Congenital lobar emphysema is a condition characterized by hyperinflation of a lobe without destruction of alveolar septa (23,24,25). Two types of lobar emphysema have been identified, one with a normal number of hyperinflated alveoli and one with an increased number of normally inflated alveoli (24). The latter variety is termed polyalveolar lobe. Bronchial obstruction, resulting from primary cartilage deficiency or dysplasia, is believed to be the cause of the overinflated lobe in most
cases. A single lobe is involved in >95% of cases, with the left upper lobe being involved in about 45% of cases, the right middle lobe in 30%, the right upper lobe in 20%, and two lobes in 5% of cases. Most patients present during the first 6 months of life with respiratory distress.

CT findings of congenital lobar emphysema are an expanded hemithorax, a hyperinflated low-attenuation lobe with pulmonary vascular pruning, atelectasis of ipsilateral adjacent lobes, and mediastinal shift into the opposite hemithorax. Although the mean lung attenuation value of the obstructed lobe is decreased, measurements are not needed for diagnosis. Visual assessment of lung density usually suffices to identify hyperinflation (Figs. 3.8 and 3.9). In the immediate postnatal period, the attenuation value of the affected lobe may increase and be closer to that of soft tissue because of impaired clearance of retained lung fluid. As the fluid is resorbed, the attenuation value of the lobe decreases.

Bronchogenic cysts are part of the spectrum of bronchopulmonary foregut malformations, which includes cystic adenomatoid malformation, bronchial atresia, and sequestration, as well as bronchogenic cyst (22,23,25,26,27). Bronchogenic cysts may be intrapulmonary or mediastinal. The cause is believed to be an error in lung bud development. Histologically, bronchogenic cysts are surrounded by fibrous walls containing cartilage and lined by ciliated, columnar epithelium. Most are unilocular and contain serous or mucoid fluid unless they are infected, and then the contents are purulent. The lesions become clinically apparent when there is superimposed infection or compression of the tracheobronchial tree.

On CT, pulmonary bronchogenic cysts are typically well-defined masses with smooth walls and attenuation equal to that of water, reflecting the presence of serous fluid. The attenuation increases and approximates that of soft tissue when the contents are viscous or mucoid. The
cyst may contain air or an air–fluid level (Fig. 3.10). Cysts complicated by infection may show wall enhancement.

Cystic adenomatoid malformation is a mass of disorganized pulmonary tissue that has a normal communication with the bronchial tree and normal vascular supply and drainage. The cause is believed to be an overgrowth of distal bronchiolar structures (23,25,28,29,30,31). Three main types of cystic adenomatoid malformation are recognized. Type I, accounting for 50% of cases, consists of variable-sized cysts, with at least one dominant large cyst measuring >2 cm in diameter. Type II, accounting for approximately 40% of cases, is composed of many thin-walled, small cysts measuring 1 to 10 mm in diameter. Type III, comprising approximately 10% of cases, appears solid on visual inspection, although microscopically there are multiple tiny cysts that are <2 mm in diameter. Cystic adenomatoid malformation is limited to a single lobe in >95% of cases, and it is bilateral in <2% of cases (31). Congenital malformations (cardiac, renal, intestinal, skeletal) are common in types II and III lesions. Most patients present within the first 6 months of life with respiratory distress. After this time period, affected children often present with cough and fever or recurrent pulmonary infections (30).

Characteristic CT findings of cystic adenomatoid malformation are multiple, thin-walled, air-filled cysts of variable size, expanding a lobe or lobes and displacing the mediastinum to the opposite hemithorax (Figs. 3.11 and 3.12). A higher attenuation equal to soft tissue or thick walls may be seen when the cysts' contents are infected or hemorrhagic (Fig. 3.13). Malignant transformation, usually to mesenchymal sarcoma, is a rare cause of increased attenuation (32). Air–fluid levels can be seen occasionally, but they do not necessarily indicate infection (30).

