2Attenuation Coefficients The relative variation of attenuation coefficients with energy and between different materials affects both the absorption of radiation dose in patients and the radiographic images produced.As the energy of the photons increase, the probability of interaction drops rapidly.
3Photon Energy Dependant Interactions Low energy photons interact with whole atom.Moderate energy photons interact with orbital electrons.High energy photons interact with nucleus.
4Coherent ScatterAKA Thomson, Rayleigh, classical, unmodified or simple scatterEnergy: very low energy photons (below 10 keV, ex: light)Interacts with: outer shell electronsIncoming photons: absorbed then released (no overall change)Interaction: excited, not ionized
5Coherent ScatterProduct: photon with same energy as incoming photon with different directionAtomic number: has no effectImportance in diagnostic: blurs shadowsImportance in therapy: none due to low probability and the fact that no energy is deposited.
6Photoelectric EffectEnergy: high energy photons (40-70 kVp), as energy increases, probability of photoelectric effect decreases.Interacts with: tightly bound inner shell electronsIncoming photons: absorbed; energy transferred to electron (released as photoelectron: Eke, mass, reabsorbs quickly)More likely to occur when x-ray photon has just slightly more energy than Eb of a K or L shell electronInteraction: ionized, ion pair formed, causes characteristic cascade
7Photoelectric EffectProduct: characteristic photons with energies equal to the differences in electron shell energiesAtomic number: probability increases as atomic number increases.Importance in diagnostic: produces shadows of high atomic number material (bone), responsible for contrast (contrast increases as energy decreases)Importance in therapy: none
8Compton EffectEnergy: high energy photons, important in orthovoltage/ megavoltage rangeInteracts with: loosely bound outer shell electronsIncoming photons: some energy absorbed by electron (released as Compton/recoil electron) & some scattered.Interaction: ionized, ion pair formed, remaining energy released as photon.Product: photons with reduced energies related to the angle of scatter, change of direction, will continue to interact until absorbed photoelectrically.
9Compton EffectAtomic number: independent of atomic number (depends on electron density- the more “free electrons” are available higher probability of effect)Importance in diagnostic: degrades image by graying film; also looked at in determining shielding requirements.Importance in therapy: best contrast obtained in areas of varying mass density.Source of occupational exposure photon possesses enough energy to be emitted from patient) and radiation fog (scatter places exposure on film unrelated to anatomy)
10Pair ProductionEnergy: threshold: at least 1.02 MeV, usually > 10 MeV; as energy increases, probability of effect increasesInteracts with: electric field of nucleusIncoming photons: absorbed by nucleusInteraction: negatron & positron produced which deposit energy as it interacts with matter Bremsstrahlung possible but unlikely due to body tissues having low Z
11Pair ProductionProduct: two photons produces in annihilation reaction (0.511 MeV each) traveling in opposite directionsAtomic number: strength of electric field is a function of the atomic number.Importance in diagnostic: noneImportance in therapy:
12Photonuclear Interaction AKA photonuclear disintegration, gamma-n interaction (γ,n)Energy: energies greater than 15 MeVInteracts with: nucleusIncoming photons: absorbed by nucleus.Interaction:
13Photonuclear Interaction Product: neutronAtomic number:Importance in diagnostic: noneImportance in therapy:
14Review: Technical Factors Increase kVp:PE absorption decreasesCompton effect increasesDecreases contrast (absorption)Increases ScatterTotal number of photons that are transmitted without interaction increases.A decrease in kVp will result in higher contrast (more absorption) and increased dose to patient.