Collision of a photon with an electron increases the kinetic energy of the electron. Thus set in motion, the electron is known as a recoil electron. The incident photon’s energy is not necessarily depleted, but it will diverge from its path and have lower energy -- i.e., it has a longer wavelength after collision.  In diagnostic radiology, such scattered photons may lower contrast and thus degrade quality of the radiographic image.

Important interaction for intermediate-energy photons (100 KeV to 10 MeV)