Differential diagnostic considerations of cystic adenomatoid malformation are lobar emphysema, bronchogenic cyst, mesenchymal cystic hamartoma, and localized persistent pulmonary interstitial emphysema. Congenital lobar emphysema and bronchogenic cyst are typically homogeneous and unilocular rather than multicystic. Mesenchymal cystic hamartoma is a multicystic mass, but it contains primitive mesenchymal cells, whereas the stroma in cystic adenomatoid malformation has predominantly fibrous tissue (33,34). The hamartoma also may be complicated by pneumothorax, hemoptysis, and malignant transformation to primitive sarcoma (34,35). A definitive diagnosis requires tissue sampling.

Localized persistent pulmonary interstitial emphysema is an acquired form of interstitial emphysema found in infants requiring ventilatory assistance. CT shows thin-walled, air-filled cystic masses containing punctate soft tissue densities representing centrilobular

vessels (Fig. 3.14). The clinical history of prolonged ventilation aids in making the correct diagnosis (36,37,38). Approximately half the lesions are limited to one lobe and half involve multiple lobes.

Pulmonary agenesis refers to absence of the lung tissue, bronchus, and pulmonary artery. Patients with bilateral involvement die almost immediately. When the involvement is unilateral, patients can present with respiratory distress immediately after delivery or they may be asymptomatic and the diagnosis made incidentally on imaging examinations obtained for other reasons. CT shows absence of the lung and absent or rudimentary pulmonary artery, shift of mediastinal structures to the affected side, and compensatory hyperinflation of the normal side (Fig. 3.15).

Pulmonary hypoplasia refers to a decreased amount of lung tissue. Pulmonary hypoplasia can be primary with no obvious cause of underdevelopment or it can occur secondary to extrathoracic compression of the fetal lung (usually owing to maternal oligohydramnios), thoracic cage compression of the fetal lung (usually owing to thoracic dystrophies), and intrathoracic fetal compression of the lung (usually owing to large diaphragmatic hernias or large congenital cystic masses). Patients may have mild respiratory distress or no symptoms at all. In comparison to those of the agenetic lung, imaging studies show some aerated lung. The hypoplastic lung is smaller than normal,
and there is associated pulmonary artery hypoplasia and some mediastinal shift (Fig. 3.16) (23,25).

Bronchial atresia (also known as congenital bronchocele or mucocele) results from abnormal development of a segmental or subsegmental bronchus with failure of the bronchus to develop or maintain its communication with the central airway. The bronchus distal to the atretic segment is patent and contains impacted mucus, whereas the lung beyond the obstruction is air-filled and overaerated as a result of collateral air drift via the pores of Kohn. Bronchial atresia rarely causes symptoms and is usually discovered on chest radiographs performed for other indications. The CT features of bronchial atresia include a dilated, mucoid-filled segmental or subsegmental bronchus surrounded by hyperinflated (oligemic) lung (Figs. 3.17 and 3.18).

Bronchopulmonary sequestration is a congenital mass of pulmonary tissue that has no normal connection with
the tracheobronchial tree and is supplied by an anomalous artery, usually arising from the descending aorta (39,40). Most sequestrations are located in the lower lobes. Although plain chest radiography may demonstrate an anomalous feeding artery and the draining vein, CT is more sensitive for identifying such vessels (23,25,39,40,41,42,43,44,45). Multiplanar reformations and 3D reconstructions are useful in demonstrating the full extent and course of the feeding artery and draining vein (42).

There are two types of sequestration: intralobar (also termed acquired) and extralobar (also termed congenital) (39,40). Intralobar sequestration is probably acquired and the sequela of chronic infection, which leads to parasitization of systemic arterial supply to the abnormal lung. Intralobar sequestration is contained within the visceral pleural covering of the normal lung and has systemic arterial supply, most commonly from the distal thoracic aortic branches. Venous drainage is usually via the pulmonary veins into the left atrium. Associated anomalies are rare. Chronic or recurrent segmental or subsegmental pneumonitis, especially at a lung base, is a common presentation. The CT findings of intralobar sequestration include an area of consolidated lung (Fig. 3.19), a round or pyramidal masslike lesion, and a cystic mass, which may exhibit air–fluid levels (Fig. 3.20). Adjacent emphysematous changes are common.

