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what is density?
amount of blackness on a given area of a radiograph
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how do you calculate density?
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what is light incident?
light striking the radiograph from the back, coming from the view box
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what is light transmitted?
light that is seen coming through the radiograph whilte it is being viewed by either the human eye or a densitometer
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how is optical density made visible?
when the crystals in the film's emulsion are converted to black metallic silver in the developer solution
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what is the reciprocity law?
any combination of mA and time that produce the same mAs value will result in the same density on the radiography
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what relationship does mAs have with density?
direct proportional
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how does kVp affect density?
directly affects it, although not in a directly proportional relationship
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what is the 15% rule?
an increase in kVp of 15% will double the density
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what is inverse square law?
the intensity of the x-ray beam is inversely proportional to the square of the distance between the source of x-rays and the image receptor
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what is the density maintenance formula?
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what type of relationship does film-screen combination have with density?
- direct proportion
- as speed (sensitivity) increases, density increases
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What do grids accomplish?
- decrease the amount of scatter radiation striking the image receptor
- decreases density unless is increased to compensate
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what do beam restriction devices do to density?
decrease density by limiting the size of the x-ray beam unleass mAs is increased to compensate
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what does filtration do to the density?
- negligible effect on density
- for radiation protection
- reduces soft, long wavelength rays
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what is contrast?
differences in adjacent densities on a radiograph
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how do grids affect contrast?
less scatter = fewer gray tones = higher contrast
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how does beam restriction affect contrast?
less radiation = less scatter = fewer gray tones = higher contrast
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how does filtration affect contrast?
decreases small wavelength x-rays = stronger beam = increase scatter = more gray tones = decrease contrast
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what is subject contrast?
patient's body controlling contrast
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what are the three controlling factors for subject contrast?
- atomic number
- tissue density
- tissue thickness
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what are the factors that affect recorded detail?
- OID
- SID
- Focal spot size
- Film-Screen combo
- Motion
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what are the two type of shape distortion?
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what are the different parts of film?
- protective coat
- emulsion (active layer)
- adhesive
- base
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what is the base of the film made out of?
polyester
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what is the anatomy of a double emulsion film?
- super coat
- emulsion
- adhesive
- base
- adhesive
- emulsion
- super coat
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what does the emulsion of a film contain?
silver halide crystals suspended in gelatin
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what are the sensitive specks on silver halide crystals for?
serve as centers for making the latent image visible
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what is the latent image?
image contained in the silver halide crystals after exposure but beforew development
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what determines the speed of a film?
- size of silver halide crystals
- number of silver halide crystals
- thickness of emulsion
- (the larger the crystals and the thicker the emulsion, the faster the film)
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how does the speed of the film affect contrast?
the faster the film the higher the contrast
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what determines the latitude of a film?
- depends of the inherent contrast
- the lower the inherent contrast the wider the latitude
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what are the other names for H&D curves?
- sensitometric curves
- characteristic curves
- D log E curves
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what is plotted on a H & D curve?
- x-axis: exposure
- y-axis: density
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what does the toe portion of the H & D curve represent?
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what does the toe portion of the H&D curve also called
base plus fog
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what does the body portion of the H&D curve also called
- straight line portion
- gamma
- slope
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what does the body portion of the H & D curve represent?
- measures usable densities
- indicates overall gray scale of the film
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what does the shoulder portion of the H & D curve represent?
- maximum density
- blackest portion
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how do you determine the speed of a film by looking at the H&D curve?
the closer the curve to the y-axis the faster the speed
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how do you determine the conrast of a film by looking at the H&D curve?
- the steeper the slope of the curve the higher the contrast
- the shallower the slope the lower the contrast
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how do you determine the recorded detail of a film by looking at the H&D curve?
- based on the speed of the film
- the faster the speed, the poorer the recorded detail
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what is the safe temperature for storing film?
no greater than 68 to 78 degrees
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how much humidity is safe to store film?
40 to 60%
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what causes the crinkle or half-moon marks on film?
bending film over fingernail during handling
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what are the different parts of an intensifying screen?
- protective layer
- phosphor layer
- reflective layer
- base
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what determines the speed of an intensifying screen?
- phosphor used
- phosphor size (larger = faster)
- active layer thickness (thicker = faster)
- efficiency of reflective layer (higher = faster)
- kvp used (higher = faster)
- prescene of yellow dye in active layer (dye absorbs light)
- conversion effciency of phosphors
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what controls what in a film-screen combination?
- speed controlled by screen
- contrast controlled by film
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how many intensifying screens are used with a single emulsion film? how many for double emulsion film?
- single = one intensifying screen
- double = two intensifying screens
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what is the intensification factor?
- ratio of exposure in mAs needed to produce image without screens compared with exposure in mAs needed to produce image with screens
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how is film-screen contact tested?
by radiographing a wide-mesh screen
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what does a grid accomplish?
reduces amount of scatter radiation reaching the image receptor
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when is a grid used?
