Define excitation
When an external stimulus gives sufficient energy to an orbiting electron to raise them from their current shell but not enough to remove it from the atom.
Define excitation
When an external stimulus transfers enough energy to the orbiting electron to overcome the binding energy and expel it from the atom.
How are x-rays produced in an x-ray machine?
- Low voltage (12V) is supplied to the filament to produce a cloud of electrons around the filament
- High voltage (70kV) is applied and the electrons accelerate towards the Tungsten target.
- Electrons make contact with the target they penetrate the surface but in doing so lose kinetic energy and are slowed down.
- The kinetic energy lost is what produces photons/quanta of x-rays.
- Process not very efficient (1-5%) the remainder produces heat.
What is the purpose of the glass envelope in the x-ray machine?
Ensures perfect vacuum so no particles get in the way of the path of the electrons as they are accelerated across.
Thus the electrons gain the maximum possible kinetic energy for the conversion of their kinetic energy into x-ray photons.
Also a heat sink.
Why is the anode copper?
- Good conductor thus electrons will circulate back round to cathode.
- Heat sink
What is line/characteristic radiation?
Radiation characteristic of the both electron bands involved in the transition and of the target/material.
What is Bremsstrahlung/ continuous radiation?
“Braking” radiation - radiation energy produced by the sudden deceleration deflection of electrons. They produce x-ray photons of different energies dependent on the distance the electron is to the nucleus.
What factors affect attenuation?
- Atomic number (higher=higher attenuation)
- Thickness/ Secondary material
- Density
- kV (energy of photons)
What is attenuation?
Fractional reduction in beam intensity as it passes through the material, expressed as a percentage of the original beam intensity.
What affect does a low kV have?
- Low energy kV have a wider angle scatter from photons original path.
- Thus will travel down through patients’ body impacting on critical organs.
What factors of total attenuation consist of?
- Absorption: x-ray photo ceases to exist all of its energy as has been given up in the interaction.
- Scatter: photon will continue to exist after the interaction but will travel in a new direction with a lower energy value.
When does absorption occur?
When incoming photon has only slightly more energy than the binding energy the electron that it interacts with has.
When does scattering occur?
When the incoming photon has a level of energy much greater than the binding energy of electron involved in the interaction (photon gives up its energy to eject recoil electron).
A Compton event occurs where the recoil electron and new photon path are equal angles compared with the original photon path.
The greater the angle of divergence from the original photon the greater % of energy the photon gives off.
What are the exposure factors?
- kV
- Exposure time
- Tube current
Why is it recommended to have a high kV?
- Same amount of scatter than low kV but it is narrower angle = reduces dose to patient!
- kV means high intensity thus shorter exposure time
- However, means contrast and sharpness reduced but lower dose to patient trumps this.
Why is it good to have a higher mA?
Higher tube current (mA) means high intensity = increase in contrast until a point where theres overexposure and decreases contrast as film under more attenuation will get more density but that under high attenuation will no so contract will reduce.
Why is does reduced sharpness and contrast occur as a result of high kV?
Narrow angle scatter blurs edges of objects and raises the overall density so contrast is reduced.
What are the advantages of extended cone paralleling?
- Maintain correct FFD
- show extent of beam at patients skin surface
Why is it better to have a rectangular collimator instead of a round one?
All radiation hits imaging plate where as a round one increaes the dose to patient as it is a lot bigger than phosphor plate.
- Reduces dose by 50% by limiting total amount of radiation hitting patient
- Improves contrast and sharpness
