INTERACTION OF X-RAYS

RATIONALE:

X-ray interaction processes are very important subjects to study to gain a full understanding of the mechanisms that lead to X-ray attenuation. Subjects such as effective atomic number, Absorption edges, the relative importance of Compton and photoelectric attenuation, secondary electrons, properties of x- and gamma rays, and attenuation of x-rays by the patient conductor, which will require a deep comprehension of radiation.

1. Core Idea (Foundation)

X-ray photons interact with matter (especially the human body) in three possible ways:

  1. Transmission – passes through (forms image)

  2. Absorption – fully absorbed (adds dose)

  3. Scattering – deflected with reduced energy (degrades image + increases exposure)

👉 These interactions are random (stochastic) but predictable in large numbers.

2. Two MOST IMPORTANT INTERACTIONS (Boards’ focus)

⚡ A. Compton Scattering

  • A photon interacts with a loosely bound (outer shell) electron

  • Only partial energy transfer

  • Results in:

    • Recoil electron (kinetic energy)

    • Scattered photon (lower energy, new direction)

💡 Key Points:

  • Causes image fog → ↓ contrast

  • Main source of radiation exposure to staff

  • Happens in ALL materials (independent of Z)

  • Depends mainly on density

📌 Behavior:

  • ↑ Photon energy → more forward scatter

  • Large angle (≈180°) → low energy photon

  • Small angle → high energy photon

⚡ B. Photoelectric Effect

  • A photon interacts with a tightly bound (inner shell) electron

  • ALL energy is absorbed

  • Electron is ejected = photoelectron

💡 Key Points:

  • Responsible for image contrast

  • Contributes to patient dose

  • Depends heavily on:

    • Atomic number (Z³)

    • Energy (1/E³)

📌 Formula Concept:

  • KE = Photon energy − Binding energy

📌 Special Feature:

  • Produces characteristic radiation

  • Can cause fluorescence

3. Comparison (VERY IMPORTANT)

Feature

Photoelectric

Compton

Energy transfer

Complete

Partial

Image quality

↑ Contrast

↓ Contrast

Depends on Z

YES (Z³)

NO

Depends on density

YES

YES

Dominates at

Low energy

High energy

Radiation hazard

Patient

Staff

4. Attenuation (BIG CONCEPT)

💡 Definition:

Reduction of X-ray intensity due to:

  • Absorption

  • Scattering

📌 Exponential Law:

Ix=I0e−μxI_x = I_0 e^{-\mu x}Ix​=I0​e−μx

Where:

  • I0I_0I0​ = initial intensity

  • IxI_xIx​ = transmitted intensity

  • μ\muμ = linear attenuation coefficient

  • xxx = thickness

👉 Meaning: Intensity decreases exponentially as it passes through matter.

5. Factors Affecting Attenuation

  1. Atomic Number (Z)
    → Higher Z = more absorption
    → ∝ Z³

  2. Density (ρ)
    → More dense = more interactions

  3. Thickness (x)
    → Thicker = more attenuation

  4. Photon Energy (E)
    → Higher energy = more penetration (↓ attenuation)

6. Linear Attenuation Coefficient (μ)

  • Probability of interaction per unit length

  • Units: cm⁻¹

👉 High μ = strong absorber (e.g., lead)
👉 Low μ = weak absorber (e.g., air)

7. Half-Value Layer (HVL)

💡 Definition:

Thickness needed to reduce intensity by 50%

HVL=0.693μHVL = \frac{0.693}{\mu}HVL=μ0.693​

📌 Meaning:

  • Large HVL → more penetrating beam

  • Small HVL → easily absorbed

8. Photoelectric vs Compton Dominance

  • Low energy + high Z → Photoelectric dominates

  • High energy + low Z → Compton dominates

👉 In the body:

  • Soft tissue → Compton

  • Bone → both

  • Contrast media (iodine, barium) → Photoelectric

9. Effective Atomic Number (Zeff)

Used for:

  • Compounds (tissue, water)

👉 Helps predict:

  • Interaction probability

  • Image contrast

10. Secondary Electrons

Produced from:

  • Photoelectric effect → photoelectrons

  • Compton effect → recoil electrons

👉 These causes:

  • Ionization

  • Biological damage

11. Electron Interactions

  • Electrons lose energy via:

    • Ionization

    • Excitation

👉 Most radiation damage comes from:
➡ Electrons, not photons

12. Electron Range

  • Distance traveled before stopping

  • Depends on:

    • Energy (↑ energy = ↑ range)

    • Density (↑ density = ↓ range)

👉 In tissue: very short (mm range)

13. Linear Energy Transfer (LET)

  • Energy deposited per distance

👉 High LET:

  • More biological damage

👉 Low LET:

  • Less concentrated damage

14. Filtration (VERY CLINICAL)

💡 Purpose:

Remove low-energy photons

👉 Why?

  • They increase the dose

  • But don’t improve the image

📌 Result:

  • ↓ Patient dose

  • ↑ Beam quality (harder beam)

15. Effects of Filtration

  • ↑ Penetration (↑ HVL)

  • ↓ Intensity

  • Removes useless radiation

  • Improves safety

🧩 Big Picture (Clinical Insight)

  • Image formation = balance of
    👉 Photoelectric (contrast)
    👉 Compton (noise)

  • Radiation safety = control
    👉 Scatter
    👉 Filtration
    👉 Exposure factors

🧠 Memory Trick (Quick Recall)

👉 “PE = Picture Enhancer” (contrast)
👉 “CS = Contrast Spoiler” (scatter)