The cardinal radiographic features of lobar collapse are increased opacity of the affected lobe and volume loss. The latter can be inferred by direct and indirect signs. Direct signs of volume loss refer to displacement of interlobar fissures, pulmonary vessels and bronchi, whereas indirect signs include compensatory shifts of adjacent structures such as hyperinflation of other lobes. The effects of a lobar collapse are often maximal on immediately adjacent structures; e.g. an upper lobe collapse often results in a shift of the superior mediastinum, whereas a lower lobe collapse often demonstrates elevation of the posterior part of the diaphragm in particular. However, the general principles and fundamental radiographic signs are similar for all lobes.
A collapsed lobe appears radiographically dense due to a combination of retained secretions or fluid within the lobe and reduction in aeration of the lobe. However, retained fluid is the dominant process, resulting in increased opacity of a partially collapsed lobe, as virtually complete collapse is required to displace sufficient air for the normally radiographically hyperlucent lung to appear dense.
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