Editors: Collins, Jannette; Stern, Eric J.
Title: Chest Radiology: The Essentials, 2nd Edition
> Table of Contents > Chapter 13 - Airways
Chapter 13
Airway disorders can be categorized into those that involve the trachea, those that involve the bronchi, and those that involve the bronchioles, the smallest branching airways leading to alveoli. Many disorders can, and frequently do, involve more than one airway compartment. Tracheal disorders will be discussed first, and disorders involving both the bronchi and bronchioles will be discussed together. The reader is referred to Chapter 1 for a discussion of normal anatomy of the airways.
Tracheal Disorders
Tracheal shape varies, depending on the phase of the respiratory cycle. The intrathoracic trachea is round or elliptic on inspiration images and flat or horseshoe shaped during and at the end of a forced exhalation as a result of anterior bowing of the posterior noncartilaginous tracheal membrane during exhalation (1). Upper limits of normal for coronal and sagittal tracheal dimensions, respectively, as determined by chest radiographs, are 25 mm and 27 mm for men and 21 mm and 23 mm for women. The lower limit of normal for both dimensions is 13 mm in men and 10 mm in women (2). Mean measurements on computed tomography (CT) of anteroposterior (AP) and transverse diameters of the extrathoracic trachea, respectively, are 20.1 mm and 18.4 mm (3); these can increase by as much as 15% in men with aging (4).
CT is superior to chest radiography for detection of abnormalities of the trachea and main bronchi; sensitivities in detecting disease on chest radiographs and CT are 66% and 97%, respectively (5). Spiral multidetector CT, which allows for the acquisition of a whole thoracic volume during a single breath-hold, eliminating respiratory motion, is the technique of choice for noninvasive imaging of the airways. Volume acquisition with multidetector CT has fostered a renewed interest in two-dimensional and three-dimensional (3D) reconstructions applied to the tracheobronchial tree. Potential clinical applications of 3D reconstructions, such as shaded surface display and volume rendering, include assisting with diagnoses, replacing bronchoscopy in some instances, and helping in surgical planning and endobronchial treatments (6). In the case of a lesion that completely obstructs an airway, CT allows visualization of the airway beyond the obstruction. However, CT's virtual bronchoscopy is currently unable to show mucosal detail, and 3D postprocessing methods are time consuming to perform and rarely performed in routine clinical practice.
Patients with tracheal disease can be asymptomatic or may present with cough, dyspnea, wheezing, or stridor. Because of the variety of conditions that can cause wheezing, a misdiagnosis of asthma is common (7). Tracheal disorders are generally organized into those that cause tracheal widening and those that cause narrowing. CT can demonstrate the degree of widening or narrowing, in addition to the location and extent of tracheal abnormality; it can also demonstrate the presence of associated extraluminal disease, postobstructive atelectasis, and pneumonia. Magnetic resonance imaging (MRI) is a valuable method for observing the trachea because of its multiplanar demonstration of the airway, the mediastinal vessels, and the other structures simultaneously, without the need for contrast medium or exposing the patient to radiation. MRI is particularly useful in children and in patients with either vascular rings or tracheal compression by the innominate artery.
Disorders That Cause Tracheal Widening
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).
   Mounier-Kuhn syndrome
      Normal aging
      Chronic airway infection
      Cigarette smoking
      Chronic bronchitis
      Cystic fibrosis
      Diffuse pulmonary fibrosis
      Playing wind instruments
      Inhalation of noxious fumes
      Chronic intubation
      Ehlers-Danlos syndrome
      Cutis laxa
      Marfan syndrome
      Ankylosing spondylitis
      Kenny-Caffey syndrome
      Brachmann-de Lange syndrome
      Connective tissue diseases
      Bruton-type agammaglobulinemia
      Light-chain deposition disease
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).
Disorders That Cause Tracheal Narrowing
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).
   Masses (e.g., thyroid, aberrant vessels, enlarged nodes)
   Fibrosing mediastinitis
   Congenital tracheal narrowing
   Granulomatous disorders (e.g., Wegener granulomatosis, sarcoidosis)
   Trauma, including intubation
   Relapsing polychondritis
   Tracheobronchopathia osteochondroplastica
   Saber-sheath trachea
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.

