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Physics FRCR 1 > Xray production > Flashcards

Xray production Flashcards

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1
Q

What do Xray tubes contain

A

Heated filament

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2
Q

Briefly, how are xrays produced

A

Electrons are drawn off in a cathode and are accelerated with high velocity to the metal anode target, this collision produces xray photons which form the image

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3
Q

Why are electrons held in a vacuum

A

to avoid collision with air molecules

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4
Q

how is the image formed

A

interaction of photons with patient

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5
Q

Cathode is positive/negative?

A

negative

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6
Q

Anode is positive/negative?

A

Positive

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7
Q

Types of xray generator

A
  • single phase generator (2 pulse)
  • 3 phase generator (6 pulse)
  • 12 pulse generator
  • medium frequency generator
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8
Q

Where are xrays generated

A

xray tube

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9
Q

where are electrons produced

A

cathode

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10
Q

what does a generator do in an xray tube

A

applies high voltage across the tube, which accelerates the electrons across the tube through the vacuum

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11
Q

where do the accelerated electrons go in the xray tube

A

they hit the target anode and interact with the tungsten metal to produce xrays which are then emitted from the tube

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12
Q

Components of the xray tube

A
  • cathode = negative heated filament
  • anode = positive surface of metal with high atomic number (tungsten)
  • evacuated glass tube
  • tube voltage 30-150kV applied between cathode and anode to accelerate electrons
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13
Q

characteristics of xray filament

A

tungsten wire 0.2mm diameter coiled into a spiral
temperature above 2200 C
size of focal spot increased with tube current
2 sizes: broad and fine focus

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14
Q

what is ‘blooming’

A
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15
Q

small filament will take less/more current, produce more/fewer xrays and require longer/shorter exposure times

A

small filament will take less current, produce fewer xrays and require longer exposure times

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16
Q

what is the line focus principle

A

if electrons are made to strike a sloping target, the apparent length of the source is smaller –> effective focal spot

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17
Q

target angle of anode

A

12-14 ‘

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18
Q

Smaller angle gives higher/lower gain, smaller/larger focal spot, but angular width of useful cone of radiation is reduced/increased

A

Smaller angle gives higher gain, smaller focal spot, but angular width of useful cone of radiation is reduced

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19
Q

2 types of anode

A

stationary and rotating

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20
Q

Properties of a stationary anode

A

thick plate tungsten bonded to copper block

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21
Q

properties of tungsten

A

high atomic number, high melting point, acceptable thermal conductivity and thermal capacity

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22
Q

where are stationary anodes used

A

dental xray sets

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23
Q

describe a rotating anode

A

anode is at the edge of a rapidly rotating disc
made of tungsten

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24
Q

which element is bonded to tungsten in a rotating anode and why

A

molybdenum - increases thermal capacity

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25
Q

what is the purpose of additional metal bonded to tungsten in anodes?

A

to dissipate heat

26
Q

prep stage in a rotating anode

A

motor is energised before xray exposure is initiated to allow anode to reach operating speed

27
Q

2 types of rotor?

A

high speed - 9000 rpm
standard speed - 3000 rpm

28
Q

maximum angle yield to cathode?

A

5-10 ‘

29
Q

decreasing anode target angle allows for greater/lesser loading for focal spot size, but reduces/increased useful area of beam

A

decreasing anode target angle allows for greater loading for focal spot size, but reduces useful area of beam

30
Q

when can the Heel effect be used

A

where there are differences in body thickness within xray beam e.g. breast imaging

31
Q

What is the anode heel effect

A
32
Q

how can damage to the anode be reduced

A

stray non-focal radiation from bombarding stray electrons can be minimised by covering the parts of the anode not used to generate xrays with a layer of carbon

33
Q

what is the glass envelope

A

highly evacuated glass vessel

34
Q

glass has a low/high coefficient of expansion

A

low

35
Q

what is tube rating

A

defines maximum input of electrical energy (load) that may be applied to an xray tube without causing damage through overheating

36
Q

Energy (J) = ?

A

Energy (J) = kV x mAs

37
Q

why does heat need to be dissipated at the anode

A

energy arrives within a fraction of a second at the focal spot, if not dissipated/removed, the temp rise would melt the focal spot

38
Q

Factors affecting tube loading

A
  • type of anode
  • composition and construction of anode
  • focal spot size
  • anode target angle
  • diameter of anode disc
  • rotation speed of anode
  • electrical power
  • rate of heat dissipation
39
Q

how to prolong xray tube life

A
  • short exposure with high tube current deposits heat more quickly and produces a greater temp raise than lower current for a longer time
  • heat radiated away more quickly
  • if temp rise not great enough to cause damage, better to use short exposures despite temp rise
  • gradually warm up xray tube
40
Q

differences in mammography xray tube

A

filtration makes beam more penetrating

41
Q

in a mammography tube, the cathode-anode distance is…

A

<1cm to ensure close control

42
Q

in a mammography tube, the focal spot is…

A

smaller to improve resolution

43
Q

in a mammography tube, the window is made of …

A

beryllium instead of glass

44
Q

Ceramic xray tube properties

A
  • better insulation
  • more compact design
  • larger anode, greater heat capacity and more efficient heat transfer
  • liquid metal lubricant to with low vapour pressure to aid cooling
  • better tube lifetime (less arcing)
  • earthed metal enclosure
  • off focus electrons attracted to grounded metal enclosure
  • rapid series of exposure
45
Q

what is arcing

A

the sign of the end of an xray tube’s life

46
Q

Detector dose (+ units)

A

certain level of dose required at image receptor to form an image (uGy)

47
Q

Entrance surface dose (ESD) (+units)

A

used to describe dose to the patient
absorbed dose in air back scatter
mGy

48
Q

Effective dose (+units)

A

used to estimate the risk associated with an exposure to radiation
- summation of absorbed tissue doses x weighting factor
- estimated using ESD and other exposure factors
uSv / mSv

49
Q

what can be changed about xray equipment

A
  • distance from xray focus to patient / detector
  • applied potential i.e. potential difference between cathode + anode kV
  • electron beam current - controlled by filament current
  • exposure time - controls total energy fluence
  • focal spot size
  • filtration
50
Q

assuming a constant dose and patient/detector distance, increasing the distance from the xray tube to the detector will result in…

A
  • increased mAs due to inverse square law
  • decreased ESD
51
Q

increased kVp (voltage) across the cathode and anode, the maximum energy…

A

increases

52
Q

increasing kVp does what to the beam

A

harder, more penetrating beam

53
Q

for a constant detector dose, increasing kV reduces/increases the ESD

A

reduces

54
Q

mA = ?

A

current through cathode filament

55
Q

increasing mA, exposure time or current-time product (mAs) will decrease/increase quantity of xrays

A

increase
(quality not affected)

56
Q

what can reduce patient dose and how

A

filtration - removing lower energy xrays (soft radiation)

57
Q

inherent filtration

A
  • target
  • exit window
  • collimator assembly
58
Q

added filtration

A
  • aluminium
59
Q

what is collimation

A

method to limit irradiated area to anatomy of interest

60
Q

light beam diaphragm (LBD)

A

see diagram

Physics FRCR 1 (5 decks)

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