On an upright chest radiograph, a free pleural effusion demonstrates a meniscus sign, which is a concave, upward-sloping interface with the lung that causes sharp or indistinct blunting of the costophrenic angle (Figs. 18-1 and 18-2). Approximately 200 mL of fluid usually is necessary to blunt the lateral costophrenic angle, although smaller amounts (>75 mL) can produce a meniscus that blunts the posterior costophrenic angle on the lateral view (Fig. 18-3). A lateral decubitus view of the chest is much more sensitive than the upright view in the detection of pleural effusion, and as little as 5 mL of fluid can be demonstrated on decubitus views (Fig. 18-4).

Figure 18-1 The frontal view shows a small pleural effusion on the left that causes blunting of the left costophrenic sulcus, producing a meniscus.

Figure 18-2 A, The frontal view shows a meniscus in the left costophrenic angle. The effusion extends upward along the left lateral chest wall. B, On the lateral view, the fluid extends anteriorly but reaches a higher level posteriorly than anteriorly.

Figure 18-3 A small pleural effusion is seen only on the lateral view. A, On the frontal posteroanterior radiograph, the costophrenic angle on the right is sharp. B, The lateral view shows minimal blunting of the right costophrenic angle (arrow).

Figure 18-4 A, The lateral view shows blunting of the costophrenic angle posteriorly (arrow). B, Left-side-down decubitus radiograph shows fluid layering along the left lateral chest wall (arrows).

In the upright position in the normal individual, a small amount of pleural fluid accumulates in a subpulmonic position. In certain individuals, a large amount of free-flowing pleural fluid may accumulate in this position before spilling into the costophrenic angles. On the frontal view, this produces a characteristic appearance with elevation of the apparent ipsilateral hemidiaphragm, flattening of the medial aspect, and displacement of the peak of the apparent diaphragm laterally. On the left side, this is easy to recognize because of separation of the stomach bubble from the apparent left hemidiaphragm (Fig. 18-5). A massive effusion produces a complete or nearly complete opacification of a hemithorax, with displacement of the mediastinum to the opposite side (Fig. 18-6). This appearance contrasts with complete atelectasis of the lung, in which the shift of the mediastinum is toward the side of the opaque hemithorax. Moderate to large amounts of pleural effusion may be missed on supine radiographs. These effusions layer posteriorly and produce a generalized increase in opacity of the hemithorax, through which the pulmonary vessels can be visualized (Fig. 18-7). There may be blunting of the costophrenic angle, and the fluid occasionally tracks over the apex of the lung, producing an apical cap.

Figure 18-5 Subpulmonic effusion. On the left, there is separation of the apparent hemidiaphragm from the stomach bubble (black arrow). There is also minimal blunting of the lateral costophrenic angle. On the right, a large effusion extends to the major fissure, subtending a lucent area that represents the superior segment of the right lower lobe (white arrow).

Figure 18-6 Massive effusion. There is a mediastinal shift to the opposite side and a completely opaque left hemithorax.

Figure 18-7 Bilateral effusions are seen with the patient in the supine position. Hazy opacification in both hemithoraces is caused by fluid layering posteriorly in the pleural spaces. The pulmonary vessels can be visualized through the fluid. Fluid is present laterally on the left (arrow) and in the minor fissure.

Fluid may occasionally accumulate within fissures, and these accumulations may produce the appearance of a mass or pseudotumor (Fig. 18-8). Differentiation from a mass can be easily made because the fluid is free and shifts on decubitus views.

Figure 18-8 Pseudotumor. Fluid is seen in the upper and lower portions of the major fissure on the right, and it extends into the minor fissure. Fluid in the fissure may have a tapered or spindle-shaped configuration, as demonstrated in the more cephalad collection.

Pleural effusions frequently loculate (Fig. 18-9); i.e., they do not shift freely in the pleural space because of adhesions between the visceral and parietal pleura. Loculation of fluid occurs in exudative effusions, particularly in patients with empyema or hemothorax.

Figure 18-9 Loculated effusion. CT shows a large collection of pleural fluid in an anterior, nondependent location (arrow). A lenticular collection of fluid lateral to the spine displaces the enhancing lung parenchyma of the left lower lobe.

