![]() 5 Pitfalls in the determination of pleural effusions, regardless of patient positioning, include overlying soft tissue, atelectasis, extrapleural fat, pericardial fat, and pleural thickening. Supine positioning causes a layering pleural effusion to distribute posteriorly, causing hazy or veil-like opacification of the hemithorax with preservation of bronchovascular structures (Figure 4). When bilateral, subpulmonic effusions may be mistaken for hypoventilation. 4 Subpulmonic effusions may demonstrate lateralization of hemidiaphragm on frontal radiograph (Figure 3). On a lateral view, a subpulmonic effusion may have an appearance likened to the “Rock of Gibraltar” - an angulated contour of the lung base with the peak at the oblique fissure (Figure 2). Subpulmonic effusions between the dome of the diaphragm and basal lung are suggested on frontal projection by elevation and lateralization of the hemidiaphragmatic peak, a paucity of vessels below the diaphragmatic margin, and increased distance between the left lung base and gastric bubble (Figure 1). 1 When an effusion is only partially well-marginated on chest radiograph, this is known as the incomplete border sign. ![]() 2 Visceral-visceral pleural separation, such as widening of the oblique fissure on lateral projection, may also indicate a pleural effusion. Parietal-visceral pleural separation may manifest in other ways, such as apical capping or thickening of the paraspinal stripe. Absence of blunting does not negate an effusion. 1 Medial blunting on frontal view may be apparent. The classic and most obvious presentation of a small pleural effusion on a radiograph is blunting of the lateral or posterior costophrenic sulcus, a form of parietal-visceral pleural separation. As the volume of an effusion increases, its appearance on imaging 1 becomes more evident (Table 2). Pleural effusions result from abnormal buildup of a thin layer of liquid that normally helps adhere and lubricate the interface between visceral and parietal pleura. In addition, we will discuss the imaging features of complex pleural-parenchymal abnormalities, with special consideration of bronchopleural fistulas, unexpandable lung, and post-pneumonectomy complications. ![]() We will review multimodality-imaging features used to establish the presence and cause of the various types of pleural pathology. Calcification or other high attenuation within the pleural space is commonly due to asbestos exposure, chemical pleurodesis, and remote trauma or infection. Pleural thickening may be focal or diffuse, benign or malignant, with characteristic imaging features that can narrow the differential diagnosis (Table 1). Similarly, gas in the pleural space, or a pneumothorax, may have both classic and elusive imaging findings, often necessitating distinction from other entities. Liquid in the pleural space, or pleural effusions, may be made conspicuous on chest radiographs by size or patient positioning, but identifying subtle findings may allow diagnosis of an effusion and its etiology. Pleural pathology varies widely and may manifest with one or more of the following: intrapleural liquid or gas, pleural thickening, and high attenuation.
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