High image contrast is most closely related to which type of x-ray interaction?

• A. the number of Compton interactions
• B. the number of scatter interactions
• C. the number of photoelectric interactions
• D. the number of pair production events

Answer: (C)

High image contrast comes from how much the X-ray beam is absorbed differently by different tissues. Photoelectric absorption is the key driver here because it increases rapidly with a tissue’s atomic number and density. This means bone (high Z) absorbs many photons compared with soft tissue, creating strong differences in attenuation that show up as sharp black-and-white contrast on the image. Scatter, from Compton interactions, adds photons to the image after they’ve changed direction, which tends to blur and reduce contrast rather than enhance it. Pair production isn't a factor at diagnostic energies. So the number of photoelectric interactions best explains why image contrast is high.