Appendix G1: Chapter 12: X-Ray Interaction with Matter Coherent (Classical: Unmodified: Thompson) scattering * Coherent interaction excites the whole atom. * Most interactions occur < 10 keV * Small % are higher and contribute to fog * Of little importance to diagnostic x-ray * Scattered & incident photon energies the same * Of no value Compton Effect * Interaction with outer shell electrons. * Energies in diagnostic range * Scatters incident photon which gives up energy * The greater the angle of deflection of the scattered x-ray, the more energy is given up (180o gives up 1/3) * Ionizes the atom * The energy given up by the incident photon = the binding energy of the target electron plus the kinetic energy gained by the newly created Compton electron * Compton electron & scattered photon may undergo many more interactions * Of negative value to x-ray imaging * Source of most occupational exposure Pair Production * Energy of at least 1.02 MeV * Incident photon heads into nucleus, disappears, and reemerges as two electons, one of which is a positron * The energy equivalence of the mass of an electron is 51 MeV, which is the minimum the electrons possess. * The electron fills a vacant hole n a the closest atom needing one. The positron combines with another free electron and the mass of the two are converted to energy in an event called annihilation radiation. Photoelectric interaction * With inner shell electrons. * Total absorption of the incident photon * Photoelectron is ejected with kinetic energy in excess of binding energy * Photoelectrons from low atomic # atoms & low binding energies (soft tissue) absorb most of the incident energy * An outer shell electron takes its place, producing a secondary photon equal to the difference in the binding energies * Secondary photons have no diagnostic value Photodisintegration * Energy above 10 MeV * Incident photon heads into the nucleus, is absorbed and emits a nucleon or nuclear fragment.

X-Ray Interaction with Matter Compton Effect: This graph demonstrates that there is little to no effect on Compton interactions from changes in atomic density, and that as x-ray energy increases Compton interactions decrease Bone Z=13.8 Soft tissue Z=7.4 The Z value of soft tissue is 7.4 and the Z of air is 7.2. Nevertheless, the differential absorption of soft tissue is 770 times that of air (3 x that of lungs) Appendix G2: Chapter 12: X-Ray Interaction with Matter Barium: Z = 56. For the Iodine: Z = 53. For the alimentary tract. Air: the vascular system. (negative contrast) is being used with barium. Left: Photoelectric and Compton effects: * The probability that a photoelectric interaction will occur in bone is about 7 times greater than in soft tissue. * At low energies the photoelectric effect predominates * At higher energies Compton scattering predominates. * The effect of Compton scatter on the image is negligible * And, at higher energies, the probability of any interaction decreases. In Summary * Two types are x-ray interaction are important: Compton and Photoelectric * But only one interaction is useful: Photoelectric * Compton scatter fogs the image and leads to occupational exposure * Photoelectric absorption is true absorption which provides differential absorption