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Huda > Chapter 1 - Xrays > Flashcards

Chapter 1 - Xrays Flashcards

1
Q

define force

A

F= ma
causes bodies to deviate (push, pull)

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2
Q

what forces causes protons and electrons to attract each other?

A

electrostatic

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3
Q

what forces hold the nucleus together

A

strong force

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4
Q

what force is involved in beta decay

A

weak force

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5
Q

what is energy

A

ability to do work, Joule

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6
Q

define electron volt

A

kinetic energy gained by electron as it is accelerated across electric potential of 1 V

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7
Q

How much energy is required to eject outer shell electron vs inner shell electron

A

outer- several eV
inner- several keV

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8
Q

How much energy is reqauired to eject nuclear particles like alpha particles?

A

several MeV

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9
Q

Name of negative and positive regions of electrical circuit

A

cathode = negative
anode = positive

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10
Q

how much power do x-ray generators use compared to average household?

A

household = few kW
generator = 100 kW, about 30 households

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11
Q

how is energy utilization in making x-rays kept low despite a higher power use?

A

-exposure time is short (abdominal x-ray exposure time is about 100 ms)

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12
Q

define matter

A

made up of atoms that contain protons, neutrons, electrons

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13
Q

atomic number of
hydrogen
carbon
nitrogen
oxygen
calcium

A

hydrogen = 1
carbon = 6
nitrogen =7
oxygen = 8
calcium= 20

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14
Q

define atomic number vs mass number

A

atomic number = Z = number of protons in nucleus
mass number = A = total number of protons and neutrons in nucleus

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15
Q

name of innermost electron sjhell

A

K shell then L shell etc.

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16
Q

what is electron binding energy

A

energy required to completely remove any electron from an atom, increases with Z

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17
Q

K shell binding energy of oxygen

A

0.5 keV, Z = 8

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18
Q

k shell binding energy of calcium

A

4 keV, Z = 20

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19
Q

k shell binding energy of iodine

A

33 keV, Z = 53

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20
Q

k shell binding energy of barium

A

36 keV, Z = 56

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21
Q

k shell binding energy of tungsten

A

70 keV, Z = 74

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22
Q

k shell binding energy of lead

A

88 keV, Z = 82

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23
Q

what is electron density

A

number of electrons per volume

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24
Q

Einstein mass energy equation

A

E = mc^2

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25
Q

Define electromagnetic radiation

A

wave associated with oscillating electric and magnetic fields
-travels at speed of light
-characterized by wavelength, frequency, and velocity

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26
Q

Z and nominal density of lung

A

Z = 7.5, 0.3 g/cm3

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27
Q

Z and nominal density of fat

A

Z = 6.5, 0.9 g/cm3

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28
Q

Z and density of water

A

Z = 7.5, 1 g/cm3

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29
Q

Z and density of soft tissue

A

Z = 7.5, 1.04 g/cm3

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30
Q

Z and density of bone

A

Z = 12, 1.7 g/cm3

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31
Q

define wave velocity

A

product of wavelength and frequency

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32
Q

what is meant by electromagnetic radiation is quantized?

A

-exists in discrete quantities called photons

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33
Q

energy of EM wave

A

E = hf, E = hc/lambda

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34
Q

low energy and high energy EM waves- give examples

A

low E = radio waves
high E = gamma waves

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35
Q

difference between gamma rays and x-rays

A

x-rays- photons produced from electrons
gamma rays- photons produced from a nuclear process

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36
Q

define excitation

A

-atomic electron is raised to higher energy shell

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37
Q

define ionization

A

electron is ejected from neutral atom, leaving behind a position ion

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38
Q

what is ionizing radiation

A

EM radiation with sufficient energy to eject atomic electrons

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39
Q

examples of non-ionizing vs ionizing radiation

A

non-ionizing- radio, microwaves, visible light
ionizing- UV, x-ray, gamma ray

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40
Q

what happens when x-rays interact with tissue?

