mAs is made up of two factors, the milliamps (mA) and the exposure time (seconds,).
mA stands for milliamps - This is the current created by the generator that ends up at the filament of the x-ray tube. When you increase the mA, you increase the current to the filament, which increases the number of electrons boiled off the filament. The result is an increase in the number of x-rays produced when they hit the target of the anode.
Exposure time - This controls the duration the x-rays are produced in an exposure. Time is in seconds. Depending on the machine, you may have to convert milliseconds to seconds in order to calculate mAs. In old machines, you may even need to convert fractions of seconds into seconds. For instance:
100 ms = 0.1 sec, 1000 ms = 1 sec.
1/2 sec = 0.5 sec
mAs is determined by multiplying mA and the time. mA x time = mAs
100 mA x 0.2 sec = 20 mAs
200 mA x 0.1 sec = 20 mAs
Both of the above will create the same number of x-ray photons.
Some control panels give you the option of choosing the mA and time separate. Some control panels only have the option of adjusting mAs. Some control panels allow you both options. This is something you will have to get familiar with when you begin taking x-rays at a facility as each machine may be different.
mAs determines the amount of x-rays that will be produced, or the QUANTITY of x-rays. The QUANTITY of x-rays produced has a very direct impact on the density, or darkness of an image. When you increase the number of x-rays produced, you will increase the number of x-rays entering the patient and the number of x-rays that make it to the image receptor. This can result in darker images and higher doses to the patient.
If you double the mAs on an x-ray system, you double the quantity of x-rays being produced. 40 mAs will create twice the x-rays of 20 mAs. In the image below, "A" has twice the mAs of "B". Increased mAs means darker images. Doubling mAs, also means doubling patient dose.
One important concept in taking x-rays is to use the shortest exposure time possible. For instance, (200mA x 0.2 sec = 40mAs) is the same as (400mA x 0.1sec = 40mAs), yet the second exposure would be the preferred one. Why is that? You can think of medical imaging in the same way that you can think of photography. In photography, if the exposure time is too long, you may end up with motion (blurry image). X-rays are the same way. If the exposure time is too long, you can end up with motion (blurry image).