Test 3 Unit 3 Density Flashcards
films overall darkness. controlled by mAs but influenced by kVp and other components
density
varying shades of gray on film. controlled by kVp
contrast
affects density and detail. focal film distance, source to image distance. inversely proportional to film density.
FFD/SID - the longer the distance, the lighter the film.
affects detail. object film distance
OFD
film too light (too little density). not enough mAs
underexposed
film too dark (too much density). too much mAs
overexposed
appears black on the film. xrays pass right through
radiolucent
appears white on film. xrays are absorbed and dont hit film
radiopaque
can be easily removed from a patient before an xray is taken
artifact
cannot be easily removed from patient
foreign body
Why do you need to know if patient has foreign body?
you may have to increase or decrease amount of xrays needed
quantity of electrons to be boiled off filament. There is a direct relationship between this and mAs.
mA
tells the cathode the time the electrons have to boil off the filament. There is a direct relationship between this and mAs.
s
the quantity of xrays produced. Responsible for density on a radiograph
mAs
composition of mAs
99% heat, % xrays
4 factors for xray production
- source of electons 2. thermionic emission of xrays 3. electrons set in motion 4. electrons stopped abruptly at target
What happens when electrons are set in motion?
Strong positive charge of anode attracts e-. E- accelerate from 0-1.5 billion meters/s.
What is the distance from the cathode to the anode?
2cm
What happens when electrons stop at target?
Sudden stop and collision with anode target converts e- energy to: xray energy 99%
50% reciprocal rule
if film is too dark (dense), cut mAs in half. If film is too light, double mAs.
30% Rule
a 30 % change in mAs is required to even see a visible change in darkness on a film. Doubling or halving the mAs will double or half the film density
Secondary factors that affect film density
kVp, FFD, patient size and shape, pathology, film processing, grids/grid ratio, film-screen speed, compensation filtration
used to maintain radiographic density when distance changes. When distance increases, triple mAs. When distance decreases, divide by 3.
Direct square law
a compound that reduces exposed silver to black atomic silver. directly proportional to concentration and temperature and time. Any decrease in one of these will result in lighter film.
developer
inside cassettes and contact with film. contain phosphor crystals. 4 layers
screen
4 layers of screen
base, undercoating, phosphor layer, and protective layer
cassette construction
cassette front (radiolucent carbon fiber), compression foam, screen, phosphor layer
more light from the phosphors emitted, less xrays required. brighter screen. less xrays needed. less detail. patient exposure reduced
fast screen (400)
less light from the phosphors emitted, requires more xrays. better detailed. increased patient exposure. Usually for extremities
slow screen (200)
large patient size
hyperstenic
average patient size
stenic
thinner than average patient size
asthenic
super thin patient size
hypostenic