A

energy is transferred to electrons and then deposited locally

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41
Q

what is characteristic x-ray

A

-vacancy in k-shell is filled by electron from higher shell, emitting chracterisit x-ray
-occurs only for discrete energy levels

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42
Q

what is energy of characteristic x-ray

A

slightly less than k shell binding energy

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43
Q

what type of power supply is used for x-ray generators?

A

3-phase (ex 0, 120, and 240 degress)- together yield a “flatter composite)- less ripple than single phase

44
Q

what does the transformer do?

A

increase or decrease voltage

45
Q

what does rectifier do?

A

convert AC to DC

46
Q

what is the max voltage in the ripple?

A

peak voltage

47
Q

equation for power dissipated in x-ray tube

A

P = IV

48
Q

typical x-ray transformer rating

A

100 kV
1000 mA
100 kW

49
Q

components of x-ray tube

A

-evacuated glass tube
-negative filament (cathode)- typically heated
positive target (anode)- usually tungsten
-voltage is applied across the tube to accelerate electrons from cathode to anode. Electrons strike the anode and yield Bremstrahlung photons

50
Q

define tube current

A

flow of electrons from cathode to anode, mA

51
Q

how much energy is converted to x-rays in a x-ray tube?

A

1%, rest is heat. Anode stores the heat

52
Q

define x-ray focal spot

A

target region producing the x-rays

53
Q

advantage of large vs small focal spot

A

small - reduce blurring
large - tolerate high power loadings, reduce exposure times

54
Q

what is bremstrahlung photons

A

x-rays produced when electrongs interact with nuclear electric fields. Electrons are decelerated by electric field and change their direction of travel. Energy lost appears as a photon.

55
Q

what is max bremstrahlung energy?

A

-x-ray tube voltage
-brems. photons are range of energies

56
Q

brems. production is proportional to what?

A

-Z and kV

57
Q

what happens to L-shell characteristic xrays?

A

-low energy and are absorbed by x-ray glass tube

58
Q

define x-ray spectruum

A

graph that shows number of x-ray photons at each energy

59
Q

average photon energy of a spectrum

A

1/3 to 1/2 the peak kV

60
Q

how much do characteristic x-rays contribute to the x-ray spectrum for radiography, fluorscopy, and CT?

A

~ 10 %

61
Q

for what imaging modality are characterisitc x-rays a major contributor to the x-ray spectrum?

A

mammo

62
Q

what are the 3 possible fates of x-rays incident on matter?

A

-scattering
-absorption
-penetration

63
Q

define coherent scatter

A

-x-ray photons is scattered without energy loss
-< 5 % of photon interactions

64
Q

In radiography, what fraction of incident photons are absorbed, scattered, and transmitted to form the image?

A

-2/3 absorbed
-1/3 scattered
< 1 % transmitted

65
Q

How many ion pairs does each Compton or photoelectron produce?

A

-hundreds or thousands

66
Q

describe photoelectric effect

A

-x-ray photons is absorbed by inner shell electron. That electron is ejected from atom (photoelectron). Outer shell electron fills the inner shell vacancy, with excess energy emitted as a characteristic x-ray.

67
Q

What does PE effect depend on?

A

Z^3/E^3
the more tighly bound the electron is, the greater the chance of PE effect if energy of electron is above k edge

68
Q

Where are PE interactions important?

A

mammo- spectra is mostly low energy photons and PE effect ~ Z^3/E^3

69
Q

define compton scatter

A

-incident photons interact with outer shell, loosely bound electrons
-results in scattered photon (less energy than incident) and scattered electron

70
Q

what does compton depend on?

A

-electron density/ energy

71
Q

where are PE and compton effects equal for soft tissue and for bone?

A

soft tissue- 25 kV
bone - 40 kV

72
Q

where are compton interactions most important?

A

chest/body radiogaphy
CT imaging

73
Q

what do scattered photons that reach the detector do to the image?

A

degrade image quality
reduce contrast resolution

74
Q

does scatter affect spatial resolution or image mottle?

