Imaging of Pleural Disease

1

Describe the normal pleural anatomy and physiologic features.

The pleural space is a potential space that contains 2 to 10 mL of pleural fluid between the visceral and parietal pleural layers that essentially represents interstitial fluid from the parietal pleura (an ultrafiltrate of plasma). The pleural space is contiguous with the interlobar fissures of the lungs. The pleura is a thin, serous layer that covers the lungs (visceral pleura) and is reflected onto the chest wall and pericardium (parietal pleura). The visceral pleura is supplied by the pulmonary arterial system and drains into the pulmonary venous system, whereas the parietal pleura is supplied by the systemic arterial system and drains into the systemic venous system. Figure 21-1 shows the normal appearance of the pleura on chest radiography.

2

List the major tumors that may affect the pleura.

• Pleural metastasis

• Solitary fibrous tumor of the pleura

• Malignant pleural mesothelioma

• Pleural lipoma

• Pleural lymphoma

3

What are the major substances that may collect within the pleural space?

• Fluid: pleural effusion (plasma ultrafiltrate, chyle, bile, urine, gastrointestinal contents, or ascites)

• Blood: hemothorax

• Gas: pneumothorax

• Pus: empyema

• Cells: pleural tumor

• Fibrosis: fibrothorax

4

What is the differential diagnosis of major causes of pleural effusion?

For the answer, see Box 21-1 .

5

List the major mechanisms of pleural effusion formation.

• Increased hydrostatic pressure in the microvascular circulation (e.g., congestive heart failure).

• Decreased oncotic pressure in the microvascular circulation (e.g., hypoalbuminemia).

• Decreased pressure in the pleural space (e.g., with pulmonary atelectasis or restrictive lung disease).

• Increased permeability of the microvascular circulation (e.g., from pleural inflammation or neoplasm).

• Impaired lymphatic drainage from the pleural space caused by blockage in the lymphatic system.

• Transit of fluid from the peritoneal cavity through the diaphragm to the pleural space.

6

What are the two major types of pleural effusions?

Transudative pleural effusions are usually caused by increased systemic or pulmonary capillary pressure and decreased oncotic pressure, resulting in increased filtration and decreased absorption of pleural fluid. Major causes include cirrhosis, congestive heart failure, renal failure, and hypoalbuminemia. Protein levels are less than 3 g/dL.

Exudative pleural effusions occur when the pleural surface is damaged with associated capillary leak and increased permeability to protein or when there is decreased lymphatic drainage or decreased pleural pressure. Major causes include infection, noninfectious inflammatory disease, malignancy, recent surgery, and acute pulmonary embolism. Protein levels are greater than 3 g/dL, the pleural protein-to-serum protein ratio is greater than 0.5, and the pleural lactate dehydrogenase-to-serum lactate dehydrogenase ratio is greater than 0.6.

7

What are the major imaging findings of simple nonloculated pleural effusions on upright chest radiography?

The most common imaging finding of a pleural effusion is blunting of the lateral costophrenic sulcus on upright frontal chest radiography or blunting of the posterior costophrenic sulcus on upright lateral chest radiography. A pleural effusion usually has a sharply marginated, concave–upward curved border between the lung and pleural space, which is known as the “meniscus” sign. Because the posterior costophrenic angle is more dependent than the lateral costophrenic angle, smaller pleural effusions are more apparent on the lateral view (with >75 mL of fluid) than on the frontal view (with >200 mL of fluid). Moderate to large pleural effusions usually obscure the ipsilateral hemidiaphragm ( Figure 21-2, A ). Less common manifestations of pleural effusions include apparent elevation and medial flattening of the hemidiaphragm with lateral displacement of the diaphragmatic apex and an increase in distance (>2 cm) between the inferior surface of the lung and the gastric bubble on the left. The lateral decubitus view is the most sensitive radiographic view for detection of a pleural effusion and can detect 5 mL of pleural fluid.

8

Describe the major imaging findings related to simple nonloculated pleural effusions on portable supine and semi-upright chest radiography, computed tomography (CT), and magnetic resonance imaging (MRI).

A little-understood fact is that blunting of the costophrenic angles is a rare manifestation of pleural effusions on portable supine and semi-upright chest radiographs. On these views, pleural effusions most often appear as increased opacity of the hemithorax without obscuration of vascular markings or of the ipsilateral hemithorax because fluid layers in the posterior pleural space dependently behind the more anterior nondependent lung. If the patient is supine, this layering appears as a uniform haze of the ipsilateral hemithorax. On semi-upright radiographs, the density produced by the pleural effusion increases from superior to inferior because free-flowing pleural fluid falls dependently into the more inferior portions of the pleural space. Rarely, if the patient is imaged in the Trendelenburg position, the pleural fluid collects as a crescentic opacity over the lung apex, which is known as an apical cap. On CT, fluid attenuation is seen within the dependent aspect of the pleural space, often with a meniscus along its non-dependent aspect ( Figure 21-2, B ). On MRI, low signal intensity fluid on T1-weighted images and very high signal intensity fluid on T2-weighted images relative to skeletal muscle without enhancement are observed.

9

What are the major imaging findings associated with complex loculated pleural effusions?

Complex pleural effusions (often seen with exudative effusions) are often located in nondependent portions of the pleural space and do not shift freely in the pleural space on lateral decubitus chest radiography because of adhesions between the visceral and parietal pleurae. CT and MRI scans may also sometimes show associated pleural thickening and enhancement, internal septations, attenuation greater than that of water (i.e., >20 HU), increased signal intensity on T1-weighted images, or decreased signal intensity on T2-weighted images. Occasionally, loculated pleural fluid in the interlobar fissure may mimic a pseudomass on chest radiography, often appearing as a poorly marginated opacity on frontal chest radiography and as an elliptical opacity in the location of a major or minor interlobar fissure on lateral chest radiography.

10

List the differential diagnosis of major causes of hemothorax.

• Trauma (blunt or penetrating or iatrogenic) (most common cause)

• Bleeding diathesis (e.g., anticoagulation, protein S or C deficiency)

• Malignancy

• Infection

• Acute pulmonary embolism

• Thoracic aortic rupture (from aneurysm, pseudoaneurysm, dissection, penetrating aortic ulcer, or transection)