Radiation output from radiographic units

This is the average x-ray output (μGy/mAs at 40″) measured on ~40 radiographic units (fixed and portable units) over about 3 years for both large and small focal spots.

Radiation output (μGy/mAs) at 40
Radiation output (μGy/mAs) at 40″

Textbooks say the radiation output should be proportional to kV2. Fitting a power law curve to the data gives the equation

Output (μGy/mAs) = 0.008373*kV1.983

which is pretty darn close to the expected square law.

There’s not as much variation as might be expected. Armed with this data, I think I’d be comfortable calculating exposure estimates for patients or image receptors for any radiographic unit.