These shades of gray are explained by summation effects. In addition, discrete organs are visible only because they are surrounded by a more opaque or less opaque medium. In order to be visible objects have to be contrasted by a medium of different opacity. The summation effect results when parts of a patient or object in different planes are superimposed. The summation image represents the degree of x-ray absorption by all of the superimposed objects.
Summation can also occur with lucent structures, rendering a hyperlucent area.
Please understand that all radiographs are comprised of summation effects. The illustration below demonstrates the summation effect. The silhouette sign The silhouette sign occurs when the margins of 2 structures of the same radiopacity are in contact . Their margins cannot be distinguished . Conversely, if 2 structures of the same radiopacity are not in contact and are separated by a substance of a differing radiopacity, their borders can be distinguished.