Line pairs per millimeter is the unit of which imaging metric?

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Line pairs per millimeter is a measure of spatial resolution—the ability of an imaging system to separate and display small, closely spaced details. Think of a bar pattern: a higher number of line pairs in each millimeter means the system can distinguish more alternating black and white lines in a given space, so finer detail appears as distinct lines rather than blur. As lp/mm goes up, the detail the image can resolve becomes sharper, which is why this unit is tied directly to resolution.

Contrast, on the other hand, describes differences in brightness between areas, and signal-to-noise ratio is about how strong the signal is compared with background noise. Image brightness relates to luminance. None of those use line pairs per millimeter as their unit, which is why lp/mm is specifically the language of spatial resolution.

In mammography, high spatial resolution is essential to detect tiny features like microcalcifications. Resolution tests and phantoms use bar patterns to express how finely details can be resolved, often analyzed in terms of lp/mm to gauge the system’s ability to reveal small structures. Related concepts like modulation transfer function describe how well contrast is preserved at different spatial frequencies, but the explicit unit for resolving detail remains line pairs per millimeter.