Milliamperage

mAs

Milliamperage per second (mAs) is a combination of mA and time (s). When trying to maintain density, an inverse relationship exists between mA and time; when time increases, mA decreases and when mA decreases, time increases. Milliamperage is a measurement of the number of electrons traveling from cathode to anode during the exposure. Milliamperage is also known as tube current. Time is a measurement of how long the x-rays are produced; the length of exposure. It is directly proportional to the number of electrons crossing the tube and is therefore directly proportional to the number of x-rays created.

30% Rule

A 30% change in mAs is needed to produce a barely noticeable difference in density. As a general rule, when only the mAs is changed, it should be halved or doubled (a 50% change). If such a change is not required, another exposure is not necessary.

The Law of Reciprocity

The Law of Reciprocity is the principle stating that optical density on a radiograph is proportional only to the total energy imparted to the radiograph. In other words, any combination of mA and time that will produce a given mAs will produce an identical radiographic density. Simply stated, as long as the mAs is the same, the density will be the same.

Patient Dose

Patient dose and mAs are directly related to each other; when mAs increases, patient dose increases, and when mAs decreases, patient dose decreases. Why does an increase in mAs increase patient dose? mAs is directly proportional to the number of electrons crossing the tube . As the number of electrons increases, the number of x-rays created directly increases. The number of x-rays created, the more grays (rads) given to the patient and thus a higher dose to the patient.