Which unit is used to express spatial frequency in radiography?

• A. Cycles per centimeter (cyc/cm)
• B. Line pairs per millimeter (lp/mm)
• C. Hertz (Hz)
• D. Counts per second

Answer: (B)

Spatial frequency describes how often a pattern repeats per unit length in an image, which reflects how fine a detail the system can reproduce. In radiography, this is expressed as line pairs per millimeter, where one line pair equals a dark line and the adjacent light space. Counting how many of these cycles fit into a millimeter directly measures the image’s resolving power—the more line pairs per millimeter, the higher the spatial resolution. Hertz would be cycles per second, which applies to time-based frequency rather than spatial detail, and counts per second relates to exposure rate, not image resolution. Cycles per centimeter is a similar concept but is not the standard unit used in radiography; millimeters are the preferred unit for describing image detail. Hence, line pairs per millimeter is the appropriate unit.