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)

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)