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Huda > Chapter 4 Patient Dosimetry > Flashcards

Chapter 4 Patient Dosimetry Flashcards

1
Q

is kV and mAs enough to characterize radiation on the patient?

A

No, also need tube design and filtration

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2
Q

what is Kair

A

intensity of x-ray beam

number of x-ray photons per square millimeter

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3
Q

how does Kair fall off with increasing distance

A

IS law

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4
Q

how does Kair changed with mAs and tube voltage?

A

proportional to mAs

supralinearly with voltage

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5
Q

how does Kair change with filtration

A

more filtration decreases Kair

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6
Q

how does adding 3 mm of Al to a 80 kV beam reduce Kair?

A

by 50%

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7
Q

2 things needed to characterize radiation incident on patients

A

quality and quantity of x-ray beam

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8
Q

what is radiation intensity incident on a patient requird to generate a good image?

A

entrance Kair

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9
Q

what does entrance Kair depend on

A

patient thickness, composition
patient size
beam quality

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10
Q

entrance Kair for skull radiograph, PA chest x-ray, and lateral lumbar spine

A

skull: 1 mGy
chest: 0.1 mGy
spine: 10 mGy

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11
Q

what is entrance Kair rate for for fluoro on 23 cm wide patient?

A

10 mGy/min

depends on FOV, frame rate, selected dose level

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12
Q

kerma area product

A

aka dose area product

  • total amount of radiation incident on patient
  • product of entrance Kair and area of cross-sectional beam
  • independent of measurement location
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13
Q

median KAP in radiographic imaging

A

1 Gy cm^2

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14
Q

KAP for fluoro guided GI studies and urologic procedures

A

20 Gy cm^2

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15
Q

KAP in interventional radiology

A

200 Gy cm^2

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16
Q

what is meant by superficial doses

A

doses absorbed by skin, scalp, and eye lens

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17
Q

why does an entrance Kair of 1 mGy result in superficial skin dose of up to 1.5 mGy?

A
  • tissues are higher Z than air (10% higher)

- backscatter can increase superficial tissue doses by up to 40 %

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18
Q

usual radiography skin dose

A

< 10 mGy

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19
Q

usual fluoro skin dose

A

< 500 mGy

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20
Q

usual interventional radiology skin dose

A

> 500 mGy

-radiation burns, epilation, cataracts are possible

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21
Q

how do organ doses change with Kair?

A

increase in proportion to Kair

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22
Q

how are organ doses affected by beam quality for a given Kair?

A

-increase with beam quality due to more penetration

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23
Q

what is embryo dose for abdominal radiograph

A

1/3 Kair for AP projection
about 1 mGy

for PA and lateral projections, 1/6 and 1/20 Kair

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24
Q

are embryo doses cumulative

A

yes

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25
Q

dose rate at embryo for fluoro

A

1.5 mGy/min

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26
Q

embryo doses in abdominal/pelvic CT

A

25 mGy

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27
Q

embryo doses in chest CT

A
  1. 1 mGy
    - mostly from internal scatter
    - lead aprons don’t provide benefit
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28
Q

what are genetically significant doses?

A

dose metrics that quantify potential genetic damage

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29
Q

GSD in US

A
  1. 3 mGy

- takes into account dose received by gonads and how many offspring an individual is likely to produce

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30
Q

downside of use of gonad shields

A

have to repeat exam if poorly placed

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31
Q

what is integral dose

A

total energy imparted to a patient

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32
Q

what is integral dose for 1 Gy imparted to 70 kg patient?

A

70 J

i.e. 1 J/kg * 70 kg

33
Q

chest x-ray integral dose

A

0.002 J

34
Q

abdominal radiograph integral dose

A

0.02 J

35
Q

head CT integral dose

A

0.15 J

36
Q

body CT integral dose

A

0.5 J

37
Q

what is plotted on cell survival curve

A

plots surviving fraction as function of radiation dose

38
Q

is energy absorbed by cells sufficient to predict biologic damage?

A

No

for example, alpha particles cause more biologic damage than x-rays

39
Q

why does some radiation cause more biologic damaga than others?

