Normal Pleura and Artifacts of Pleural Ultrasonography The normal pleura is composed of two membranes comprising the opposed visceral and

parietal layers, which are seen as a highly echogenic curvilinear structure [Fig. 1(A)].

Between these two layers there are small hypoechoic inhomogeneities. It is not always possible to visualize sonographically both layers and the hypoechoic space between them (5). The echogenic visceral pleura line moves during respiratory excursions.

This has been termed as the “lung sliding” sign (12). Sometimes, the parietal pleura is accompanied by a thin hypoechoic layer and nodular hypoechoic spreading, which represents subpleural fat (7).

At the interphase between the pleura and the ventilated lung tissue, intensive band-like reverberation echoes (comet-tail artifacts) are seen during the breathing movements (6,7) [Fig. 1(A)]. This artifact can be evoked only at the boundary between the visceral pleura and ventilated pulmonary alveoli. Mirror artifacts are commonly seen as duplication of structures external to the pleura projected over the lung because of total reflection of the sound waves at the pleural surface when the US beam strikes the lung surface at certain angles [Fig. 1(B)].

(A) (B)

Figure 1 Sonographic normal pleura and artifacts. (A) Normal pleura and comet-tail artifact.

Linear transverse US scan shows highly echogenic band of normal pleura opposed parietal and visceral pleura and interface reflection of aerated lung (arrows), which can be distinguished from the thoracic wall (TW). A vertical reverberation echo, comet-tail artifact (black arrow) evoked at the boundary between the visceral pleural and the ventilated lung is also seen. (B) Mirror artifact. Linear transverse US scan shows two echogenic bands, one is normal pleura (N) and the other one is a mirror image (M) of normal pleura in the lung. Duplication of structures outside the pleural space is projected into the lung due to total reflection of sound waves at the lung surface.

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IV. Pleural Effusion

The conventional chest radiographic findings of pleural fluid collections are variable and depend on the amount and the age of the fluid collection. These findings range from complete opacification of the hemithorax to less striking but puzzling areas of increased opacity, especially in patients with loculated empyema or associated peripheral pulmonary lesions (3). The presence of pleural fluid may not be identified with chest radiographs when there is extensive pulmonary consolidation or collapse. If there is any doubt whether a pleural effusion exists, US allows easy distinction of pleural fluid from increased opacity due to pulmonary parenchymal lesion (Fig. 2).

On US imaging through an intercostal approach, pleural fluid between the parietal and visceral pleurae is demarcated with a sharp echogenic line delineating the visceral pleura and lung. Sonography can also demonstrate subpulmonic effusion in patients with an apparently elevated hemidiaphragm on plain chest radiographs (Fig. 3), which is particularly useful to detect hemothorax in trauma patients, accurate, and significantly faster than supine and decubitus portable chest radiography (13).

At US, pleural fluid may be characterized as a simple or complicated nature of the fluid. Yang et al. (14) assessed the value of sonography in determining the nature of pleural effusions of various causes. A simple effusion appears as clear anechoic or cloudy hypoechoic fluid that may be transudates or exudates. Transudates are almost always echo-free, whereas about half of the exudates are echogenic (15). Most exudative pleural effusion is of infectious origin, which is known as parapneumonic effusion or empyema (16). Hypoechoic and echogenic fluid may contain diffusely distributed swirling or floating echogenicities that reflects particles in the fluid, for example, cells, protein, fibrin, or blood (5). Homogeneous echogenic effusions may be due to hemorrhagic effusion or empyema.

(A) (B)

Figure 2 Sonographic distinction of pleural fluid from pneumonia. (A) Chest radiograph shows an area of increased opacity in the left lower lobe that shows meniscus shape suggesting massive pleural effusion. (B) Linear longitudinal US scan through the intercostal space at the left lower lobe shows scanty amount of hypoechoic effusion (arrows), which is not feasible for aspiration. Most of the opacity shown on the radiography is the echogenic area of pneumonic consolidation (L) and the spleen (S).

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(A) (B)

Figure 3 Elevated hemidiaphragm due to extensive subpulmonic pleural effusion. (A) Chest radiograph shows the elevated right hemidiaphragm associated with pleural effusion along the major fissure. (B) Sector oblique longitudinal US scan shows extensive pleural effusion in the subpulmonic and lateral pleural space with collapse of the right lower lobe (L). The right hemidiaphragm (arrow) is not elevated but everted due to subpulmonic effusion.

As exudative fluid collections organize, mobile, linear structures of fibrin bands or to-and-fro motion of septa, which are typical for inflammatory effusions, tend to occur (17). This complicated effusion appears as septated or multiloculated, hypoechoic fluid.

In some empyemas, the septa are so profuse that they produce a honeycomb appearance (Fig. 4). The septated or multiloculated nature of pleural fluid may not be visible with computed tomography (CT). The lung could be captured by inflammation and may not slide up and down during the respiratory cycle and no clear demarcation is visualized

(A) (B)

Figure 4 Complicated pleural effusion with multiple loculi: (A) the pleural space is filled with profusely septated fluid, which has a honeycomb appearance; (B) extensive thickening of the parietal (P) and visceral (V) pleura encircles the thickened fluid, which is not amenable for aspiration.

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between the lung and pleural components (14). Sonographic findings of thickening of the parietal and visceral pleurae and associated parenchymal lesions in the lung are most likely indicative of empyemas. Septated fluids caused by fibrous strands are mainly observed not only in infected exudative fluid, but also in malignant effusion (18,19), and may rarely be found in patients with tuberculous pleurisy (20,21).

With advance of pleural fluid organization, extensive pleural thickening or fibrotho-rax may ensue. This appears as echogenic, solid-appearing pleural plaque with or without some loculation of fluid that is predictive of significant difficulties with thoracocentesis (3). US performed with a high-resolution transducer is sensitive in demonstrating the inter-nal derangement about the nature of pleural changes and provides detailed information to determine the planning of therapeutic approaches.

As pleural scars and thickening appear as echogenic rind of pleural plaque, the discrimination between echogenic pleural fluid and solid pleural thickening may be diffi-cult. It is important to determine by the nature of pleural changes whether thoracocentesis is feasible. Characterization of pleural changes with US is very informative in guiding thoracocentesis.

Sonographic differentiation between benign and malignant pleural fluid is possible only if solid nodular structures are visible (15,18). US detection of large, confluent pleural masses greater than 1 cm is indicative of malignant effusion (18). Pleural thickening may occur diffusely in both benign and malignant effusions. According to Leung et al. (22), parietal pleural thickening of more than 1 cm on CT was specific for malignancy in 94%. In benign exudative effusions, associated pleural thickening is usually of less than 1 cm and combined mostly with a lung parenchymal change. Akhan et al. (20) reported thickening of the pleura in one-third of their patients with nonmalignant pleural effusions, especially tuberculous effusions. In almost all cases, thickness was less than 1 cm and decreased with treatment within six months. Therefore, pleural thickening of more than 1 cm should arouse a high suspicion of malignancy.