Congenital or nonacquired diffuse tracheal widening is much less common than tracheal narrowing and has a more limited differential diagnosis. The Mounier-Kuhn syndrome, which affects primarily men in the fourth and fifth decades, accounts for almost all cases of nonacquired tracheal widening (8). Thought to be congenital (9), it is an abnormality of the trachea and main bronchi characterized by atrophy or absence of elastic fibers and thinning of muscle, which allows the trachea and main bronchi to become flaccid and markedly dilated on inspiration, with narrowing or collapse on expiration or cough. The abnormal airway dynamics and pooling of secretions in broad outpouchings, or diverticula, of redundant musculomembranous tissue between the cartilaginous rings predispose patients to the
development of chronic pulmonary suppuration, bronchiectasis, emphysema, and pulmonary fibrosis (10). The trachea is involved from the subglottic region to the carina. A tracheal diameter greater than 3 cm is required for diagnosis, and tracheal widths up to 5.5 cm have been recorded (8). The radiographic and CT features of the condition include marked dilatation of the trachea and mainstem bronchi, tracheal diverticulosis, and a variable incidence of bronchiectasis and chronic pulmonary parenchymal disease (11,12).

Several conditions can result in acquired tracheobronchomegaly that may closely resemble that seen in Mounier-Kuhn syndrome (Table 13-1). Some degree of tracheal dilatation may be seen with aging (4,13) and in musicians who play wind instruments (13). Chronic infection, cigarette smoking, chronic bronchitis, emphysema, cystic fibrosis (CF), inhalation of noxious fumes, chronic intubation, and diffuse pulmonary fibrosis can also result in tracheobronchomegaly (10,14,15). Other conditions associated with tracheal widening, which may in fact be related to Mounier-Kuhn syndrome, are Ehlers-Danlos syndrome and cutis laxa (16,17). Although tracheal narrowing is the usual end result in relapsing polychondritis - a disorder of cartilaginous inflammation involving the nose, ear, trachea, and joints - diffuse tracheal widening may develop occasionally (18). In contrast, nasal and ear cartilage abnormalities are absent in Mounier-Kuhn syndrome. Additional causes of secondary tracheobronchomegaly are listed in Table 13-1. Nevertheless, the majority of cases appear to be sporadic, predominantly occurring in men in the third and fourth decades of life (9).

Tracheal narrowing is seen with a variety of disorders and can be idiopathic (7,19,20,21) (Table 13-2). Strictures of the trachea are usually caused by damage from a cuffed endotracheal or tracheostomy tube or trauma to the neck (22). Postintubation tracheal injuries remain the most common indication for tracheal resection and reconstruction, despite identification of the causes of these lesions and development of techniques for their avoidance (23). Tracheomalacia, which is diagnosed when the trachea collapses more than 50% on expiration, can also be caused by trauma, and it may be recognized only on dynamic or expiratory CT scanning (1,24).

Saber-sheath trachea is diffuse intrathoracic narrowing of the trachea, with the coronal diameter reaching two thirds (or less) of the sagittal diameter when measured 1.0 cm above the top of the aortic arch (25). More than 95% of patients with this deformity have clinical evidence of chronic obstructive pulmonary disease (COPD), and saber-sheath trachea is considered an insensitive but specific sign for COPD on standard chest radiography (Fig. 13-1) (25,26).

Relapsing polychondritis is a systemic autoimmune connective tissue disease in which cartilage is affected diffusely by recurrent episodes of inflammation. The pinnal, nasal, laryngeal, and tracheal cartilages are most commonly involved. The major airways are involved in more than 50% of cases, and recurrent pneumonia is the most common cause of death in these patients (8,27). CT shows diffuse or multifocal fixed narrowing of the tracheobronchial lumen, with associated thickening of the wall (28,29). Dense calcium deposits may be seen in the thickened tracheal cartilage (30).

Amyloidosis of the respiratory tract, both primary and secondary, is a rare condition that produces focal or diffuse irregular narrowing of the airway by submucosal deposits of amyloid (22). Both radiography and CT of the chest can demonstrate diffuse narrowing or show nodular protrusions into the tracheal lumen that can be calcified (Fig. 13-2) (8).

Tracheobronchopathia osteochondroplastica is a benign condition characterized by multiple submucosal osteocartilaginous growths along the inner anterolateral surfaces of the trachea (31,32). Although the etiology is unknown, theories have linked this disorder to chronic inflammation, degenerative processes, amyloidosis, and neoplasia (33,34,35). Radiography and CT of the chest show multiple sessile nodular tumors, with or without calcification, extending over a long segment of the trachea and into the main bronchi. In contrast, the nodules in amyloidosis may be circumferential and can be distinguished from those of tracheobronchopathia osteochondroplastica, in which there is always sparing of the posterior membranous wall.

