X-Ray

1
X-Ray ?-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 gt
  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 (?,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.