Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

ability of occurrence in the diagnostic energy range. In soft tissue, Rayleigh scatteringaccounts for less than 5% of x-ray interactions above 70 keY and at most onlyaccounts for 12% of interactions at approximately 30 keY. Rayleigh interactions arealso referred to as "coherent" or "classical" scattering.Compton scattering (also called inelastic or nonclassical scattering) is the predominantinteraction of x-ray and gamma-ray photons in the diagnostic energy rangewith soft tissue. In fact, Compton scattering not only predominates in the diagnosticenergy range above 26 keY in soft tissue, but continues to predominate wellbeyond diagnostic energies to approximately 30 MeY. This interaction is most likelyto occur between photons and outer ("valence") shell electrons (Fig. 3-7). The electronis ejected from the atom, and the photon is scattered with some reduction inenergy. As with all types of interactions, both energy and momentum must be conserved.Thus the energy of the incident photon (Eo) is equal to the sum of theenergy of the scattered photon (Ese) and the kinetic energy of the ejected electron(Ee-), as shown in Equation 3-2. The binding energy of the electron that was ejectedis comparatively small and can be ignored.(I)~(I)---(I) ~/' ~(I) /(I)~ -\Valence electrons(/ @ \.\\ *\ K)- j(1)\ '-. : ./ _ L lMM""'- ~ • Compton~,=,______!iIIII' / ~ electron (Ee-)Incidentphoton(Eo)1/"1 ~ ", ", e Angle of deflection-1