A

for example, alpha particles result in a more concentrated pattern of energy deposition than x-rays which produce a more diffuse pattern
i.e. alpha particles have higher linear energy transfer (denser pattern of energy deposition)

40
Q

unit of LET

A

keV/um

41
Q

LET of alpha particles

A

100 keV/um

42
Q

LET of x-rays, gamma rays, beta particles

A

1 keV/um

43
Q

radiation weighting factor

A

higher LET radiation have higher weighing factors

higher Wr= more biologic damage at the same dose

44
Q

Wr for x-rays, gamma rays, beta particles

A

1

45
Q

Wr of protons

A

2

46
Q

Wr of alpha particles

A

20

47
Q

Wr of neutrons

A

1-20 depending on energy

48
Q

equivalent dose

A

absorved dose X radiation weighting factor
lets you compare different types of radiation
-expressed in Sv

49
Q

radiation detriment

A

defined by International Commission on Radiologic Protection (ICRP)
-judges relative importance of fatal cancers, non fatal cancers, and genetic effects in future generations

50
Q

tissue weighting factor Wt

A

fractional contribution of each organ to the total detriment (uniform whole-body radiation)
Wt values are age and sex averages
indicators of radiosensitivity of an organ

51
Q

what are remainder organs

A 
adrenals
gall bladder
heart
kidneys
pancreas
prostate
small intestine
thymus
uterus/cervix
52
Q

Wt for red bone marrow, lung, stomach, breast, remainder organs

A

0.12

detriment is cancer

53
Q

Wt for gonads

A

0.08

detriment is hereditary

54
Q

Wt for bladder, liver, esophagus, thyroid

A

0.04

detriment is cancer

55
Q

Wt for skin, bone surfaces, salivary glands, brian

A

Wt is 0.01

detriment is cancer

56
Q

what is effective dose

A

accounts for equivalent dose to every organ as well as each organ’s relative radiosensitivity

  • presented in mSv
  • multiply equivalent dose (H) to an organ by the organ weighting factor (Wt) summed for all irradiated organs
  • effective dose is uniform whole body dose that results in same patient detriment
57
Q

effective dose for lateral skull radiograph

A

0.03 mSv

58
Q

effective dose for PA chest radiograph

A

0.015 mSv

59
Q

effective dose for AP Abdomen radiograph

A

0.5 mSv

60
Q

effective dose for head CT

A

1.5 mSv

61
Q

effective dose for chest CT

A

5 mSv

62
Q

effective dose for abdomen/pelvic CT

A

6 mSv

63
Q

very low vs low vs moderate vs high effective dose ranges

A

very low: < 0.1 mSv
low: 0.1 - 1 mSv (most radiographic exams)
moderate: 1 -10 mSv (GI studies, some CT exams)
highL > 10 mSv : interventional studies, some CT exams

64
Q

uniquitous natural background radiation

A

includes cosmic, internal, and terrestrial activity

-~ 1 mSv/yr in US

65
Q

effective dose from cosmic radiation

A

0.4 mSv/y

66
Q

effective dose from internal primordial radionuclides (40K and 14C)

A

0.4 mSv/y

67
Q

effective dose from terrestrial activity

A

0.3 mSv/yr

68
Q

dose from transcontinental US flight

A

0.03 mSv

69
Q

additional dose to air crews

A

5 mSv/yr

70
Q

space travel dose

A

0.01 mSv/yr

71
Q

what does radon emit

A

alpha particles
progeny of radon are radioactive and attach to aerosols that are inhaled and deposited in the lungs
-15% of lung cancers due to exposure to radon

72
Q

average annual effective dose from radon in US

A

2 mSv/yr

depends on location

73
Q

total yearly natural background effective dose US

A

3 mSv

74
Q

population-averaged incidence of fatal cancer from radiation

A 
4 %/Sv
cancer incidence (fatal + non fatal) is 10%/Sv
75
Q

nominal cancer detriment

A

5.5 %/Sv

76
Q

total radiation detriment (cancer + hereditary effects)

A

6 %/Sv

77
Q

what does effective dose not account for?

A

age sex

risk is higher for newborn than in retiree

78
Q

mid organ radiation vs exit for abdomen

A

middle radiation about 10% of entrance Kair, , exit is 1 % of entrance Kair

79
Q

are absorbed organ doses (mSv) always numerically equal to organ equivalent dose (mGy)?

A

yes

Huda (20 decks)

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