This patient suffered complete collapse of the right lower lobe. The major fissure is clearly displaced inferiorly, the atelectatic lobe projects as a flat opacity, and the right hemidiaphragm is “silhouetted” (see below). Indirect signs of atelectasis include a right shift of the mediastinum. Other indirect signs such as compensatory hyperinflation, elevation of the right hemidiaphram, and hilar shift are not seen here. This study is an excellent example of the “silhouette” sign. In a normal chest x-ray, the diaphragm and mediastinal structures are visible because of the difference in radiodensity between lung and these structures (i.e there is an “interface” between the tissues). The “silhouette” sign refers to a loss of normally appearing interfaces, implying opacification due to consolidation (most common), atelectasis, mass, etc., in adjacent lung. In this case, the collapsed right lower lobe “silhouettes” the right hemidiaphragm. Note that the right heart border, adjacent to the right middle lobe, is clearly visible.
