Which of the following best describes 15 lp/mm in the context of screen-film mammography?

• A. A density value
• B. An exposure metric
• C. A measure of geometric distortion
• D. A measure of spatial resolution in line pairs per millimeter

Answer: (D)

Spatial resolution is about how well an imaging system can distinguish fine details. In radiography, this capability is quantified by spatial frequency, specifically how many line pairs per millimeter the system can resolve. A value of 15 line pairs per millimeter means the screen-film mammography setup can separate about 15 alternating dark and light lines within each millimeter, reflecting its ability to reproduce small structures such as microcalcifications or delicate tissue details. This directly describes how sharp and detailed the image can be.

Other concepts describe different things: density value relates to how dark the film becomes, tied to exposure and processing rather than detail; an exposure metric measures the amount of radiation used; geometric distortion concerns how accurately image geometry matches the true object geometry. None of these directly capture the system’s ability to resolve fine detail like line pairs per millimeter.