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1015 Medical Radiation Sciences > x-ray tube parts 1 & 2 > Flashcards

x-ray tube parts 1 & 2 Flashcards

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USMLE® Step 1 flashcards
1
Q

what are the 3 major support types

A

ceiling
floor
C-arm

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2
Q

what is a ceiling support system

A

2 sets of rails and a telescoping column

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3
Q

what is the most frequently used support system

A

ceiling

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4
Q

what is a floor mounted support system

A

single column with rollers, limited movement

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5
Q

what two things can you use a floor mounted support system with

A

table or wall buvky

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6
Q

what is a C-arm support system

A

where the IR and tube are connected
flexible movement, limited SID

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7
Q

what is a variant of the C-arm

A

the U-arm

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8
Q

where is C-arm support system used

A

in interventional radiography suites

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9
Q

what 4 types of protection does the tube housing provide

A

radiation, electrical, thermal, physical

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10
Q

how doe the housing provide radiation protection

A

lead lining with a window for the useful radiaion

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11
Q

how does the tube provide electrical protection

A

uses high voltage receptacles (glass and oil)

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12
Q

what are the high voltage receptacles

A

glass and oil

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13
Q

how does the tube provide thermal protection

A

oil and a cooling fan

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14
Q

how does the tube provide physical protection

A

housing protects from bumps

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15
Q

what is contained by the glass tube

A

the cathode and anode

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16
Q

what type of glass is the tube and why is it useful

A

pyrex
high heat capacity and a good insulator

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17
Q

what is the purpose of the vacuum in the tube

A

prevents collisions
prevents the oxidation of electrodes

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18
Q

why don’t you want electrodes to oxidize?

A

because it means losing electrons, aka what we really don’t want

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19
Q

why are newer tubes being made with metal

A

less tube wear and therefore longer lifespan

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20
Q

what is the filament made of

A

a tungsten coil, 1-2 % thorium

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21
Q

how big is the filament

A

2mm in diameter
1-2cm in length

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22
Q

what is the purpose of the filament

A

it is the source of electrons

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23
Q

why is tungsten so popularly used?

A

high melting point (prevents wire from melting)
low vapour pressure (does not evaporate easily)

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24
Q

what is thermionic emission

A

where current creates heat
heat gives more energy to e-
e- can escape their binding forces

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25
Q

what is the rough threshold of thermionic emission

A

2200 degrees C

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26
Q

what occurs after the threshold of thermionic emission

A

a small filament increase creates a large tube increase

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27
Q

what is space charge

A

electrons forming a cloud around the filament

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28
Q

what are compensation circuits

A

compensates for space charge, ensures that the mA we select is actually correct

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29
Q

what does a high kVp do in the context of compensation circuits

A

makes it easy to “pull” mA across the tube, lowers the filament heating surrent and reduces space charge size

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30
Q

what does a low kVp do in the context of compensation circuits

A

makes it harder to “pull” mA across the tube, raises the filament heating current and increases space charge size

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31
Q

what is the difference between the filament current and the tube current

A

the filament current creates a space charge, and occurs while pressing the button to prep
tube current allows mA to flow across the tube, and happens only when the exposure button is pressed

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32
Q

what is the focusing cup?

A

it supports the filaments

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33
Q

what is the focusing cup made of and why

A

nickel
its a good conductor, but a bad thermioninc emitter

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34
Q

how does the focusing cup work

A

it is negatively charge, so “pushes” the electrons to the target focal spot

35
Q

why does the focusing cup hold 2 filaments

A

one for each focal spot size

36
Q

how big is the small fs filament

A

0.1-1mm

37
Q

how big is the large fs filament

A

0.3-2mm

38
Q

which focal spot has higher mA and lower resolution

A

large

39
Q

which focal spot has lower mA and higher resolution

A

small

40
Q

what is the relation type between mA and the filament current

A

inverse relation

41
Q

what mA limits are there for each focal spot size

A

<300mA = small
400mA< = large

42
Q

what is kVp

A

the potential difference

43
Q

describe the energy of electrons as determined by the kVp

A

1eV per V is the energy AVAILABLE to become x-rays

44
Q

what does increasing kVp do and not do

A

increases energy of photons
increases the # of photons
does not change the # of ELECTRONS

45
Q

what does the anode do?

