General principles

Abstract

The contrast in an X-ray film is caused by the varying absorption of X-rays by the material being irradiated. This absorption is dependent on the atomic number of the atoms present in the molecules, on the concentration of these molecules and on the thickness of the irradiated slice. In the case of chest X-rays, the bones, the airiferous lungs, the heart and other tissues provide adequate natural contrast. In other cases, however — e.g. in the abdomen-, the composition of the organs is so similar that the differences in absorption are too small; they are radiographically anonymous and must be made visible by additional measures (fig. 1).