Tissue Interactions

X-Ray Beam Terminology

  • Primary Beam

    • Definition: The x-ray beam as it exits the x-ray tube.

  • Remnant Radiation

    • Definition: The beam as it exits the patient.
    • Other term: Exit Radiation.
    • Note: Remnant radiation is a charge vocabulary term and may appear repetitively in discussions.

Components of the X-Ray Tube

  • Anode

    • Definition: The specific part of the x-ray tube where x-rays are emitted.

  • Focal Spot

    • Definition: The area under bombardment by electrons (also referred to as the target).
    • Important Related Terms:
    • Focal Track
    • Focus

Structure and Materials

  • Tungsten and Rhenium

    • Tungsten is embedded in Rhenium.

  • Filaments

    • The typical x-ray tube contains two filaments (dual focus).

Filtration in X-Ray Tubes

  • Filtration

    • The tube itself has a standard of 2.5 mm of aluminum equivalent filtration.

  • Lead Housing

    • Purpose: Protects against radiation allowing only intended beams of x-rays to exit in the desired direction.
    • Filtration typically consists of aluminum, while lead is utilized for protection from radiation leakage.

Anode Heel Effect

  • Definition: Describes the variation in radiation intensity on either side of the x-ray beam due to the angle of the anode.
  • Key points:
    • More absorption occurs on the anode side than on the cathode side.
    • The beam's intensity is affected by the angle of the anode.
    • Specific anatomical region absorption:
    • Steeper angles result in higher absorption (heel effect).
  • Energy influence on attenuation:
    • Lower MA and KV settings lead to greater absorption due to weaker photons produced.

Attenuation and Photon Interactions

  • Attenuation

    • Definition: The process where the x-ray beam loses energy as it traverses through matter.
    • It begins at skin entrance exposure, leading to photon energy loss as it penetrates tissue.

  • Compton Scattering

    • Definition: Occurs when photons collide with outer shell electrons, transferring some energy.
    • Typical energy range: Predominates around 80 keV.
    • Key Elements:
    • Incident photon loses energy and changes direction.
    • The electron ejected from the atom is referred to as a Compton electron or recoil electron.
    • Further interactions:
    • Can also lead to backscatter interactions, where photons change direction after colliding with electrons.

Photoelectric Effect

  • Definition: An interaction where an incident photon is fully absorbed by an inner shell electron, leading to ionization.
  • Energy Requirement: The incident photon must exceed the binding energy of the inner shell electron.
  • Key Characteristics:
    • This energy must be slightly higher than the binding energy for the effect to occur (e.g., at 35 keV).
  • Secondary Effects:
    • Cascading effect of electrons may follow, leading to characteristic radiation.
    • This remains critical in diagnostic imaging to mitigate biological risks.

Summary of Photon Interaction Types

  1. Compton Scattering

    • Most significant scattering type in diagnostic radiology.
    • Predominantly occurs at about 80 keV.

  2. Photoelectric Absorption

    • Dominant up to around 35-70 keV.
    • Key for ionization and its subsequent biological impacts, especially in high atomic number areas like bones and contrast agents.

  3. Coherent Scattering

    • Occurs at low energy levels, typically under 10 keV.
    • The scattered photon changes direction without losing energy, primarily resulting in low-energy interactions.

Impact of Secondary Characteristic Radiation

  • Secondary characteristic radiation can result from the photoelectric effect.
  • Can lead to local absorption within the patient or exit if the radiation is weak.
  • Risk Assessment:
    • It includes understanding patient exposure, especially in sensitive areas (e.g., reproductive organs).
    • Historical epidemiological studies (like from Hiroshima) inform about potential long-term health risks from radiation exposure.

Conclusion

  • Understanding these various concepts of radiation is crucial for safe practice and patient care in radiography.
  • Students are encouraged to connect these concepts to prior knowledge and ask questions for clarification.
  • Review assignments are due in the upcoming weeks, emphasizing key terms and objectives covered recently.