X-ray physics notes curriculum
Fundamentals of radiation
The X-ray machine
Production of X-rays
Interaction of radiation with matter (current module)
X-ray detection and image formation
Image quality
Radiation safety in X-ray imaging
Fluoroscopy
Mammography
Coherent scatter, also known as Rayleigh scatter, is a type of photon–atom interaction that occurs at very low photon energies (typically below 10 keV). Remember, with inherent and added filtration, photons <10keV rarely reach the patient.
It is an elastic scattering process, meaning the photon changes direction, but no energy is lost.
Although it contributes very little to image formation or patient dose in diagnostic radiology, it’s included here for completeness and exam awareness.
Simply put, coherent scatter occurs when an incident X-ray photon interacts with the entire atom (not a single electron). The photon’s electric field causes the atom’s electrons to oscillate briefly in phase, and the atom then re-emits a photon of identical energy but in a different direction.
Coherent scatter, also known as Rayleigh scatter, is a type of photon–atom interaction that occurs at very low photon energies (typically below 10 keV). Remember, with inherent and added filtration, photons <10keV rarely reach the patient.
It is an elastic scattering process, meaning the photon changes direction, but no energy is lost.
Although it contributes very little to image formation or patient dose in diagnostic radiology, it’s included here for completeness and exam awareness.
Simply put, coherent scatter occurs when an incident X-ray photon interacts with the entire atom (not a single electron). The photon’s electric field causes the atom’s electrons to oscillate briefly in phase, and the atom then re-emits a photon of identical energy but in a different direction.
Step-by-Step Mechanism
- Incident photon approaches atom
- Low-energy photon (<10 keV) interacts with all electrons collectively.
- The photon’s wavelength is relatively long compared with atomic dimensions.
- Temporary excitation
- The atom absorbs the photon’s energy momentarily, causing all its electrons to vibrate in phase.
- Re-emission of photon
- The atom immediately re-emits a photon of the same energy and wavelength, but at a different angle.
- Resulting photon
- Energy unchanged → no ionisation, no dose contribution.
- Only direction altered → a very small component of scattered radiation.
Rayleigh (coherent) scatter vs Compton scatter
| Feature | Coherent (Rayleigh) | Compton Scatter |
|---|---|---|
| Type of interaction | Elastic (no energy loss) | Inelastic (energy transfer) |
| Photon energy range | <10 keV | 30–150 keV |
| Photon energy change | None | Decreased (E′ < E) |
| Target | Entire atom | Single outer electron |
| Ionisation | No | Yes |
| Image effect | Minimal | Major (reduces contrast) |
| Dose contribution | Negligible | Moderate |
| Relative importance | <5% of total interactions in diagnostic imaging | Dominant in most imaging energies |
Key takeaways and exam tips:
- Coherent scatter = elastic interaction with the whole atom.
- Photon changes direction but not energy (no ionisation).
- Occurs at very low photon energies (<10 keV).
- Probability ∝ Z² and ∝ 1/E².
- Negligible contribution to image formation and dose at diagnostic energies.
- Common exam question: “What is coherent (Rayleigh) scatter and how does it differ from Compton scatter?”
Up Next
Next, we’ll move on to Comparing Interaction Probabilities, where we’ll bring together the photoelectric, Compton, and Rayleigh interactions to show how their relative importance changes with photon energy and atomic number. This is the key to understanding tissue contrast and image optimisation.