What is Compton scattering?
Compton scattering is an interaction in which an X-ray photon collides with a loosely bound outer-shell electron, transferring part of its energy to the electron and being deflected in a different direction. The photon continues with reduced energy, while the ejected electron is called a Compton electron.
Compton scattering occurs when an X-ray photon transfers part of its energy to an outer-shell electron and is deflected with reduced energy, producing scatter radiation.
Compton scattering is the dominant interaction in soft tissues at typical diagnostic X-ray energies and is a major source of scatter radiation, which reduces image contrast.
Understanding the physics
In Compton scattering, an incoming X-ray photon interacts with an outer-shell electron that is only weakly bound to the atom. Because the binding energy of these electrons is small compared with the photon energy in diagnostic imaging, the electron can be treated as essentially free.
During the interaction, the photon transfers part of its energy to the electron. This causes the electron to be ejected from the atom as a recoil (Compton) electron, while the photon continues travelling in a different direction with lower energy.
The change in photon energy depends on the angle through which the photon is scattered. Larger scattering angles result in greater energy transfer to the electron and therefore lower energy of the scattered photon.
The energy–angle relationship is described by the Compton equation, which relates the change in wavelength of the photon to the scattering angle:
me
where h is Planck’s constant, me is the electron mass, is the speed of light, and θ is the scattering angle.
Unlike the photoelectric effect, the probability of Compton scattering depends primarily on the electron density of the material rather than its atomic number. Because most soft tissues have similar electron densities, Compton scattering produces relatively small differences in attenuation between tissues.
The probability of Compton scatter decreases very slowly with increasing photon energy (unlike the exponential decline seen in the probability of the photoelectric effect).
Where this matters clinically
Compton scattering is a major source of scatter radiation in radiographic imaging. Scattered photons may still reach the detector, but because they have been deflected from their original paths they do not accurately represent the location of the structures that produced them.
This scattered radiation adds unwanted signal to the image and reduces contrast between structures. Techniques such as anti-scatter grids, collimation, and air gaps are used to reduce the amount of scatter reaching the detector.
Compton interactions also contribute significantly to radiation dose to tissues, since the scattered photons and recoil electrons deposit energy in the patient.