Milliampere-seconds (mAs)

Milliamperage-seconds more commonly known as mAs is a measure of radiation produced (milliamperage) over a set amount of time (seconds) via an x-ray tube. It directly influences the radiographic density, when all other factors are constant.

An increase in current (mA) results in a higher production of elections that are inside the X-ray tube which will, therefore, increase the quantity of radiation; more radiation will cause more photons reaching the detector and hence theapparent structural density will decrease, yet the signal intensity will increase. The The time factor (s) is a measure of the electrons production duration in the tube; meaning 's' prescribes how long mA will last.

For example:   

mA x T = mAs 600 mA x 0.1 sec. = 60 mAs

Increasing either the current or time will increase the quantity of radiation; therefore the amount of radiation in an examination is represented as mAs.

The reciprocity law 

The reciprocity law states that a reaction of a photogenic emulsion to light will be equal to the products of the intensity of that light and the time of the exposure ¹. This law pertains to mAs in the sense that all combinations of mA x T that amount to an equal quantity will produce the same amount of density.

50 mA x 1/5 sec. = 10 mAs is equal to 300 mA x 1/30 sec. = 10 mAs

It is due to this law that radiographers will have to take into consideration all other factors (mA, focal spot, SID, kVp) to reduce time to avoid motion blur.