Rounded atelectasis

The mechanism of formation is not certain. One proposal is that passive atelectasis of lung occurs adjacent to a pleural effusion; fibrinous adhesions the develop between the adjacent visceral and parietal pleura and as the effusion resolves the atelectatic lung is trapped and becomes folded on itself. An alternative suggestion is that a sheet of pleural fibrosis develops which contracts as it matures, resulting in deformity of the adjacent lung.

The chest radiograph shows a mass-like lesion 27 cm in diameter which is peripherally sited adjacent to the pleura, most commonly in the posterior aspect of a lower lobe, or occasionally anteriorly in the middle lobe or lingula. The pleura is thickened adjacent to the lesion and there is often associated blunting of the costophrenic angle and volume loss of the affected hemithorax. The CT features are usually characteristic. There is a juxtapleural mass with associated pleural thickening and distorsion of adjacent vessels and bronchi producing a "comet tail" appearance as they lead towards the mass (Fig.1). The mass makes acute angles with the pleural surface, is usually well defined, except on the margin pointing towards the hilum and often contains air bronchograms (best visualized on high resolution CT). It enhances homogeneously after the administration of intravenous contrast medium. Round atelectasis usually remains static in size; however, both increase and decrease in size and even complete resolution may occur. On MR imaging the appearance are similar to those seen with CT; however, MR may show the densely fibrotic nature of the adjacent thickened pleura, differentiating this from tumour infiltration.

CF - HM