Milliampere second (mA or mAs) is the primary controlling factor of radiographic density, kVp also affects the radiographic density in a round about way. As the energy (kVp) of the stream of electrons in the x-ray tube increases, the more likely the x-ray photons created from those electrons will penetrate the cells of the body and reach the image receptor (film or plate), resulting in increased radiographic density (compared to lower energy beams that may be absorbed in the body on their way to the Image Receptor). However, "scatter radiation" also contributes to increased radiographic density; in that, the higher the kVp of the beam, the more scatter will be produced. Scatter is unwanted density (that is, density created without bringing any pertinent information to the image receptor). This is why kVp is not primarily used to control density - as the density resulting from increasing kVp passes what is needed to penetrate a part, it will only add useless information to the image.

Increasing mAs causes more photons (radiation) of the particular kVp energy, to be produced. This is helpful when larger parts are imaged, because they require more photons. The more photons you can get to pass through a particular tissue type (whose kVp is interacting at the cellular level) will result in a statistically increased amount of photons reaching the image receptor. The more photons that pass through a part, and reach the image receptor with pertinent information - the more useful the density is created on the resulting image. Conversely, lower mAs creates less photons, which will decrease density, but is helpful when you image smaller parts.

For example: If the intensity of a 70 kVp exposure at 20 mAs is 100 mR. What would be at 5 mAs? Answer 1/4 or 25 mR (intensity/mAs equation)

Factors Affecting the X-Ray Emission Spectrum[]

mA is the "Quantity of X-Rays - the amount of x-rays is influenced by many factors. It is the area under the curve for X-Ray emission spectrum. The larger the area the more x-rays. The chart below shows some of the factors that can affect the size and shape of the X-Ray Emission Spectrum. The higher the Emission Spectrum the more x-rays there are and the more the peak is to the right the better the quality of the x-rays (energy).

Effect of mA and mAs[]

A change in mA or mAs results in a proportional change in the amplitude of the x-ray emission spectrum at all energies. Looking at this diagram if you have a setting for 200 mA then switch to 400 mA the area under the curve doubles. Also, notice that the highest part of the graph does not move left or right, but stays in the same relative area.

The higher the mA, the higher the radiation dose to the patient. One of the factors that affect image quality is the number of x-rays reaching the film. Image contrast is the difference in the number of photons that get through the various parts of the body being imaged. The higher the kVp, the more photons that get through, resulting in less differentiation between tissues (contrast). The goal is to keep the mA as low as possible and the kVp as high as possible to achieve a compromise between the the number of photons reaching the film and optimum image (contrast).