Extralobar sequestration is probably a true congenital anomaly of tracheobronchial branching. It has its own separate pleural investment, and it has systemic arterial supply, most commonly from the upper abdominal or lower thoracic aortic branches. Venous drainage is usually to the inferior vena cava or the azygous system and occasionally the coronary artery or portal veins (46,47). Associated anomalies are common in extralobar sequestration, including diaphragmatic eventration and

hernia, fistulas between the sequestered lung and gastrointestinal tract, and skeletal and cardiac anomalies. Patients are usually asymptomatic, and the anomaly is detected incidentally on prenatal sonography or on chest radiography, sometimes obtained for evaluation of other congenital anomalies. On CT, extralobar sequestration typically appears as a solid round, ovoid, or triangular mass, usually near the medial bases of the lower lobes (Figs. 3.21 and 3.22). It almost never contains cystic spaces or air. The presence of a mass with
cystic spaces and solid tissue with associated anomalous arterial supply should suggest a sequestration with an associated cystic adenomatoid malformation (Fig. 3.23) (43).

Hypogenetic lung syndrome, also known as venolobar syndrome and scimitar syndrome, is characterized by anomalous pulmonary venous return, hypoplasia of the lung, and hypoplasia of the ipsilateral pulmonary artery (48,49). It is almost always on the right. The cause is unknown, but it is thought to represent an error in development of the entire lung bud early in embryogenesis. The anomalous venous return is most often to the inferior vena cava, but it may be to a portal vein, hepatic vein, or right atrium. Associated congenital heart disease occurs in approximately 25% of patients, the most common being atrial septal defect, followed by ventricular septal defect, tetralogy of Fallot, and patent ductus arteriosus. Bronchial anomalies are also common, particularly isomerism. The presence or absence of symptoms relates to the degree of pulmonary hypoplasia and the complexity of the cardiac malformations.

Characteristic CT findings of hypogenetic lung syndrome are a small right lung, ipsilateral mediastinal shift with dextroposition of the heart, a small pulmonary artery, and partial anomalous pulmonary venous return, usually to the inferior vena cava (Fig. 3.24) (48,49). Less common anomalies include systemic arterial supply to the hypoplastic lung, accessory diaphragm, and horseshoe
lung. In horseshoe lung, the posterobasal segments of both lungs are fused behind the pericardial sac.

Pulmonary arteriovenous malformation (AVM) is characterized by a direct communication between a pulmonary artery and vein without an intervening capillary bed. It is usually supplied by one artery and drained by one vein, although multiple feeding arteries and draining veins may be present. Pulmonary AVMs are multiple in approximately 35% to 50% of patients and are bilateral in about 10% to 20% (50,51). Most occur in the lower lobes, and both lungs are involved with equal frequency. Approximately 60% of pulmonary AVMs are associated with Osler–Weber–Rendu syndrome (hereditary hemorrhagic telangiectasia). Conversely, only about 15% of patients with this condition have pulmonary arteriovenous malformations. Symptoms include cyanosis, dyspnea, hemoptysis, and clubbed fingers, caused by abnormal oxygenation and secondary embolic phenomena.

On CT, AVMs can appear as a well-defined smooth, round, or tubular nodule representing a single vascular sac or as a lobulated or serpiginous mass representing a tangle of dilated tortuous vessels (Figs. 3.25 and 3.26) (50,51,52,53). Most are subpleural or peripheral in location. Contrast-enhanced CT shows a dilated feeding pulmonary artery and draining pulmonary vein and marked opacification enhancement of the lesion(s) just after enhancement of the right ventricle. Maximum-intensity projection (MIP) and 3D reconstructions are helpful in
precisely depicting the architecture, size, and number of malformations and the feeding arteries and draining veins (Figs. 3.25B and 3.26B).

Pulmonary varix refers to abnormal dilatation of a segmental pulmonary vein. It may be congenital or acquired (54). Acquired varicosities are associated with mitral valvular disease. Varices are usually asymptomatic lesions. Rare complications include rupture and intracranial embolism. The characteristic CT finding is a soft tissue attenuation nodule in the medial third of the lung, most often in the area subtended by the lingular vein or the inferior pulmonary vein in the right lower lobe. The dilated vein enhances with or immediately following enhancement of the central pulmonary veins (Fig. 3.27).

