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diagnostic radiology (ionising) > x ray production > Flashcards

x ray production Flashcards

1
Q

two methods of x ray production

A

bremsstrahlung (breaking radiation) or characteristic (fluorescence)

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2
Q

features of the general x ray energy spectrum (3)

A

low energy part (limited by self absorption), characteristic lines, intensity proportional on KV^2

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3
Q

high energy electron interaction with bound electron

A

characteristic x-rays

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4
Q

high energy electron interaction with the nucleus

A

highest energy bremsstrahlung

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5
Q

high energy electron interaction near the nucleus

A

high energy bremsstrahlung

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6
Q

high energy electron interaction further away from the nucleus

A

low energy bremsstrahlung

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7
Q

what do clinical x-ray beams (excl. mammo) mostly consist of and what percentage

A

~ bremsstrahlung (90%), characteristic (10%)

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8
Q

x ray generation: components needed (3)

A

cathode: electron source

applied kV: to accelerate electrons

anode: made of a material where electrons interact and their energy converted into x rays which rotates to cool it down (99% of electron energy is converted into heat)

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9
Q

what encloses the vacuum of the xray tube

A

glass (pyrex) or metal/ceramic envelope

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10
Q

what is contained in the cathode (2)

A

multiple tungsten filaments

focusing cups

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11
Q

how does rotating anode cool it down

A

spreads heat over large area by rotating up to 3500 rpm

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12
Q

purpose of tungsten filaments in the cathode

A

source of electrons

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13
Q

what kind of tungsten are the filaments

A

thoriated (alloyed with thorium - greater thermionic emission)

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14
Q

thermionic emission process to produce electrons

heated ___
___ escape
forming ___ ___

A

Electron emission from heated surface.
cathode
Electrons
space charge

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15
Q

purpose of focusing cups

A

Focus Electron Beam which strikes the focal spot

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16
Q

Diagnostic kV (accelerating potential) Range

A

Typically spans 40 kV to 150 kV.

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17
Q

purpose of stator windings

coils of wire around the ___
produce ___ ___ which ___ the anode rapidly by ___ ___

A

tube
magnetic field
spins
em induction

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18
Q

rotating anode:
made of ___ because of it’s high __ and __ ___.

___ disk to help ____ ___
___ mount for the ___ and ___

A

tungsten
Z - 74
melting point = 3400 deg C
pmolybdenum
dissipate heat
graphite
mo disk and W target, helps dissipate heat)

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19
Q

x ray tube housing made of ___ with ___ lining and containing ___ ___

A

steel
lead
insulating oil

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20
Q

what is blurring at the penumbra

A

Blurred or indistinct edge in a radiographic image, reducing image sharpness and detail/clarity, specifically at the edges of the object

. This is most easily seen at the penumbra which is the blurring specifically at the edges of the object image. When the spot is small, the radiation effectively originates from a point source, thus the boundary of the image is very sharp, and hence the penumbra does not appear. In contrast, when the spot is large, the radiation path depends on the location in the spot from which it originates, causing the edges to be less well-defined, creating a large penumbra as seen in Figure 1. Typically the size of the spot, and hence of the penumbra, increases with the power of the electron beam.
Causes: e.g. X-ray beam divergence, and scatter.
Mitigation: proper positioning, collimation, and exposure techniques.
Significance: Clear visualization of anatomical structures is essential for accurate diagnosis in medical imaging.

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21
Q

what causes penumbra blurring

A

x-ray beam divergence: Focusing the electron beam results in an X-ray spot instead of a point source so when the X-rays interact with the object, and are then recorded by the detector, blurring in the image can be observed

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22
Q

equation that links effective focal spot size to penumbra size, how to minimise penumbra

A

X1/X2=Y1/Y2, where X1 is the effective focal spot, X2 is the penumbra at the image plane, Y1 is the distance between the target and object, Y2 is the distance between the object and image plane.

penumbra minimised for smaller X1 (focal spot) and Y2 (distance to image plane)

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23
Q

a cathode is _______ charged and the anode is _________ charged.

