Sarcoidosis is a systemic disease of unknown etiology that is characterized by widespread development of noncaseating granulomas. More complete discussions of this disorder are found in chapters 3, 6, and 10. This section will be limited to a discussion of those patients with sarcoidosis who have so-called "alveolar" opacities on chest radiography or CT.

Although the term alveolar sarcoidosis is used, the process is predominantly interstitial, with compression and obliteration of alveoli creating the appearance of alveolar filling on radiologic imaging. Histologically, these lesions are seen to represent confluent interstitial granulomas. The radiographic appearance of airspace (alveolar) opacities develops in 10% to 20% of patients with sarcoidosis. The radiologic appearance is that of bilateral, multifocal, poorly defined opacities that range in size from 1 to 10 cm and show a predilection for the peripheral middle lung, sparing the costophrenic angles (1,2,3) (Fig. 12-3). The peripheral distribution is better seen and on occasion appreciated only on CT. Air bronchograms are common. Associated CT findings of reticulonodular opacities, especially in a lymphatic distribution, and mediastinal and hilar lymphadenopathy provide clues to the correct diagnosis. Most patients with alveolar sarcoidosis have accompanying lymphadenopathy (4) (Fig. 12-4). When only peripheral opacities are seen, the appearance can be indistinguishable from those of COP or EP. Although all three disorders can present with blood eosinophilia, the degree of eosinophilia is most pronounced with EP. The peripheral opacities of alveolar sarcoidosis can clear rapidly with or without steroid treatment (1).

The term pulmonary eosinophilia, synonymous with pulmonary infiltration with eosinophilia, describes a group of diseases in which blood and/or tissue eosinophilia affects major airways and lung parenchyma (5). Blood eosinophilia, however, is not necessary to make a diagnosis of eosinophilic lung disease. The number of diseases included under the umbrella term pulmonary eosinophilia are numerous. A simple mnemonic, "NAACP," and the term idiopathic can be used to remember the major classifications of disorders that constitute pulmonary eosinophilia (Table 12-2). This section will focus on parasitic infections and idiopathic pulmonary eosinophilia.

Tropical pulmonary eosinophilia is a systemic disease caused by hypersensitivity to microfilariae, which are the early larval forms of various filarial nematodes, most notably Brugia malayi and Wuchereria bancrofti (6). The disease is endemic in the Indian subcontinent, Southeast Asia, the South Pacific, North Africa, and South America. In nonendemic areas, the disease is seen in immigrants. The predominant respiratory symptom is chronic cough, which is often worse at night. Patients have marked blood eosinophilia, elevated immunoglobulin E (IgE) levels, and a high titer of antifilarial antibody. The chest radiograph is abnormal in the majority of patients, with the most common abnormality being fine linear opacities that are distributed diffusely and symmetrically. Small nodules, ranging in size from 1 to 5 mm, are seen in about half of cases. Hilar lymphadenopathy is uncommon and, when present, is mild (7). An important diagnostic criterion is rapid response to treatment with diethylcarbamazine. Chronic interstitial fibrosis can develop in some patients (8).

The larval stages of a number of worms other than filarial nematodes can pass through the lung and cause EP. These include Ascaris lumbricoides, Strongyloides stercoralis, Toxocara canis, Trichuris trichiura, and Schistosoma sp. The radiologic pattern is usually identical to that of acute EP.

Cryptogenic EP can be acute or chronic, depending on whether the condition lasts more or less than 1 month (9). The 1-month criterion is arbitrary, and the distinction between acute and chronic EP is not always clear. Acute EP, also referred to as Löffler syndrome, is characterized by blood eosinophilia;
mild or absent symptoms and signs (cough, fever, dyspnea); one or more nonsegmental mixed interstitial and alveolar pulmonary opacities that are transitory or migratory; and spontaneous clearing of opacities (within 1 month). The pulmonary opacities have a tendency to be predominantly peripheral in distribution. One way of remembering this peripheral distribution is to think of EP as being the opposite of PE (pulmonary edema). The classic distribution of pulmonary edema is a central batwing or butterfly pattern of alveolar lung disease, which is just the opposite of the peripheral alveolar opacities seen with EP. Pleural effusions and lymphadenopathy are not features of acute EP.