Based on their pattern of distribution, nodules can be classified as peribronchial (also known as perilymphatic), centrilobular, and random (2,3,71) (Fig. 3.44). Peribronchial nodules (Table 3.2) are usually well defined and have a patchy or asymmetric distribution. They can involve parahilar areas, interlobular septa, and pleural surfaces and fissures (Fig. 3.45).

Centrilobular nodules (Table 3.3) are distributed predominantly within the center of the secondary pulmonary lobule; they spare the pleural surfaces, usually
being centered 5 to 10 mm away from the pleural surface and interlobar fissures (91). They can range in size from a few millimeters to >1 cm and may be well defined or ill defined. They tend to be evenly spaced and of similar size.

Centrilobular nodules may have a branching configuration, termed tree-in-bud pattern. Tree-in-bud morphology reflects the presence of mucus, fluid, and/or pus in dilated centrilobular bronchioles (Fig. 3.46). These branching nodules are visible in the lung periphery and tend to be centered 5 to 10 mm from the pleural surface (2,3). The presence of the tree-in-bud pattern indicates the presence of small airway disease, almost always caused by infection (e.g., bronchopneumonia, Mycobacterium avian-intracellulare) or mucus impacted centrilobular bronchioles (e.g., cystic fibrosis or asthma) (92,93,94).

Random nodules show no definite relationship to the secondary lobule or other structures of the lung (i.e., interlobular septa, small vessels, pleura). They usually are well defined, diffuse, and symmetric in distribution. Subpleural nodules are common. Common causes include miliary tuberculosis, fungal infections, hematogenous metastases, and Langerhans cell histiocytosis (Fig. 3.47).

Subpleural nodules (e.g., in contact with the pleural surface or fissures) should suggest either a peribronchial or random pattern. If the nodules have a patchy distribution, they are peribronchial in location. It the nodules are evenly scattered throughout the lung parenchyma, they are random in location. If subpleural nodules are absent, then the nodules are centrilobular in location. If centrilobular nodules have a tree-in-bud configuration, the diagnosis is likely infection.

Ground-glass opacity refers to hazy increased lung attenuation with preservation of the margins of underlying
vessels and bronchi. It typically has a patchy distribution (Fig. 3.48). Ground-glass opacity may be caused by airspace filling, alveolar wall thickening, and/or interstitial thickening. Although it is a nonspecific finding, it indicates the presence of active disease and can be seen in nearly all the difuse diseases described in following sections (95,96).

Airspace consolidation (Table 3.4) is characterized by increased lung attenuation associated with obscuration of underlying vessels and air bronchograms (Fig. 3.49) (71). It indicates filling of the airspaces by fluid, cells, protein, or other material (95,96).

Lung cysts (Table 3.5) are thin-walled air-containing lesions ranging between 3 mm and 10 mm in diameter (2,3). Bullae are round, thin-walled, air-filled spaces measuring >1 cm in diameter (Fig. 3.50). Bullae and cysts are seen in cystic lung disease, such as Langerhans cell histiocytosis, tuberous sclerosis, Marfan syndrome, and neurofibromatosis and in association with Pneumocystis carinii or Staphylococcus aureus pneumonia.

Emphysema refers to irreversible abnormal enlargement of the airspaces distal to the terminal bronchioles, accompanied by destruction of alveolar walls (71,89,98,99). CT findings of emphysema are areas of decreased attenuation (several millimeters to several centimeters in diameter) without definable walls and pruning of pulmonary vessels (Fig. 3.51). Three main patterns of emphysema have been described: centrilobular, paraseptal, and panacinar. The latter form is seen most often in children. The other two types of emphysema are related to cigarette smoking and are common in adults.

