In diagnostic radiology, X-ray photons interact with matter primarily through photoelectric absorption, Compton scatter, and (to a much lesser degree) coherent (Rayleigh) scatter. The likelihood of each process occurring depends on the photon energy (E) and the atomic number (Z) of the material.

Understanding how these interactions vary helps explain differences in image contrast, patient dose, and beam behaviour across imaging techniques.

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Dependence on Photon Energy and Atomic Number

Interaction	Energy Dependence	Atomic Number (Z) Dependence	Dominant Range (keV)	Description
Photoelectric	∝ 1/E³	∝ Z³	<50	Strong absorption in high-Z materials; basis of image contrast
Compton	∝ 1/E	Independent of Z (depends on electron density)	50–150	Dominant in soft tissue at typical diagnostic energies
Coherent (Rayleigh)	∝ 1/E²	∝ Z²	<10	Minor elastic scatter; negligible in diagnostic imaging

Interpretation:

At low energies and high-Z materials, photoelectric absorption dominates.
As photon energy increases, photoelectric effect decreases rapidly and Compton scatter becomes dominant.
The crossover point occurs at higher energies because the photoelectric effect remains more probable due to the Z³ dependence.
Coherent scatter contributes only at very low energies and is clinically insignificant.

Combined factors that influence overall attenuation

Factor	Effect on Attenuation	Explanation
Photon energy (kVp)	↑ Energy → ↓ Attenuation	High-energy photons are less likely to interact.
Atomic number (Z)	↑ Z → ↑ Attenuation (∝ Z³ for PE)	Higher electron density and binding energies.
Density (ρ)	↑ Density → ↑ Attenuation	More atoms per cm³ increase probability of interaction.
Thickness (x)	↑ Thickness → ↑ Attenuation (exponentially)	Longer path → more opportunities for interaction.
Filtration	↑ Filtration → ↓ Attenuation	Removes low-energy photons → beam hardening.

Impact on image contrast

Photoelectric absorption drives subject contrast, because its probability changes strongly with Z.
Compton scatter contributes mainly to image noise (non-differential scatter), minimal contribution to contrast because largely independent of Z.
As kVp increases:
- Overall attenuation decreases (beam more penetrating).
- Photoelectric probability drops faster than Compton.
- Image contrast therefore decreases.

This explains why:

Low kVp → higher contrast (photoelectric-dominant, higher dose per photon).
High kVp → lower contrast (Compton-dominant, lower dose per photon).

Why does increasing kVp decrease dose? That seems wrong

I see many people getting confused with kVp and dose (I often get myself in a twist when trying to explain it). There are two related reasons why dose decreases with increasing kVp. First is the equation for photoelectric effect probability, we know that increasing photon energy decreases the likelihood of attenuation and more photons are transmitted to the detector. X-ray systems try to maintain detector exposure. To do this the mA would need to be reduced. This is the second reason for reduced dose. This is simple on a per photon basis. I think we can all agree that an individual photon with higher energy will be less likely to impart dose.

However, where confusion can set in is looking at absolute number of photons. Surely an increase in kVp increases beam quantity (the total number of photons) which leads to more attenuation events. This is true. But, crucially this is a relative number game. Lets take a look at a very crude example.

Let’s say a 100 kVp beam has 100 photons in it. 50 are attenuated and 50 reach the detector.

Let’s now have a 115 kVp beam (this would have 200 photons in it at the same mA as above – 15% rule*). In this case ~70 photons are attenuated and ~130 are transmitted. The change in ratio of transmitted:attenuated is due to the 1/E³ relationship between the photoelectric effect and photon energy.

If we increase kVp we want to keep detector exposure the same.

To compensate we halve mA (15% rule)

Now the 115kVp beam has 100 photons. 35 will be attenuated and 65 transmitted.

Can you see how there are only 35 attenuation events (at 115 kVp) compared to the 50 attenuation events (at 100kVp). There’s less dose in the second example and higher detector exposure.

I hope that makes sense! It certainly helped me reason out why dose decreases with increasing kVp.

Key Takeaways and Exam Tips

The photoelectric effect dominates at low energies and high-Z materials.
Compton scatter dominates at higher energies and low-Z tissues.
Coherent scatter is negligible above 10 keV.
Image contrast arises from the balance between photoelectric absorption (contrast) and Compton scatter (noise).
kVp selection is a trade-off between contrast and dose.
Common exam question: “Describe how the probabilities of the photoelectric effect and Compton scatter vary with photon energy and atomic number, and explain how this affects image contrast.”

Up Next

Next, we’ll move on to Factors Affecting Attenuation and Contrast. We’ll link these interaction probabilities to real imaging parameters (kVp, Z, density, tissue thickness, and filtration) and explore how they influence image contrast and patient dose in clinical practice.