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

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.

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

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

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.

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

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

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.

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)

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

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:

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:

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

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.