02  -interaction_of_radiation_with_matter_i
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02  -interaction_of_radiation_with_matter_i 02 -interaction_of_radiation_with_matter_i Presentation Transcript

  • Interaction of Radiation with Matter I
  • Particle interactions
    • Energetic charged particles interact with matter by electrical forces and lose kinetic energy via:
      • Excitation
      • Ionization
      • Radiative losses
    • ~ 70% of charged particle energy deposition leads to nonionizing excitation
  • Specific Ionization
    • Number of primary and secondary ion pairs produced per unit length of charged particle’s path is called specific ionization
      • Expressed in ion pairs (IP)/mm
    • Increases with electrical charge of particle
    • Decreases with incident particle velocity
  • Specific ionization for 7.69 MeV alpha particle from polonium 214
  • Charged Particle Tracks
    • Electrons follow tortuous paths in matter as the result of multiple scattering events
      • Ionization track is sparse and nonuniform
    • Larger mass of heavy charged particle results in dense and usually linear ionization track
    • Path length is actual distance particle travels; range is actual depth of penetration in matter
  • Path lengths vs. ranges
  • Linear Energy Transfer
    • Amount of energy deposited per unit path length is called the linear energy transfer (LET)
    • Expressed in units of eV/cm
    • LET of a charged particle is proportional to the square of the charge and inversely proportional to its kinetic energy
    • High LET radiations (alpha particles, protons, etc.) are more damaging to tissue than low LET radiations (electrons, gamma and x-rays)
  • Bremsstrahlung
  • Bremsstrahlung
    • Probability of bremsstrahlung production per atom is proportional to the square of Z of the absorber
    • Energy emission via bremsstrahlung varies inversely with the square of the mass of the incident particle
      • Protons and alpha particles produce less than one-millionth the amount of bremsstrahlung radiation as electrons of the same energy
  • Bremsstrahlung
    • Ratio of electron energy loss by bremsstrahlung production to that lost by excitation and ionization = EZ/820
      • E = kinetic energy of incident electron in MeV
      • Z = atomic number of the absorber
    • Bremsstrahlung x-ray production accounts for ~1% of energy loss when 100 keV electrons collide with a tungsten (Z = 74) target in an x-ray tube
  • Neutron interactions
    • Neutrons are uncharged particles
    • They do not interact with electrons
      • Do not directly cause excitation or ionization
    • They do interact with atomic nuclei, sometimes liberating charged particles or nuclear fragments that can directly cause excitation or ionization
    • Neutrons may also be captured by atomic nuclei
      • Retention of the neutron converts the atom to a different nuclide (stable or radioactive)
  • Neutron interaction
  • X- and Gamma-Ray Interactions
    • Rayleigh scattering
    • Compton scattering
    • Photoelectric absorption
    • Pair production
  • Rayleigh Scattering
    • Incident photon interacts with and excites the total atom as opposed to individual electrons
    • Occurs mainly with very low energy diagnostic x-rays, as used in mammography (15 to 30 keV)
    • Less than 5% of interactions in soft tissue above 70 keV; at most only 12% at ~30 keV
  • Rayleigh Scattering
  • Compton Scattering
    • Predominant interaction in the diagnostic energy range with soft tissue
    • Most likely to occur between photons and outer (“valence”) shell electrons
    • Electron ejected from the atom; photon scattered with reduction in energy
    • Binding energy comparatively small and can be ignored
  • Compton Scattering
  • Compton scatter probabilities
    • As incident photon energy increases, scattered photons and electrons are scattered more toward the forward direction
    • These photons are much more likely to be detected by the image receptor, reducing image contrast
    • Probability of interaction increases as incident photon energy increases; probability also depends on electron density
      • Number of electrons/gram fairly constant in tissue; probability of Compton scatter/unit mass independent of Z
  • Relative Compton scatter probabilities
  • Compton Scattering
    • Laws of conservation of energy and momentum place limits on both scattering angle and energy transfer
    • Maximal energy transfer to the Compton electron occurs with a 180-degree photon backscatter
    • Scattering angle for ejected electron cannot exceed 90 degrees
    • Energy of the scattered electron is usually absorbed near the scattering site
  • Compton Scattering
    • Incident photon energy must be substantially greater than the electron’s binding energy before a Compton interaction is likely to take place
    • Probability of a Compton interaction increases with increasing incident photon energy
    • Probability also depends on electron density (number of electrons/g  density)
      • With exception of hydrogen, total number of electrons/g fairly constant in tissue
      • Probability of Compton scatter per unit mass nearly independent of Z
  • Photoelectric absorption
    • All of the incident photon energy is transferred to an electron, which is ejected from the atom
    • Kinetic energy of ejected photoelectron (E c ) is equal to incident photon energy (E 0 ) minus the binding energy of the orbital electron (E b )
    • E c = E o - E b
  • Photoelectric absorption (I-131)
  • Photoelectric absorption
    • Incident photon energy must be greater than or equal to the binding energy of the ejected photon
    • Atom is ionized, with an inner shell vacancy
    • Electron cascade from outer to inner shells
      • Characteristic x-rays or Auger electrons
    • Probability of characteristic x-ray emission decreases as Z decreases
      • Does not occur frequently for diagnostic energy photon interactions in soft tissue
  • Photoelectric absorption (I-131)
  • Photoelectric absorption
    • Probability of photoelectric absorption per unit mass is approximately proportional to
    • No additional nonprimary photons to degrade the image
    • Energy dependence explains, in part, why image contrast decreases with higher x-ray energies
  • Photoelectric absorption
    • Although probability of photoelectric effect decreases with increasing photon energy, there is an exception
    • Graph of probability of photoelectric effect, as a function of photon energy, exhibits sharp discontinuities called absorption edges
    • Photon energy corresponding to an absorption edge is the binding energy of electrons in a particular shell or subshell
  • Photoelectric mass attenuation coefficients
  • Photoelectric absorption
    • At photon energies below 50 keV, photoelectric effect plays an important role in imaging soft tissue
    • Process can be used to amplify differences in attenuation between tissues with slightly different atomic numbers, improving image contrast
    • Photoelectric process predominates when lower energy photons interact with high Z materials (screen phosphors, radiographic constrast agents, bone)
  • Percentage of Compton and photoelectric contributions
  • Pair production
    • Can only occur when the energy of the photon exceeds 1.02 MeV
    • Photon interacts with electric field of the nucleus; energy transformed into an electron-positron pair
    • Of no consequence in diagnostic x-ray imaging because of high energies required
  • Pair Production