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FRCR Physics > Film Screen Radiography > Flashcards

Film Screen Radiography Flashcards

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1
Q

what is the emulsion made of?

A

this is a suspension in gelatine of silver halide crystals generally silver iodobromide with 90% Br and 10% I.

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2
Q

how is the film and screens are protected from abrasion and damage?

A

both films and screens have a very thin transparent antistatic super coat to protect against abrasion, and are designed not to curl.

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3
Q

the small proportion of I relative to Br distorts the …., which allows the silver ions to move through the lattice.

A

lattice

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4
Q

how is the latent image produced?

A

light photon is absorbed by the crystal–> e is liberated and migrated to the sensitivity speck.-> when enough e accumulated they attract mobile silver ions from within the crystal to join them and neutralise them.
the distribution within the emulsion forms a latent image in the film that is awaiting development

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5
Q

what are the 3 stages of film processing?

A
  1. development
  2. fixation
  3. washing
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6
Q

how is the film developed?

A

the film is developed by immersion in an alkaline solution of a reducing agent (e donor) which is able to enter the crystal at the site of latent image
it reduces positive silver ions to silver atoms and the latent image grows into a grain of metallic silver.

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7
Q

T/F?

The developer affects all crystals.

A

false

the unexposed crystals which carry no latent image are unaffected by the developer.

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8
Q

he amount of background fog produced by the development of the unexposed crystal is a function of …

A

development time , strength and temperature

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9
Q

what happens in fixation?

A

the film is fixed by an acid solution of thiosulphate (hypo), which dissolves out the unaffected silver ions so that the image is stable and unaffected by the light.

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10
Q

what is the result of inadequate washing?

A

he retained hypo turns the film brown/yellow in time, causing the stored film to have a familiar vinegary smell.

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11
Q

what is the OD?

A

the OD/blackening of an area of the film depends on the number of silver grains per unit area.

OD = log(incident light / transmitted light)

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12
Q

how do you measure the OD?

A

Densitometer, this is a small light source, 1-2mm in diameter and a light detector.

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13
Q

what is the density of a complete transparent film/

A

0

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14
Q

T/F?

densities are additive

A

true

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15
Q

what is intensifying screen in film screen radiography?

A

its a sheet of material that is the same size as the film.
it has a polyester base o.25mm thick, , coated a dense layer of fine phosphor crystal bound by transparent resin.
the crystal absorbs x rays and emits light proportional to intensity of x rays.

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16
Q

name the traditional phosphor used for radiography;

A

calcium tungstate CaWO4
emits blue light when irradiated with x rays

the use of calcium tungstate has been superseded by use of rare earth screen.

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17
Q

what are the features (atomic number) of rare earth materials.

A

atomic number 57-70 and k edge 39-61 kev.
they are more efficient than calcium tungstate into converting x rays into light.
They also require shorter exposure times –> lower patient dose.

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18
Q

Name the most common rare earth materials.

A

lanthanum oxybromide
lanthanum oxysulphide
gadolinium oxysulphide

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19
Q

please read…

A

x ray film is usu double coated with an emulsion on each side of the base and it is used with a pair of screens
~ 1/3 of the x ray falling on the front screen, closer to patient, s absorbed and about the half the light so produced travels forward and exposes the nearer emulsion (front)
the rear screen absorbs about half of x ray transmitted by the front screen, ie 2/3 of total fluence is absorbed by the two screens in combination.light from the back screen exposes the rear facing emulsion.

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20
Q

the screen materials are chosen so that they do not emit delayed fluorescence, why is that?

A

because these would retain a memory of the previous exposure. On reloading the cassette the previous image would be recorded on the new film and superimposed on the image of a subsequent exposure.

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21
Q

what is an x ray cassette?

A

this is a flat, light tight box with internal pressure pads designed to keep its content (film) in close and uniform contact with the two intensifying screen.

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22
Q

what is the structure of the x ray cassette?

A

the front, closer to the tube, is made of a low atomic number material, possibly carbon fibre, Z =6, in order to minimise the attenuation of the beam and to reduce the patient exposure.

the back, incorporates a thin lead sheet to minimise any backscattered radiation

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23
Q

the use of low atomic number material bwn the patient and the intensifying screen is important at lower kv such as those used for……. and ……

A

mammography and extremity radiography.

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24
Q

the response of a film and screen combination to x ray is described by its…….

A

characteristic curve.

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25
Q

what is the characteristic curve?

