X-ray production Flashcards
How are x-ray photons created by electrons?
- 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
What are the main components of Dental X-ray unit?
- Tubehead (with X-ray tube)
- Collimator
- Positioning arm
- Control panel
- Circuitry
What are the main components within the X-ray tube?
- 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
How does the cathode work?
- 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
What is Filament made of and why?
- 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
What is the Focusing cup of the Cathode?
- 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
What is the Focusing cup of the Cathode made of?
- Made of molybdenum
- High melting point 2623℃
- Relatively poor thermionic emitter
Why is the cathode-anode relationship important?
- 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
How is the voltage obtained in a tubehead?
- 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
What are Electron Volts (eV)?
- 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
What is the Target within the anode?
- Metal block bombarded by electrons
- Produces photons and lots of heat
- Off-angle in relation to filament
What is anode target made of?
- Tungsten
- High melting point 3422℃
- Produces X-ray photons of useful energies
What is the Focal spot in relation to anode target?
- Precise area on target where electrons collide and x-rays are produced
- i.e the X-ray source
What is the Heat-dissipating block in anode?
- 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
What is the Heat-dissipating block in anode made of?
- Copper
- High melting point 1085℃
- High thermal conductivity
What is omnidirectional emission of divergent X-ray photons?
- Photons are Transmitting in all different directions
What is the Penumbra effect?
- Blurring of radiographic image due to focal spot not being a single point (small area instead)
- Minimised by shrinking size of focal spot
How is the Penumbra effect reduced?
- 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
What is the Glass envelope?
- Air tight enclosure
- Supports cathode and anode
- Maintains a vacuum so electrons able to travel from cathode to anode unhindered by gas molecules
What other material is the Glass envelope made of?
- 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
What are the main components of Tubehead?
- 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)
What is metal shielding in tubehead made of?
- Usually lead
- Absorbs x-rays
- Has window where x-ray beam exits
What is the oil used for in tubehead?
- Dissipates heat produces by x-ray tube by thermal convection
Why is Filtration important in the tubehead?
- 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
How does Aluminium help to remove lower energy x-rays during filtration?
- 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
Why is spacer cone important?
- 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
Why is focus to skin distance in a spacer cone important?
- 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
What should the Focus to Skin distance be?
- > = 60kV = 200mm (i.e. modern equipment)
- Measurement taken from focal spot where x-ray photons originate
What is the fate of the x-ray photons emitted from focal spot?
- 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)
How does a Collimator work?
- 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
Why is Rectangular collimation strongly recommended?
- Reduce effective does to patient by approx 50%
- Improves image contrast by reducing scatter
What is a negative of rectangular collimation?
- Increases risk of collimation errors
- This can be minimised using good radiographic technique
What is on the control panel?
- On/off switch and light
- Electronic timer
- Exposure time selector and presets
- Warning light and noise for when x-rays being generated
- Kilovoltage selector
What are the two consequences of electrons bombarding target?
- Heat production (99% interactions)
- X-ray production (<1% interactions)
What are heat-producing interactions of x-ray production?
- 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
What are the two X-ray producing interactions?
- Continuous radiation interactions (majority)
- Characteristic radiation interactions
What is continuous radiation? (aka Bremsstrahlung)
- 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
What is an aspect of continuous radiation spectrum in regard to photons?
- 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
What is an aspect of continuous radiation spectrum in regard to photons?
- 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
When is maximum energy achieved on the continuous radiation spectrum?
- 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
What is filtration of continuous spectrum?
- Removal of lower energy as lower energy is non-diagnostic x-ray photons from beam
What is characteristic radiation?
- 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
What is the characteristic radiation spectrum?
- 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
Continuous radiation vs Characteristic radiation point 1
- Continuous produces continuous range of x-ray photon energies
- Characteristic produces specific energies of x-ray photon, characteristic to element used for the target
Continuous radiation vs Characteristic radiation point 2
- Continuous has maximum photon energy which matches peak voltage
- Characteristic has photon energies depending on the binding energies of electron shells
Continuous radiation vs Characteristic radiation point 3
- Continuous bombarding electron interacts with nucleus of target atom
- Characteristic bombarding interacts with inner shell electrons of target atom
What is the dental x-ray beam spectrum?
- x-ray beam = continuous radiation + characteristic radiation - filtered photons
What is the Collimator?
- Lead diaphragm attached to end of spacer cone
- Reduces patient dose
