What is the most important factor in the production of scattered radiation?

• A. Kilovoltage
• B. Part thickness
• C. Field size
• D. Tissue density

Answer: (C)

Scattered radiation arises from photons interacting with tissue inside the irradiated volume, mainly via Compton scattering. The total amount of scatter reaching the image receptor depends most strongly on how much tissue is irradiated. A larger field size means more tissue is exposed, creating more opportunities for scattering events and more scattered photons that can contribute to the image. While kilovoltage, tissue density, and part thickness influence scatter in other ways (energy of scatter, per-volume scatter, etc.), the overall quantity of scatter is driven primarily by the irradiated volume. Therefore, reducing the field size through collimation markedly decreases scatter and improves image contrast.