A

negatively, positively

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24
Q

the source of electrons for x-ray production comes from the ______ ________

A

cathode filament

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25
Q

What purpose does the focusing cup serve being negatively charged?

A

it helps to “herd” negatively charged electrons into a space cloud

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26
Q

what is the boiling off of electrons from the filament called?

A

thermionic emission

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27
Q

the cathode focusing cup is made of?

A

molybdenum (or nickel)

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28
Q

What part of the cathode holds the filament?

A

focusing cup

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29
Q

What is the target material made of and why is it used?

A

tungsten, it has a high melting point

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30
Q

the induction motor is made up of what 2 parts?

A

the rotor and stator

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31
Q

Why must the target be angled?

A

in order for the x-ray photons to exit the window

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32
Q

what is the negative aspect of angling the target called?

A

Heel effect

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33
Q

what causes Heel effect

A

self absorption within the heel of the angled target depending on the angle of the target

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34
Q

consequences of Heel effect

A

% of central x ray energy varies with the target angle, may limit minimum target angle in certain circumstances e.g. radiotherapy

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35
Q

is the beam intensity higher on the cathode or anode side

A

cathode side - greater self absorption at lower angles on anode side

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36
Q

Greater than ____% of heat is produced in x-ray production and less than ____% of x-rays are produced.

A

99, 1

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37
Q

When using techniques of under _____ kvp you will have 100% ______ production.

A

70, brems

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38
Q

Characteristic radiation only occurs when using techniques over _kVp.

A

70

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39
Q

What is the binding energy of a tungsten atom?

A

69.5 (which is why we need at least 70 kVp to knock an inner shell electron from orbit

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40
Q

purpose of light beam diaphragm

A

enables the radiographer to visualise and control the field size and pathway of the x ray beam. the “gatekeeper” for the X-ray beam, ensures that only the necessary portion of the beam reaches the patient, optimizing safety and diagnostic accuracy

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41
Q

what determines the max spectral energy

A

the generating voltage (keV=kV)

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42
Q

bigger patients need more/less kV

A

more (penetration)

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43
Q

_____ _____ _____ and _____ _____ go up with kV

A

maximum spectral energy, output intensity

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44
Q

why do lower energies need to be filtered

A

they increase skin dose

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45
Q

what material is used for filtration

A

aluminium

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46
Q

why is aluminium used for filtration

A

low atomic number (atomic number 13).
absorbs most low-energy photons.
allows transmission of most high-energy photons

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47
Q

What is the equation for total filtration in an X-ray machine

A

Total filtration = Inherent Filtration + Added Filtration

48
Q

what are the three key radiographic settings
what do they determine

A

kV (determines penetration) beam current mAs (beam current and exposure time ->no, of xrays produced/dose)

49
Q

relationship between kV and mA

A

inverse

50
Q

Give the properties of the glass envelope

what is it made of and why

what is the window

contributes to what

A

made of pyrex = withstands a lot of heat

maintains a vacuum

window:
- segment of glass thinner than rest of glass envelope

  • contributes to inherent filtration (.5mm Al equivalent)
51
Q

what does the rotating anode contain
what is it made of

A
  • contains rotating disks of 5-13cm in diameter
  • made of Molybdenum
52
Q

what are two advantages of using a rotating anode ?

A

greater target area

greater heat dissipation

53
Q

Describe the focal spot
portion of the – where – – – – will impact

size compromise
smaller =?
larger =?

A

the portion of the anode where the high voltage electron stream will impact

must be small enough to produce sharp images but large enough to absorb heat without melting

54
Q

What is the effective focal spot?

