1. X-Ray & γ-Ray Interactions with Matter
Chapter 5

2. Attenuation 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.

3. Photon Energy Dependant Interactions
Low energy photons interact with whole atom.
Moderate energy photons interact with orbital electrons.
High energy photons interact with nucleus.

4. Coherent Scatter
AKA Thomson, Rayleigh, classical, unmodified or simple scatter
Energy: very low energy photons (below 10 keV, ex: light)
Interacts with: outer shell electrons
Incoming photons: absorbed then released (no overall change)
Interaction: excited, not ionized

5. Coherent Scatter
Product: photon with same energy as incoming photon with different direction
Atomic number: has no effect
Importance in diagnostic: blurs shadows
Importance in therapy: none due to low probability and the fact that no energy is deposited.

6. Photoelectric Effect
Energy: high energy photons (40-70 kVp), as energy increases, probability of photoelectric effect decreases.
Interacts with: tightly bound inner shell electrons
Incoming 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 electron
Interaction: ionized, ion pair formed, causes characteristic cascade

7. Photoelectric Effect
Product: characteristic photons with energies equal to the differences in electron shell energies
Atomic 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

8. Compton Effect
Energy: high energy photons, important in orthovoltage/ megavoltage range
Interacts with: loosely bound outer shell electrons
Incoming 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.

9. Compton Effect
Atomic 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)

10. Pair Production
Energy: threshold: at least 1.02 MeV, usually > 10 MeV; as energy increases, probability of effect increases
Interacts with: electric field of nucleus
Incoming photons: absorbed by nucleus
Interaction: negatron & positron produced which deposit energy as it interacts with matter  Bremsstrahlung possible but unlikely due to body tissues having low Z

11. Pair Production
Product: two photons produces in annihilation reaction (0.511 MeV each) traveling in opposite directions
Atomic number: strength of electric field is a function of the atomic number.
Importance in diagnostic: none
Importance in therapy:

12. Photonuclear Interaction
AKA photonuclear disintegration, gamma-n interaction (γ,n)
Energy: energies greater than 15 MeV
Interacts with: nucleus
Incoming photons: absorbed by nucleus.
Interaction:

13. Photonuclear Interaction
Product: neutron
Atomic number:
Importance in diagnostic: none
Importance in therapy:

14. Review: Technical Factors
Increase kVp:
PE absorption decreases
Compton effect increases
Decreases contrast (absorption)
Increases Scatter
Total 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.