The milliampere-second (mAs) value is the primary exposure factor controlling the total quantity of X-rays produced during an imaging procedure. mAs stands for Milliamperage-seconds, representing the combination of electrical current flowing through the X-ray tube and the duration of the exposure. This combined value determines the overall darkness or lightness, known as density, of the resulting X-ray image. Calculating mAs precisely is necessary to ensure the final image is clear for diagnosis while limiting the patient’s radiation exposure.
Defining the Variables of X-ray Exposure
The mAs value is the product of two distinct, independent variables selected by the operator: Milliamperage (mA) and Time (s). Milliamperage (mA) measures the electrical current flowing through the X-ray tube’s filament. This current heats the filament, which determines the number of electrons available to produce X-rays. Essentially, the mA setting controls the rate at which X-ray photons are generated.
The second variable, Time (s), is the duration for which the Milliamperage current is applied to the X-ray tube. This factor is measured in seconds and determines how long the X-ray beam is active. Since the mA controls the rate of X-ray production, the time controls the total length of that production period. Because exposure times are often very short, the time setting is frequently displayed in milliseconds (ms) on the control panel. Any time value in milliseconds must be converted to seconds before it is used in the mAs calculation.
Step-by-Step mAs Calculation
Calculating the milliampere-seconds value involves a straightforward multiplication of the two component factors. The fundamental formula for this calculation is mAs = mA x s, where mA is the Milliamperage and s is the time in seconds. This calculation provides the total quantity of radiation used for the image.
The first step is always to ensure the time factor is correctly expressed in seconds. If the time is given in milliseconds (ms), it must be divided by 1,000 to convert it to seconds (s). For example, if an exposure uses a current of 200 mA and an exposure time of 0.5 seconds, the calculation is simple: 200 mA x 0.5 s = 100 mAs.
A slightly more complex scenario involves a time setting given in milliseconds, such as 400 mA and 150 ms. The conversion is necessary first: 150 ms / 1,000 = 0.15 s. The final calculation is then 400 mA x 0.15 s = 60 mAs.
Practical Application: Adjusting mAs for Image Control
The mAs value has a direct and proportional relationship with the radiographic exposure, which controls the image’s overall darkness or density. A higher mAs value means more total X-ray photons are produced, leading to a darker image, while a lower mAs value results in a lighter image. This relationship allows operators to control the visual quality of the image by manipulating the mA and time settings.
A principle known as the Law of Reciprocity states that any combination of mA and time that yields the same mAs product will produce the same total exposure or image density. For instance, 400 mA x 0.1 s and 200 mA x 0.2 s both result in 40 mAs and should produce images with the same darkness. This flexibility is used to select a shorter exposure time with a higher mA to minimize the chance of motion blur on the image.
The mAs value is also directly tied to the radiation dose received by the patient. Increasing the mAs necessarily increases the patient’s radiation exposure. Because of this, technical settings aim to use the lowest possible mAs that still produces a diagnostic-quality image.
A general clinical rule for adjusting image density is the requirement to change the mAs by at least 30% to produce a visible difference in the final image. Doubling the mAs will approximately double the image density, and halving the mAs will reduce the density by half. This rule guides the operator in making precise adjustments to the mAs when a repeat image is necessary due to under- or overexposure.