What units are used to express spatial frequency?

• A. Coulombs per kilogram (C/Kg)
• B. Line pairs per millimeter (lp/mm)
• C. Joles per kilogram (J/Kg)
• D. Lead lines per inch (LL/in)

Answer: (B)

Spatial frequency is about how often image details repeat per unit distance, reflecting how finely the image can resolve patterns. In radiography, this is quantified as line pairs per millimeter, because one line pair equals a complete cycle of a line and the adjacent space. A higher lp/mm value means finer detail and better resolution of the image. The other units listed don’t describe spatial detail: coulombs per kilogram is a dose measurement, joules per kilogram is energy per mass, and lead lines per inch isn’t the standard way to express spatial frequency in radiography.