Dmi 100 Review image evaluation
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what is density?
amount of blackness on a given area of a radiograph
how do you calculate density?
what is light incident?
light striking the radiograph from the back, coming from the view box
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
how is optical density made visible?
when the crystals in the film's emulsion are converted to black metallic silver in the developer solution
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
what relationship does mAs have with density?
how does kVp affect density?
directly affects it, although not in a directly proportional relationship
what is the 15% rule?
an increase in kVp of 15% will double the density
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
what is the density maintenance formula?
what type of relationship does film-screen combination have with density?
- direct proportion
- as speed (sensitivity) increases, density increases
What do grids accomplish?
- decrease the amount of scatter radiation striking the image receptor
- decreases density unless is increased to compensate
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
what does filtration do to the density?
- negligible effect on density
- for radiation protection
- reduces soft, long wavelength rays
what is contrast?
differences in adjacent densities on a radiograph
how do grids affect contrast?
less scatter = fewer gray tones = higher contrast
how does beam restriction affect contrast?
less radiation = less scatter = fewer gray tones = higher contrast
how does filtration affect contrast?
decreases small wavelength x-rays = stronger beam = increase scatter = more gray tones = decrease contrast
what is subject contrast?
patient's body controlling contrast
what are the three controlling factors for subject contrast?
- atomic number
- tissue density
- tissue thickness
what are the factors that affect recorded detail?
- Focal spot size
- Film-Screen combo
what are the two type of shape distortion?
what are the different parts of film?
- protective coat
- emulsion (active layer)
what is the base of the film made out of?
what is the anatomy of a double emulsion film?
- super coat
- super coat
what does the emulsion of a film contain?
silver halide crystals suspended in gelatin
what are the sensitive specks on silver halide crystals for?
serve as centers for making the latent image visible
what is the latent image?
image contained in the silver halide crystals after exposure but beforew development
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)
how does the speed of the film affect contrast?
the faster the film the higher the contrast
what determines the latitude of a film?
- depends of the inherent contrast
- the lower the inherent contrast the wider the latitude
what are the other names for H&D curves?
- sensitometric curves
- characteristic curves
- D log E curves
what is plotted on a H & D curve?
- x-axis: exposure
- y-axis: density
what does the toe portion of the H & D curve represent?
what does the toe portion of the H&D curve also called
base plus fog
what does the body portion of the H&D curve also called
- straight line portion
what does the body portion of the H & D curve represent?
- measures usable densities
- indicates overall gray scale of the film
what does the shoulder portion of the H & D curve represent?
- maximum density
- blackest portion
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
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
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
what is the safe temperature for storing film?
no greater than 68 to 78 degrees
how much humidity is safe to store film?
40 to 60%
what causes the crinkle or half-moon marks on film?
bending film over fingernail during handling
what are the different parts of an intensifying screen?
- protective layer
- phosphor layer
- reflective layer
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
what controls what in a film-screen combination?
- speed controlled by screen
- contrast controlled by film
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
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
how is film-screen contact tested?
by radiographing a wide-mesh screen
what does a grid accomplish?
reduces amount of scatter radiation reaching the image receptor
when is a grid used?
- when part thickness is 10 cm or greater
- 70 kvp or higher
how is a grid constructed?
lead strips separated by aluminum interspacers
what is grid ratio?
- height of lead strips divided by the distance between them
- grid ratio = H/D
what is grid frequency?
- number of lead strips per inch or cm
- lines per inch or lines per cm
what happens to the lead strip thickness when grid frequency increases?
- becomes less visible
what are the different types of grids?
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
what is a focused grid?
- lead strips are angled to conincide with divergence of beam
- used with specific ranges of SID
what is grid radius?
- distance at which focused grid may be used
- also called focal distance or focal range
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
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
what is grid selectivity?
ratio of primary radiation transmitted through the grid to secondary radiation transmitted through the grid
what happens to selectivity when the grid frequency and grid ratio are high?
what does high selectivity mean?
indicates high efficiency of scatter cleanup
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
what are the grid conversion factors?
- 5:1 - 3
- 8:1 - 4
- 12:1 - 5
- 16:1 - 6
upside down focused grids will produce what type of grid error?
normal density in the middle with decreased density on the sides
off level focused grids will produce what type of grid error?
decreased density over the entire image
lateral decentering of focused grids will produce what type of grid error?
cutoff visible to one side of the image
focused grid decentering will produce what type of grid error?
normal density in the middle with cutoff visible on the sides
air gap technique is similar to what grid ratio?
what does a grid do to contrast?
increases contrast because it absorbs scatter radiation which decreases the amount of gray tones
each step on the density control for AEC represents how much change in density?
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
what are the different types of agents in the developer?
- reducing agents
what are the reducing agents and what do they accomplish?
- phenidone- quickly build gray tones
- hydroquinone- slowly fill in black tones
what does an activator do during processing?
soften and swells film emulsion
what does a hardener do?
controls swelling of film emulsion to allow safe transport through the processor
what does a restrainer do?
prevents reducing agents from producing fog, which is created when unexposed silver bromide crystals develop
what does a preservative do?
slows oxidation of reducing agents by room air
what is the solvent in the processor?
water which is the medium in which chemicals are dissolved
what does the fixer accomplish?
clears and removes unexposed silver bromide crystals
what does the transport system do?
- moves film through processor
- agitates chemistry
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
what does a crossover accomplish?
moves film from the developer tank into the fixer tank and from the fixer tank to the wash tank
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
replenishment rates in a typical workday are based on what size of film?
what is the temperature maintained in inside the developer?
90 to 95 degrees F
what is the temperature in the dryer?
120 degrees F
Maintenance of processor: what is recommended daily?
- wash crossover racks twice daily
- drain wash tank
Maintenance of processor: what is recommended weekly?
clean deep racks
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
When is sensitometric testing done for automatic processors?
what does sensitometric testing check?
- base plus fog
what does a densitometer measure?
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
increased developer temperature does what to a film?
- chemical fog
- increased density
what type of malfunction to the processor causes light films?
developer too low
what type of malfunction to the processor causes film to appear milky?
poor fixer replenishment
what type of malfunction to the processor causes dark flakes to appear?
algae from wash water
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
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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