Dental x ray set, conventional image receptors and processing

Question 1

what is used for filtration of x ray beam? (be specific)

Answer 1

1. 5mm aluminium below 70kV

2. 5mm aluminium above 70kV

Question 2

purpose of filtration 2

Answer 2

• removes low energy photons

- decreases dose to pt

Question 3

function of rectangular collimator

Answer 3

matches beam size to image receptor

–> reduces dose by 50%, stops beam being wasted

Question 4

5 things on x ray control panel

Answer 4

• on/off switch
• timer
• exposure time selection
• warning lights/audible signals
• exposure button

Question 5

what determines

a. quality
b. quantity of x ray photons

Answer 5

a. quality: kV

b. quantity: mA (tube current)/time

Question 6

effect of higher kV (higher power given to electrons) 3

Answer 6

• decrease pt dose
• decrease contrast (more electrons pass through dense tissue so less contrast with soft tissue)
• increase scatter

Question 7

effect of higher mA or time 2

Answer 7

• increase pt dose

- increase film blackening

Question 8

should x rays use AC or DC ? why?

Answer 8

DC

must have positive voltage so electrons all flow in same direction and have high energy

Question 9

4 advantages of constant potential

Answer 9

• xray production per unit time more efficient
• more high energy photons per exposure
• fewer low energy harmful photons produced
• shorter exposure times

Question 10

what shape of beam is used and why

Answer 10

neat parallel beam (not divergent)

–> minimal magnification, smaller area irradiated

Question 11

2 types of conventional image receptors

Answer 11

• 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

Question 12

what size x ray film to use for

a. anterior teeth
b. posterior teeth
c. occlusal radiography

Answer 12

what size x ray film to use for

a. anterior teeth: 0,1
b. posterior teeth: 2
c. occlusal radiography: 4

Question 13

2 functions of lead foil

label film packet (see lecture)

Answer 13

reduce scatter

reduce dose

Question 14

what does dot on x ray tube show and why

Answer 14

location of x ray tube

must have 20cm between target and tube

Question 15

explain scatter

Answer 15

x rays pass through desired dense tissue, bounces back off other structures –> poor quality image

Question 16

cross section of x ray film

Answer 16

protective coating
emulsion
adhesive
transparent plastic base
adhesive
emulsion

Question 17

2 mixtures used for the emulsion

Answer 17

• silver halide crystals in gelatin matrix (90%)

- silver iono-bromide (10%)

Question 18

effect of silver iono bromide

Answer 18

increases sensitivity

Question 19

how does the emulsion work

Answer 19

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

Question 20

order of letters of film speed and colour

Answer 20

D speed (blue) –> E speed (1/2 dose of D speed) –> F speed (60% dose of pink speed) –> digital (50% dose of F speed)

Question 21

which of these is currently used for intraoral x rays?

Answer 21

F speed

Question 22

what effect does film speed have on image quality and why

Answer 22

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

Question 23

how do indirect action films work

Answer 23

x ray photons interact with an intensifying screen –> light produced –> interacts with film (film is sensitive to light)

Question 24

describe what an indirect action cassette looks like/made of and what each part does

Answer 24

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

Question 25

when are indirect action films used and why

Answer 25

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

Question 26

what are intensifying screens made of and why

Answer 26

fluorescent phosphors (emit light when excited by x rays) embedded in a plastic matrix

Question 27

effect of intensifying screens on pt dose and why

Answer 27

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)

Question 28

disadvantage of indirect action film/ intensifying screens

Answer 28

decreased image resolution

Question 29

define image resolution

Answer 29

ability to differentiate between different structures that are close together on a radiograph

Question 30

resolution of

a. direct action film
b. indirect action film

Answer 30

a. direct action film: 10 line pairs per mm

b. indirect action film: 5 line pairs per mm

Question 31

how to calculate intensification factor

Answer 31

exposure required when screens not used/

exposure required with screens

Question 32

compare rare earth screens and calcium tungstate screens

Answer 32

rare earth screens: 5x faster than calcium tungstate screens –> lower dose to pt

Question 33

why must film be matched to screen with rare earth screens?

Answer 33

different phosphors emit different coloured lights, so film must be able to detect type/colour of light emitted

Question 34

what does film processing do?

Answer 34

turn invisible latent image –> visible radiographic image

Question 35

advantages of automatic processing over manual processing 4

Answer 35

• time saving (20 mins –> 5 mins)
• no darkroom rqd
• controlled standardised processing conditions
• replenishment of chemicals is automatic

Question 36

5 stages of film processing

Answer 36

1. development
   (2. clearing)
2. fixation
3. washing
4. drying

Question 37

explain the development stage

Answer 37

sensitised silver halide crystals in the emulsion are converted to black metallic silver to produce the black/grey parts of the image

Question 38

what can cause overdevelopment? 3

Answer 38

• conc of soln too strong
• too long
• too hot

Question 39

what can cause underdevelopment? 4

Answer 39

-conc of soln too weak, because it is oxidized by air over time –> less effective
(too old)
-too cold
-not long enough

Question 40

how often should developing soln be changed and why

Answer 40

every 14 days

alkaline soln pH 10.5 is oxidized by air, weakening soln –> underdevelopment

Question 41

describe appearance of

a. overdeveloped
b. underdeveloped film

Answer 41

a. overdeveloped: black, can’t see image

b. underdeveloped film: white, image unclear

Question 42

explain the clearing stage of processing

Answer 42

-unsensitised silver halide emulsion is removed to reveal the transparent/white parts of image and emulsion is hardened

Question 43

fixation stage of processing and conditions required

Answer 43

-fixer anchors silver grains to film base

acidic pH 4-4.5

Question 44

compare clearing and fixing time and fixing time for manual processing

Answer 44

fixing time is double clearing time (usually 8-10 mins for manual processing)

Question 45

describe under-fixed film

Answer 45

greenish yellow/ milky

may discolour to brown over time

Question 46

why must film be washed

Answer 46

remove any residual fixer (otherwise film becomes stained brown)

Question 47

what causes chemical splashes 2

Answer 47

(usually in manual processing)

• fixer splashes prior to development
• developer splashes

Question 48

what causes films to stick together in automatic processing

Answer 48

not leaving long enough before putting films in processor

Question 49

what causes light fogging?

Answer 49

opening film packet in light