Wegener granulomatosis is characterized by granulomatous vasculitis of the upper and lower respiratory tract, usually in conjunction with renal and other organ involvement. The CT appearance of airway involvement includes circumferential narrowing of the airway lumen, abnormal soft tissue within the tracheal rings, and dense irregular calcification of the tracheal cartilages (36). Sarcoidosis is another granulomatous disorder that may rarely involve the trachea and bronchi. Granulomatous sarcoid lesions may exist intrinsically in the airway, or enlarged hilar nodes may compress the bronchi extrinsically (37,38).

Many viral, bacterial, or fungal diseases can involve the trachea. In North America, most cases of laryngotracheobronchitis are viral in nature; subglottic or laryngeal narrowing is
common, but radiographically demonstrable tracheal narrowing is unusual (7).

In adults, tracheobronchial filling defects are usually produced by mucus (Fig. 13-3) or neoplasms (Figs. 13-4, 13-5, 13-6, 13-7). Less common causes include papilloma, infections (Fig. 13-8), foreign bodies (Fig. 13-9), broncholiths (Fig. 13-10), and other miscellaneous disorders. If mucus is suspected on a CT scan, it can be helpful to selectively repeat the scan at the region of interest after the patient has coughed.

Tracheoesophageal fistulas in adults are almost exclusively acquired lesions. They occur as a complication of intrathoracic malignancies (accounting for 60% of cases), infection, and


trauma (39,40). The diagnosis is usually made with a fluoroscopic contrast study but can be made, in some cases, with CT. In addition to demonstrating the site of a fistula, CT can suggest the etiology and detect pulmonary and mediastinal complications (41).

Congenital tracheobronchial anomalies can present as life-threatening emergencies at birth, or they may go undiagnosed for years. Clinical symptoms are often nonspecific, and radiographic evaluation is frequently required to localize and characterize the lesion before endoscopy, surgery, or medical management. The radiologist must be on the alert for unsuspected additional associated anomalies involving airways, lungs, great vessels, and the esophagus, which occur with relative frequency.

Tracheal webs produce localized areas of narrowing with no associated deformity of the underlying cartilage. The thickness of the webs determines the severity of obstruction and the therapeutic approach (42). Congenital tracheal stenosis may occur in any portion of the trachea, usually involving more length and depth of the trachea than webs, and is more likely to require resection rather than dilatation alone. Stenosis secondary to long-term compression by a dilated esophagus, abnormal great vessels, or cervicomediastinal masses results in a focal fibrous and cartilaginous deformity that persists for some time after the mass is removed. Congenital tracheal stenosis is frequently associated with bronchial stenosis; pulmonary hypoplasia or agenesis; tracheal bronchus; tracheoesophageal fistula; tracheomalacia; anomalies of vertebrae, ribs, and thumbs; and cardiac anomalies.

Tracheomalacia is an abnormally flaccid trachea that may involve all or part of the trachea and results in abnormal anteroposterior tracheal collapse during expiration of ≥50% of cross-sectional area. The innominate artery compression syndrome can result in secondary tracheomalacia, in which there is persistent narrowing of the anterior tracheal wall at the level of the thoracic inlet. Short trachea, which occurs when there are 15 or fewer tracheal rings, can be diagnosed on CT when the tracheal bifurcation lies above the fourth thoracic vertebral body in children younger than 2 years old or above the fifth thoracic vertebra thereafter (43).

Aberrant tracheal bronchus (so-called "pig bronchus," or bronchus suis), the most common anomalous airway pattern, is reported in 2% of children during bronchoscopy examination (44). It occurs most commonly in boys, arising most often from the right lateral tracheal wall within 2 cm of the carina. It can be asymptomatic, or it may result in right upper lobe infection, atelectasis, or bronchiectasis, usually from a stenotic bronchial segment and poorly cleared secretions. The CT appearance is that of a bronchus arising from the trachea in a section more cephalad than the carina (Fig. 13-11) (45).