Ultrasound

Ultrasound may be used to detect pleural abnormalities and to differentiate solid pleural masses from pleural effusions. However, ultrasound is most frequently used for severely ill patients and for patients in the intensive care unit because of the ready availability for bedside imaging. Ultrasound is useful for detecting the presence of pleural fluid and as a guide to aspiration.

Pleural fluid collections may be anechoic or echoic, and they may change shape during respiration. Most collections are anechoic (Fig. 18-10) and are delineated by an echogenic line of visceral pleura and lung. Anechoic effusions are usually transudates, whereas effusions that contain septations represent exudates in approximately 80% of cases (Fig. 18-11).

Figure 18-10 Anechoic pleural fluid collection. The sonogram shows a collapsed right lower lobe surrounded by a large pleural effusion.

From McLoud TC, Flower CD: Imaging of the pleura: sonography, CT and MR imaging. AJR Am J Roentgenol 156:1145-1153, 1991.

Figure 18-11 Empyema with multiple loculations. The transonic space is divided into multiple secondary loculations by curvilinear septa.

From McLoud TC, Flower CD: Imaging of the pleura: sonography, CT and MR imaging. AJR Am J Roentgenol 156:1145-1153, 1991.

Computed Tomography

Free-flowing pleural fluid produces a sickle-shaped opacity in the most dependent part of the thorax posteriorly on CT scanning (Fig. 18-12). CT allows very small amounts of pleural fluid to be detected. Loculated fluid collections are seen as lenticular opacities in fixed positions (see Fig. 18-9). CT is of limited value in differentiating transudates from exudates or in the diagnosis of chylous pleural effusions. Although most chylous effusions are indistinguishable from other causes of pleural effusion on CT, low attenuation consistent with fat or a fat-fluid level in the pleural collection is rarely seen. Acute pleural hemorrhage, however, can be identified by the presence of a fluid-fluid level or by increased attenuation of the pleural fluid collection (Fig. 18-13).

Figure 18-12 Contrast-enhanced CT shows bilateral pleural effusions; the right is greater than the left. Both have low attenuation and form sickle-shaped opacities posteriorly.

Figure 18-13 Hemothorax after a stab wound to left chest wall. Many areas of increased attenuation can be seen in the fluid collection (arrow). Notice the area of active extravasation of contrast (arrowhead) due to injury to left intercostal vessel.

Case courtesy of Justin Kung, MD, Beth Israel Deaconess Medical Center, Boston, MA.

Pleural fluid can be distinguished from ascites by several CT features (Box 18-3), including the displaced crus sign, the interface sign, the diaphragm sign, and the bare area sign. If the diaphragmatic crus is displaced away from the spine by an abnormal fluid collection, the fluid is located in the pleural space (Fig. 18-14). The location of ascites is lateral and anterior to the crus. The interface sign describes a sharp interface that can be identified between fluid and the liver or spleen when ascites is present. In cases of ascites, the interface is sharp, whereas in cases of pleural effusion, the interface is ill defined (Fig. 18-15). If the diaphragm is identifiable adjacent to an abnormal fluid collection in the right upper quadrant, the diaphragm sign is probably the most reliable means of differentiating fluid from ascites (Fig. 18-16). The location of the diaphragm is readily visible in patients with ascites but may not be identified in patients with pleural effusions. Pleural effusion is visualized outside the hemidiaphragm, whereas ascites is seen within the hemidiaphragmatic contour. The bare area is the portion of the right lobe of the liver that lacks peritoneal covering. Restriction of peritoneal fluid by the coronary ligaments from that area is another useful distinguishing sign. To differentiate pleural effusions from intra-abdominal fluid collections, all four signs should be assessed in each case. The attempt to differentiate ascites from pleural effusion may have pitfalls. A large pleural effusion, particularly on the left side, may cause inversion of the diaphragm, resulting in the pleural fluid being located centrally rather than peripherally.

Box 18-3 Computed Tomography: Pleural Fluid versus Ascites

Pleural Fluid	Ascites
Displaced crus
Ill-defined interface with liver and spleen	Sharp interface with liver or spleen
Fluid outside diaphragm contour	Fluid inside diaphragm contour
	Fluid excluded from bare area of liver