FIGURE 13-1. Saber-sheath trachea. A: Posteroanterior (PA) chest radiograph of a 67-year-old man with emphysema shows narrowing of the coronal diameter of the trachea (arrow). B: On the lateral view, the tracheal diameter (arrow) is normal or slightly enlarged.
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).
FIGURE 13-2. Tracheobronchial amyloidosis. A: CT of a 44-year-old woman with dysphagia and dyspnea on exertion shows circumferential thickening and calcification of the tracheal wall (arrow). B: CT at a more inferior level shows thickening of the walls of the main bronchi (arrow). C: CT at a level inferior to (B) shows thickening of the wall of the right upper lobe bronchus (solid arrow) and left main bronchus (dashed arrow).
FIGURE 13-3. Mucous plugging. A: PA chest radiograph of a 45-year-old woman with shortness of breath after cervical spine fusion shows abnormal opacities in both lower lobes and volume loss in the left lower lobe. B: CT shows low-attenuation material in both lower lobe segmental bronchi (solid arrows), postobstructive atelectasis of the left lower lobe (A), and posteromedial displacement of the left major fissure (dashed arrow).
Tracheobronchial Filling Defects
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
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).
FIGURE 13-4. Tracheal adenoid cystic carcinoma. A: PA chest radiograph of a 59-year-old man with recurrent right middle lobe pneumonia and cough shows a linear band of opacity in the right middle lung (arrow). B: Lateral view shows an oblique band of opacity paralleling the inferior aspect of the major fissure (arrow). (Continued ) C: Coronal reformatted CT shows a lobular soft tissue mass (arrow) almost completely obstructing the lumen of the trachea just above the level of the carina. D: Axial CT shows that the mass (arrow) almost completely fills the lumen of the trachea. E: CT at the level of the inferior pulmonary veins shows postobstructive atelectasis and pneumonia in the right middle lobe.
FIGURE 13-5. Metastatic endometrial carcinoma. A: PA chest radiograph of a 75-year-old woman shows a mass adjacent to the right hilum. B: Lateral view shows the mass projected over the heart. The contours of the mass suggest that it is related to the inferior edge of the major fissure. C: Axial CT shows low-attenuation material obliterating the lumen of the right middle lobe bronchus (arrow). Note that the bronchial wall is outlined by calcium. Tumor is growing through the bronchus into the right middle lobe. The opacity seen on the chest radiograph represents tumor and collapsed right middle lobe.
FIGURE 13-6. Endobronchial non–small cell carcinoma. CT of a 63-year-old woman with cough shows a soft tissue mass that almost completely obliterates the lumen of the bronchus intermedius (arrow).
FIGURE 13-7. Tracheal metastasis. CT of a 64-year-old woman with renal cell carcinoma shows a soft tissue mass adjacent to the anterior wall of the trachea (arrow) representing one of many biopsy-proven metastases to the trachea. Mucus could have a similar appearance, but it will usually clear with a repeated scan after the patient clears the throat.
FIGURE 13-8. Endobronchial blood clot. CT of a 10-year-old girl with leukemia and Rhizopus necrotizing pneumonia shows a filling defect occluding the bronchus intermedius (arrow). The right middle and lower lobes were surgically resected; pathology showed pulmonary artery and vein thromboses, diffuse pneumonia and pulmonary hemorrhage, and clotted blood in the bronchus intermedius.
FIGURE 13-9. Endobronchial foreign body. CT shows a radiopaque foreign body (arrow) in the left main bronchus. Note the hyperlucency and hyperinflation of the left upper and lower lobes secondary to air trapping. A chicken bone was removed from the airway.
Congenital Tracheobronchial Anomalies
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).
FIGURE 13-10. Broncholithiasis. CT of a 72-year-old man with cough, wheezing, and increasing shortness of breath shows large calcifications in the left hilum and left lower lobe bronchus (arrows). Note postobstructive atelectasis of the left lower lobe.

FIGURE 13-11. Aberrant tracheal bronchus. CT of a 58-year-old woman with cough and recurrent pneumonia shows a so-called "pig bronchus" (arrow) arising from the right lateral wall of the trachea, above the level of the carina.
