interactions of xrays with matter (compton scatter)

Question 1

what is compton scatter

Answer 1

compton scatter occurs at photon energies above 30kvp and it contributes to:
- scatter accross the xray room
- pateient does
- degrading image quality

it happens when an incoming xray photon ionises an outer shell electron and this can only happen as long as the energy of the xray photon is much greater than the binding energy of the electron

Question 2

explain how compton scatter occurs

Answer 2

an incoming xray photon (with much more energy than the binding energy of the electron ) ejects the outer shell electron of an atom in the medium

a proportion of enerrgy possessed by the xray photon is donated to the ejected electron as kinetic energy and then this ejected electron gets involved with other interactions until all donated energy is used up.

the xray photon still has energy remaining after this donation how ever its energy has decreased so it experiences a change in direction and theis is called the angle of scatter which is relative to the amount of energy donated to the electron

Question 3

what can affect the angle of scatter

Answer 3

the angle of scatter is greater at lower photon energies and smaller at higher photon energies

at lower photon energies more energy is donated to the ejected electron resulting in a larger deflection - this is backwards scatter

at higher photon energies, less energy is donated to the electron therefore ythere is a weaker deflection - this results in forward scatter towards the receptor

Question 4

factors affecting compton scatter

Answer 4

medium thickness(there will be a higehr tendancy for scatter to occur when imaging thicker parts of the body such as the pelvis)

beam energy(higher beam energy means less scatter )