As well as determining the side and distribution of an abnormality it is often helpful to consider its anterior-posterior position. A lateral view may help, but 3D location may also be possible on a posterior-anterior (PA) view if you have a knowledge of chest X-ray anatomy and an understanding of the silhouette sign.
The silhouette sign
Normal adjacent anatomical structures of differing densities form a crisp contour or 'silhouette'. Loss of a specific contour can help determine the position of a disease process. This phenomenon is known as the silhouette sign.
The silhouette sign is a misnomer - it should really be called the 'loss of silhouette' sign.
For example, the heart (a soft tissue density structure - near white) lies adjacent to lung tissue (near air density - near black). A crisp contour or 'silhouette' is formed at the interface of these two tissue densities. Loss of clarity of the right heart contour (formed by the right atrium) implies disease of the right middle lobe which lies next to the right atrium. Loss of distinction of the left heart contour indicates an abnormality of the lingula (part of the left upper lobe which wraps over the left ventricle).
If you find a 'loss of silhouette' it may not be possible to determine the tissue from which the abnormality arises. Often the lung is the origin of disease but abnormalities of other structures such as the pleura, mediastinum, vessels or heart may mimic lung disease.
