Interaction Processes Flashcards
Explain the PE effect? What factors does it vary with? (5 marks)
Photon liberates an electron from the inner shell of an atom, hence the name. Outer shell electron transitions into the hole and emits characteristic radiation. E is dependent on the atomic structure.
Prob = k Z^3 / E^3
Dominates at lower energies, gives contrast in an image.
Give two contrast agents which are used in DR? (4 marks)
Barium (Z=56) used in Barium swallows.
Iodine (Z=53) used in coronary angiography exams.
Both used because they have high Z and therefore high absorption.
Explain Compton scattering? (6 marks)
- Inelastic scattering of photons (they lose energy) with free or loosely bound electrons.
- After collision electron transfers a variable amount of energy and has a change in direction as a result of the scattering.
- Contributes to staff and patient dose and increases scatter in an image.
- Want to minimise.
- Prob = 1/E above 100 keV, does not vary much over the diagnostic energy range (30-100 keV)
Explain inelastic scattering? (4 marks)
- Photon is absorbed by the whole atom and re-emitted with the same energy in the same direction (hence inelastic)
- Not of much importance from a practical perspective.
- Prob = k Z^2 / E
What is meant by the term fluorescent yield and how does this relate to Auger electrons? (4 marks)
Fluorescent yield = w_k = no of k x-ray photons / no of k shell vacancies
If w_k equal to 0 then instead of x-ray photons being emitted, they are emitted as Auger electrons.
In lower Z materials the fluorescent yield is lower, hence more Auger electrons.
How does the mass attenuation coefficient vary with energy in water? And which interaction processes dominate as the energy is changed? (3 marks)
Exponentially decreasing plot, with PE effect dominating at low energies and compton dominating at higher energies.
How are k-edges used in imaging to decrease the dose to the patient? As well as the choice of contrast agents and filters. (8 marks)
The k-edge refers to the binding energy of the inner most orbital, or k-shell.
There is a sudden increase in the absorption coefficient at this energy due to the photo-electric effect being possible in the k-shell electrons.
The average photon energy is around 40 keV for most DR x-ray spectrums, therefore if we can use a detector and or contrast agent which has a k-edge in this region, then we can produce images with less dose to the patient. Due to the increased absorption in signal in the detector.
Lanthunum is an ideal detector as it has a k-edge at 40 keV therefore will absorb a larger proportion of photons around this energy, when compared to a similair Z detector without a k-edge.
Iodine (Z=53, k-edge = 33 keV) also is ideal as a contrast agent for the same reason. Barium is also ideal (Z=56, k-edge = 37 keV).
Irbium filters can also be used (Z=68, k-edge = 58 keV) to preferentially absorb the higher energy photons in a spectrum which are mainly scattered. This can reduce the staff and patient doses for the same image quality.
Same reason why lead is penetrated by beams just below 88 keV, as this is where leads k-edge is.
Explain what is meant by pair production? (3 marks)
- Only occurs above 1.02 MeV
- Photon travels close to the nucleus and is converted into a electron and a positron (a pair)
- Positron will quickly annihilate with an electron producing two gammas
- Prob = k (1.02 - E MeV)
Which processes attenuate and which processes absorb? (4 marks)
They all attenuate.
Elastic does not absorb.
PE does absorb due to characteristic radiation and electron ejected.
Compton has some absorption due to energy imparted to scattered electrons.
Pair production imparts all but 1.02 MeV due to creation of particles.
Where do neutron interactions arise and how can sheiding be implemented? (2 marks)
Heads of linacs due to material being made radioactive, which can produce neutrons as part of the decay process.
Can shield by using low Z materials as they lose more energy in low Z materials like Boron.
