Pulmonary gangrene and the air crescent sign.

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Abstract
A study was carried out to increase familiarity with the aetiology, pathogenesis, and radiographic features that characterise pulmonary gangrene. Four patients with one of the disorders vasoinvasive aspergillosis, infarcted tuberculous cavity, chronic necrotising aspergillosis, and gangrene due to Pseudomonas aeruginosa were selected because they showed the variations of the typical radiographic pattern and illustrated the pathogenesis. A fifth case is also presented, in which pulmonary gangrene was simulated by the invagination of a loculated pleural effusion into the wall of a contiguous lung abscess. Evolution of a crescent or rim of air within a homogeneous shadow is the feature that both heralds the development and facilitates the recognition of pulmonary gangrene. It is most often the result of vascular thrombosis induced by the infecting organism. The outcome of treatment is often unfavourable, principally because of the severity of the predisposing systemic or local underlying disorder, although a delay in diagnosis, possibly due to unfamiliarity with the radiographic pattern, may have contributed to the adverse outcome in some instances.