Panacinar or (panlobular) emphysema results in large areas of low attenuation, usually diffuse and most severe in the lower lobes. This form of emphysema is typical of α₁-antitrypsin deficiency (Fig. 3.51). Centrilobular emphysema is characterized by focal areas of low attenuation within the central lung (centrilobular region), usually with an upper lobe predominance. Paraseptal emphysema is characterized by subpleural areas of low-attenuation areas adjacent to the chest wall and mediastinum.

The terms mosaic lung attenuation and mosaic perfusion are used to describe the regional differences in parenchymal attenuation, which results from decreased perfusion secondary to vasculitis or small airway obstruction (100,101). In pulmonary vascular obstruction, there is redistribution of blood flow away from the areas of vascular obstruction. In small airway disease, the subtended lung is poorly ventilated and poorly perfused because of reflex vasoconstriction. In both scenarios, there is redistribution of blood flow to areas of normal lung parenchyma. The result is that the oligemic lung has decreased attenuation and a decreased number and size of pulmonary vessels compared with normal lung. The normal lung may have a normal or slightly increased attenuation due to shunting of blood from the abnormal oligemic areas. Common causes of small airway obstruction include reactive airway disease (e.g., asthma) and irreversible small airway disease (e.g., cystic fibrosis, bronchiolitis obliterans, Swyer–James syndrome).

Distinguishing between the two causes of mosaic lung attenuation can be difficult on inspiratory CT scans. The distinction, however, usually can be made on expiratory CT (Fig. 3.38). In patients with airway obstruction, the low-attenuation areas become more pronounced relative to normal lung because of air trapping on expiratory CT. In patients with primary vascular disease, the attenuation of oligemic and hyperemic or normal lung increase to a similar degree on expiratory scans.

Hydrostatic pulmonary edema results from near drowning, congestive heart failure, or volume overload and it may have an interstitial or airspace pattern on CT. Although CT is not required for diagnosis, knowledge of the CT findings is important as pulmonary edema may be encountered during an examination performed for other clinical problems. HRCT findings of interstitial pulmonary edema include smooth septal thickening, peribronchial cuffing, ground-glass opacity, consolidation, pleural effusions, and thickened pleural fissures (102,103) (Fig. 3.40). Left atrial and ventricular enlargement may also be noted.

Pulmonary lymphangiectasia is a congenital anomaly characterized by dilated lymphatic channels in the interlobular septa and pleura (104). Most affected patients have respiratory distress in the first day of life (104). Rarely, lymphangiectasia is associated with Noonan syndrome; the disease in this group of patients usually does not present until later childhood. HRCT shows smooth or nodular septal thickening, peribronchial cuffing, and peribronchial nodules. Pleural fluid and ground-glass opacities may be present (Figs. 3.41 and 3.52).

Bronchopulmonary dysplasia is the result of neonatal lung injury caused by oxygen toxicity and/or barotrauma. HRCT findings include areas of emphysema, septal thickening, and ground-glass opacity (Figs. 3.43 and 3.53) (105). Bronchiectasis, bronchial wall thickening, and pleural thickening are usually absent. Lung volumes are increased.

The pulmonary abnormalities associated with sickle cell disease include pneumonia, acute chest syndrome, and sickle cell lung disease. Pneumonia and acute chest syndrome manifest as airspace disease, and the diagnosis usually is made on plain chest radiographs. Sickle cell lung disease is a chronic interstitial lung disease with a prevalence of about 5% (106). Postmortem studies have shown widespread pulmonary fibrosis and obliterated pulmonary arterioles. HRCT findings include interlobular septal thickening, most commonly in a basal distribution, traction bronchiectasis, and architectural distortion.

The idiopathic interstitial pneumonias are a group of diffuse lung diseases associated with interstitial inflammation and fibrosis. Although there is a relatively extensive list of idiopathic interstitial pneumonias (2,3), only a few of these are seen in children. These include usual interstitial pneumonia (UIP), idiopathic pulmonary fibrosis (IPF), and nonspecific interstitial pneumonia (NSIP). UIP may be associated with collagen vascular diseases, drug toxicity, radiation, and chronic hypersensitivity pneumonia. When idiopathic, UIP is termed idiopathic pulmonary fibrosis. HRCT findings include honeycombing, irregular interstitial thickening, and traction bronchiectasis. The disease predominates in the lung bases subpleurally. Ground glass opacities are rare (Fig. 3.43). Mediastinal lymph node enlargement also may be noted (2,3).

