Interaction of Radiation with Matter I

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Interaction of Radiation with Matter I
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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

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Excitation
De-excitation
Ionization and production of delta rays
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Specific Ionization

• Number of primary and secondary ion pairs
  produced per unit length of charged particles
  path is called specific ionization
• Expressed in ion pairs (IP)/mm
• Increases with electrical charge of particle
• Decreases with incident particle velocity

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Specific ionization for 7.69 MeV alpha particle
from polonium 214
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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

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Path lengths vs. ranges
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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)

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Bremsstrahlung
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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

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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

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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)

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Neutron interaction
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X- and Gamma-Ray Interactions

• Rayleigh scattering
• Compton scattering
• Photoelectric absorption
• Pair production

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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

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Rayleigh Scattering
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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

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Compton Scattering
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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

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Relative Compton scatter probabilities
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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

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Compton Scattering

• Incident photon energy must be substantially
  greater than the electrons 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

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Photoelectric absorption

• All of the incident photon energy is transferred
  to an electron, which is ejected from the atom
• Kinetic energy of ejected photoelectron (Ec) is
  equal to incident photon energy (E0) minus the
  binding energy of the orbital electron (Eb)
• Ec Eo - Eb

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Photoelectric absorption (I-131)
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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

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Photoelectric absorption (I-131)
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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

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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

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Photoelectric mass attenuation coefficients
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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)

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Percentage of Compton and photoelectric
contributions
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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

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Pair Production