Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 5-4. Generation of a characteristicx-ray in a target atomoccurs in the following sequence:(1) The incident electron interactswith the K-shell electron via arepulsive electrical force. (2) The K-shell electron is removed (only ifthe energy of the incident electronis greater than the K-shell bindingenergy), leaving a vacancy in the Kshell. (3) An electron from the adjacentL shell (or possibly a differentshell) fills the vacancy. (4) A Kacharacteristic x-ray photon is emittedwith an energy equal to thedifference between the bindingenergies of the two shells. In thiscase, a 59.3-keV photon is emitted.ous bremsstrahlung spectrum. The most prevalent characteristic x-rays in the diagnosticenergy range result from K-shell vacancies, which are filled by electrons fromthe L, M, and N shells. Even unbound electrons outside of the atom have a smallprobability of filling vacancies. The shell capturing the electron designates the characteristicx-ray transition, and a subscript of a or ~ indicates whether the transitionis from an adjacent shell (a) or nonadjacent shell (~). For example, K a refers to anelectron transition from the L to the K shell, and K~refers to an electron transitionfrom M, N, or 0 shell to K shell. A K~ x-ray is more energetic than a K a x-ray.Within each shell (other than the K shell), there are discrete energy subshells, whichresult in the fine energy splitting of the characteristic x-rays. For tungsten, threeprominent lines on the bremsstrahlung spectrum arise from the K a1 , Kaz, and K~ltransitions, as shown in Fig. 5-5. Characteristic x-rays other than those generatedCharacteristicRadiation Spikes/ (Tungsten)40 50 60Energy (keV)FIGURE 5-5. The filtered spectrumof bremsstrahlung andcharacteristic radiation from atungsten target with a potentialdifference of 90 kVp illustratesspecific characteristicradiation energies from Ka andK~ transitions. Filtration (thepreferential removal of lowenergyphotons as they traversematter) is discussed inChapter 3, section 3.