There is no universally accepted definition of asthma; it may be regarded as a diffuse, obstructive lung disease with hyperreactivity of the airways to a variety of stimuli and a high degree of reversibility of the obstructive process, which may occur either spontaneously or as a result of treatment. Asthma is a complex disorder involving biochemical, autonomic, immunologic, infectious, endocrine, and psychologic factors in varying degrees in different individuals. Both large and small airways may be involved, again to varying degrees. The three elements that contribute to airway obstruction in asthma are: (i) spasm of smooth muscle; (ii) edema and inflammation of the mucous membranes lining the airways; and (iii) intraluminal exudation of mucus, inflammatory cells, and cellular debris. Asthma can be a benign, self-limiting problem; it can lead to acute respiratory failure; or it can be a chronic, recurrent disease that leads to debilitating, irreversible airflow obstruction and COPD. Emphysema is not a prominent finding in the lungs of nonsmoking patients with asthma, even in those with severe disease (46).

Chest radiographs of patients with asthma can be normal, show increased lung markings and hyperinflation, or show low lung volumes and multifocal atelectasis. CT findings can include bronchiectasis involving mostly subsegmental and distal bronchi, bronchial wall thickening, small centrilobular opacities, and decreased lung attenuation (47). Allergic bronchopulmonary aspergillosis (ABPA) occurs with a greater prevalence in patients with asthma and CF (48) (Figs. 13-12 and 13-13). Central bronchiectasis on CT is the hallmark of ABPA.
Bronchial wall thickening in asthma (which is assessed subjectively on CT) may reflect bronchial and peribronchial inflammation as well as increased smooth muscle, mucous gland, cartilage, and submucosal area (49,50). Areas of hyperlucency are caused by decreased lung perfusion secondary to reflex vasoconstriction in hypoventilated areas, and by air trapping (51) (Fig. 13-14). Emphysema seen on the CT scans of patients with asthma is attributed to cigarette smoking (52). Small centrilobular opacities may correspond to plugging or to thickening of the bronchiole walls (49). Because central airway lesions and mitral stenosis can produce symptoms attributed to asthma, the airways, cardiac silhouette, and pulmonary vasculature should always be evaluated closely on every chest radiograph when the clinical history is "asthma." The chest radiograph should also be assessed for evidence of pneumonia, which is known to exacerbate asthma, and pneumomediastinum (Fig. 13-15) and pneumothorax, as evidence of alveolar rupture that can be caused by wheezing and coughing (Table 13-4).

Bronchiectasis, defined as irreversible dilatation of the bronchial tree, can cause chronic sputum production and hemoptysis, and it can be described morphologically as cylindric, varicose, cystic, or traction in type (53). Cylindric bronchiectasis, the mildest form, is characterized by smooth, uniformly dilated bronchi (Figs. 13-16 and 13-17). Sectioned lengthwise, these bronchi resemble nontapering "tram tracks"; sectioned crosswise, the bronchi appear round or oval. Beaded dilatation of bronchi describes the varicose type (Fig. 13-18); cystic bronchiectasis, the most severe type, is characterized by cysts in clusters, often with air–fluid levels (Figs. 13-19 and 13-20). Traction bronchiectasis refers to irreversible dilatation of bronchi and bronchioles in areas of pulmonary fibrosis. It occurs predominantly in the peripheral portions of lung, where bronchi contain less supporting cartilage (see Fig. 3-3B) (54). There are numerous causes of bronchiectasis; these can be remembered with the mnemonic "BRONCHIECTASIS" (Table 13-5).

Although patients with bronchiectasis rarely have a normal chest radiograph (55), the chest radiographic findings are neither sufficiently sensitive nor specific enough to be of value
in the accurate assessment of bronchiectasis, and they are unreliable in determining the severity and extent of the disease (55,56,57). The common radiographic findings are loss of definition and increase in number and size of the bronchovascular markings (caused by peribronchial inflammation/fibrosis and the presence of retained secretions), tram tracking, tubular or ring-shaped opacities with central lucency if the airways are air filled, central opacity if there is mucoid impaction, and cystic spaces that can be up to 2 cm in diameter.

Early studies assessing the accuracy of conventional CT in diagnosing bronchiectasis resulted in sensitivities of 60% to 80% and specificities of 86% to 100% (57,58,59,60). With the use of 1.5-mm collimation at 10-mm intervals, sensitivity with CT improved to a range of 96% to 98%, with specificity of 93% to 99% (61,62). Thin-section CT is now the accepted gold standard for diagnosing bronchiectasis. With current state-of-the-art multidetector CT, routine imaging of the lungs with 3-mm collimation and 1.25-mm reformatting will allow for diagnosis of most cases of bronchiectasis, even very mild cases. The most reliable finding for the diagnosis of cylindric bronchiectasis is visualization of bronchi within 1 cm of costal or paravertebral pleura, or visualization of bronchi abutting the mediastinal pleura. Although lack of normal bronchial tapering and increased bronchoarterial ratios are helpful in the diagnosis of
bronchiectasis, these findings can also be seen in 10% to 20% of healthy subjects.

