to visible light.
What is the protective layer made of? Plastics and protects fragile phosphor material beneath it.
What will happen if there is damage to the protective layer? May result in the removal of underlying phosphors. This could create a negative density which causes a white area on the film.
Dirt/dust on the screen results in what type of negative density? white dots/specks
the negative density of white lines on a film are the result of? Scratches that result in loss of phosphors in that area of the screen.
What are the 3 characteristics of phosphor material? 1.high atomic number 2.conversion efficiency 3.luminescence
Why is a high atomic number required for phosphor material? to ensure interaction with x-ray photons
what is conversion efficiency? Refers to the number of light photons that are generated for every x-ray photon.
What is luminescence? The production of light from the phosphor upon interaction with a x-ray photon.
What are the 2 types of luminescence? 1.fluorescence (good) 2.phosphorescence (bad)
What is fluorescence? The desired type of luminescence. The ability of phosphors to instantaneously produce visible light when exposed to x-rays.then stops when exposure ceases.
What is phosphorescence? When screen phosphors continue to emit light after x-ray exposure stops. known as screen lag/afterglow.
how does afterglow affect density on the image? increases density
What is spectral emission? Color of light emitted by a particular intensifying screen due to the phosphors used. Different phosphors will emit different colors when stimulated.
what is spectral sensitivity? The color of light to which a particular film is most sensitive.
What type of film is sensitive to blue light? panchromatic
What type of film is sensitive to green light? orthrochromatic
What is spectral matching? Correctly matching the color sensitivity of film to color emission of intensifying screens.
If a blue emitting intensifying screen is paired with orthochromatic film,what will occur to the image after exposure? No image will be recorded
what is the chemical equation for calcium tungstate phosphors? CaWO4
What color light does Calcium tungstate phosphors emit? blue light
what % of x-rays are converted into light by calcium tungstate phosphors? 3-5%
what % of x-rays are converted into light by rare earth screens? 15-20%
what are the 3 types of rare earth screens? 1.gadolinium 2.lanthanum 3.yttrium
What type of light does lanthanum and yttrium emit? blue light
what type of light does gadolinium emit? green light
What is the intensification factor? Refers to a measurement of an intensification screen's ability to intensify the x-ray beam. It can also be used to determine how much patient dose is reduced.
what is screen speed/relative speed? The ability of the intensifying screen to produce light and therefore produce light and therefore produce radiographic density.
what is the relationship between screen speed and recorded detail? inverse-increased screen speed = decreased recorded detail
what are the 10 factors effecting screen speed? 1.size of phosphor crystals 2.thickness of phosphor layer 3.materials used 4.distribution 5.kvp 6.temperature 7.presence of reflecting layer 8.light absorbing layer/dyes 9.absorption efficiency 10.conversion efficiency
what does the size of the phosphors do to recorded detail? the larger the phosphor, the more divergence of light,therefore there is a decrease in recorded detail.
what does the thickness of the phosphor layer mean for screen speed? The thicker the phosphor layer means that there are more phosphor crystals present= more efficient at producing light= increases screen speed
how does distribution of the phosphors effect the intensifying screen? smaller phosphors and a more dense concentration limits the amount of divergence of light which will increase recorded detail.
how does kvp effect intensifying screens? phosphors within the intensifying screens have high atomic numbers, therefore increasing kvp will increase light producing interaction with the phosphors.
what effect does temperature have on intensifying screens? increasing temperature causes significant damage to the phosphors and will impede light production.
What does the reflecting layer in a screen do? reflects light back toward film and increases speed of screen
what does the absorbing layer in a screen do? absorbs light that would otherwise reach film and decreases speed of scree. which would increase recorded detail.
what is absorption efficiency? the ability of the screen to absorb the incident photon.
increasing the atomic number of the phosphors in screens does what to absorption? increases absorption of incident photons
what is the absorption percentage for rare earth? 60%
what is the absorption percentage for calcium tungstate? 30-40%
What is conversion efficiency? refers to how well screen phosphors convert incident photons to light
what produces more light rare earth or calcium tungstate? Rare earth because it produces 3-4 times more light
what does decreasing the thickness of the phosphor layer do to recorded detail? increases recorded detail
what is the average fast screen speed? 400SS
What is the average slow screen speed? 100SS
What is the relationship between mAs and screen speed? inversely related.
what is the mAs conversion formula? mas1/mas2=SS2/SS1
What type of screens are AECs usually calibrated to work with? generally fast screen speeds
what would happen if a detail screen was used with AEC? Decreased density since AEC usually work with fast screen a when paired with a slow speed the image would come out under developed.
What is quantum mottle/image noise when very low number of photons are used to produce image density, image appears mottled/blotchy.
how does increased screen speed affect quantum mottle and recorded detail? increases quantum mottle and decreases recorded detail
what can you do to correct quantum mottle? increase mAs while using a slower screen speed OR increase mAs and decrease kvp while using increased screen speed.
5 qualities of the cassette/container for film and screen 1.front is made of material with low atomic numbers to allow x-rays to pass 2.light proof 3.light weight 4. rigid/sturdy 5.promotes good film screen contact
what is used to test good film and screen contact? wire mesh tool
what is the photographic formula? PE= mA X time X kvp^2 X SS / SID^2 X GCF
Created by: lchadderton