Inhomogenuity of Densities
The following densities are homogenous and symmetrical:
Naturally, the cardiac shadow is more dense over the spine and descending aorta. Densities of the left and right pulmonary artery should be the same. If you see inhomogenuity, it suggests that there is an additional density at that site either in front , behind, or at that level. Similarly asymmetry in clavicular densities should suggest retroclavicular lung lesion.