A

the projection perpendicular to the central x-ray which is its apparent area

the focal spot produced as a result of angulating to target material

55
Q

what is the target rating chart used for

A

by manufacturers to programme x ray sets

56
Q

what information does the target rating chart give

A

max mA for any kV and exposure time

57
Q

what could go wrong (5)

A
  1. anode damage from heating
  2. failure of filaments, high voltage wiring etc
  3. failure of heat sensors, oil pumps
  4. oil leaks
  5. loss of insulation on x ray housing
58
Q

developments (2)

A
  1. larger/faster anode discs -> greater thermal capacity
  2. metal-ceramic x-ray tubes -> greater resilience
59
Q

Define the use of the collimator

A

used to restrict the size and shape of the x-ray beam to reduce patient exposure (scatter radiation)

60
Q

why is tungsten thoriated

A

increase thermionic emission

61
Q

KV selection based on desired __ and __ ___.

A

contrast, tissue penetration

62
Q

diagnostic KV for soft tissue imaging, like chest X-rays.

A

Lower kV (40-70 kV)

63
Q

diagnostic KV for denser structures, such as bones.

A

Higher kV (80-150 kV)

64
Q

what does rotating anode allow

A

continuous X ray generation by spreading heat

65
Q

What does the x-ray tube insert provide

A

the electron source, vacuum environment, target electrode

66
Q

what does the external power source provide

A

high voltage potential difference to accelerate the electrons

67
Q

what does the x-ray tube housing that the insert is mounted within include

A

metal enclosure
protective radiation shielding
X-ray beam filters
collimators

68
Q

what is the purpose of x-ray beam filters

A

to shape the x-ray spectrum

69
Q

what is the purpose of collimators

A

to define the size and shape of the x-ray field

70
Q

what does the x-ray generator supply

A

the cheaper to ensure to accelerate the electrons
a filament circuit to control tube current
and and exposure time

71
Q

definition of exposure

A

dosimetry quantity describing the amount of charging coulombs release in a known mass of air in coulombs per kilogram

72
Q

definition of exposure rate

A

the increment of exposure in a time interval in the units of coulombs per kilogram per second

73
Q

definition of electron volt

A

the energy obtained by an electron after it is accelerated across the potential difference of 1 V. One electron volt is equal to 1.6 X 10 ^ -19 Jules

74
Q

and applied x-ray tube potential of 50 kV accelerates the electrons to a kinetic energy of __

A

50 K EV

75
Q

what happens during most interactions

A

99% of interactions are collision or with other electrons produce nothing but heat

76
Q

what is bremsstrahlung radiation

A

radiative emission of an x-ray photon Caused by
electrical forces attracting and decelerating an electron changing its direction and velocity

77
Q

What factor determines the X-ray energies of the resulting bremsstrahlung radiation

A

The Coulombic attraction
Which is proportional to the inverse square of the distance

78
Q

What happens at closer interaction distance

A

increase coulombic force causes greater electron deceleration and conversion to hire x-ray energies

79
Q

what happens when there is a direct impact of an electron of the target nucleus

A

The target nucleus stops the electron and convert all its kinetic energy into an equivalent x-ray photon
the highest bremsstrahlung x-ray energy

80
Q

what is a bremsstrahlung spectrum

A

the probability distribution of x-ray photon fluence produced as a function of energy expressed in K EV

81
Q

What is filtration

A

the removal of x-rays by attenuation

82
Q

what Material causes inherent filtration

A

materials that are inherent in the x-ray tube such as the glass or metal window of the cheap insert

83
Q

What Material is used for added filtration

A

Materials intentionally placed in the beam
thin aluminium and copper sheets which preferentially attenuate lower energy x-rays

84
Q

what are the two main factors that affect x-ray production efficiency

A

the kinetic energy of the incident electrons
The atomic number of the target material

85
Q

The blank is directly related to the tube potential

A

the kinetic energy of the incident electrons

86
Q

what is characteristic radiation

A

discrete x-ray energy peaks

87
Q

what do continuous x-ray energies depend on (2)

