What is mAs in radiology?

Answer 1

mA (millamps) x Time (seconds) = mAs

mA Tube Current

The second control of the output of the x-ray tube is called the mA (milliamperage) control. This control determines how much current is allowed to flow through the filament which is the cathode side of the tube. If more current (and therefore more heating) is allowed to pass through the filament, more electrons will be available in the "space charge" for acceleration to the target and this will result in a greater flux of photons when the high voltage circuit is energized. The effect of the mA circuit is quite linear. If you want to double the number of "x" photons produced by the tube, you can do that by simply doubling the mA. Changing the number of photons produced will affect the blackness of the film but will not affect the film contrast.

S Time

The third control of the x-ray tube which is used for medical imaging is the exposure timer. This is usually denoted as an "S" (exposure time in seconds) and is combined with the mA control. The combined function is usually referred to as mAs or milliampere seconds so, if you wanted to give an exposure using 10 milliampere seconds you could use a 10 mA current with a 1.0 second exposure or a 20 mA current for a 0.5 second exposure or any combination of the two which would result in the number 10. Both of these factors and their combination affect the film in a linear way. That is, if you want to double film blackness you could just double the mAs.