Chronic Obstructive Pulmonary Disease
COPD refers to a group of disorders characterized by chronic or recurrent obstruction to airflow. Five principal disorders fall under this heading (Table 13-3), although the term is commonly used clinically to refer to emphysema. Because these disorders are sometimes difficult to distinguish from one another on chest radiography, the term COPD should not be restricted to emphysema.
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).
Chronic bronchitis/bronchiolitis
Cystic fibrosis
FIGURE 13-12. Allergic bronchopulmonary aspergillosis. PA chest radiograph of a 64-year-old woman with a long history of asthma shows multiple tubular opacities in the left upper lobe (arrows) representing dilated bronchi filled with mucus, debris, and fungal hyphae.
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).
FIGURE 13-13. Allergic bronchopulmonary aspergillosis. CT shows central bronchial dilatation and impaction (solid arrow). Peripheral nodular and ground-glass opacities (dashed arrow) represent impaction of small airways and peribronchiolar inflammation.
FIGURE 13-14. Asthma. A: Inspiratory CT of a 34-year-old woman with steroid-dependent asthma is normal. Note the round contour of the trachea. B: Expiratory image at the same level as (A) shows areas of lucency (L), representing air trapping. Note the flattened posterior tracheal contour on expiration.
FIGURE 13-15. Pneumomediastinum. PA chest radiograph of a patient with asthma shows mediastinal air outlining the heart (dashed arrow) and extending into the neck bilaterally (solid arrows).
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.
Mucous plugging (ABPA)
Mitral stenosis (patients can present with symptoms of "asthma")
Endotracheal lesion (e.g., patients with carcinoid tumor can present with symptoms of "asthma")
ABPA, allergic bronchopulmonary aspergillosis.
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.
FIGURE 13-16. Cylindric bronchiectasis. Coronal reformatted CT shows smooth, uniformly dilated bronchi (arrows), predominantly in the lower lungs.
FIGURE 13-17. Atypical mycobacterial bronchiolitis. A: CT shows tree-in-bud opacities in the periphery of the right lower lobe (arrows). B: Thin-section CT (1.25 mm) shows cylindric bronchiolectasis in the right lower lobe (arrow). Note bronchiolar opacities in the left lower lobe as well.
FIGURE 13-18. Varicose and cystic bronchiectasis. CT of a 66-year-old man shows dilated bronchi and bronchioles. In profile, some of the bronchiectatic airways have the "beaded" appearance of varicose bronchiectasis (straight arrow); in cross section, some are grouped together like a "cluster of grapes," as is seen with cystic bronchiectasis (curved arrow). The bronchial and bronchiolar walls are thickened. Some of the dilated bronchioles are filled with mucus, forming peripheral nodular opacities (arrowhead).
FIGURE 13-19. Cystic bronchiectasis. CT of a 65-year-old woman with a history of Pasteurella multocida infection of the lungs shows dilated bronchi and bronchioles, forming a "cluster of grapes" in the right lower lobe (arrows).
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.
FIGURE 13-20. Cystic bronchiectasis. A: PA chest radiograph of a 78-year-old woman shows increased "interstitial markings" in the lower lungs. There is a prominent thin-walled "ring shadow" in the right middle lobe (arrows). B: CT shows thick-walled, dilated bronchioles forming a “cluster of grapes” in the right lower lobe and a large dilated airway in the right middle lobe (arrow). Patchy areas of dense airspace opacity in the right lower lobe were thought to represent acute pneumonia.
Retraction of parenchyma (fibrosis)
Obstruction by foreign body
Neoplastic obstruction
Cartilage deficiency (Williams-Campbell syndrome)
Cilia syndrome (Kartagener syndrome)
Host defenses down (agammaglobulinemia)
Cystic fibrosis
Chronic granulomatous disease
Allergic bronchopulmonary aspergillosis
Swyer-James syndrome
Inhalation injury (ammonia, gastric acid)
Sarcoidosis (uncommon)

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.
Cystic Fibrosis
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
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.
