K-absorption edge

Last revised by Dr Daniel J Bell on 23 Apr 2022

The K-absorption edge (K-edge) refers to the abrupt increase in the photoelectric absorption of x-ray photons observed at an energy level just beyond the binding energy of the k-shell electrons of the absorbing atom.

K-shell binding energies are specific to each element. As the atomic number (Z) of an element increases, so does its corresponding k-shell binding energy, and therefore the greater the photon energy at which the K-edge occurs. Binding energies are expressed in kiloelectronvolts (keV).

The most abundant elements in human tissue (hydrogen, carbon, oxygen, nitrogen) have K-edges that are too low to be detectable (<1 keV).

Elements with larger K-edge values, that are within the useful portion of the x-ray spectrum, are of greater interest in radiology. The k-edge properties of certain materials can be specifically chosen for their use in contrast media, intensifying screens and beam filters.

For example:

  • iodine (Z = 53, K-edge = 33.2 keV) is commonly used as a contrast agent in radiography
  • molybdenum (Z = 42, K-edge = 20.0 keV) is commonly used as a beam filter in mammography

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