In practice, screen-film mammography resolution of about 15 lp/mm means that approximately how many line pairs are visible per millimeter?

• A. 15 lp/mm
• B. 5 lp/mm
• C. 25 lp/mm
• D. 50 lp/mm

Answer: (A)

Line pairs per millimeter measure how many alternating dark and light stripes a system can resolve in one millimeter. Each line pair is one dark line plus the adjacent light space. So if the system resolves about 15 line pairs per millimeter, you can distinguish 15 such dark-light cycles within each millimeter. In other words, the visible line pairs per millimeter is 15. If you counted individual lines, that would be about 30 lines per millimeter, but the given measure is in line pairs. This reflects a fairly high level of detail for mammography, where higher lp/mm means finer detail can be seen.