The reciprocity law in radiography states that for a given total mAs, different combinations of mA and time yield the same image density under controlled conditions. When may it not hold?

• A. Under any circumstances; it always holds.
• B. Only when using a single exposure technique chart.
• C. In some detectors or in the presence of motion.
• D. When using the inverse square law.

Answer: (C)

Reciprocity means the image density you get depends on the total exposure (mA × time), not on how you split that exposure, as long as kVp, filtration, geometry, and the detector respond linearly and there’s no motion or other complicating factors. It holds when the detector’s response is linear and the subject is stationary, so different mA-time combinations that give the same mAs deposit the same amount of energy into the receptor.

It may not hold in some detectors or with motion because those conditions introduce nonlinearity or timing-related effects. Some detectors respond nonlinearly to dose rate or have dose-rate effects, so two exposure sets with the same mAs can produce different densities or noise characteristics. Motion during exposure also alters the signal captured in each area, causing blur and density differences that break the expected equivalence, even though the total mAs is the same.