An update on bronchiolitis obliterans organizing pneumonia.

Learning objectives: To describe the diverse imaging patterns of bronchiolitis obliterans organising pneumonia (BOOP) as well as the less frequently recognised appearances that have recently been described. Background(s): BOOP was first described in the early eighties as a clinicopathological syndrome characterised by a subacute respiratory illness and histopathologically by the presence of granulation tissue in the bronchial lumen, alveolar ducts and alveoli. It represents a nonspecific pathological pattern of response to injury by the lung and may occur in isolation (cryptogenic organising pneumonia) or in association with a myriad of conditions such as infections and connective tissue diseases. Imaging findings: Typically BOOP manifests as consolidation and ground glass opacity predominantly in a subpleural or peribronchial distribution. This pattern however is seen in only 60% of cases. The consolidation may be more often seen in the mid to lower zones of the lungs and is usually bilateral. Ill-defined peribronchial nodules or larger mass-like lesions are occasionally observed. Linear and nodular opacities, bronchial dilatation and small bilateral pleural effusions may also be seen. In recent years, the perilobular pattern of opacity and the 'reversed halo' sign have been described. It has also recently been established that in many cases of BOOP, an interstitial fibrosis pattern is observed on follow up CT studies, often resembling non-specific interstitial pneumonia (NSIP). Conclusion(s): The imaging features of BOOP are many and varied. The exhibit illustrates established patterns as well as new findings of BOOP which have been described in recent years. The usefulness of these radiographic signs in diagnosing and predicting outcome of BOOP is explored.