2. Radiation Interactions
Understanding how radiation interacts with matter is fundamental to imaging physics and radiation protection.
Learning Objectives
- Describe the photoelectric effect and its clinical significance
- Explain Compton scattering and its impact on image quality
- Understand pair production in PET imaging
- Apply knowledge to optimize imaging and minimize dose
Photoelectric Effect
The photoelectric effect occurs when a photon is completely absorbed by an inner-shell electron:
- Energy requirement: Photon energy > binding energy of electron
- Result: Ejection of photoelectron + characteristic X-rays
- Probability: Proportional to Z³/E³
- Clinical significance: Attenuation in dense tissues (bone), basis for contrast
Key Point: Photoelectric absorption is the dominant interaction at low energies (<100 keV) and in high-Z materials.
Compton Scattering
Compton scatter involves partial energy transfer between photon and outer-shell electron:
- Energy range: 100 keV - 2 MeV (most nuclear medicine energies)
- Result: Scattered photon + recoil electron
- Impact on imaging: Degrades contrast and resolution
- Minimization: Energy windows, scatter correction algorithms
Pair Production
Occurs when photon energy exceeds 1.022 MeV:
- Only relevant for PET (511 keV is below threshold, but high-energy therapy nuclides)
- Result: Electron-positron pair created
- Location: Near nucleus to conserve momentum
Attenuation
Linear attenuation coefficient (μ) describes overall photon absorption:
Half-Value Layer (HVL)
The thickness that reduces intensity by 50%:
Content under development: Attenuation correction, scatter correction algorithms