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