Nonspecific interstitial pneumonia may be associated with collagen vascular diseases, hypersensitivity pneumonia, drug reaction, infection, or immunodeficiency (2,3). HRCT findings include irregular septal thickening and ground-glass opacity with a basilar predominance (Fig. 3.54). Honeycombing is rare.

Lymphangitic carcinoma refers to spread of tumor to pulmonary lymphatics. Characteristic CT findings are smooth or nodular septal thickening and peribronchial nodules. The HRCT findings are virtually identical to those of lymphangiectasia (Fig. 3.41). Associated findings include hilar and mediastinal lymphadenopathy and pleural effusion.

Sarcoidosis is a systemic disease process characterized by the presence of noncaseating granulomas. There are four stages of sarcoidosis: I, isolated adenopathy; II, lymphadenopathy plus pulmonary disease; III, parenchymal disease alone; and IV, pulmonary fibrosis. The characteristic HRCT finding of active sarcoidosis (stages I to III) is small (2- to 3-mm) peribronchial nodules (Figs. 3.45 and 3.55). Sometimes, HRCT shows areas of consolidation and ground-glass opacities (107,108). In end-stage disease associated with fibrosis, the findings include irregular septal thickening, architectural distortion with displacement of interlobar fissures, traction bronchiectasis, and honeycombing, (109). The upper long zones are predominantly involved. Enlarged hilar and mediastinal lymph nodes are also commonly seen on CT.

Wegener granulomatosis is the most common necrotizing systemic vasculitis in childhood. There are other conditions with various degrees of vasculitis and granulomatous reaction, including allergic angiitis and granulomatosis (Churg–Strauss syndrome), lymphoid granulomatosis, bronchocentric granulomatosis, and necrotizing sarcoid granulomatosis, but they are extremely rare in children (110,111,112). Pathologic examination shows necrosis and granulomatous reaction in small and medium-sized arteries and veins, resulting in vessel thrombosis with resultant ischemic damage to the pulmonary parenchyma. The lungs are involved in 95% of patients. The characteristic HRCT finding is multiple lung nodules, which are often bilateral and without a zonal predominance (2,3,110,111,112). The nodules range from 2 mm to several centimeters in diameter,
are round or oval, and may be well defined or poorly defined. Cavitation occurs in about 50% of nodules, usually those >2 cm in diameter (Fig. 3.56). Other CT findings include areas of consolidation, attributed to pulmonary hemorrhage and/or infarction; endobronchial lesions with distal atelectasis; tracheal or bronchial stenosis; and effusions (111,113). Mediastinal adenopathy is absent.

As noted above, hydrostatic pulmonary edema may have an interstitial or airspace pattern on CT. HRCT findings of airspace edema include areas of ground-glass opacity and/or consolidation, typically in the perihilar and dependent portions of the lungs. Associated findings include septal thickening in a central distribution, pleural effusions, and thickened fissures (102,103) (Fig. 3.57).

Acute respiratory distress syndrome (ARDS) is the result of capillary leak. HRCT usually demonstrates areas of ground-glass opacity and consolidation corresponding to edema or superimposed infection, respectively. Pleural effusions, pneumothoraces, and pneumomediastinum may also be seen.

Pulmonary alveolar proteinosis is a disease of unknown cause, characterized by the intra-alveolar deposition of lipid-rich, proteinaceous material, which stains with the use of the periodic acid-Schiff method. There are two forms (114): a congenital form due to a deficiency of surfactant protein B, which presents in full-term neonates soon after birth; and a late-onset form, which presents several months to several years after birth. Both forms result in a clinical picture of respiratory failure. HRCT findings in both forms include bilateral ground-glass opacities and smooth septal thickening (crazy-paving pattern) (Fig. 3.58). Airspace consolidation may be seen in some cases. The disease usually has a diffuse distribution (114,115).

