"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)."
"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"