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CSA SR > optimisation in radiography e lec > Flashcards

optimisation in radiography e lec Flashcards

to look at methods of dose reduction in radiography

1
Q

what are the three principles of radiation protection

A

justification
optimisation
limitation

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2
Q

how to reduce dose

A 
is called optimisation- this happens by
selection criteria 
avoiding lapses in quality assurance 
unnecessary duplication of x rays  
equipment and technique factors
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3
Q

what is the x ray tube made up from

A

sealed glass envelope
vacuum
anode(copper) with tungsten inset
cathode filament of tungsten wire

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4
Q

where do electrons move

A

from negative to positive

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5
Q

what is the mains supply to the x ray tube

A

240 volts

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6
Q

how do we generate an x ray

A

we can generate a high potential difference kV to accelerate electrons across the x ray tube using a step up transformer
we can also provide low current to heat up the tube filament using a step down transformer

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7
Q

why is a low current needed

A

to heat up the tube filament

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8
Q

what is the low current caused by

A

by a step down transformer

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9
Q

how can we make this more efficient

A

constant potential or DC unit

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10
Q

what is the advantages of using a constant potential

A 
keeps the KV at its peak throughout exposure
more high energy useful x rays produced 
fewer low energy useless x rays produced
shorter exposure time
eliminates the filament warm up time
reduces dose by 0.8
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11
Q

what is the disadvantage of a constant potential

A

decreased contrast

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12
Q

what is the effect on dose altering kvp

A

at a lower dose the photoelectric effect predominates which is pure absorption
lower energy electrons produced which may reach the patient
by increasing the KVP from 50 to 65, it can reduce thr effective dose by half

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13
Q

what happens at a lower dose

A

the photoelectric effect predominates which is pure absorption

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14
Q

what tube potential do we use for intraoral radiography tube

A

no lower than 50 kV

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15
Q

what do we do if the tube potential is under 45 kV

A

withdrawn immediately

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16
Q

what should the potential of new equipment be

A

60-70 kV

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17
Q

what haooens when the kV is increased

A

it reduces the dose to the patient

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18
Q

what happens to the film in low kV

A

a very contrasty film

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19
Q

what type of filtration do we have

A

inherent and added

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20
Q

how much aluminium do we need for 70 kv

A

1.55mm

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21
Q

what should be the length of aluminium above 70kV

A

2.5mm of which 1.5mm should be permanent

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22
Q

what happens if you add too much filtration

A

then the exposure times get too long

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23
Q

what does collimation do

A

shapes and limits the size of the beam

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24
Q

what can we use as a collimater

A

metal disc or cylinder

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25
Q

what was previously recommended as a collimater

A

circular and as 6cm

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26
Q

what do we now use as a collimatar

A

size 2 film rectangular collimation

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27
Q

what happens to the dose when you use a rectangular collimater instead of a circular

A

you can half the dose

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28
Q

what is the spacer cone length

A

dose follows the inverse square law

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29
Q

what does the intensity equal

A

1/d2 where d is the distance

30
Q

how do we reduce magnification

A

we can reduce magnification by the focal spot to object distance to be as long as possible- but the x ray beam intensity reduces

31
Q

what is the long cone paralleling technique

A

produces images with minimal distortion

32
Q

what is the minimum focus to skin distance if greater than 60kV

A

20cm

33
Q

what is the focus to skin distance if it is less than 60kV

A

10cm minimum

34
Q

what happens to the dose when you have a short cone

A

if you are greater than 60kV and a short cone it increases the dose by 1.5

35
Q

which spacer cone shape is better

A

the open ended cone is better than the pointer cone

36
Q

which is the fastest conventional film for intraoral

A

F speed

37
Q

how much faster is E speed from D speed

A

twice as fast

38
Q

how much % decrease is digital image receptors

A

dose reductions of upto 90% but usually in comparison of D speed

39
Q

how much is rare earth intensifying screens reduce dosage in extraoral image receptors

A

REDUCED BY 50% compared with calcium tungstate

40
Q

what do we use for intra oral x rays

A

film holders

41
Q

what is the adv of film holders

A

reduces retakes
more reproducible
the projection geometry was optimal

42
Q

what is recommended with the panoramic technique

A

field trimming and sectional option

43
Q

what are diagnostic reference levels

A

national levels which you can compare equipment on

may have ti set your own standards

44
Q

what is the achievable dose

A

intraoral radiography
1.8mG
on a 70kV with E speed film

45
Q

what is the diagnostic reference level of panoramic x ray

A

66.7mGy/mm

46
Q

what do we do if we operating at or above the diagnostic reference level

A

withdraw ASAP doesn’t matter about operating kV

47
Q

what do we do if we operating 2x above diagnostic reference level

A

withdraw immediately doesn’t matter about operating kV

48
Q

what is the equivalent background radiation of 2 bitewings

A

8-16 hrs

49
Q

what is the equivalent background radiation of panoramic

A

2-5 days

50
Q

what is the equivalent background radiation of the skull

A

40 hours

51
Q

what is the equivalent background radiation of barium meal

A

15 months

52
Q

what is the effective dose in microSv for intraoral radiograph

A

1-8.3 microSv

53
Q

what is the effective dose in microSv for anterior maxillary occlusal

A

8 microSv

54
Q

what is the effective dose in microSv for panoramic

A

3.85-30 microSv

55
Q

what is the effective dose in microSv for lateral cephalometric radiograph

A

2-3 microSv

56
Q

what is the effective dose in microSv for cross sectional tomography

A

1-189 microSv

57
Q

what is the risk of fatal cancer from a intraoral radiograph

A

0.02-0.6 per million

58
Q

what is the risk of fatal cancer from a panoramic

A

0.21-1.9 per million

59
Q

what is the risk of fatal cancer from a anterior maxillary occlusal

A

0.4 per million

60
Q

what is the risk of fatal cancer from a lateral cephalometric radiograph

A

0.34 per million

61
Q

what is the risk of fatal cancer from a cross sectional tomography

A

1-14 per million

62
Q

what is the risk in relation to age under 10

A

multiplication factor for risk is x3

63
Q

what is the risk in relation to age 10-20

A

multiplication factor for risk is x2

64
Q

what is the risk in relation to age 20-30

A

multiplication factor for risk is x1.5

65
Q

what is the risk in relation to age 30-50

A

multiplication factor for risk is x0.5

66
Q

what is the risk in relation to age 50-80

A

multiplication factor for risk is x0.3

67
Q

what is the risk in relation to age 80+

A

negligible risk

68
Q

what are other forms of lead protection

A

thyroid collar

69
Q

what do we use a thyroid collar for

A

our lateral cephalometric radiograph

70
Q

why do we use a thyroid collar

A

the thyroid is in the field of view and the younger patients have multiplicand associated risk

71
Q

what is CBCT

A

CONE BEAM computed tomography

CSA SR (52 decks)

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