Dental x ray set, conventional image receptors and processing Flashcards
what is used for filtration of x ray beam? (be specific)
- 5mm aluminium below 70kV
2. 5mm aluminium above 70kV
purpose of filtration 2
- removes low energy photons
- decreases dose to pt
function of rectangular collimator
matches beam size to image receptor
–> reduces dose by 50%, stops beam being wasted
5 things on x ray control panel
- on/off switch
- timer
- exposure time selection
- warning lights/audible signals
- exposure button
what determines
a. quality
b. quantity of x ray photons
a. quality: kV
b. quantity: mA (tube current)/time
effect of higher kV (higher power given to electrons) 3
- decrease pt dose
- decrease contrast (more electrons pass through dense tissue so less contrast with soft tissue)
- increase scatter
effect of higher mA or time 2
- increase pt dose
- increase film blackening
should x rays use AC or DC ? why?
DC
must have positive voltage so electrons all flow in same direction and have high energy
4 advantages of constant potential
- xray production per unit time more efficient
- more high energy photons per exposure
- fewer low energy harmful photons produced
- shorter exposure times
what shape of beam is used and why
neat parallel beam (not divergent)
–> minimal magnification, smaller area irradiated
2 types of conventional image receptors
- direct action film: x ray photons interact with film
- indirect action film: x ray photons interact with intensifying screen producing light which then interacts with film
what size x ray film to use for
a. anterior teeth
b. posterior teeth
c. occlusal radiography
what size x ray film to use for
a. anterior teeth: 0,1
b. posterior teeth: 2
c. occlusal radiography: 4
2 functions of lead foil
label film packet (see lecture)
reduce scatter
reduce dose
what does dot on x ray tube show and why
location of x ray tube
must have 20cm between target and tube
explain scatter
x rays pass through desired dense tissue, bounces back off other structures –> poor quality image
cross section of x ray film
protective coating emulsion adhesive transparent plastic base adhesive emulsion
2 mixtures used for the emulsion
- silver halide crystals in gelatin matrix (90%)
- silver iono-bromide (10%)
effect of silver iono bromide
increases sensitivity
how does the emulsion work
x ray/light photons sensitize silver halide crystals that they strike forming a latent image
sensitized crystals reduced to black metallic silver in the developer
order of letters of film speed and colour
D speed (blue) –> E speed (1/2 dose of D speed) –> F speed (60% dose of pink speed) –> digital (50% dose of F speed)
which of these is currently used for intraoral x rays?
F speed
what effect does film speed have on image quality and why
faster film–> lower quality image
because film speed ∝ size (and number) of silver halide crystals in soln
so larger crystals –> faster film but less fine image
how do indirect action films work
x ray photons interact with an intensifying screen –> light produced –> interacts with film (film is sensitive to light)
describe what an indirect action cassette looks like/made of and what each part does
light-tight aluminium or carbon fibre casings. contains sandwich:
1st intensifying screen, picks up lower energy photons
cassette film
2nd intensifying screen, picks up higher energy photons
when are indirect action films used and why
when there’s a thicker part of body to penetrate (eg jaw, EXTRAORAL X RAYS) so would have to increase exposure too much if direct action film was used
what are intensifying screens made of and why
fluorescent phosphors (emit light when excited by x rays) embedded in a plastic matrix
effect of intensifying screens on pt dose and why
decreases pt dose:
one x ray photon –> many light photons (so less x ray photons required to produce an image than with direct action film)
disadvantage of indirect action film/ intensifying screens
decreased image resolution
define image resolution
ability to differentiate between different structures that are close together on a radiograph
resolution of
a. direct action film
b. indirect action film
a. direct action film: 10 line pairs per mm
b. indirect action film: 5 line pairs per mm
how to calculate intensification factor
exposure required when screens not used/
exposure required with screens
compare rare earth screens and calcium tungstate screens
rare earth screens: 5x faster than calcium tungstate screens –> lower dose to pt
why must film be matched to screen with rare earth screens?
different phosphors emit different coloured lights, so film must be able to detect type/colour of light emitted
what does film processing do?
turn invisible latent image –> visible radiographic image
advantages of automatic processing over manual processing 4
- time saving (20 mins –> 5 mins)
- no darkroom rqd
- controlled standardised processing conditions
- replenishment of chemicals is automatic
5 stages of film processing
- development
(2. clearing) - fixation
- washing
- drying
explain the development stage
sensitised silver halide crystals in the emulsion are converted to black metallic silver to produce the black/grey parts of the image
what can cause overdevelopment? 3
- conc of soln too strong
- too long
- too hot
what can cause underdevelopment? 4
-conc of soln too weak, because it is oxidized by air over time –> less effective
(too old)
-too cold
-not long enough
how often should developing soln be changed and why
every 14 days
alkaline soln pH 10.5 is oxidized by air, weakening soln –> underdevelopment
describe appearance of
a. overdeveloped
b. underdeveloped film
a. overdeveloped: black, can’t see image
b. underdeveloped film: white, image unclear
explain the clearing stage of processing
-unsensitised silver halide emulsion is removed to reveal the transparent/white parts of image and emulsion is hardened
fixation stage of processing and conditions required
-fixer anchors silver grains to film base
acidic pH 4-4.5
compare clearing and fixing time and fixing time for manual processing
fixing time is double clearing time (usually 8-10 mins for manual processing)
describe under-fixed film
greenish yellow/ milky
may discolour to brown over time
why must film be washed
remove any residual fixer (otherwise film becomes stained brown)
what causes chemical splashes 2
(usually in manual processing)
- fixer splashes prior to development
- developer splashes
what causes films to stick together in automatic processing
not leaving long enough before putting films in processor
what causes light fogging?
opening film packet in light
