Empyema is inflammatory fluid and debris in the pleural space. It results from an untreated pleural space infection that progresses from free-flowing pleural fluid to a complex collection in the pleural space. The diagnosis is made when the pleural fluid is grossly purulent, when organisms are identified on the basis of Gram stain or culture, or when the fluid has a white blood cell count greater than 5  ×  10⁹ cells/l (Light 1990). Radiographic findings are almost always the same as for any pleural collection, but certain CT findings are suggestive of empyema. The typical empyema is lenticular. Nonenhanced CT scans can demonstrate atypical pleural effusions along the mediastinum, thickened pleurae, loculations in the fissures, septa, or gas bubbles in the pleural space. Gas bubbles in the pleural space strongly suggest an empyema in the proper clinical context (i.e., in the absence of recent thoracentesis). Lung windows can demonstrate pneumonia adjacent to the abnormal pleural collection. With most empyemas, CT scans after intravenous injection of contrast medium demonstrate enhancement of the parietal and visceral pleura, the so-called split pleura (Fig. 17.5), thickening of the extrapleural subcostal tissues and increased attenuation of the extrapleural fat (Williford and Godwin 1983). It is necessary to perform a differential diagnosis with an abscess when the collection is peripheral: an empyema generally has a lenticular shape with obtuse margins toward the chest wall, whereas a peripheral lung abscess usually has a spherical shape and forms acute angles with the chest wall. An empyema has thin, smooth walls along its inner margin; lung abscess usually has thick, irregular walls, especially internally. Finally, the adjacent lung is frequently compressed by an empyema, resulting in displacement of peripheral pulmonary vessels and bronchi, while the parenchyma surrounding a lung abscess is often infected, and the pulmonary vessels and bronchi extend directly toward the periphery of the lesion. Empyemas and associated parapneumonic effusions with pH below 7.0 or pleural fluid glucose level less than 40 mg/dl must be treated with a chest tube, inserted surgically or by image guidance with sonography or CT (McLoud and Flower 1991a). The success rates of imaging-guided drainage ranges from 70 to 90 % and are higher than those for surgical tubes guided only by the findings on the chest radiograph (Silverman et al. 1988).

Pneumothorax is defined as the presence of air or gas in the pleural cavity. Spontaneous pneumothorax is very rare in the elderly population but more frequent in young men; it generally occurs in people without underlying lung disease and in the absence of an inciting event. However, many patients, whose condition is labeled as primary spontaneous pneumothorax, have subclinical lung disease, such as pleural blebs, that can be detected by CT scanning. Pneumothorax secondary to pulmonary pathology is most common in elderly patients with chronic obstructive pulmonary disease, asthma, or chronic infiltrative lung disease (including idiopathic pulmonary fibrosis, sarcoidosis, eosinophilic granuloma, lymphangioleiomyomatosis). Other causes are radiation pneumonitis, silicosis, and tuberous sclerosis.

Pleural plaques constitute the most frequent benign pleural pathology in asbestos-exposed patients and are radiologically detectable in 20–60 % of workers exposed to high concentrations of asbestos. A history of asbestos exposure may be demonstrated in over 80 % of subjects with bilateral pleural plaques. Pleural plaques represent circumscribed collections of hyalinized collagen in the submesothelial layer of the parietal pleura of chest wall or diaphragm; they rarely affect mediastinal pleura. Pleural plaques almost always involve the parietal pleura alone, but occasionally they may be seen in the visceral pleura and in the interlobular fissures. They generally appear 15–20 years after the initial exposure. The pathogenesis of asbestos-related pleural plaques is unknown; it is presumed that asbestos fibers penetrate the visceral pleura and irritate the parietal pleura, inciting collagen deposition. The thickness of plaques can vary from 1 mm to more than 10 mm. Plaques are thicker over the ribs than over the intercostal spaces.

Fibrothorax, almost always due to previous empyema, is easily detected as a thick, calcified pleural rind on both plain radiographs. Calcified fibrothorax in older patients is particularly likely to be tuberculous in origin. The list of possibilities can be narrowed down given a history of therapy for tuberculosis, although such a history is frequently lacking (Hulnick et al. 1983). Rarely, tuberculous pleurisy progresses to become chronic tuberculous empyema, which may be defined as persistent, grossly purulent pleural fluid containing numerous tubercle bacilli. Since the advent of chemotherapy, this is usually seen secondary to incomplete medical treatment, which sometimes leads to the development of resistant organisms (Gallardo et al. 2000). Tuberculous empyema is often asymptomatic and sometimes does not become apparent until many years after the episode of acute pleurisy. During this period, tubercle bacilli remain dormant within the calcified pleural space and are later reactivated. CT, which differentiates fluids from soft tissues and calcifications, can identify collections of pleural fluid within a calcified fibrothorax. These findings are the most accurate indicator of active infection. In addition, CT can be of value in diagnosing complications that may arise from chronic tuberculous empyema, specifically bronchopleural fistula (BPF) and empyema necessitatis (EN). Although BPF is decreasing in frequency, it is still a serious complication of pulmonary resection and inflammatory diseases of the lungs. Tuberculous residual derangement in pulmonary or pleural architecture, spontaneous or iatrogenic, predisposes to the development of BPF. Fibrothorax appears to be the most common source of late BPF formation, while, during the active disease, rupture of a pulmonary cavity in the pleural space is more common. BPF may present clinically in various forms, ranging from incidental radiographic discovery to life-threatening acute pneumothorax. Radiographically, BPF usually presents as a new pleural air-fluid level, and sometimes with a tension pneumothorax. Diagnosis is often made by the correlation of sequential changes in chest radiographs. The presence or absence of an active infection is crucial in selecting appropriate medical treatment. The introduction of a drainage tube alone can lead to closure of the fistula. If the fistula does not close with tube drainage in a few weeks, a definitive operation is recommended (Hulnick et al. 1983). EN is an uncommon ­complication of an empyema in which the inflammatory mass spontaneously bores its way from the pleural cavity into the soft tissues of the thoracic wall, forming a subcutaneous abscess, which may or may not open to the skin. The most common etiology is tuberculosis. The most common clinical presentation is a mass in the thoracic wall that is sometimes accompanied by a cutaneous fistula. The chest radiograph usually shows signs of chronic pleural disease with pleural thickening, which may be calcified. A subcutaneous mass associated with pleural empyema should be considered suspicious for EN. CT is very useful and is diagnostic when it shows a pleural collection with thickened often calcified pleural wall, associated with a mass in the thoracic wall (Fig. 17.8).

Malignant pleural disease is a common clinical problem, as about 25 % of all pleural effusions in elderly patients have a neoplastic origin Feragalli B et al. 2003).

The pleura may be involved by one of several primary or secondary malignant tumors. Primary tumors account for about 10 % of pleural malignancies and are malignant pleural mesothelioma, localized fibrous tumor, and pleural sarcoma. Secondary tumors account for about 90 % of pleural neoplasms; the principal causes are adenocarcinoma or, less frequently, lymphoma and thymoma.

Depending on the location, size, and underlying histologic features, pleural tumors may produce a spectrum of findings; CT is the best method for characterizing location and composition of pleural masses and is particularly useful in distinguishing pleural from peripheral pulmonary lesions.