- when part thickness is 10 cm or greater
- 70 kvp or higher
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how is a grid constructed?
lead strips separated by aluminum interspacers
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what is grid ratio?
- height of lead strips divided by the distance between them
- grid ratio = H/D
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what is grid frequency?
- number of lead strips per inch or cm
- lines per inch or lines per cm
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what happens to the lead strip thickness when grid frequency increases?
- decreases
- becomes less visible
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what are the different types of grids?
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what is a linear grid?
- lead strips are parallel to one another
- x-ray tube may be angled along the length of the grid w/o cutoff
- used primarily with large SID or small field
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what is a focused grid?
- lead strips are angled to conincide with divergence of beam
- used with specific ranges of SID
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what is grid radius?
- distance at which focused grid may be used
- also called focal distance or focal range
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what is a crossed grid?
- consist of two linear grids placed perpendicular to each other
- superior scatter cleanup
- allow no angulation of x-ray beam
- require perfect positioning and centering
- primary use is biplane cerebral angiography
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what is contrast improvement factor?
- measure of a grid's ability to increase contrast
- ratio of the contrast with a grid to the contrast without a grid
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what is grid selectivity?
ratio of primary radiation transmitted through the grid to secondary radiation transmitted through the grid
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what happens to selectivity when the grid frequency and grid ratio are high?
more selectivity
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what does high selectivity mean?
indicates high efficiency of scatter cleanup
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what is grid conversion factor?
amount of exposure increase necessary to compensate for the absorption of image forming rays and scatter in the cleanup process
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what are the grid conversion factors?
- 5:1 - 3
- 8:1 - 4
- 12:1 - 5
- 16:1 - 6
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upside down focused grids will produce what type of grid error?
normal density in the middle with decreased density on the sides
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off level focused grids will produce what type of grid error?
decreased density over the entire image
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lateral decentering of focused grids will produce what type of grid error?
cutoff visible to one side of the image
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focused grid decentering will produce what type of grid error?
normal density in the middle with cutoff visible on the sides
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air gap technique is similar to what grid ratio?
10:1
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what does a grid do to contrast?
increases contrast because it absorbs scatter radiation which decreases the amount of gray tones
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each step on the density control for AEC represents how much change in density?
25%
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what does the developer do during processing of film?
- converts exposed silver halide (bromide) crystals to black metallic silver
- or convert latent image to manifest image
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what are the different types of agents in the developer?
- reducing agents
- activator
- hardener
- restrainer
- preservative
- solvent
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what are the reducing agents and what do they accomplish?
- phenidone- quickly build gray tones
- hydroquinone- slowly fill in black tones
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what does an activator do during processing?
soften and swells film emulsion
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what does a hardener do?
controls swelling of film emulsion to allow safe transport through the processor
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what does a restrainer do?
prevents reducing agents from producing fog, which is created when unexposed silver bromide crystals develop
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what does a preservative do?
slows oxidation of reducing agents by room air
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what is the solvent in the processor?
water which is the medium in which chemicals are dissolved
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what does the fixer accomplish?
clears and removes unexposed silver bromide crystals
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what does the transport system do?
- moves film through processor
- agitates chemistry
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what do deep racks do?
- move film into and through solutions in developer, fixer, and wash tanks and between drying tubes in dryer section
- has turnaround assembly at the bottom
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what does a crossover accomplish?
moves film from the developer tank into the fixer tank and from the fixer tank to the wash tank
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when does replenishment of chemicals occur inside the processor?
- for each film fed into the processor
- activated by micro switch at end of entrance roller
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replenishment rates in a typical workday are based on what size of film?
14x17
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what is the temperature maintained in inside the developer?
90 to 95 degrees F
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what is the temperature in the dryer?
120 degrees F
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Maintenance of processor: what is recommended daily?
- wash crossover racks twice daily
- drain wash tank
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Maintenance of processor: what is recommended weekly?
clean deep racks
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Maintenance of processor: what is recommended monthly?
- drain and clean by hand all tanks and dryer
- put in fresh developer and fixer
- add starter solution to developer chemicals
- change developer filter
- change water wilters
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When is sensitometric testing done for automatic processors?
daily
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what does sensitometric testing check?
- speed
- contrast
- base plus fog
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what does a densitometer measure?
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what are pi line artifacts?
- small marks 3.14 inches apart
- caused by raised nick on roller scratching film
- also by chemical stain or dirt
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increased developer temperature does what to a film?
- chemical fog
- increased density
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what type of malfunction to the processor causes light films?
developer too low
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what type of malfunction to the processor causes film to appear milky?
poor fixer replenishment
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what type of malfunction to the processor causes dark flakes to appear?
algae from wash water
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what type of malfunction to the processor causes dark films?
- developer temp too high
- developer overreplenishment
- fixer contamination of developer
- white light leak
- crack in safelight
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what type of malfunction to the processor causes film fog?
- developer contamination
- developer overreplenishment
- crack in safelight
- developer temp too high
- outdated film
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