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Coherent scattering (also called Rayleigh scattering or classical scattering) is an interaction where an incident low-energy X-ray photon (<10 keV) interacts with an atom as a whole.

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• The photon is deflected (changes direction) but retains its energy (no ionization, no energy loss).

![Illustrative summary of x-ray and γ-ray interactions. (A) Primary, unattenuated beam does not interact with material. (B) Photoelectric absorption results in total removal of incident x-ray photon with energy greater than binding energy of electron in its shell, with excess energy distributed to kinetic energy of photoelectron. (C) Rayleigh scattering is interaction with electron (or whole atom) in which no energy is exchanged and incident x-ray energy equals scattered x-ray energy with small angular change in direction. (D) Compton scattering interactions occur with essentially unbound electrons, with transfer of energy shared between recoil electron and scattered photon, with energy exchange described by Klein–Nishina formula.

Seibert JA, Boone JM. X-Ray Imaging Physics for Nuclear medicine Technologists. Part 2: X-Ray Interactions and Image Formation. Journal of Nuclear Medicine Technology. Published March 1, 2005.](attachment:9038dd57-0643-414e-b937-bffec2706b74:image.png)

Illustrative summary of x-ray and γ-ray interactions. (A) Primary, unattenuated beam does not interact with material. (B) Photoelectric absorption results in total removal of incident x-ray photon with energy greater than binding energy of electron in its shell, with excess energy distributed to kinetic energy of photoelectron. (C) Rayleigh scattering is interaction with electron (or whole atom) in which no energy is exchanged and incident x-ray energy equals scattered x-ray energy with small angular change in direction. (D) Compton scattering interactions occur with essentially unbound electrons, with transfer of energy shared between recoil electron and scattered photon, with energy exchange described by Klein–Nishina formula.

Seibert JA, Boone JM. X-Ray Imaging Physics for Nuclear medicine Technologists. Part 2: X-Ray Interactions and Image Formation. Journal of Nuclear Medicine Technology. Published March 1, 2005.

Mechanism

1. Incident photon interacts with the entire electron cloud of an atom.
2. Energy is insufficient to eject an electron (no ionization).
3. The atom absorbs the photon’s energy transiently → re-emits a photon of identical energy but in a different direction.
4. Net effect = change in direction only.

Characteristics

• Occurs at very low photon energies (<10 keV).
• Probability increases with:
  • Low photon energy (diagnostic range: negligible above 30 keV).
  • High atomic number (Z) (e.g., bone, contrast agents).
• No energy transfer → no patient dose contribution from coherent scattering itself.

Radiological Importance

• Minimal contribution to image formation in diagnostic radiology (accounts for ~5% or less of interactions).
• Contributes slightly to image noise / fog but not significant compared to Compton scatter.
• Becomes relevant in:
  • Mammography (low kVp, 20–30 keV range).
  • Soft tissue imaging with low energies.

Comparison with Other Scattering