Radiation Interactions with Matter

Photon Interaction with Matter

• Creation of photons occurs at the target which interact with the patient.
• The two processes involved are attenuation and absorption.
• Attenuation: Loss of primary photons due to absorption or scatter.
• Absorption: When a photon is absorbed, resulting in complete energy transfer to matter.

Understanding Attenuation

• As X-ray beams travel through matter, they undergo attenuation.
• Distinction between attenuation and absorption:
• Attenuation involves loss of photons through either absorption or scattering.
• Absorption denotes a complete transfer of energy, resulting in the photon no longer existing.

Interactions During X-ray Transmission

• Possible interactions with matter:

1. Absorption - photon is absorbed: no longer exists.
2. Scatter - photon changes direction but continues to exist.
3. Remnant Radiation - photons that interact with the image receptor (IR).
   • Terms for Remnant Radiation: remnant, exit, transit.

Interaction Mechanisms

• Primary interactions relevant for diagnostic imaging are:

1. Photoelectric Absorption
2. Compton Scattering

• Additional interactions include:
• Coherent Scattering: involves low energy photons; effects often absorbed before reaching the patient.
• Pair Production and Photodisintegration: primarily occur at high energy; not typically relevant in diagnostic radiology.

Atomic Structure Review

• Importance of understanding atomic structure in relation to X-ray interactions.
• Positive nucleus (protons and neutrons).
• Negative electrons in shells around the nucleus.
• Z Number: refers to the atomic number (number of protons).
• Changes in atomic structure:
• Losing an electron results in an ion.
• Changing the number of neutrons results in an isotope.
• Changing the number of protons results in a new element.

Electron Shells and Their Behavior

• K, L, M, N shells.
• Formula for maximum electrons in a shell: 2n^2, where n = shell number.
• Binding energy: energy required to remove an electron from a shell. Higher binding energy implies tighter binding of the electron.

Interactivity of Photons with Electrons

• Interactions mainly with orbital electrons.

• Photoelectric Absorption occurs when an incident photon interacts with an inner shell electron, resulting in ionization.

  • The ejected electron is referred to as the photoelectron.
  • Energy dynamics: E{ ext{incident}} = E{ ext{binding}} + E_{ ext{kinetic}}, where
    • E_{ ext{incident}} is the incident photon energy,
    • E_{ ext{binding}} is the inner shell binding energy, and
    • E_{ ext{kinetic}} is the kinetic energy of the ejected photoelectron.

• Compton Scattering involves interactions with outer shell electrons leading to lower energy scattered photons. This phenomenon increases occupational exposure and is responsible for radiation fog.

Dosage and Safety Considerations

• Photoelectric absorption contributes to patient dose, while Compton scattering relates to occupational dose.
• The relationship between photon energy and binding energy influences interaction likelihood:
• Higher photon energies decrease the likelihood of photoelectric interactions.
• Practical implications for radiographic procedures, including the principle of using the lowest necessary kVp to reduce patient dose while maintaining image quality.

Reducing Scatter and Enhancing Image Quality

• Scatter does not contribute to diagnostic information and can degrade image quality, hence minimizing scatter is essential.
• Grids can absorb scatter and help improve image quality by increasing contrast without adding extra exposure to the patient.

Important Interaction Dynamics

• Summary of interactions:
• Photoelectric Absorption | Inner shell electrons | Significant in dense tissues like bone | Causes ionization | Results in secondary radiation
• Compton Scattering | Outer shell electrons | Significant in soft tissues | Results in scatter radiation | Responsible for occupational exposure

Conclusion

• Knowledge of these interactions is crucial for effective practice in diagnostic imaging and radiation safety, ensuring effective patient care while minimizing risks associated with exposure.