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
Once X-rays are produced and leave the X-ray tube, they travel through the patient before reaching the detector.
During this passage, photons may be transmitted, absorbed, or scattered depending on their energy and the composition of the tissue they encounter. These interactions determine both the appearance of the image and the radiation dose received by the patient.
Understanding how X-rays interact with matter forms the foundation of diagnostic radiology.
Possible Photon Outcomes in Tissue
When an X-ray photon encounters matter, three main outcomes are possible:
- Transmission:
The photon passes through the tissue without interaction.
→ Reaches the detector and contributes to image formation. - Absorption:
The photon’s energy is completely absorbed by the tissue (photoelectric effect).
→ Contributes to image contrast but also to patient dose. - Scatter:
The photon changes direction after interacting with an electron (Compton or Rayleigh scatter).
→ Reduces image contrast and may contribute to staff exposure.
The relative proportion of these outcomes depends on photon energy and atomic composition of the material.
Factors Determining Interaction Type
Two physical factors determine how X-rays interact with matter:
- Photon energy (E):
- Lower energy photons are more likely to be absorbed (photoelectric effect) vs scattered.
- Higher energy photons are more likely to be scattered (Compton) vs absorbed.
- Atomic number (Z) of the absorber:
- High-Z materials (bone, iodine, barium) → greater proportion of photoelectric absorption.
- Low-Z materials (soft tissue, fat) → relatively more Compton scatter events compared to photoelectric absorption.
As photon energy increases, photoelectric probability drops rapidly, and Compton scattering becomes the dominant interaction.
These relationships can be confusing. When discussing interaction type, we are referring to the relative likelihood of one process compared with the others. As photon energy increases, the absolute number of both photoelectric and Compton interactions decreases (the beam is less attenuated overall).
However, the photoelectric effect decreases far more rapidly with increasing energy than Compton scatter does. Therefore, the proportion of Compton interactions (relative to photoelectric absorption) increases as photon energy rises.
Ultimately, the image detected represents the balance between transmitted photons (both primary and scattered) and absorbed photons. This is why, as we’ll explore later, increasing kVp tends to produce a lower-contrast image, as a greater fraction of the transmitted signal is composed of scattered photons.
Attenuation of the X-ray Beam
Keep in mind, this is an overview. We will discuss these concepts in more depth later.
As X-rays pass through tissue, their intensity decreases. A process called attenuation.
I = I0e−μx
Where:
- I0: incident intensity
- I: transmitted intensity
- μ: linear attenuation coefficient (cm⁻¹)
- x: tissue thickness (cm)
Attenuation is the combined effect of absorption (photoelectric) and scatter (Compton + Rayleigh). The rate of attenuation depends on tissue composition, thickness, and photon energy.
Brief summary of interactions
| Interaction | Energy Dependence | Z Dependence | Outcome | Clinical Impact |
|---|---|---|---|---|
| Photoelectric effect | ∝ 1/E³ | ∝ Z³ | Photon fully absorbed | High contrast, high dose |
| Compton scatter | ∝ 1/E | Independent of Z (dependent on electron density) | Photon scattered with energy loss | Low contrast, increased noise |
| Rayleigh scatter | ∝ 1/E² | ∝ Z² | Photon scattered elastically | Negligible in diagnostic imaging |
At typical diagnostic energies (60–100 keV), both photoelectric and Compton effects occur simultaneously, with Compton dominating in soft tissue and photoelectric in bone or contrast agents.
Key takeaways and exam tips:
- In the diagnostic range, the three key processes are photoelectric absorption, Compton scatter, and (minimally) Rayleigh scatter.
- Attenuation = absorption + scatter.
- Differential attenuation creates radiographic contrast.
- Photoelectric ∝ Z³ / E³ → dominant at low energies, responsible for contrast.
- Compton ∝ electron density → dominant at high energies, causes scatter.
- HVL is a practical measure of beam attenuation and quality.
- Common exam question: “Describe the main interactions between diagnostic X-rays and matter, and explain their relevance to image formation.”
Up next:
Next, Attenuation of X-rays. Let’s explore the mathematics of attenuation, the exponential attenuation law, the meaning of the linear and mass attenuation coefficients, and how these relate to beam quality, contrast, and HVL.