X-Ray Production Flashcards
what is included in the production of a radiographic image
• Source of X-rays - x-ray machine - production of X-rays
• Object
○ Teeth and jaws
○ Interaction of X-rays with matter
○ No interaction means we are not going to get an image
• Image receptor
○ Digital or film
• Processing
○ Conversion of latent image to permanent visible image by computer technology or chemical
when are x-rays produced
X-rays are produced when fast moving electrons are rapidly decelerated
what is an electron
Negatively charged particle in an atom (negative)
where are electrons found
Conceptually sited in orbits about the nucleus (positive) - the Bohr model
what are the components of the x-ray machine
• Wall mounted
○ Can also be mounted from the ceiling or mobile on something that has wheels
- Tube head - contains x-ray tube
- Jointed, positioning arm
• Control panel
○ Can either be attached or separate ~ Allows you to choose the right exposure for the radiographic image you are creating
what is in a siemens heliodent MD tubehead
• Tubehead
• Spacer cone
○ Either cylinder or rectangular
• Rectangular collimation
○ Built in or inserted
○ This controls the shape of the beam [All machines have this]
what are the components of the x-ray tube head
- Filament - cathode
- Transformer
• Target - anode
○ This is what we bombard the electrons at
○ Positively charged part of the tube
- Target surround
- Evacuated glass envelope
- Shielding
- Filtration
- Collimator
- Spacer cone
what is the filament
- Negative
- Made of tungsten
• Filament circuit (step down transformer)
○ Low voltage, high current
○ An electrical circuit runs through the filament and it follows the current coming out of the wall going through a step down transformer to produce a low voltage, high current which is then going to make something happen at that filament
• Tiny little bit of coiled wire, difficult to see, embedded within
what is tungsten
• Symbol: W
• Z = 74 ~ number of protons / electrons ~ these are the same because it is a stable atom
○ High atomic number - this is useful
• Melting point = 3410˚C
what is one of the reasons we use tungsten
One of the reasons we use tungsten is because of its really high melting point which means we can use the filament again and again and it won’t degrade or melt - it will maintain it’s integrity
what is the function of the filament
- Filament - cathode
- Low voltage current passed through filament circuit
- Filament heats up to incandescence
• Electrons form a cloud around filament
○ Some of the electrons come out of the atoms and these form the cloud
○ We have the electrons and they are around the filament
○ Similar to steam coming out of a hot cup of coffee
• So what we want to do is effectively pull the electrons over towards the positive side of the x-ray tube but to do this we need to have a very high voltage - this is where the step-up transformer comes into play
what does the transformer do
• Step up
○ Copper coils inside the tube head, at the front of the tube
○ Hollow centre
○ Transformer in front of the tube allows the whole tube head to be a bit smaller
• 240 eV domestic input
○ This is our electricity supply
• 60-70 keV high voltage output
○ We want to put out 60-70,000 volts
○ We need to make a very big change
○ Apply this to our x-ray tube which then creates the potential difference between the negative filament and the positive target which creates this very powerful force which will attract the negative electrons to the positive target
- Huge attraction of negative electrons (mA) from cathode towards positive anode (target)
- Flow of electrons ~ 7 to 15 mA
new x-ray equipment should operate within what range
New equipment should operate within the range 60 to 70 kV ….