Despite the ease with which bronchiectasis can be identified on CT in most cases, there are a number of potential pitfalls (63). These include artifacts from both respiratory and cardiac motion, and inappropriate collimation and electronic windowing. A number of diffuse lung diseases can simulate bronchiectasis, especially cystic bronchiectasis; these include Langerhan cell histiocytosis, lymphangioleiomyomatosis, cystic changes related to connective tissue diseases or lymphocytic interstitial pneumonitis or in patients with acquired immunodeficiency syndrome and Pneumocystis jiroveci pneumonia, emphysema, and cystic metastases. The characteristic combination of "cyst" paired with the accompanying pulmonary artery is sometimes helpful in confirming bronchiectasis, as is variation in the sizes of bronchiectatic "cysts" with inspiration and expiration, a feature that is not usually seen with other types of cystic lesions. Following cystlike lesions from one CT scan section to another and noting their relationship to central airways and their "tubular" nature also allow for accurate distinction between bronchiectasis and other cystic diseases, in most cases. This can be facilitated by reconstructing multidetector CT data to create maximum-intensity-projection images and coronal reformations.

The dyskinetic cilia syndrome, first described in 1976 (64), represents a spectrum of genetically determined defects in ciliary structure and function that interfere with mucociliary clearance. Although the term immotile cilia syndrome has been used, in many cases the cilia demonstrate some motility (although dyskinetic). Described conditions include (a) situs inversus, paranasal sinusitis, and bronchiectasis (the three major components of Kartagener syndrome) (Figs. 13-21 and 13-22); (b) recurrent upper and lower respiratory tract infections; (c) and immotile sperm and infertility.

CF is a relatively common genetic disorder that affects the upper and lower respiratory tracts, pancreas, liver and gallbladder, intestines, and genital tract. Approximately one in 1,600 live births is affected by this autosomal recessive disease, which occurs predominantly in Caucasians. In 1985, the CF defect was determined to be located on chromosome 7 (65), and 4 years later the CF gene was identified by positional cloning (66,67,68). This new knowledge gave rise to new therapies, including in vivo gene therapy (69). The median survival age rose from about 18 years in 1976 to 29 years in the early 1990s (70,71).

Chest radiographic findings in adult patients with CF include hyperinflation and atelectasis as well as bronchiectasis (72). CT shows the presence, severity, and extent of bronchiectasis, peribronchial thickening, mucous plugging (Fig. 13-23), abscesses, bullae, lung collapse, and dense parenchymal opacification (73) (Fig. 13-24). In early stages of the disease, the upper lungs are involved to a greater extent than the lower lungs (Fig. 13-25). As the disease progresses, the process becomes more diffuse and an upper lung–predominant pattern may not be appreciated.

Chronic bronchitis is defined clinically as a chronic or recurrent increase in the volume of mucoid bronchial secretions sufficient to cause expectoration, occurring on most days during at least 3 consecutive months for no less than 2 consecutive years (74). It is common among cigarette smokers. The diagnosis is based on the presence of chronic productive cough in the absence of any specific cause, such as bronchiectasis or chronic infection. The radiographic features are nonspecific and include tubular shadows, thickened bronchial walls, hyperinflation of the lungs, and areas of pulmonary oligemia. The term dirty lung has been used to describe the increase in bronchovascular markings. Hyperinflation and oligemia are probably a result of associated pulmonary emphysema. Findings of centrilobular emphysema can predominate on CT of patients with chronic bronchitis. Radiographic and CT features are insensitive and nonspecific, and a high degree of interobserver variability further limits the diagnostic capabilities of imaging.

Evaluation of the bronchioles, defined as peripheral airways that do not contain cartilage, requires an understanding of the anatomy of the secondary pulmonary lobule, the smallest portion of lung that is surrounded by connective tissue septa. The lobular bronchioles measure no more than 1 mm in diameter (75), and their walls are less than 0.1 mm thick. Normal bronchioles are generally not seen on thin-section CT. However, bronchiolar abnormalities may be detected when there is thickening of the bronchiolar wall, peribronchiolar inflammation and fibrosis, and bronchiolectasis with or without filling of the dilated bronchiole with secretions (76).

