Understanding Interactions with Matter in Radiology

Photoelectric Absorption

• Interaction with matter that mainly contributes to patient dose.
• Patient absorbs the radiation.
• Important for imaging contrast; helps visualize structures.

Compton Scatter

• Responsible for occupational dose; leads to exposure of radiographers.
• Occurs when an X-ray photon interacts with a loosely bound outer shell electron.
• Results in ionization (ejected electron) and a scattered photon with less energy going in a different direction.
• Least desirable interaction as it decreases image quality by adding scatter noise.

Room Protocols

• Technologists must avoid being in the path of the primary beam.
• Use lead shields to protect against scatter radiation.

Contrast

• Refers to varying shades of gray in images.
• Low contrast shows more shades of gray; high contrast shows fewer shades (black and white).
• Low contrast is often more clinically useful for diagnostic purposes.

Coherent Scattering

• Involves low energy photons (< 10 keV).
• Two types: Thompson and Rayleigh scattering.
• Thompson: Excites a single outer shell electron, no ionization.
• Rayleigh: Excites multiple electrons, no ionization.
• Generally negligible in diagnostic imaging due to low energy photons being absorbed by the tube.

Pair Production

• High-energy photon interaction (≥ 1.02 MeV) with the nucleus generates a positron and an electron.
• Positron-electron annihilation generates two 0.51 MeV photons.
• Not significant in diagnostic imaging; relevant in PET scans.

Photo Disintegration

• Extremely high-energy photon interaction (≥ 10 MeV) absorbed by the nucleus.
• Produces nuclear fragments (protons, neutrons, alpha particles).
• Not relevant in diagnostic imaging.

Technical Factor Selection

• Only photoelectric absorption and Compton scattering significantly affect X-ray imaging.
• Increasing kVp decreases photoelectric interactions and increases Compton scattering:
• Higher kVp = higher energy = more chance to pass through matter.
• Photoelectric interactions occur at lower kVp due to insufficient energy to escape.

Influences on Compton Scattering

• More matter (thicker patients) leads to more Compton scatter; thus lower image contrast.
• Tissue density also affects scatter; denser tissues produce more scatter.
• The field size (collimation) affects the amount of matter exposed; larger fields produce more scatter.
• Higher kVp increases scatter and reduces total image contrast.

Reducing Compton Scatter

• Strategies include reducing part thickness and using optimal collimation.

Conclusion

• Understanding interactions with matter is crucial for effective imaging and patient safety.
• Key interactions to focus on for exams: photoelectric absorption, Compton scatter, coherent scatter, pair production, and photo disintegration.