Pulmonary hemorrhage in the pediatric population is most often caused by idiopathic pulmonary hemosiderosis. Less common causes include Goodpasture syndrome (anti–glomerular basement membrane disease), Wegener granulomatosis, collagen vascular diseases, drug reaction, and thrombocytopenia.

Idiopathic pulmonary hemosiderosis is a disease of unknown origin characterized by recurrent episodes of pulmonary hemorrhage with hemoptysis and iron deficiency anemia. The pathologic finding in acute disease is alveolar hemorrhage. At this stage, CT findings include ground-glass opacity and consolidation, often in a central distribution (Fig. 3.59) (116). With recurrent bouts of hemorrhage, hemosiderin-laden macrophages are deposited in the interlobular and intralobular septa and in the bronchiolar and arterial walls; ultimately fibrosis develops. In advanced stages, HRCT demonstrates septal thickening, centrilobular nodules, and honeycombing.

Hypersensitivity pneumonias, also known as extrinsic allergic alveolitis, is an immune-mediated granulomatous reaction caused by inhalation of antigens in organic dust. In the acute and subacute stages, HRCT findings include bilateral ground-glass opacities and small (1- to 3-mm), poorly-defined centrilobular nodules representing alveolitis
and bronchiolitis, respectively (Fig. 3.60) (117). The disease frequently has middle and lower lung zone predominance. Mosaic perfusion on inspiratory CT and air trapping on expiratory CT also may be noted in the subacute phase (weeks to months after exposure). In the chronic phase, manifested by fibrosis, HRCT findings include smooth or nodular septal thickening, centrilobular nodules, patchy areas of ground-glass opacity, and honeycombing (118). The chronic form of the disease has a middle lung zone predominance with relative sparing of the lung bases (unlike the basal predominance seen in idiopathic pulmonary fibrosis). Lung volumes are normal or increased.

Collagen vascular diseases include scleroderma, lupus erythematosis, and juvenile rheumatoid arthritis (119,120,121). Early disease process is characterized by the findings of nonspecific interstitial pneumonia—ground-glass opacity and septal thickening (Figs. 3.54 and 3.61). In advanced disease associated with fibrosis, HRCT findings include ground-glass opacity, irregular septal thickening, honeycombing, and traction bronchiectasis. Pleural effusions, which are usually small and unilateral, and pericardial effusion may also be noted (119,120,121). The distribution of disease is typically lower lobe.

Cryptogenic organizing pneumonia (COP), also known as bronchiolitis obliterans organizing pneumonia (BOOP) is a condition characterized histologically by the presence of patchy organizing pneumonia involving alveoli and alveolar ducts. The classic HRCT findings are patchy bilateral airspace consolidation in a subpleural and/or peribronchial distribution. Other CT findings include areas of ground-glass attenuation, small ill-defined centrilobular nodules, and small bilateral pleural effusions.

Tuberous sclerosis is an autosomal dominant genetic disorder associated with the triad of mental retardation, seizures, and adenoma sebaceum. About 1% of patients have lung disease (122). HRCT findings include cysts, bullae, areas of emphysema and interlobular septal thickening (Fig. 3.62) (123). Lung volumes are usually normal, although they may be hyperinflated in advanced disease. Similar findings can be seen in Ehlers–Danlos syndrome (Fig. 3.63) (123) and lymphangioleiomyomatosis. The latter disease usually affects young women of child-bearing age and may be hormone-related.

Langerhans cell histiocytosis, previously known as eosinophilic granuloma, is a chronic disorder of unknown cause, characterized by proliferation of Langerhans cells in the interstitial tissues (124). Pathologic findings in early disease include peribronchial and peribronchiolar granulomas containing Langerhans cells and eosinophils. Findings in
later stages of disease include destruction of the bronchiolar walls, leading to cyst formation, and fibrosis. The lung disease may be isolated or associated with skeletal and multisystem disease. HRCT findings consist of small centrilobular and peribronchial nodules (Fig. 3.47) and thin-walled cysts (Fig. 3.64) (124,125,126,127). The nodules are usually <5 mm in diameter, but they may exceed 1 cm. Larger nodules may have hypoattenuating centers, representing either cavitation or dilated bronchioles. The predominantly nodular pattern is seen in the early stages of disease and the predominantly cystic pattern in the later stages. Other findings include septal thickening, ground-glass opacity, mediastinal lymphadenopathy, and pneumothorax secondary to cyst rupture (124,125,126,127). The disease has an upper and middle lung zone predominance with relative sparing of the lung bases.

