2 - X-ray production Flashcards
What are the components of a dental x-ray unit?
- tube head with X-ray tube
- collimator
- positioning arm
- control panel
- circuitry
What are the components of the X-ray tube?
- glass envelope (vacuum inside)
- cathode
- anode
What are the components of the cathode?
- negative end
- filament
- focussing cup
What are the components of anode?
- positive end
- target
- heat dissipating block
Describe how the filament works.
- coiled metal wire
- low voltage, high current electricity passed through wire
- heats up to 2200c
- electrons released from wire by thermionic emission
- cloud of electrons form around cathode
What is the filament made of?
Tungsten
What are the properties of tungsten?
- high melting point (3422c)
- high atomic number (74)
- malleable and ductile
Describe the focussing cup.
- curved metal plate around filament
- negatively charged so directs electrons at target
- made of molybdenum
Describe the relationship between the cathode and anode.
- electrons formed at the cathode are directed at the anode by the focussing cup
- electrons with high kinetic energy collide the anode target
Describe the target.
- located in the anode
- metal block made of tungsten
- bombarded with electrons to produce photons
- angled increase SA but direct photons in correct direction
Describe the heat dissipating block.
- made of copper
- lost of excess heat produced by electrons hitting target
- reduces risk damage to target
What are the properties of copper?
- high melting point (1085c)
- high thermal conductivity (removes heat away from target)
Describe the emission of x-ray photons.
- omnidirectional
- divergent
What is the penumbra effect?
- blurring of a radiographic image due to focal spot not being a single point
- x-rays diverge from several points
- reduced by reducing size of focal spot
Describe the glass envelope.
- air tight vacuum enclosure
- supports cathode and anode
- made of leaded glass to absorb x-ray photons, unleaded window in desired direction of photons
- vacuum prevents air molecules interfering with movement of electrons
What are the components of the tubehead?
- X-ray tube
- metal sheilding
- aluminium filtration
- oil
- spacer cone
Describe the aluminium filtration.
- removes the lower energy x-rays
- aluminium is able to absorb photons (attenuate)
- aluminium must be at least 1.5mm when x-ray machine operates at < 70 kV
Why are lower energy x-rays problematic?
- non diagnostic
- cannot travel through desired tissues to receptor
- photons will still be absorbed by tissues and therefore increased dose
Describe the spacer cone.
- dictates distance between focal spot and patient
- focus to skin distance (fsd)
What is the required fsd?
> 60 kV = 200mm (modern technology)
Describe the collimator.
- lead diaphragm attached to the end of the spacer cone
- reduces patent dose
- crops x-ray beams to size and shape of receptor
How does rectangular collimation reduce x-ray dose?
- reduces dose by nearly 50%
- can lead to collimation errors although these are avoided by good technique
What are the components of the control panel?
- on/off switch
- electronic timer
- exposure time selector and presets
- warning light and noise
- kV selector
What are the outcomes of electrons bombarding the target?
- heat production (99% of interaction)
- x-ray production (1%)
How is the heat energy dissipated in the X-ray tube?
- tungsten target
- copper block
- oil in tubehead
- air
What are the x-ray producing interactions?
- continuous radiation interaction (majority)
- characteristic radiation interaction
Describe continuos radiation interactions.
- bombarding electrons pass close to nucleus of target atom
- electron rapidly decelerates and is deflected
- releases lost kinetic energy as x-ray photons
Describe the continuous radiation spectrum.
- the closer the electron passes to the nucleus, the greater the energy released
- greater proportion of lower energy electrons
- greatest energy achieved when electron collides with nucleus and stops completely
Describe characteristic radiation interactions.
- bombarding electron collides with electron in shell and displaces
- can displace into different shell (excitation)
- can displace entirely (ionisation)
- energy released based on electrons dropping down the lower shell, values depending on atom