A

the elemental composition of the anode
the applied x-ray tube voltage

88
Q

What is the order of shells from inner to outer

A

KLMN

89
Q

which Shell has the high select from binding energy

A

the innermost shell K

90
Q

how is a vacancy caused in a target atom

A

the kinetic energy of an incident electron exceeds the binding energy of the electron shell so the interaction can inject an electron

91
Q

why can an outer shell electron feel the vacancy by immediately transitioning

A

because it has less binding energy

92
Q

what is the energy of the characteristic x-ray equal to

A

the difference in the electron binding energies of the two shells

93
Q

when are characteristic K x-rays produced

A

only when the impinging electrons on the target exceed the binding energy of a K shell electron

94
Q

Major components of the x-ray tube

A

cathode
anode
Rotor/stator
glass or metal envelope
Tube port
cable sockets
cheap housing

95
Q

what is the x-ray tube voltage for mammography

A

25 to 49 kV

96
Q

what is the x-ray tube current

A

the number of electrons per second flowing from the cathode to the anode

97
Q

What does the cathode comprise of

A

An electron emitter and focusing cup

98
Q

how are filaments positioned in most x-ray tubes for diagnostic imaging

A

Two filaments of different lengths positioned in a slot Which is machined into the focusing cup

99
Q

What is Thermionic emission

A

electrical resistance heating the filament to a temperature determined by the amplitude of the current resulting in a release of electrons from the filament surface

100
Q

when is a space charge( static electron cloud) formed around the filament

A

when the repulsive force of the negative charge of emitted electrons equalling the thermionic emission force

101
Q

What is the function of the focusing cup

A

to surround the filament emitter with a slotted half cylindrical structure to reduce the spread of electrons during exposure

102
Q

what determines the focal spot dimensions (2)

A

the length of the filament in one direction
the width of the focusing cup in the perpendicular direction

103
Q

What anode materials are used for mammography x-ray tubes And why

A

molybdenum and radium because they provide useful characteristic x-rays for breast imaging

104
Q

what are the two anode configurations

A

Stationary and rotating

105
Q

what are the advantages of rotating anodes

A

high x-ray output

106
Q

what spins the anode in a rotating anode

A

the stator/rotor induction motor which is energised

107
Q

Why is there a short delay before x-ray exposure when the button is pushed for General radiography x-ray systems

A

the x-ray tube generator is designed so the x-ray tube voltage is applied only when the anode is at full speed

108
Q

Why are Rotor bearings often the cause of x-ray tube failure

A

they are heat sensitive

109
Q

how to achieve thermal insulation from the hot anode

A

Using a connector stem to the rotor made of molybdenum which is a poor heat conductor

110
Q

what is the heel effect

A

reduction in the x-ray beam fluence on the anode side of the x-ray field due to the steep anode angle causing x-rays directed towards the aThisnode side of the beam attenuated by greater path lengths through the anode material

111
Q

purpose of x-ray tube housing

A

mechanically support ,
Electrically and thermally insulate ,
and protect the x-ray tube insert

112
Q

Purpose of the lead shielding inside the x-ray tube housing

A

attenuate nearly all x-rays not directed to the tube port

113
Q

The purpose of automatic exposure control

A

to measure the actual amount of radiation incident on the image receptor during the acquisition and terminate x-ray production when the optimal radiation levels are obtained

114
Q

What does automatic exposure control compensate for

A

patient thickness and other variations in attenuation

115
Q

what is the advantage of automatic exposure control over manual exposure time settings

A

more consistent exposures

116
Q

how does the anti-scatter good work

A

place between the detector and patient
allows all primary radiation incident upon it to pass while absorbing all of the scattered radiation
Obliquely angled photons from scattered x-rays emanating from the interior of the patient have a higher probability of striking the highly absorbing grid septa

117
Q

what is the design of the scatter grid

A

open into space regions and alternating x-ray absorbing septa are aligned with the x-ray tube focal spot

diagnostic radiology (ionising) (7 decks)

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