Properties of CA

• Types of agents used in medical imaging:

  • Positive (+) Iodine:
  • Attenuation of tissue, blood, and outline spaces as a contrast medium
  • Properties: high density, suitable binding, and pharmacologically inert
  • Negative (-) Air:
  • Attenuation by occupying space as a contrast medium
• Physical properties related to subject contrast:

  • Subject contrast: ratio of transmitted radiation intensity between 2 adjacent body parts
  • Influenced by x-ray attenuating characteristics of subjects and the setup of the x-ray beam, including factors like tissue thickness and tissue atomic number
• Radiation quality (KVP):

  • Involves differential attenuation with increasing beam energy
  • Low kV results in high contrast as low-energy radiation is more easily attenuated, while high kV leads to low contrast with more transmission instead of attenuation
• Attenuation/absorption in medical imaging:

  • Reduction of intensity of the x-ray beam as it passes through matter
  • Interactions include Compton scatter and photoelectric absorption (PE), with the introduction of positive contrast enhancing PE
• PE interaction:

  • Incident x-ray interacts with inner electron, usually a K-shell electron, causing it to be ejected as a photoelectron
  • Strong when the incident photon energy is just slightly higher than the binding energy of the electron
• The K-edge effect:

  • X-ray absorption increases when photon energy matches the binding energy of a K-shell electron
  • Barium and Iodine have a K-edge in the middle of the spectrum, with differences more apparent at low energies around 80 KVP