Chronic EP is most prevalent in the third to seventh decades of life, with women outnumbering men by 2 to 1 (10). Symptoms of dyspnea, cough, wheeze, malaise, weight loss, fever, and night sweats can be mild or severe. Blood eosinophilia occurs in the majority of patients. Serum IgE is normal or only mildly elevated, which is helpful in distinguishing the condition from allergic bronchopulmonary aspergillosis and tropical and parasitic pulmonary eosinophilias, in which serum IgE levels are markedly elevated. The classic findings on chest radiograph and CT are nonspecific peripheral, nonsegmental, homogeneous alveolar opacities, often with air bronchograms (10,11) (Figs. 12-5 and 12-6). In a minority of patients, the opacities are central in distribution or both central and peripheral (Fig. 12-7). Chronic EP is sensitive to steroid therapy; rapid clearing of radiologic abnormalities is usually seen within a few days, with complete clearing by 1 month. Relapse is common, and the majority of patients need long-term low-dose steroids, distinguishing this disease from acute EP (12). The radiologic manifestations can be migratory, occurring in new locations with relapse.

Only 15% or fewer of thromboemboli cause pulmonary infarction (13). It is unknown why some emboli cause infarction

and others do not, but it is likely a result of compromise of both the pulmonary and bronchial arterial circulation. This is most likely to occur with peripheral emboli and in patients with left heart failure or circulatory shock (14). It is known that bronchial circulation alone can sustain the lung parenchyma without infarction occurring (15).

No chest radiographic sign is specific for pulmonary embolism or infarction, and the sensitivity of chest radiography for these conditions is poor. Even with large pulmonary artery clot burden, the chest radiograph can be normal (16). The main role of the chest radiograph, therefore, is to exclude other diagnoses that might mimic pulmonary embolism clinically, such as pneumonia or pneumothorax.

Pulmonary infarction results in airspace opacities that are usually multifocal and predominantly in the lower lungs. They usually appear within 12 to 24 hours after the embolic event. The opacities are classically peripheral, with a triangular or rounded shape (thus the term "Hampton hump"), and they are always in contact with the pleural surfaces (Fig. 12-8). The apex or hump of the opacity is directed toward the lung hilum. Occasionally, lobar consolidation resembling pneumonia can occur. Air bronchograms are rarely present. It is important to
note that the opacities can be a result of a combination of atelectasis and pulmonary hemorrhage without infarction, in which case clearing occurs within a week. Infarction takes several months to resolve, often with residual scarring (Fig. 12-9). As infarcts resolve, they melt away "like an ice cube" (giving rise to the melting ice cube sign; see Fig. 2-16). The opacity clears from the periphery first, whereas in pneumonia the opacity clears homogeneously (both centrally and peripherally) at the same time. Cavitation can occur within infarcts but is rare without coexisting infection, either secondary infection of an infarct or a result of septic emboli or vasculitis. Pleural effusions related to pulmonary emboli are usually small, unilateral, and associated with pulmonary infarction.

COP, previously referred to as bronchiolitis obliterans organizing pneumonia, is a clinicopathologic entity of unknown cause characterized by nonspecific clinical symptoms of cough and dyspnea and by equally nonspecific chest radiographic findings of patchy, peripheral airspace opacities. COP should not be confused with bronchiolitis obliterans, also known as obliterative bronchiolitis or constrictive bronchiolitis, a separate entity that is discussed in Chapter 13. Although COP is idiopathic in etiology, several precipitating conditions leading to organizing
pneumonitis have been identified, including infections (most notably viral), connective tissue disorders (Fig. 12-10), drug toxicity (Fig. 12-11), inhalation of noxious fumes, and lung and bone marrow transplantation (Fig. 12-12). Patients with COP are generally 55 to 60 years of age, and half of them have a history of an influenza-like prodrome followed by an illness of about 3 months characterized by cough, exertional dyspnea, malaise, fever, and weight loss (17).

Radiographs and CT scans of the chest resemble those of many other types of pneumonia and show bilateral, patchy, nonsegmental, airspace opacities with a peripheral and basilar predominance (18). The opacities may contain air bronchograms. On occasion, the opacities are not peripheral in distribution but are central and centered along bronchovascular bundles. Although COP is usually a bilateral process, occasionally only one lung is involved or is much more involved than the other lung (Fig. 12-13). Areas of ground-glass attenuation are common and may be the only finding on CT.