Marfan syndrome is a connective tissue disorder that typically affects the skeletal, ocular, and cardiovascular systems. HRCT findings of pulmonary disease include bullae, cysts, areas of emphysema, and bronchiectasis (Fig. 3.65) (128).

Neurofibromatosis type I is the most common phakomatosis or neurocutaneous disorder. Histologic findings include interstitial fibrosis at the lung bases and cysts or bullae predominantly at the lung apices. HRCT include septal thickening and thin-walled cysts or bullae (129).

Cystic fibrosis is the most common cause of chronic pulmonary disease and insufficiency in childhood. It is an autosomal recessive condition that results from a defect in the structure of the cystic fibrosis transmembrane regulator protein that leads to abnormal chloride transport across epithelial membranes (130). The genetic error results in decreased mucous clearance, airway plugging, an increased incidence of bacterial airway infection, and ultimately irreversible bronchiectasis.

The CT findings of cystic fibrosis include bronchiectasis, peribronchial wall thickening, centrilobular nodular and tree-in-bud opacities, mucus plugging, cystic or bullous lung lesions, and mosaic attenuation (Fig. 3.2). Focal areas of consolidation and ground-glass attenuation also may be present. Air trapping is especially well seen on expiratory scans. Hilar and mediastinal lymph node enlargement also may be seen, owing to chronic infection. The disease has a predilection for the upper lobes, although all lobes are involved. Systemic CT scoring systems have been devised to evaluate the severity of cystic fibrosis and patient response to therapeutic regimens, but these have not gained widespread use (130,131,132).

α₁-Antitrypsin deficiency is characterized by enlarged airspaces as a result of proteolytic digestion of lung parenchyma. HRCT findings are emphysema and associated vascular distortion. There usually is lower lobe predominance (133) (Fig. 3.51).

Bronchiolitis is a disease of the small airways, ≤3 mm in diameter (i.e., the terminal and respiratory bronchioles). Histologically, there are two major classes of disease: obliterative and proliferative (134,135,136).

Obliterative or constrictive bronchiolitis is characterized by inflammation and fibrosis of the submucosal and peribronchial tissues of the terminal and respiratory bronchioles, leading to airway narrowing and obliteration (136). Bronchiolitis obliterans can be idiopathic or it can follow various insults, including viral or bacterial infections, toxic fume inhalation, collagen vascular diseases, and
bone marrow and lung or heart–lung transplants. HRCT findings include a pattern of mosaic attenuation due to air trapping and reflex vasoconstriction, and bronchiectasis (Fig. 3.38) (137,138). The mosaic attenuation manifests as areas of decreased attenuation associated with relatively small-caliber vessels alternating with areas of relatively increased attenuation associated with larger-caliber vessels (78,79,101). Air trapping may be noted on expiratory scans (139).

Proliferative bronchiolitis is characterized by the presence of bronchiolar and peribronchiolar inflammatory cell infiltrate associated with intraluminal inflammatory exudates. This form of bronchiolitis can be idiopathic or secondary to collagen vascular disease and hypersensitivity pneumonia. HRCT findings include tiny centrilobular or peribronchial nodules, tree-in-bud pattern, interlobular septal thickening, and dilated bronchioles (91,92,93).

Swyer–James syndrome is the result of bronchiolitis obliterans occurring as the sequela of a viral respiratory infection in early childhood. HRCT findings are a small unilateral hyperlucent lung, bronchiectasis, and air trapping on expiratory CT scans (Fig. 3.66). In one series, approximately 60% of patients also showed patchy hyperlucent areas in the contralateral lung (140).