A

No

75
Q

what does scatter radiation from the patient do in fluoro procedures?

A

exposes the operator to radiation

76
Q

definie linear attenuation coefficient

A

fraction of incident photons removed from a beam in traversing a unit distance (cm^-1)
-accounts for al interactions, including coherent

77
Q

what does linear attenuation coefficient depend on?

A

-physical density, Z
-decreases with energy (exception is K-edge)

78
Q

what is transmission of primary beam through average patient for skull rdiography?

A

1 %

79
Q

what kV would you use with iodine contrast?

A

70 kV to get 35 keV photons and make use of 33 keV k-edge of iodine

80
Q

what filters are typically used in mammo to transmit low energy photons and absorb higher energy photons with PE efect?

A

Mo- k edge 20 keV
rhodium - k edge 23 keV
silver- k edge 25 keV

81
Q

what is kerma

A

kinetic energy released per unit mass
-J/kg (or Gy)
-obeys IS law

82
Q

what is entrance Kair

A

measure of x-ray intensity incident on the patient undergoing an x-ray exam
-a few mGy

83
Q

what is Kair at image receptor?

A

-measure of x-ray intensity that is used to generate an image
-a few micro Gy

84
Q

what reduces Kair at image receptor?

A

patient attenuation
grid losses
distance from x-ray tubes

85
Q

what is HVL

A

thickness of material that attenuates beam by 50 %

86
Q

what is HVL of soft issue in mammo (30 kV) vs abdo radiography (80 kV) vs CT (120 kV)

A

1 cm vs 3 cm vs 4 cm

87
Q

define beam quality

A

average x-ray beam energy
defined by HVL (thickness of aluminum)

88
Q

beam quality in Al of mammo (30 kV) vs abdo radiography (80 kV) vs CT (120 kV)

A

0.5 vs 3 vs 6-10 mm Al

89
Q

beam intensity vs tube current, exposure time

A

-directly proportional to exposure time and mA

90
Q

what does changing mAs affect in the beam?

A

beam intensity (quantity) but not quality

91
Q

if you need more photons what would you do?

A

-increase mA
-increase exposure time would increase photons but also motion blur

92
Q

what does changing tube voltage impact?

A

-beam quality and quantity
-beam intensity is proportional to square of voltage for radiography, kV^2.6 for CT (not as simple for mammo)
-increasing kV increases average photon energy

93
Q

rule of thumb for mAs to maintain Kair at image receptor when changing kV

A

-in radiography, increasing kV by 15% requires halving of the mAs to maintain Kair at receptor

94
Q

what does increase in distance do to beam quality and quantity?

A

-decreases quantity, no change to quality

95
Q

what does filtration do to quantity and quality?

A

decrease quantity, increase quality

96
Q

what do low energy photons do to patient and image?

A

irradiate patient and add nothing to image

97
Q

what are filters for?

A

absorb low energy photons which add nothing to image and add patient dose

98
Q

what are most filters made from

A

3 mm Al
-chest radiography is performed at higher k V and may use Cu filtering

99
Q

why does CT use very high filtration

A

reduce beam hardening artefacts

100
Q

define beam hardening

A

preferential loss of low energy photons by an absorber

101
Q

where does beam hardening not occur?

A

monochromatic x-ray beams

102
Q

2nd HVL is always what than first HVL?

A

larger

103
Q

when the linear attenuation coefficient is 0.1 cm^-1, the fraction of photons transmitted through 2 cm is most likely?

A

0.8

104
Q

% of x-ray photon energy incident on a patient that is most likely absorbed within the patient

A

65%

105
Q

tissue HVL for typical xray beam

A

3 cm

106
Q

when the tissue linear attenuation coefficient is 0.1 cm^-1, the fraction of photons transmitted through 2 cm is?

A

2 cm absorbs about 0.2 so 0.8 is transmitted

107
Q

radiation intensity Kair at image receptor for typical radiograph

A

3 uGy

exposure index = 300

Huda (20 decks)

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