X-ray Production Flashcards
What are the basics of X-ray production?
Electrons accelerate towards atoms at high speed
On collision, the kinetic energy of the electrons is converted to heat and EM radiation (ideally X-ray photons)
The X-ray photons are aimed at a subject
What are the main components of the dental X-ray unit?
Tubehead (with X-ray tube)
Collimator
Positioning arm
Control panel
Circuitry
What is found inside the X-ray tube?
Glass envelope with vacuum inside
Cathode (-ve)
Anode (+ve)
What does the cathode filament do?
Low voltage, high current electricity is passed through the wire heating it up
Heat and electrons are released
What does the cathode focusing cup do?
Repels electrons released at the filament
Focuses the electrons at a small point on the anode target
What is the cathode made of?
Filament - tungsten
Focusing cup - molybdenum
Describe the relationship between the cathode and anode
High-voltage electricity passes through the X-ray tube
Electrons released from the filament are repelled from the cathode and attracted to the anode
Electrons have high kinetic energy upon colliding with the anode target
What do the transformers do?
Take mains electricity supply and convert it by changing the voltage and current
What are electron volts (eV)?
Unit used to measure the kinetic energy gained by electrons as they accelerate from cathode to anode
1eV = kinetic energy gained by 1 electron moving across a potential difference of 1 volt
What is the anode made from?
Target - tungsten
Heat-dissipating block - copper
What is the anode target?
Metal block bombarded by electrons to produce photons and heat
What does the anode heat-dissipating block do?
Surrounds the target
Heat produced in target from electron collisions dissipate into the block by thermal conduction to reduce risk of overheating
What is the penumbra effect?
Blurring of a radiographic image due to focal spot not being a single point but rather a small area
Why is focal spot angulation used?
We need a small focal spot but this means less kinetic energy is converted to photons and more to heat. A larger focal spot solves this but decreases image quality
Angling the target increases the actual surface area where electrons impact (increasing heat tolerance) while reducing the apparent surface area from the where the X-ray beam is emitted (reducing penumbra effect)
Describe the glass envelope
Air-tight enclosure to support the cathode and anode while maintaining a vacuum
Made of leaded glass to absorb X-ray photons so only the photons travelling in the desired direction can escape the X-ray tube
What are the main components of the tubehead?
X-ray tube
Metal shielding - usually lead
Aluminium filtration
Oil
Spacer cone
What does filtration do?
Removes lower energy X-rays from the beam - these would increase dose but not contribute to image
What does the spacer cone do?
Dictates the distance between the focal spot of target and the patient
What sizes of spacer cones are needed?
If <60 kV then 100mm
If >60 kV then 200mm
What thickness aluminium is needed for filtration?
If <70 kV then 1.5mm
If >70 kV then 2.5mm
What is the collimator?
Lead diaphragm attached to the end of the spacer cone
Crops the X-ray beam to match the size and shape of the X-ray receptor
Why is rectangular collimation used?
Reduces effective dose to patient by about 50%
Describe continuous radiation
Produces a continuous range of X-ray photon energies
Maximum photon energy matches the peak voltage
Bombarding electron interacts with nucleus of target atom
Describe characteristic radiation
Produces specific energies of X-ray photon, characteristic to the element used for the target
Photon energies depend on the binding energies of electron shells
Bombarding electron interacts with inner-shell electrons of target atom
Describe the dental X-ray beam spectrum
X-ray beam = continuous radiation + characteristic radiation - filtered photons