A

this is the graph of OD as a function of exposure plotted on a log scale.

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26
Q

what are the 3 components of characteristic curves?

A
  1. the region of correct exposure is the straight line ( steepest gradient), diagnostic interest.
  2. the toe is the low density region- slope of the curve- shallow
  3. the shoulder is at higher densities and has a shallow slope
    after shoulder–> flattens = saturation (Ag+ –>Ag atom)
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27
Q

what does the speed class depend on?

A

phosphor type
thickness of phosphor
type of film

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28
Q

what is the intensification factor?

A

ratio of air kerma required to produce OD 1 for film alone : the air kerma required with the film + screen.

typically in the range of 30-100

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29
Q

what is the effect of increasing developer temp, [ ] or developing time on gamma and fog level?

A

increasing the developer temp = increase the rate of chemical reaction= increases speed but also fog level= increases film gamma initially but above the temp recommended by the manufacturer the increase in fog level will reduce the average gamma
Increasing [ ] or developing time has similar effects.

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30
Q

T/F?

The film gamma and latitude are inversely related

A

true

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31
Q

give an example of wide latitude and high film gamma

A

wide latitude in chest radiography

High gamma is used in mammography because of low subject contrast

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32
Q

Define film latitude;

A

this is the range of exposures that produce net densities lying in the useful density range between 0.25 and 2

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33
Q

what is the film gamma?

A

this is the average slope of the characteristic curve
it influences the amount of contrast displayed in the radiographic image.

ie the steeper the curve the higher the contrast

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34
Q

what does the value of gamma depend on?

A

it depends on the RANGE of crystal sizes and not their average size.

35
Q

what is the base plus fog level (B + F)?

A

this is the density of processed but unexposed film.

additional fog may arise during storage esp if temp and humidity are too high or if the film is old.

36
Q

what is the effect of fog on contrast

A

fog reduces contrast and should be as low as possible.

37
Q

hat instrument is used to derive the characteristic curve?

A

sensitometer

38
Q

what is the film consist of in film screen radiography?

A

its made of a polyester base- 0.2mm thick which is coated usu on both sides with a thin photographic emuslion- 5-10 µm thick.

39
Q

the contrast seen in the film is due to both…………and………

A

subject contrast and film gamma

gamma amplifies contrast

40
Q

what is the contrast in film screen radiography?

A

this is the OD between the adjacent areas on the film.

41
Q

what is the disadvantage of having a high gamma film/higher contrast?

A

more contrasty film has lower latitude , so image detail is lost in darker or lighter areas of the image

42
Q

T/F?

in film screen radiogrphy, the degree of unsharpness increases with phosphor thickness

A

true

the thicker the phosphor the greater the sensitivity or speed of the screen.

43
Q

what are the +ve and -ve of thicker phosphor?

A 
thicker ph would increase speed and therefore less dose this also causes increased unsharpness
thick ph for abdomen and pelvis
tin ph (called definition screen) for extremities
44
Q

what could be added to the screen base at the outer surface of ph to increase sensitivity?

A

a reflective layer

45
Q

what could be incorporated in the ph layer to reduce unsharpness?

A

to reduce unsharpness coloured dyes may be incorporated in the ph layer to absorb some of the light as it passes through the ph.

46
Q

T/F?

light photons emitted at a wider angle are more likely to be attenuated, thus reducing unsharpness.

A

true

this is at the cost of reducing sensitivity of the system.

47
Q

T/F?

cross over causes blurring

A

true

48
Q

list factors contributing to unsharpness

A
  • reflective layer
  • cross over between screen and emulsion
  • parallax in double emulsion film has 2 images, esp when the film is viewed obliquely
  • film screen contact, poor contact can be seen as an area of blurring
49
Q

what factors responsible for amount of radiation reaching the film?

A

Kv
mA
exposure time

50
Q

patient dose …… with increasing kv

A

decreases

because the more penetrating beam at higher kv requires less radiation to produce a satisfactory exposed film.

51
Q

what are the advantages of high kv?

A
  1. increased penetration leading to reduced patient dose
  2. increased exposure latitude of exposure resulting in a larger range of tissue displayed.
  3. reduction in mA, allowing shorter exposure time.
52
Q

what is the disadvantage of high kV?

A

reduced subject contrast

53
Q

what is the key parameter determining the image quality and patient dose?

A

choice of kv

54
Q

what does the mAs depend on?

