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BDS3 Radiology > X-ray Production > Flashcards

X-ray Production Flashcards

1
Q

What are the components of an X-ray tube?

A
  • glass envelope
    • vacuum inside
  • cathode (-ve)
    • filament
    • focussing cup
  • anode (+ve)
    • target
    • heat dissipating block
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2
Q

What is a filament?

A
  • coiled metal wire
  • low voltage, high current electricity passed through wire
    • heats up until incandescent
      - ~2200 degrees
    • electrons released from atoms in wire
      - thermionic emission
    • cloud of electrons forms around cathode
  • increasing current increases heat and number of electrons
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3
Q

What are filaments made of?

A
  • tungsten
    • high melting point
      - 3422 degrees
      - able to withstand high temperatures
    • high atomic number
      - 74
      - lots of electrons per atom
    • malleable
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4
Q

What is a focussing cup?

A
  • metal plate shaped around filament
    • negatively charged
      - repels electrons released at filament
    • shaped to focus the electrons at small point on anode target
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5
Q

What are focussing cups made of?

A
  • molybdenum
    • high melting point
      - 2623 degrees
    • relatively poor thermionic emitter
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6
Q

What is the cathode-anode relationship?

A
  • high voltage electricity passed through X-ray tube
  • electrons released at filament are repelled away from cathode
    • attracted to anode (target)
    • accelerate at high speeds over short distance
      - up to half the speed of light
    • increased voltage increases acceleration and kinetic energy
  • electrons have high kinetic energy upon collision with anode target
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7
Q

What is the target in the anode?

A
  • metal block bombarded by electrons
    • produces photons (+ heat)
  • off-angle in relation to filament
  • focal spot
    • precise area on target where electrons collide
    • x-rays are produced
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8
Q

What are targets made of?

A
  • tungsten
    • high melting point
      - 3422 degrees
      - produces x-ray photons of useful energies
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9
Q

What is a heat-dissipating block?

A
  • block into which target is embedded
    • heat produced dissipates into block
      - thermal conduction
      - reduces risk of overheating which may damage target
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10
Q

What are heat-dissipating blocks made of?

A
  • copper
    • high melting point
      - 1085 degrees
    • high thermal conductivity
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11
Q

What is the penumbra effect?

A
  • blurring of radiographic image due to focal spot not being a single point
    • minimised by shrinking size of the focal spot
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12
Q

What is focal spot angulation?

A
  • used to achieve a small focal spot
    • decreasing focal spot side increases image quality
      - also increases heat concentration
  • angled target used to increase surface area where electrons impact
    • better heat tolerance
    • reduces apparent surface area from where x-ray beam is emitted
    • reduced penumbra effect
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13
Q

What is a glass envelope?

A
  • air tight enclosure
    • supports cathode and anode
    • maintains a vacuum
      - electrons able to travel from cathode to anode
      - unhindered by gas molecules
  • leaded glass to absorb x-ray photons
    • except for an un-leaded window
    • only x-ray photons travelled in desired direction can escape
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14
Q

What are the main points of an X-ray tube head?

A
  • X-ray tube
  • metal shielding
    • usually lead
    • absorbs x-rays
    • window through which x-rays escape
  • aluminium filtration
  • oil
    • dissipates heat produced by X-ray tube by thermal convection
  • spacer cone
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15
Q

What is aluminium filtration?

A
  • removes lower energy (non-diagnostic) x-rays from beam
    • low energy photons fully absorbed by tissue
    • would increase dose but not contribute to image
  • aluminium able to absorb low energy photons
    • minimum thickness
      - <70kV = 1.5mm (modern operates at 60-70kV)
      - >70kV - 2.5mm
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16
Q

What is a spacer cone?

A
  • dictated the distance between the focal spot and patient
    • focus to skin distance (fsd)
    • set distance helps ensure a consistent radiographic technique
  • indicates the direction of the beam
  • may be detachable
17
Q

What is focus to skin difference?

A
  • distance between focal spot of target and patient
    • altering affects degree of divergence of beam
  • increasing fsd reduces divergence, magnification and intensity
  • 200mm
    • > 60kV
  • measurement taken from focal spot, marked on tube head
18
Q

What happens to x-ray photons emitted from the focal spot?

A
  • attenuated by lead shielding
  • attenuated by aluminium filtration
  • exit tubehead to form x-ray beam
19
Q

What is a collimator?

A
  • lead diaphragm attached to end of spacer cone
    • reduces patient dose
  • crops x-ray beam to match size and shape of x-ray receptor
    • tube heads create a circular beam
    • collimators chance cross section to rectangular
  • should come standard on new equipment
    • can be retrofitted to older equipment
20
Q

what is rectangular collimation used for?

A
  • size 2 receptors
    • 50-40mm collimation
    • 45-35mm collimation preferable
  • reduces surface area irradiated by almost 50%
  • improves image contract by reducing scatter
21
Q

What are the disadvantages of collimation?

A
  • increased risk of collimation errors
    • minimised using good radiographic technique
22
Q

What does the control panel of an x-ray unit contain?

A
  • on/off switch and light
    • light indicates when operational
  • electronic timer
    • duration of exposure
  • exposure time selector and presents
    • manual change also possible
    • select for teeth being irradiated
    • some teeth require longer exposure
      - bone teeth are contained in
      - width of teeth
  • warning light and noise
    • for when x-rays are being generated
    • alerted to accidental exposure
    • indicates when patient can move again
  • kilovoltage selector
    • usually at set number
    • 70kV in dental hospital
23
Q

What are the consequences of electrons bombarding the target?

A
  • heat production
    • involves outer-shell electrons of tungsten atoms at target
    • 99% of interactions
      - more common due to plentiful outer shell electrons
  • x-ray production
    • involves inner shell electrons and nuclei of tungsten atoms
    • 1% of interactions
24
Q

What is involved in heat-producing interactions?

A
  • bombarding electrons reach tungsten outer shell electrons
    • comes into close proximity and is decelerated and deflected
      OR
    • collides and is deflected
  • bombarding electron loses kinetic energy which is converted to heat
  • heat energy dissipated
    • tungsten target -> copper block -> oil in tube head -> air
25
Q

What is involved in x-ray producing reactions?

A
  • continuous radiation interactions
    • majority
    • bombarding electron passes close to target nucleus
    • electron is rapidly decelerated and deflected
    • kinetic energy lost released as x-ray photons
  • characteristic radiation interactions
    • bombarding electron collides with an inner shell electron
    • displaces to a more peripheral shell (excitation)
      OR
    • removes it completely (ionisation)
    • remaining orbiting electrons rearrange themselves
    • refill innermost shells
      - energy released when drops to lower shell
      - emitted as photon

BDS3 Radiology (11 decks)

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