What unit is used to express spatial frequency in radiography?

• A. pixels
• B. lp/mm
• C. Hz
• D. mm

Answer: (B)

Spatial frequency describes how often a repeating pattern occurs within a given distance. In radiography, that is expressed as line pairs per millimeter, abbreviated lp/mm. A line pair means one dark line and one adjacent light line, so measuring how many such pairs fit into one millimeter directly quantifies how finely details can be resolved. Higher lp/mm indicates finer spatial detail that the imaging system can distinguish. The other options aren’t about how often patterns repeat per distance: pixels are image sampling elements, Hz is cycles per second (a temporal frequency), and millimeters are a unit of length, not frequency.