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BDS3 Radiology > X-ray production > Flashcards

X-ray production Flashcards

1
Q

How are x-ray photons created by electrons?

A
  • Electrons accelerated towards atoms at very high speed
  • On collision the KE of these electrons is converted to heat and electromagnetic radiation (ideally X-ray photons)
  • X-ray photons aimed at subject
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2
Q

What are the main components of Dental X-ray unit?

A
  • Tubehead (with X-ray tube)
  • Collimator
  • Positioning arm
  • Control panel
  • Circuitry
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3
Q

What are the main components within the X-ray tube?

A
  • Glass envelope with a vacuum inside
  • Negative cathode side with a filament and Focusing cup
  • Positive Anode on other side with a Target and Heat-dissipating block
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4
Q

How does the cathode work?

A
  • Coiled metal wire
  • Sits in cathode facing Anode
  • Low voltage high current electricty passed through wire
  • Heats up until incandescent (approx 2200℃)
  • Electrons released from atom in wire by thermionic emission
  • Cloud of electrons forms around cathode
  • Increase current in filament = increase heat and increase electrons
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5
Q

What is Filament made of and why?

A
  • Made of Tungsten
  • Due to high melting point of 3422℃ so able to withstand high temps
  • Also due to high atomic number of 74 so has lots of electrons per atom
  • And its malleable
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6
Q

What is the Focusing cup of the Cathode?

A
  • Metal plate shaped around Filament
  • Negatively charged so repels electrons released at filament
  • Shaped to focus the electrons at a small point on the anode target
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7
Q

What is the Focusing cup of the Cathode made of?

A
  • Made of molybdenum
  • High melting point 2623℃
  • Relatively poor thermionic emitter
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8
Q

Why is the cathode-anode relationship important?

A
  • High volatge electricty passed through X-ray tube
  • High potential difference between -ve cathode and +ve anode
  • Electrons released at filament are repelled away from cathode and attracted to anode target
  • It accelerates at very high speed over very short distance
  • Increase potential difference = Increase acceleration = increase KE
  • Electrons have high KE upon colliding with anode target
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9
Q

How is the voltage obtained in a tubehead?

A
  • Transformers take mains electrical supply (220-240V) and convert it by changing voltage and current
  • 2 Transformer present in Tubehead
  • Step up transformer which increased potential difference across X-ray tube to 60,000-70,000V
  • Step down transformer which decreases potential difference across filament to approx 10V
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10
Q

What are Electron Volts (eV)?

A
  • Unit used to measure KE gained by electrons as they accelerate from cathode to anode
  • 1eV = KE gained by 1 electron moving across potential difference of 1 volt
  • E.g. If potential difference across X-ray tube is 70kV, each electron has 70keV of KE upon reaching anode
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11
Q

What is the Target within the anode?

A
  • Metal block bombarded by electrons
  • Produces photons and lots of heat
  • Off-angle in relation to filament
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12
Q

What is anode target made of?

A
  • Tungsten
  • High melting point 3422℃
  • Produces X-ray photons of useful energies
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13
Q

What is the Focal spot in relation to anode target?

A
  • Precise area on target where electrons collide and x-rays are produced
  • i.e the X-ray source
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14
Q

What is the Heat-dissipating block in anode?

A
  • Surrounds target as a larger block of metal
  • Heat produced in target by electron collisions dissipates into this block by thermal conduction
  • Reduces risk of overheating which may damage target
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15
Q

What is the Heat-dissipating block in anode made of?

A
  • Copper
  • High melting point 1085℃
  • High thermal conductivity
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16
Q

What is omnidirectional emission of divergent X-ray photons?

A
  • Photons are Transmitting in all different directions
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17
Q

What is the Penumbra effect?

A
  • Blurring of radiographic image due to focal spot not being a single point (small area instead)
  • Minimised by shrinking size of focal spot
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18
Q

How is the Penumbra effect reduced?

A
  • By using Focal spot angulation
  • Small focal spot is required
  • Problem is heat as approx 99% KE converted to heat
  • So if you decrease focal spot size = increase image quality BUT increase heat conc
  • Solution is using angles target
  • Increase ACTUAL SA where electrons impact
  • Increase heat tolerance
  • Reduce APPARENT SA from where X-ray beam emitted
  • Decrease penumbra effect
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19
Q

What is the Glass envelope?

A
  • Air tight enclosure
  • Supports cathode and anode
  • Maintains a vacuum so electrons able to travel from cathode to anode unhindered by gas molecules
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20
Q

What other material is the Glass envelope made of?

A
  • Leaded glass
  • Absorbs the x-ray photons due to omnidirectional emission
  • Not leaded at small window so only the x-ray photons travelling in desired direction can escape x-ray tube
21
Q

What are the main components of Tubehead?

A
  • X-ray tube
  • Metal shielding (outer of tubehead not inc spacer cone)
  • Aluminium filtration (where window is)
  • Oil (surrounding the x-ray tube)
  • Spacer cone (at front of tubehead, cylindrical part)
22
Q

What is metal shielding in tubehead made of?

A
  • Usually lead
  • Absorbs x-rays
  • Has window where x-ray beam exits
23
Q

What is the oil used for in tubehead?

A
  • Dissipates heat produces by x-ray tube by thermal convection
24
Q

Why is Filtration important in the tubehead?

A
  • Removes lower energy (non diagnostic) X-rays from beam
  • Low energy photons would be fully absorbed by patients tissues and increase patient dose but not contribute to image
25
Q

How does Aluminium help to remove lower energy x-rays during filtration?

