[CT findings of peripheral atelectasis associated with pleural effusion: in association with the mechanism of rounded atelectasis formation].

The role of pleural effusion in the formation of rounded atelectasis (RA) was evaluated using CT in 159 patients with 210 pleural effusions. The forms of peripheral atelectasis associated with pleural effusion (PA-PE) were classified into five types: type 1: no atelectasis, type 2: flat or crescent type, type 3: convex type, type 4: mass-like type, type 5: consolidation type. Atelectasis (type 2-5) accounted for 78% (164/210) of all lesions. Type 2 was the commonest type of PA-PE, followed by type 5. Types 2 and 5 accounted for 79% (130/164) of all lesions with PA-PE. In most lesions of types 2 and 5, the collapsed pulmonary parenchyma re-expanded following decrease or disappearance of pleural effusion. Types 3 and 4 accounted for only 15% (25/164) and 5% (9/164), respectively. Most lesions of type 4 satisfied the criteria of CT findings of RA. Type 3 was similar to type 4 except for the obtuse angle between the collapsed pulmonary parenchyma and the pleura. As a consequence, it is reasonable to regard type 4 as RA, type 3 as the similar lesion of RA. Most lesions of types 3 and 4 were found in patients with small or moderate inflammatory exudate and pleural thickening, and most of them remained the same type in the follow-up studies. Three lesions of type 3 changed to type 4. This study showed that RA was mainly formed in the patients with inflammatory exudate and thickened pleura, rarely in the patients with transudate. It is concluded that inflammatory pleural effusion plays an important role in the formation of RA formation.