Imaging FindingsThe appearance is that of a rounded subpleural opacity that forms acute angles with the pleura. It is usually seen in the lower lobes posteriorly, and there is almost always adjacent pleural thickening. As in other forms of atelectasis, there is volume loss in the affected lobe. There may be air bronchograms within the mass, giving the appearance of cavitation (pseudocavitation).
The aunt Minnie is the comet tail sign, which is formed by vessels and bronchi of the affected collapsed lung that are pulled in as the lung collapses.
TheoriesTwo main theories attempt to explain how round atelectasis happens after a pleural effusion. One posits that round atelectasis is caused by infolding the visceral pleura into and surrounding an area of atelectasis. This somehow causes the lung to curl on itself. Then fibrous adhesions form that suspend the atelectatic segment and usually tilt the lung cranially. When the effusion resolves, aerated lung fills in the space between the area of round atelectasis.
Another theory proposes that local pleural irritation (e.g., by asbestos) cause contraction and thickening of the pleura in the setting of a pleural effusion. The underlying lung then shrinks, and a round area of atelectasis develops.
The main differential diagnosis is bronchogenic carcinoma.