X-ray scatter

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Scatter radiation is radiation that has changed direction after interaction with matter (usually the patient) during an X-ray exposure.

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It has lower energy than the primary beam and contributes to image fogging, reduced contrast, and increased radiation dose to patient and staff.

![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.

Origin / Mechanisms

The main interaction in diagnostic energy range (30–150 keV):

Compton Scattering

Dominant in soft tissues at diagnostic energies.
X-ray photon interacts with outer-shell electron → ejects electron (Compton electron) → photon scattered in different direction with reduced energy.
Major source of scatter → occupational exposure + degraded image quality.

Coherent (Rayleigh) Scattering

Photon changes direction without energy loss.
Occurs at lower energies (<10 keV).
Contributes less to dose and image degradation.

Factors Affecting Scatter

Factor	Effect
kVp	Higher kVp → more scatter (higher photon penetration, more Compton interactions)
Field size / collimation	Larger field → more tissue irradiated → more scatter
Patient thickness	Thicker parts → more scatter
Tissue density / composition	Denser tissues produce more scatter
Distance	Scatter intensity decreases with distance (inverse square law)

Effects of Scatter

On image quality: