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.