Spark image

X ray image contrast

The third control of the X-ray tube that 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.

Remember, image contrast is controlled by the energy of the "X" photon beam. Therefore, high kV techniques result in low contrast images (the assumption is always made that the image will have approximately the same average film density so if kV is increased, there must be a compensation in mAs to keep film density constant). To increase image contrast in situations where there is low tissue contrast, a low kV, high mAs technique should be used. This is obvious for mammography but you should also remember this possibility for other special situations such as looking for low-density foreign bodies embedded in soft tissue. To improve film contrast for mammograms we would need to use a very low energy X-ray beam.

Mammograms are frequently done with beams in the 25 keV range. For the chest X-ray, we would like to use a low contrast technique that requires a relatively high-energy beam. Chest X-rays are frequently done with beam energies above 100 keV. You should understand that for similar film densities, the high keV technique usually results in lower patient radiation exposure. Think about this long enough to clearly understand why less radiation is absorbed in the patient when a high-energy beam is used.


© Keith Gibbs