Old machines may operate at 50 kV and this has an impact on patient dose and has an impact on how the x-ray beam will interact with matter
what is the target
Target - anode
- Positive
- Tungsten
- Effective area 0.7mm2
• 20˚ slope (ie not parallel to filament)
○ This is important as it is a way of having a target that is in fact a rectangle (which we refer to as a square) called the effective area
○ Also a way to increase the efficiency of what we are doing
• Referred to also as focus or focal spot
what happens when electrons interact with the target (which is a solid metal)
• Heat production - 99%
○ Majority of electrons will undergo an interaction which will result in heat production
○ This is a very inefficient way of producing x-rays but it is the way it has to be done
○ 99% of the energy carried by the fast moving electrons is converted into heat
• X-ray production - <1%
○ Continuous spectrum
§ Term spectrum means we are talking about energy values
○ Characteristic spectrum
§ Characteristic to tungsten the material where the interactions are happening
○ Only a little under 1% of the energy is actually converted into x-rays
what happens with regards to heat production produced by target interactions
• Incoming electron (e-)
○ Deflected by cloud of outer-shell tungsten electrons, or collides with an outer shell electron, displacing it
- Small loss of energy (E)
- Loss of energy in form of heat
• Removed through copper block, oil, then air
○ Small tungsten target is set into a block of copper which is very good at conducting heat away
○ Then the x-ray tube is surrounded by oil which is also good at conducting heat away
○ Can get quite warm around an x-ray machine if they are used again and again and again as there is always this heat being produced
what is the target surround
• Copper - Cu
○ Same colour as coils - like a pinky colour
• Z = 29
• Melting point= 1080˚C
○ Not nearly as high as tungsten
• Effective heat conductor
○ It is there to get rid of the heat
○But also is there so there is something for the target to be set into because it is so small
what happens during target interactions that causes x-ray productions on the continuous spectrum
X-ray production: continuous spectrum = interactions that does result in x-rays
(Bremsstrahlung = braking / white radiation)
• Incoming e- passes close to nucleus of a target atom
○ Gets through the outer cloud and gets quite close to the nucleus
• E- rapidly decelerated and deflected
○ Has its direction changed as well
• Amount of deceleration and deflection proportional to E loss
○ These 2 things together give an indication as to how much energy is lost
• E loss in form of electromagnetic radiation as a continuous spectrum of energies
○ There is a range of energy that can be lost
○ It is dependent on what is the applied kV
• Maximum E is applied kV (eg 70 kV)
○ So a 70kV machine means the maximum energy would be 70 and the minimum would be down just above 0
why are low energy x-rays not useful to us in image production
they don’t have enough energy to actually get through the tissues to help to create an image - we need to get rid of them later because it is the higher ones that we actually want
what happens to target interactions the cause x-ray production: characteristic spectrum
X-ray production: characteristic spectrum
Characteristic to a particular element - in this case it is characteristic to tungsten
• Incoming e- collides with an inner shell (orbit) target e-
○ So the incoming electron gets through outer shell but doesn’t get as far as the nucleus, it then collides with an electron already on an inner shell
• Target e- displaced to an outer shell or completely lost from atom
○ Because of the collision the inner shell electron will either be displaced and move further away from the nucleus or it might be knocked right out
• Target atom unstable
○ It is unstable whether the electron is displaced or lost
○ Atoms don’t like being unstable so they are going to rearrange the electrons instantly to get back their stability
• Orbiting e-s re-arranged to fill vacant orbital slots to return atom to neutral state
○ Rearrange themselves automatically in order to have the right number of electrons in each of the electron shells
○ If it is completely lost then there will be some loose electrons nearby that can just drop into a shell
• Difference in E between orbits is released as characteristic radiation, of known E values
○ Electrons are in shells of different binding energy values
○ The difference between that energy is released when it is re-arranged and that is what is called characteristic radiation
• Same mechanism as photoelectric absorption - covered in lecture on interaction with matter
○ Very important mechanism
what is tungsten characteristic radiation
Characteristic radiation of tungsten has values of approximately:
○ 8kV - L shell ~ second shell out from the nucleus
○ 58kV - K shell
○ 68kV - K shell
§ These are useful energies = good
what is the glass envelope
- Evacuate glass
- Vacuum prevents risk of interaction of electrons with air atoms prior to meeting target
The vacuum is inside the glass
what is generated at the target
x-rays that go off in all different directions
why is there lead
to absorb x-rays that are trying to get out of the tube head
ie the x-rays we are not using
