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1. Generally, absorption decreases as photon energy increases.
2. Orbiting electrons can only absorb energy from a photon when the photon has an energy greater than its binding energy (hence, this phenomenon is seen in the photoelectric effect).
3. The probability of an interaction between a photon and an electron increases dramatically when the photon has just a little bit more energy than the binding energy.
4. This probability falls again as the photon energy increases.
5. Maximum absorption occurs when the photon has just enough energy to eject a bound electron.
6. This sudden jump in the attenuation coefficient is known as the K-edge (a less important L-edge also exists).
7. As Z ↑, K-shell binding energy ↑ and K-edge ↑
8. K-edges are important for contrast agents (Iodine 33 keV, Barium 37 keV, Lead 88 keV).


1. Bushberg, J. T. (2012). The essential physics of medical imaging. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. Find it at Amazon
2. Heggie, J. C., Liddell, N. A., & Maher, K. P. (1997). Applied imaging technology. Melbourne: St. Vincents Hospital.

Ⓒ A. Manickam 2018

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