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

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