ch 10 part two

X-ray Imaging Interactions

• Photodisintegration and Nuclear Interaction

  • High energy process; unlikely to occur in standard X-ray imaging.

  • Incoming X-ray photon interacts with the nucleus.

  • Can lead to absorption and release of nuclear fragments.

• Differential Absorption

  • Key concept for image formation.

  • Results from different interactions: Compton scattering and the photoelectric effect leading to varying absorption levels.

  • High X-ray absorption materials are termed radiopaque (e.g., bone - appears white on X-ray).

  • Low absorption materials are termed radiolucent (e.g., air in lungs - appears black on X-ray).

• Percentage of X-rays Reaching Image Receptor

  • Only 1% of delivered X-rays reach the image receptor.

  • Of that, 0.5% contribute to image creation.

  • Understanding KVP (kilovolt peak) is essential for managing image quality.

• KVP and Differential Absorption

  • Increasing differential absorption involves reducing KVP.

  • This is important for achieving good image contrast.

  • The energy of X-ray photons influences interaction outcomes with tissue.

Radiopaque vs Radiolucent Materials

• Definitions:

  • Radiopaque: Materials that absorb X-ray photons (appear white in images).

  • Radiolucent: Materials that allow X-ray photons to pass through (appear black in images).

• Examples:

  • Bone (radiopaque) absorbs X-rays, appears white.

  • Air in lungs (radiolucent) does not absorb X-rays, appears black.

Scatter and Image Quality

• Effects of Scatter:

  • Small Angle Scatter: Still reaches the image receptor, decreases quality.

  • Large Angle Scatter: Exits patient and may pose a risk to personnel, increases patient and occupational dose.

• Compton Scattering:

  • Independent of atomic number.

  • Lower energy interactions favor photoelectric effect (higher energy favors Compton).

Contrast in Imaging

• Contrast Agents:

  • Positive Contrast Agents (high atomic number): e.g., Barium and Iodine.

  • Negative Contrast Agents: e.g., Air.

  • Used to enhance visibility of structures not typically seen on standard X-rays.

Mass Density and Atomic Number

• Definitions:

  • Mass Density: Quantity of matter per unit volume, affects interaction probability.

  • Higher atomic number materials have more electrons, leading to better absorption (example: bone vs soft tissue).

Attenuation Explained

• Definition of Attenuation:

  • Reduction in X-ray beam intensity due to absorption and scattering.

  • Influenced by part thickness, tissue density, atomic number, and beam energy.

• Effects of Thickness:

  • Greater thickness results in increased attenuation due to more matter for X-rays to interact with.

• Effects of Density and Atomic Number:

  • Higher density materials (e.g., abdomen vs chest) attenuate more radiation.

  • Higher atomic number materials like bone attenuate more than soft tissue.

• Beam Energy Relation:

  • Higher KVP results in more energetic photons and therefore less attenuation (more penetration).

  • Compton interactions increase with higher KVP.

• Illustrative Example:

  • If 5% of an X-ray beam is transmitted, 95% is considered attenuated (either absorbed or scattered).

Summary of Key Concepts

• Important Points on Interactions:

  • Photoelectric absorption, coherent scattering, and Compton scattering impact beam intensity.

  • Understanding differential absorption is critical for effective imaging and patient safety.

  • Employing low KVP techniques can enhance image quality, specifically for soft tissue differentiation.