Production of X-rays Flashcards
X-rays are produced when what happens?
when fast moving electrons are rapidly decelerated.
What are the components of a dental x-ray machine?
- Wall mounted
- Tubehead (contains X-ray tube)
- Jointed, positioning arm
- Control panel
What are the components of the X-ray tube head? (9)
- Filament – cathode (negatively charged part)
- Transformer
- Target – anode (positively charged part)
- Target surround
- Evacuated glass envelope
- Shielding
- Filtration
- Collimator
- Spacer cone
The filament has what charge?
It is the negative side of the x-ray tube
What is the filament?
A small coiled wire made from tungsten
What properties of tungsten make it a good material for the filament to be made of and why?
- high atomic number
- High melting point (3410 degrees) which means that the filament can be reused and it will maintain its integrity
What are the 2 types of transformers in the x-ray tube head?
- Step-down transformer
- Step-up transformer
What does the step-down transformer do?
converts the electricity from the mains electricity to a low voltage current which is passed through the filament circuit
What happens to the filament when electricity is passed through it?
- It causes the filament to heat up and incandescence (emit light as a result of being heated).
- This causes electrons to form a cloud around the filament (the electrons have come out from atoms in the filament).
What do you need to do to extract the electrons in a cloud around the filament?
Need a very high voltage - this is where the step up transformer comes into play
What does the step-up transformer do?
It takes the domestic input from the mains electricity supply (approx. 240eV) and converts it to a high voltage output of 60-70 keV (60-70 thousand).
The high voltage that has come from the step-up transformer does what?
causes a huge attraction of the -ve electrons from the cathode (the negative part of the filament) towards to the positive anode (the target).
The flow of electrons (current) from the cathode to the anode is what?
7-15 milliamps
What is the anode (target) and what is it made from?
- is the positively charged part of the x-ray tube
- tungsten
What happens when the electrons reach the anode?
The electrons will interact with the anode and either:
- undergo a reaction that either produces heat
- undergo a reaction that produces x-rays
1% of the electrons will be turned into X-rays and the other 99% will be transformed into heat
The heat from the interaction between the anode and electrons is dissipated by what?
The copper stem on the anode
Where do the x-rays produced from the electron interation with the anode go?
The x-rays exit the unleaded glass window towards the aluminium disc (filtration) then pass through the collimator
What is the glass envelope made from and what does it do?
This is made from evacuated glass.
It acts as a vacuum preventing the risk of interaction of electrons (from the filament) with air atoms prior to meeting the target.
What shielding is there within the x-ray tube and why do we have it?
We want the x-rays created to go towards the patient but only the patient
We don’t want the x-rays going anywhere else so there is lead that acts as a shield.
Why is lead good for shielding?
Lead has a high atomic number (82) and is very good at absorbing x-rays.
It makes sure the dose rate in the vicinity Is not > than 7.5 uSvh-1 i.e. it means that people working with the x-rays will not get doses greater than what is legally allowed.
Why do we want to filter x-rays?
We only want high energy x-rays to meet the patient as these are able to penetrate the tissues and still reach the image receptor.
Low energy x-rays will just be absorbed by the tissue so we want to filter these out
How are x-rays filtered in an x-ray tube head? How does it work?
We use a disk of aluminium
aluminium has a low atomic number meaning it is good at interacting with low energy x-rays but high energy x-rays will still get through it.
How thick should the disk of aluminium be for
a) < or + 70kV
b) >70kV
a = 1.5mm thick
b = 2.5 mm thick (panoramics)
Describe a collimator (what it does and what its made from).
It is made from lead and is circular or rectangular
It reduces the dose of X-ray’s given to the patient
What is the max diameter allowed for circular collimation at the patient end of the spacer cone?
60mm
Should rectangular or circular collimation be used and why?
Rectangular as it reduces the dose further (less mm2)
30% reduction in x-ray dose
What are spacer cones/beam-indicating devices and what do they do?
These can be circular or rectangular and controls the target (focus) to skin distance (fsd).
What do we want the focus to skin distance to be?
10cm for <60kV and 20cm for > or = 60kV (should therefore always be 20cm)
Note: there is an external marker on the cone to measure this
Why do you want to use a long x-ray focus-skin distance?
- in order to reduce magnification.
- The longer the distance the more parallel beams so less magnification and distortion.
Note: a ‘long spacer cone’ = a long fsd.
