Organizing pneumonia: typical and atypical HRCT patterns.

Organizing pneumonia (OP) is a clinical-pathological entity that, if idiopathic, makes part of the idiopathic interstitial pneumonias classification of the American Thoracic Society/European Respiratory Society (ATS/ERS) of 2002 (50% of the cases, called cryptogenic organizing pneumonia, or COP). In the remaining 50% it is secondary, representing one modality of response of the lung to various forms of injury. Typical computed tomography (CT) pattern has been widely described and consists of peripheral parenchymal consolidations with air bronchogram with or without surrounding ground-glass-like opacities. The purpose of this article is to describe the less frequent imaging pattern of this disease represented by single or multiple focal lesions (nodules or masses that place diagnostic problems with malignancy), bronchocentric pattern (parenchymal consolidations with peribronchovascular distribution), atoll sign (central area of ground-glass-like density and peripheral area of consolidation), nodular lesions (poorly defined micronodular pattern), linear and band-like opacities (subpleural linear opacities that can have disposition parallel or perpendicular in relation to the pleura), perilobular pattern (thickening of the interlobular septa with reticular pattern) and progressive fibrotic pattern (irregular thickening of the interlobular septa with associated ground-glass-like appearance and consolidations).