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 wich will, therefore, increase the quantity of radiation; more radiation will cause more photons reaching the detector and hence the density will increase. The time factor (s) is a measure of the electrons production duration in the tube; meaning 's' prescribes how long mA will last.
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.
- 1. Bunsen R. W. and H. E. Roscoe. "Photochemical Researches.--Part V. On the Measurement of the Chemical Action of Direct and Diffuse Sunlight.". Proceedings of the Royal Society of London 12, no. 0 (1862): 306-312. . doi:10.1098/rspl.1862.0069.
- 2. Bushong Stewart C. Radiologic science for technologists. St. Louis: Elsevier Mosby, 2013.