Lower lobe collapse due to pleural effusion: a CT analysis.

In the presence of a pleural effusion, volume loss is seen in the adjacent lung, primarily in the lower lobe. With a modest effusion a compliant lower lobe may show displacement and generalized volume loss without focal atelectasis, but more commonly segmental atelectasis does occur, typically in the posterior basal segment. Large effusions result in a major degree of lower lobe collapse. The collapsed lobe tends to be uplifted by the fluid and compressed toward the hilum with consequent stretching and attenuation of the inferior pulmonary ligament, rather than collapsing posteriorly and medially.