X-Ray Interactions in Radiology

Overview of X-Ray Interactions

• Key interactions: Photoelectric Effect, Compton Scattering, and Coherent Scattering.
• Photoelectric: Main contributor to image contrast.
• Compton: Leads to artifacts in images.
• Coherent: Minimal impact on most x-ray/CT procedures.

Physics Behind X-Ray Interactions

• X-rays interact with the body to form diagnostic images.
• Two dominant interactions: Photoelectric Effect and Compton Scattering.
• Understanding these effects is crucial for selecting appropriate technical parameters.

The Photoelectric Effect

• Dominant in creating image signals by locally depositing x-ray energy.
• Occurs when x-ray is absorbed, transferring energy to an electron which is ejected.
• Inner shell electrons have higher interaction likelihood, especially in materials with high atomic number (Z).
• More likely at lower x-ray energies (1/E^3).

Key Points:

• Better contrast with high Z elements.
• Lower energy photons often preferred for imaging tasks.

Compton Scattering

• Second major effect, involves x-ray interacting with outer shell electrons.
• Interaction likelihood is independent of Z.
• Scattered photon retains energy, can cause further scatter or be detected.

Key Points:

• X-ray scatters with an electron, both continue in opposing directions.

Coherent (Classical) Scatter

• Also known as Elastic or Rayleigh Scattering.
• X-ray interacts with electron cloud and scatters without energy loss.
• Occurs at very low energies (<10keV), minimal impact in typical diagnostic imaging.

Key Points:

• Less relevant due to low occurrence in diagnostic energy spectra.

Energy Dependence of Interactions

• Photoelectric effect and Compton scattering vary with energy and material.
• Photoelectric effect dominant at low energies; Compton at higher energies.
• In water, photoelectric is dominant up to 26 keV; in bone, up to 45 keV.

Key Points:

• Transition between effects is at higher energy for bones due to high Z elements.

Summary Table

• Coherent Scatter:
  • Products: Scattered x-ray (same energy).
  • Impact: Minimal.
• Photoelectric Effect:
  • Products: Electron, characteristic x-ray (low energy).
  • Impact: Primary contrast source.
• Compton Scatter:
  • Products: Electron, scattered photon (lower energy).
  • Impact: Background haze.

Impact on Image and Dose

• Image Contrast:
  • Photoelectric provides primary contrast.
  • Compton can lead to artifacts.
• Patient Dose:
  • Photoelectric and Compton significantly contribute to patient dose due to energy deposition.
• Staff Dose:
  • Compton is a significant source of stray radiation in the room.

Study Tips

• Use the summary table to create flashcards.
• Test your understanding of how interactions affect dose and contrast in imaging.