If a screen-film system resolves 15 lp/mm, how many line pairs are visible within one millimeter?

• A. 10 line pairs per mm
• B. 12 line pairs per mm
• C. 15 line pairs per mm
• D. 20 line pairs per mm

Answer: (C)

Resolution in radiography is described as line pairs per millimeter (lp/mm), which tells how many dark-and-light line pairs fit into each millimeter. If the system resolves 15 lp/mm, that means in every millimeter you can distinguish 15 such line pairs. So within one millimeter you would see 15 line pairs.