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62 Cards in this Set
- Front
- Back
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*What is spatial resolution?
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Image detail, the ability to distinguish 2 things of the same opacity as different
-Determined using line pairs per mm |
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*What is contrast resolution?
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Ability to distinguish 2 things as different opacity that are in close proximity
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What 5 factors affect image detail?
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1) Motion
2) Speed of film/screen combination 3) Focal spot size 4) Focal spot-film and object-film distance 5) Grid |
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What are 2 ways to prevent motion when taking a radiograph?
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1) mAs
-use a short exposure length and increase the mA so get same amount of photons but less time for motion 2) Sedation |
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When buying films, what is one thing you must check to make sure it is compatible with your x-ray machine?
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The color of the film must match the color of the screen on cassette
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How is detail related to film/screen speed?
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Speed & detail=Inversely proportional
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What is the film?
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Size and thickness of silver halide crystals
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What is the screen?
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Size and thickness of phosphor crystals
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*What are the effects of bigger crystals in the screen?
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Bigger crystals=faster speed=lower detail
-More light diffusion and blur w/ faster speed |
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*What are the advantages of intensifying screens?
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Decrease x-rays needed to make a radiograph
-Less radiation dose to patient & personnel -Allows use of shorter exposure times=less motion -Can use low output x-ray machines for large body parts |
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What are the disadvantages of intensifying screens?
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Loss of detail bc of light diffusion
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What are non-screen radiographs used for?
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Superior detail so used for dental radiography
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How does the technique differ for non-screen radiographs?
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Requires much higher mAs values
-very long exposure so anesthesia required and have to leave room |
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What determines the focal spot size?
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Filament size in cathode
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What is the advantage of a small focal spot? What is it used for? Disadvantages?
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Better detail so use for small things like a chihuahua carpus
-Have to use lower mA values because its a smaller coil so more prone to catch fire |
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*What's the advantage of a large focal spot? disadvantages?
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Higher mA values are possible so used for example for a horse carpus.
-Poorer detail |
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How can the focal spot be manipulated?
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Changing the angle of anode and size of cathode
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What is penumbra?
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A region of geometric unsharpness at the edge of the film
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How is the size of the focal spot related to penumbra?
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Smaller focal spot=smaller penumbra
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True or false. You can tell what size of focal spot was used by the penumbra.
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True
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*How does focal spot-film distance affect the image detail?
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Long FFD=decreased penumbra=high detail
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How does the technique need to be adjusted for a long focal spot-film distance?
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Requires higher mAs
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*What is the equation that determines the mAs required for a certain focal spot-film distance?
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mAs1/mAs2=D2^2/ D1^2
-Inverse square law |
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*What happens to the penumbra when there's a long object film distance? Detail?
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Increases the penumbra
-Decreases detail |
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True or false. A long object film distance magnifies the object, increasing detail.
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False, it magnifies the object but decreases detail
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What is the function of the grid?
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Grid absorbs scatter therefore increasing the detail
-Scatter decreases detail |
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How does the technique have to be altered if using a grid?
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Absorbs some of primary beam so have to increase exposure
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What are 2 types of grids?
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Potter-bucky=moving grid
Stationery grid |
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What are grids composed of?
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A rectangular plate that has alternating lead and aluminum strips
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What is the advantage of a potter-bucky grid?
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Moves grid during exposure so don't see grid lines
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What is the bucky (grid) factor?
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The ratio of radiation falling on the grid: radiation passing through grid
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What does the bucky factor measure?
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Measures ability of grid to absorb scatter
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*What is the application of the bucky factor?
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Indicates how much increase in technique is needed when changing from non-grid to a grid technique
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What is contrast?
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Difference in film blackness across an image
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*Contrast depends on what 3 factors?
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1) Subject contrast
2) Film contrast 3) Fog and scatter |
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*What 4 characters of tissue affect the subject's contrast?
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1) Thickness
2) Tissue composition (fat VS soft tissue) 3) Atomic number 4) kVp |
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*Do high or low kVp x-rays produce more contrast?
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Low kVP=higher contrast
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Do more x-rays pass through the subject with a high or low kVp?
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High kVp becasue they are more powerful rays so more pass through without being absorbed, this results in a lower contrast.
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*How do you get the same exposure after increasing the mAs?
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Decrease the KvP
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True or false. With a high kVp a lower contrast results so the soft tissue and trabecular pattern of bone is less apparent.
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False, soft tissue is more apparent with a higher kVp
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What happens to the film of a radiograph when only the mAs is increased?
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Increase the blackness
(more x-rays are being produced so more hit film) |
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*What are the 3 settings that could result in a technique using 4 mAs?
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100 mA @ 0.04 s
400 mA @ 0.01 s 1000 mA @ 0.004 s |
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What is the issue with using a mAs of 4 with 100 mA @ 0.04 s VS using 400 mA @ 0.01 s?
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Increased time=increased risk of motion
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Would you want high contrast (low kVp) for a thoracic radiograph or abdominal radiograph?
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Abdomen because it has low contrast, whereas the thorax is inherently contrasted with lots of air and soft tissue so can use high kVp with low contrast.
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*What do you do if a radiograph has insufficient contrast but is properly exposed?
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Decrease kVp by 15% and double the mAs.
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**What alters film contrast? What alters film blackness?
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kVp=contrast
mAs=blackness |
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**How does kVp affect the # of x-rays? mAs?
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kVp=proportion of electrons accelerated from cathode to anode
mAs=size of e- cloud produced at cathode |
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What determines the film contrast?
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Inherent property of film, cannot be varied for a given film
-usually use mid range contrast film |
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What determines the film contrast with computed radiography or digital radiography?
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Inherent property of detector(s)
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What 4 things can cause fog?
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1) Exposure of film to pressure or high temperature
2) Exposure of film to light -Darkroom safelight -Bad seal on door of darkroom 3) Outdated film 4) Scatter |
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*True or false. The primary beam is a typical source of personnel radiation exposure.
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False, scatter radiation is the major source of personnel exposure (Compton scatter mainly)
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What is the primary beam?
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X-rays that originate from the anode
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*What is scatter radiation?
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X-rays that have been deflected by interaction with the patient fly in all directions
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*What happens to the scatter radiation (where does it go)?
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Hits the "wrong" location on the film
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How does scatter radiation effect the radiographic image?
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Causes random low-level exposure over the entire film
-Darkens image -Reduces contrast -Lose detail |
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*How can you minimize scatter radiation when taking a x-ray?
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1) Collimate-narrow primary beam
2) Use high kVp exposures 3) Use a grid |
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Why do we use collimation? What does collimation change?
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Because it minimizes the volume or tissue radiated
-minimizes scatter radiation Changes the size of the primary beam |
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*What does collimation decrease? (2)
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Scatter radiation
Penumbra |
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What technique has the most scatter radiation? least?
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Highest=low kVp, high mAs
Lowest=high kVp, low mAs *Remember high kVp=less contrast so its a trade off! |
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What rays does a grid block? Why is this important?
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The grid blocks the scatter radiation but also part of the primary beam so the technique needs to be increased
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Is there more scatter radiation with a thick or thin patient?
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Thick patient-more opportunity for x-rays to interact and scatter
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*When should a grid be used when taking a radiograph?
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When tissue thickness is more than 10 cm, less than 10 cm and a grid should not be used!
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