Another CT feature of bronchiolar (small-airway) disease is a mosaic pattern of lung attenuation, which can also be seen with pulmonary vascular and infiltrative lung diseases. In cases of small airway disease, areas of variable lung attenuation that form a mosaic pattern, which is accentuated during forced exhalation, represent air trapping, hypoxic vasoconstriction, and mechanical pressure on blood vessels (77). On CT obtained at end-exhalation, air trapping is recognized as a lack of increase in attenuation or a lack of decrease in volume of areas of abnormally lucent lung. In small airway disease, the size and number of vessels in the abnormally lucent area of lung are decreased relative to areas of higher-attenuation lung. Air trapping can

also be seen with pulmonary vascular diseases but not with infiltrative diseases.

Obliterative bronchiolitis (OB) is defined pathologically as irreversible fibrosis of small-airway walls that causes the airway lumina to become narrow or obliterated. Constrictive bronchiolitis, which emphasizes the fibrotic and extrinsic nature of the lesion, is a synonym for OB and is favored by some authors since it avoids confusion with bronchiolitis obliterans organizing pneumonia, a condition now more commonly referred to as cryptogenic organizing pneumonia (78). The clinical criteria
used for the diagnosis of OB are irreversible airflow limitation, with a forced expiratory volume in 1 second (FEV₁) that is less than 60% of the predicted value, in the absence of emphysema, chronic bronchitis, asthma, or other cause of airway obstruction (79). OB and cryptogenic organizing pneumonia are not thought to be related, although they can occur as a result of similar etiologic factors. Both are commonly idiopathic. OB is a common sequela of heart or lung transplantation, representing chronic rejection in lung transplantation, and bone marrow transplantation, where OB represents chronic graft-versus-host disease. OB is a component of Swyer-James syndrome related to childhood viral infection (Fig. 13-26).

Chest radiographs are usually normal in OB but can show slowly progressive hyperinflation. The CT findings include bronchiolectasis, centrilobular branching structures and nodules caused by peribronchiolar thickening and bronchiolectasis with secretions (80), and mosaic lung attenuation (80,81,82,83) (Fig. 13-27). Interpretation of air trapping must be made with caution, because occasional isolated areas of lobular air trapping can be seen in healthy individuals (51).

Pathologic changes occur in the small airways of essentially all smokers (84,85). Respiratory bronchiolitis, also referred to as smoker's bronchiolitis (86,87), involves the respiratory bronchioles and is characterized by mild chronic inflammation of the bronchioles associated with accumulation of pigmented macrophages in respiratory bronchioles and adjacent alveoli. The condition may be severe enough to produce clinical symptoms of cough and shortness of breath and to produce CT abnormalities, including areas of ground-glass attenuation, centrilobular micronodules, and air trapping (88,89). The abnormalities usually involve predominantly the upper lungs (a distribution similar to that of smoking-related centrilobular emphysema) but may be diffuse.

Diffuse panbronchiolitis is an inflammatory lung disease of unclear etiology that is prevalent in Asians and rare in Europeans and North Americans. Histologically, there is thickening of the walls of respiratory bronchioles and associated peribronchiolitis, and, in advanced stages, bronchiolectasis (90). The chest radiograph can show disseminated small nodular opacities up to 2 mm in size (91). The findings on CT have been classified into four types: (a) nodules alone, (b) nodules associated with branching linear opacities, (c) nodules with ring-shaped or small tubular opacities (probable bronchiolectasis), and (d) large cystic opacities accompanied by dilated proximal bronchi (90,92) (Figs. 13-28 and 13-29).

Bronchopneumonia, regardless of the type of infectious agent, can result in centrilobular nodules or branching structures on CT, which are related to peribronchiolar consolidation or pus-filled small airways (91), and it is the most common cause of the "tree-in-bud" pattern seen on CT (93,94) (Figs. 13-30, 13-31, 13-32, 13-33, 13-34). Aspiration of infected or other material into the small airways is another common cause of the tree-in-bud pattern (Figs. 13-35 and 13-36).

Pulmonary emphysema, as defined by the National Heart, Lung, and Blood Institute, is “an abnormal permanent enlargement of the airspaces distal to the terminal bronchioles, accompanied by destruction of the alveolar walls, and without obvious fibrosis” (95). Three different morphologic subtypes of emphysema have been described according to their location in the secondary pulmonary lobule: centrilobular, panlobular, and paraseptal (distal lobular) (Table 13-6). A fourth type of emphysema, paracicatricial emphysema, results from and is
always associated with pulmonary fibrosis and therefore does not meet the strict definition of emphysema.

