What is coherent (Rayleigh) scattering?
Coherent scattering, also known as Rayleigh scattering, is an interaction in which an X-ray photon interacts with an atom and is scattered without loss of energy. The photon changes direction but retains the same energy after the interaction.
Coherent scattering is a low-probability interaction in which an X-ray photon is deflected without losing energy or causing ionisation.
This process does not cause ionisation and contributes only minimally to attenuation in diagnostic radiology.
Understanding the physics
In coherent scattering, the incoming X-ray photon interacts with the entire atom rather than with an individual electron. The electric field of the photon briefly distorts the electron cloud surrounding the atom, causing it to oscillate.
This oscillation results in the emission of a new photon with the same energy and wavelength as the original photon, but travelling in a different direction. Because no energy is transferred to the atom, the photon energy remains unchanged and no electrons are ejected.
The probability of coherent scattering is relatively low at the photon energies typically used in diagnostic radiography. It occurs more frequently at lower photon energies and in materials with higher atomic numbers, but even under these conditions it contributes only a small fraction of total photon interactions.
As photon energy increases, coherent scattering becomes much less likely compared with Compton scattering and the photoelectric effect.
Where this matters clinically
Coherent scattering contributes only a small proportion of interactions in diagnostic imaging and generally has little impact on image formation. However, because the photon is deflected from its original path, it can contribute slightly to scattered radiation reaching the detector.
In most diagnostic energy ranges, coherent scattering accounts for only a small percentage of interactions, and its contribution to image degradation is much less significant than that of Compton scatter.