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image receptors
chapter 8 fosbinder
| Question | Answer |
|---|---|
| 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
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