A

produces the x-rays (through 2 processes)

46
Q

what is stationary about a stationary anode

A

the target does not move

47
Q

what are the parameters allowed for a stationary anode

A

10-15mA
55-90kVp

48
Q

what is the stationary anode made of

A

the stem is copper
the target is tungsten

49
Q

why is the stationary anode stem made of copper

A

high thermal capacity
high conductivity
low melting point

50
Q

what is the atomic number of tungsten

A

74

51
Q

unequal thermal expansion and contraction causes what

A

pitting

52
Q

how does the process of x-ray production work

A

electrons bump out valence electrons, or are drawn to the nucleus and slowed down
both of these would lose energy, but that isn’t possible. So, the excess becomes photons

53
Q

what does a rotating anode do better

A

more effective heat dissipation

54
Q

what are the components of a rotating anode (5)

A

disc
focal track
stem
rotor
stator

55
Q

what makes a focal track

A

focal spot + rotation

56
Q

how much of the focal track is made of rhenium

A

10%

57
Q

what is the anode disc backing made up of

A

molybdenum and carbon (ie. graphite)

58
Q

why are different material used for anode target backings

A

holds heat more as you get deeper, which is a good thing because you don’t want heat traveling all the way back to the anode. Could wreck the bearing and stop the disc from spinning as well

59
Q

what is the stem of a rotating anode do and what is it made of

A

thin piece that connects the disc to the rotor
molybdenum (poor conductor)

60
Q

what does the rotor do, and what is is made of

A

rotates on the bearings
copper (high conductivity)

61
Q

what does the stator do

A

causes the rotor to spin

62
Q

how fast does the spinning of a rotating anode go

A

3000-12000 rpm

63
Q

how long is the rotating anode coast time

A

roughly 60 seconds

64
Q

what 3 parts of the tube are at high risk of wear

A

filament
anode
bearings

65
Q

what causes wear to happen faster

A

higher exposure ratings and longer prep times

66
Q

what causes more filament wear

A

high mA and longer prep times

67
Q

what are the consequences of filament wear

A

tungsten plating evaporates and settles on tube, causing arcing
the filament can break

68
Q

what causes anode wear

A

uneven thermal expansion (pitting) and high exposure ratings

69
Q

what are the consequences of anode wear

A

pitting and unequal x-ray intensity

70
Q

what are the mm^2 for stationary vs rotating targets, respectively

A

4mm^2 vs 1800mm^2

71
Q

what causes bearing wear

A

heavy use and long prep times

72
Q

what are the consequences of bearing wear

A

slower rotation and more pitting
usually emits a grinding noise

73
Q

what causes housing damage

A

physical damage and overheating

74
Q

what are the consequences of housing damage

A

oil leakage (allowing it to evaporate and stop absorbing heat, further damaging the housing)

75
Q

how to prevent tube wear

A

lower technique
short prep
tube warm up

76
Q

what are tube rating charts for

A

showing safe and unsafe combinations of kVp and mA

77
Q

what is the heat storage capacity

A

how many exposures you can do without cooling
(most important for the bearings)

78
Q

what are HU/S

A

the amount of electrons emitted, and the energy of those hitting the target per second
(more important for filament and target)

79
Q

what are anode cooling charts

A

show the heat capacity of the anode and the length of time it takes to cool

80
Q

what is the formula for heat units

A

HU = kVp x mAs

81
Q

what is the formula for heat units per second

A

HU / s = kVp x mA

82
Q

what is the biggest way that cooling in the tube happens

A

the tube is in contact with oil, which quickly dissipates heat

83
Q

what is a rotating envelope tube

A

where the whole tube spins, not just the anode
better at cooling

1015 Medical Radiation Sciences (13 decks)

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