A
  • Aluminium of adequate thickness able to absorb photons so X-ray beam contains mostly diagnostic x-ray photons
  • Min thickness of aluminium required for <70kV is 1.5mm
  • Min thickness required for >= 70kV is 2.5mm
26
Q

Why is spacer cone important?

A
  • Dictates distance between focal spot of target and patient
  • ‘Focus to skin distance’ fsd
  • Indicates direction of the beam (aka bean-indicating device)
  • May be detachable
27
Q

Why is focus to skin distance in a spacer cone important?

A
  • Altering fsd affect degree of divergence of x-ray photons in x-ray beam
  • Affects intensity of x-ray beam
  • Increasing fsd reduces divergence of x-ray beam
  • Therefore reduces mag of image and intensity of x-ray beam
  • A set distance helps ensure consistent radiographic technique
28
Q

What should the Focus to Skin distance be?

A
  • > = 60kV = 200mm (i.e. modern equipment)
  • Measurement taken from focal spot where x-ray photons originate
29
Q

What is the fate of the x-ray photons emitted from focal spot?

A
  • Some attenuated by lead shielding (not diagnostic)
  • Some attenuated by aluminium filtration (not diagnostic)
  • Some hit outer casings in all directions (not diagnostic)
  • Some exit tubehead and successfully pass to form x-ray beam (diagnostic)
30
Q

How does a Collimator work?

A
  • Crops X-ray beam to match size and shape of x-ray receptor
  • Tubeheads create a circular beam
  • Collimators change circular cross-section to rectangular cross section to match x-ray receptor
31
Q

Why is Rectangular collimation strongly recommended?

A
  • Reduce effective does to patient by approx 50%
  • Improves image contrast by reducing scatter
32
Q

What is a negative of rectangular collimation?

A
  • Increases risk of collimation errors
  • This can be minimised using good radiographic technique
33
Q

What is on the control panel?

A
  • On/off switch and light
  • Electronic timer
  • Exposure time selector and presets
  • Warning light and noise for when x-rays being generated
  • Kilovoltage selector
34
Q

What are the two consequences of electrons bombarding target?

A
  • Heat production (99% interactions)
  • X-ray production (<1% interactions)
35
Q

What are heat-producing interactions of x-ray production?

A
  • Bombarding electron reaches tungsten outer-shell electron and will do one of two things
    1. Comes into close proximity and decelerated and deflected as both -ve charge
    2. Collides and is deflected
  • Bombarding electron loses KE which is converted to heat
  • Heat energy dissipated
  • Goes from Tungsten target to Copper block to Oil in tubehead to Air
36
Q

What are the two X-ray producing interactions?

A
  • Continuous radiation interactions (majority)
  • Characteristic radiation interactions
37
Q

What is continuous radiation? (aka Bremsstrahlung)

A
  • Bombarded electron passes close to target nucleus which is +ve
  • Causes it to be rapidly decelerated and deflected
  • Lost KE released as x-ray photons
38
Q

What is an aspect of continuous radiation spectrum in regard to photons?

A
  • Photons produced over wide range of energies
  • Increase proximity of electron to nucleus
  • Leads to increase deceleration and deflection
  • Lead to increase energy released
  • There is a greater proportion of lower energy photons
39
Q

What is an aspect of continuous radiation spectrum in regard to photons?

A
  • Photons produced over wide range of energies
  • Increase proximity of electron to nucleus
  • Leads to increase deceleration and deflection
  • Lead to increase energy released
  • There is a greater proportion of lower energy photons
40
Q

When is maximum energy achieved on the continuous radiation spectrum?

A
  • Max energy when electron collides directly with nucleus and stops completely
  • Rare
  • Numerically identical to potential difference (voltage) across x-ray tube
  • e.g. 70kV = 70keV photon produced
41
Q

What is filtration of continuous spectrum?

A
  • Removal of lower energy as lower energy is non-diagnostic x-ray photons from beam
42
Q

What is characteristic radiation?

A
  • Bombarding electron collides with an inner shell electron
  • Displaces it into more peripheral shell (excitation)
  • or removes it completely (Ionisation)
  • Remaining orbiting electrons rearrange themselves to re-fill innermost shells (occurs until all inner shells filled)
  • When an electron drops to lower shell it loses energy
  • This energy emitted as photon of specific energy
  • Values depend on element involved
43
Q

What is the characteristic radiation spectrum?

A
  • Photon energy equals the difference in the binding energies of the 2 shells involved (specific to that element)

Tungsten example:
K shell binding energy = 69.5keV
L shell binding energy = 10.2keV
M shell binding energy = 2.5keV

Dental x-ray tubes operate at 70kV
- Bombarding electrons have sufficient energy to displace out inner most k shell electrons and cause characteristic radiation

44
Q

Continuous radiation vs Characteristic radiation point 1

A
  • Continuous produces continuous range of x-ray photon energies
  • Characteristic produces specific energies of x-ray photon, characteristic to element used for the target
45
Q

Continuous radiation vs Characteristic radiation point 2

A
  • Continuous has maximum photon energy which matches peak voltage
  • Characteristic has photon energies depending on the binding energies of electron shells
46
Q

Continuous radiation vs Characteristic radiation point 3

A
  • Continuous bombarding electron interacts with nucleus of target atom
  • Characteristic bombarding interacts with inner shell electrons of target atom
47
Q

What is the dental x-ray beam spectrum?

A
  • x-ray beam = continuous radiation + characteristic radiation - filtered photons
48
Q

What is the Collimator?

A
  • Lead diaphragm attached to end of spacer cone
  • Reduces patient dose

BDS3 Radiology (11 decks)

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