X-Ray & γ -Ray Interactions with Matter

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 • AKAphotonuclear 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.