Emphysema is found at autopsy in up to 66% of adult patients (96,97), but clinical detection of disease during life is difficult unless the condition is advanced. The presence of airflow obstruction alone is a sensitive indicator of the presence of emphysema but is not specific, since asthma, irreversible small-airway disease, and certain forms of interstitial lung disease may also result in decreased FEV₁ (95,98,99). Evidence of impairment in gas transfer, as assessed with carbon monoxide diffusing capacity, is more sensitive than abnormal spirometry for the diagnosis of emphysema; it is also nonspecific, however, and patients may have up to 30% of their lung involved with emphysema but have no evidence of functional impairment (100). The accuracy of diagnosis based on findings from chest radiographs depends on the severity of parenchymal destruction (101,102). CT findings correlate with the presence and severity of morphologic emphysema better than chest radiographic findings or results of pulmonary function tests (103,104), although several studies that assessed CT with 10-mm and 1-mm collimation concluded that CT consistently underestimates the extent of centrilobular and panlobular emphysema and the severity of emphysema when compared with pathologic assessment (105,106,107,108). In spite of these limitations, CT is currently the best way to detect emphysema in living patients.

The most common form of emphysema, centrilobular emphysema, is strongly associated with cigarette smoking, with the severity of emphysema increasing with the number of cigarettes smoked (109,110). Centrilobular emphysema results from destruction of alveoli around the proximal respiratory bronchiole and characteristically has a predominantly upper lung distribution. Although the upper lungs are more severely affected by emphysema, the degree of emphysema in the lower lungs has a stronger correlation with pulmonary function abnormalities. This indicates that the upper lungs are physiologically a relatively silent region, where extensive destruction may occur before functional abnormalities become detectable (111).

Panlobular emphysema has a characteristic lower lobe–predominant distribution; this is the type of emphysema seen in patients with α-1-antitrypsin deficiency (Fig. 13-37). In panlobular emphysema, the alveoli are destroyed throughout the secondary pulmonary lobule. The same findings of basilar

emphysema can be seen in patients who intravenously inject methylphenidate (crushed Ritalin tablets) (Fig. 13-38) (112).

Paraseptal emphysema is a focal or multifocal abnormality involving the periphery of the pulmonary lobule that is almost always seen in the periphery of the lung along the fissures and at sharp pleural reflections. Coalescence of paraseptal emphysema leads to the formation of bullae and is important in the development of spontaneous pneumothorax (113,114). Paraseptal emphysema should not be confused with honeycombing, which has thicker walls and is associated with fibrosis (115).

Criteria for chest radiographic diagnosis of emphysema include two or more of the following:

Other findings include increased AP diameter of the chest, saber-sheath configuration of the trachea, narrow cardiomediastinal silhouette, and enlargement of the central pulmonary arteries and right ventricle when pulmonary artery hypertension and cor pulmonale are present, respectively.

Thin-section CT shows centrilobular emphysema as focal areas of low attenuation up to 1 cm in diameter within a homogeneous background of lung parenchyma; occasionally, this results in a “Swiss cheese” appearance. These areas of low attenuation are usually round or oval, have no definable wall, and are often associated with a small centrilobular "dot" representing the normal centrilobular core structures (Fig. 13-40). The appearance of panlobular emphysema on CT is large, extensive areas of uniform low attenuation with a lower lobe–predominant distribution associated with a reduction in the size and number of pulmonary vessels. No peripheral preservation of the lobule occurs, and therefore no striking difference in density exists between affected lobules and a homogeneous background of normal pulmonary parenchyma. Because of this, mild to moderate disease can be easily missed and the extent of disease underestimated (116). Paraseptal emphysema appears as multiple small subpleural airspaces ranging from a few millimeters to 1 cm in diameter (117).

Bullae, a term used synonymously with blebs, are described as air-filled structures greater than 1 cm in diameter, with thin walls, occurring in a subpleural or intraparenchymal location. They are usually multiple or associated with paraseptal, centrilobular, or panlobular emphysema (118). Giant bullous emphysema, or vanishing lung syndrome (Fig. 13-41), is
characterized by large bullae that are several centimeters in diameter and in some cases large enough to fill an entire hemithorax. When giant bullae impair pulmonary function and are associated with compressed lung on CT, the usual method of treatment is surgical resection (bullectomy) (119). On occasion, bullae can become infected and present as cystic masses with air–fluid levels (Figs. 13-42 and 13-43).