A
  • quality of radiation beam, not only kv but filtration and waveform
  • focus film distance
  • attenuation in the tabletop, grid
  • the speed of film- screen combination
  • the region of patient being examined
  • patient thickness
55
Q

larger fatter patients may require the use f high mAs resulting in unacceptable tube loading and exposure time, what is the alternative?

A

increased kv might be used.

56
Q

define the reciprocity law;

A

in Rx it is assumed that the film density depends only on the total quantity of radiation- mAs- and not on the combination of mA and exposure time. this is known as the reciprocity law.
this is true for x rays ie without screens.

57
Q

name a situation where reciprocal law cannot be ignored?

A

mammography where the exposures are longer

58
Q

where is AEC positioned?

A

AEC is a thin radiation detector ( flat parallel plate ionisation chamber) positioned in front of the cassette and behind the grid

59
Q

The use of AEC is limited to those films taken in the …..

A

bucky

its use is also compromised because the detector have a fixed geometry that may not be ideal for all beam sizes, therefore may have limited use in paediatric.

60
Q

what is the atomic number of glandular and adipose tissue?

A

7.4 and 6.5 respectively

61
Q

to max subject contrast between glandular and adipose tissue, it is necessary to use a ………. tube potential

A

low

62
Q

what are the two most common target material in mammo/

A

molybdenum ( Z = 42) and Rhodium (Z = 45) , former being most common.

63
Q

what is the k edge for molybdenum?

A

20 kev, it produces characteristic radiation at 17.4 and 19.6 kev

64
Q

what is the purpose f beryllium window?

A

to minimise attenuation

in order to achieve a mono-energetic beam the emerging spectrum is filtered by the same material- mo 0.03mm thick

65
Q

what is the other name for magnification radiography?

A

macroradiography

66
Q

what is the mean glandular dose inmammo?

A

1.5-3 mGy per film.

at a dose of 2 mGy the fatal cancer risk is ~ 1 in 50,000 for women in the age range 50-65

67
Q

what sort of kv is used for mammo?

A

16-22 kev

68
Q

what type of screens are used in mammo?

A

rare earth screen

69
Q

what is the principle interaction in rare earth screen in mammo?

A

photoelectric absorption with the L shell electron

K edge for screens are 50 kev

70
Q

how would you eliminate cross over and parallax effect in mammo?

A

by using single screen with single sided emulsion.they are on the distal side of the film. ( this is because most interactions are on the side closest to the film)

71
Q

what is the limiting resolution of mammo?

A

15 lp/mm or better

72
Q

what is the arrangement of the anode cathode axis in mammo?

A

it is perpendicular to the chest wall with the cathode on the chest wall of the patient due to anode heel effect. ( it reduces the output on the anode side ie nipple side of the breast)

73
Q

in order to minimise the beam divergence into the patient where would the target be located?

A

the target is located directly above the chest wall edge of the platform.

74
Q

what are the two focal spot size used in mammo

what is the tube current? what is the operating potential?

A

0.1-0.4

tube current 100 mA

operating potential 24-35 kv

75
Q

what are the benefits of breast compression?

A

it reduces geometrical unsharpness
reduces breast thickness
dose is reduced
immobilises the patient.

76
Q

where is the AEC positioned in mammo?

A

behind the film cassette

77
Q

what is tomography? what happens in tomography?

A

only structures in a selected slice of the patient parallel to the film are imaged sharply, everything else will be blurred out.
the blurring is produced by: tube, film, patient

78
Q

the level of cut or cut height is adjusted by raising or lowering the…..

the thickness of cut is controlled by adjusting the …….

A

a) pivot

b) tomographic angle , angle ø

79
Q

what is the effect of changing the angle on image?

A

as the angle is reduced the degree of blurring is reduced therefore, increasing the thickness of the structures appearing to be in focus.

80
Q

what would happen when the angle ø is 0

A

conventional x ray - all structures would appear sharp

81
Q

what is the typical angle in tomography?

A

ø is 40, resulting in a cut thickness of 3mm

contrast is low and a reasonable low kv is used consistent with penetrating the patient

82
Q

what type of speed class film screen use?

A 
they are commonly 400 -speed class,
speed class being defined as 1000/dose (µGy) required to produce a net optical density of 1.
83
Q

what type of filter would you use for larger breast?

A

rhodium- Rh ( K edge- 23.3)

84
Q

increasing thickness of the ph in the screen causes what?

A

it increases sensitivity at the cost of increased screen unsharpness

FRCR Physics (9 decks)

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