Interaction Processes Flashcards
What is absorption?
The transfer of energy from the radiation to the medium
What is scatter?
The change in direction of a photon (potential energy lost)
Signal vs noise
Signal= the real image
Noise= scatter
TLAC
Total Linear Attenuation Coefficient
Fraction of x-rays removed from the beam per unit thickness of attenuating material
Compares attenuation of real structures
(Dependent of density)
TMAC
Total Mass Attenuation Coefficient
Fraction of x rays removed from the beam per unit mass of a medium
Compares basic attenuating properties of materials
(Independent of density)
How is the type of interaction process determined?
Energy of photon
Material through which it is travelling
Explain elastic scattering
Happens at lower energies (mammography)
No energy transfer
Less than 10% of transfers
Scatters in a forward direction= use grid
Explain photoelectric absorption
Happens when energy is greater than or equal to BE of electron in atom
Incident electron interacts with an electron, ALL energy is transferred
Electron is ejected, may further ionise
Vacancy, therefore outer shell electron falls into inner shell
Emission of characteristic electromagnetic radiation, which is equal to the difference in shell BE
PE absorption increases as atomic number increases as high atomic number materials have many electrons with many different BE
Incidence decreases as kVp increases
Explain Compton scatter
Energy of the photon is reduced by the interaction and electron gains kinetic energy
Incident photon is deflected off its original path and is scattered
Up to 100keV, CS is relatively constant
Above 100keV, CS decreases gradually as photon energy increases, due to photons having so much energy they go in a forward direction
CS is directly proportional to electron density
Explain pair production
When incident photon energy is greater than or equal to 1.02 MeV
Photon interacts with nucleus which recoils and the photon ceases to exist
Instantaneously, an electron-positron pair is created
Positrons cannot exist outside a nucleus, and it disappears in an interaction with an electron (annihilated)
Pair production increases with photon energy or atomic number
How to protect from low energy radiation
high Z material used
How to protect from high energy radiation
effect of Z is diminished, so high density is used (thickness)
What materials are used in doors for protection?
Lead 1st, to absorb high energy
Concrete 2nd to absorb low energy
What is attenuation?
A decrease in the intensity of a beam due to both absorption and scatter