FIGURE 13-21. Kartagener syndrome. A: PA chest radiograph of a woman with sinusitis shows dextrocardia, a right-sided stomach bubble (S), and subtle bilateral lower lobe "interstitial" opacities. B: CT shows bilateral lower lobe cystic bronchiectasis.
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.
FIGURE 13-22. Kartagener syndrome. A: PA chest radiograph of a 56-year-old man shows dextrocardia and a right-sided stomach bubble. B: CT confirms dextrocardia. C: CT at a level inferior to (B) shows situs inversus, with the liver on the left and the spleen on the right. D: CT with lung windowing shows bibasilar bronchiectasis (arrows).
FIGURE 13-23. Cystic fibrosis. A: PA chest radiograph of a 28-year-old woman with cystic fibrosis shows bilateral bronchiectasis, along with tubular and nodular opacities representing mucus-impacted airways. B: CT shows mucous plugging of distal airways (arrow).
FIGURE 13-24. Cystic fibrosis. A: CT of a 22-year-old man with cystic fibrosis shows extensive bronchiectasis and bronchial wall thickening in the upper lobes. B: CT at a more inferior level shows extensive involvement of the small airways.
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).
FIGURE 13-25. Cystic fibrosis. A: PA chest radiograph of a 19-year-old man with cystic fibrosis shows diffuse bilateral bronchiectasis, bronchial wall thickening, and areas of mucoid impaction. B: CT of the upper lungs confirms the chest radiographic findings. C: CT at a more inferior level shows normal lung bases. Early in the course of the disease process, it is typical for the upper lungs to be more involved than the lower lungs. As the disease progresses, an upper lung predominance may not be appreciated.
FIGURE 13-26. Swyer-James syndrome. CT shows bronchiectasis, bronchiolectasis, tree-in-bud opacities, and bullae in the right lung. The right lung is hyperlucent as a result of air trapping related to obliterative bronchiolitis. The right pulmonary artery is diminutive, another feature of Swyer-James syndrome.
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).
FIGURE 13-27. Obliterative bronchiolitis. A: Inspiratory CT of a 40-year-old woman with a transplanted heart is normal. B: Expiratory CT shows diffuse areas of abnormal parenchymal lucency, representing air trapping. Note anterior bowing of posterior membranous trachea on expiration (arrow).
FIGURE 13-28. Diffuse panbronchiolitis. CT of a 61-year-old nonsmoking man with chronic sinusitis, cough, fever, shortness of breath, scant sputum production, and mixed restrictive and obstructive pattern on pulmonary function tests shows cylindric bronchiectasis and bronchiolectasis (arrows), along with peripheral nodular and linear branching opacities representing dilated, impacted bronchioles (arrowheads).
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.

FIGURE 13-29. Diffuse panbronchiolitis. CT shows diffuse bronchiectasis, bronchiolectasis, airway wall thickening, and tree-in-bud opacities.
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).
FIGURE 13-30. Infectious bronchiolitis. Maximum-intensity-projection CT of an 81-year-old man with cough and weight loss shows diffuse bronchiolitis, with prominent tree-in-bud opacities in the periphery of the lungs.
FIGURE 13-31. Atypical mycobacterial bronchiolitis. CT of an 88-year-old woman with chronic cough shows varicose and cystic bronchiectasis in the right middle lobe and tree-in-bud opacities in the right lower lobe (arrow).
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.
FIGURE 13-32. Atypical mycobacterial bronchiolitis. CT of a 69-year-old woman with fever and cough shows tree-in-bud opacities in the right middle lobe (solid arrow) and cylindric bronchiectasis in the lingula (dashed arrow).
FIGURE 13-33. Mycobacterium avium complex infection. A: CT of a 58-year-old woman shows cylindric bronchiectasis in the right middle lobe. B: Extensive bronchiectasis, bronchiolectasis, airway wall thickening, and tree-in-bud opacities are seen in the lower lobes.
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.
FIGURE 13-34. Infectious bronchiolitis. CT of a 72-year-old man with methicillin-resistant Staphylococcus aureus pneumonia shows tree-in-bud opacities in the right lower lobe and dense airspace opacity and volume loss in the left lower lobe.
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).
FIGURE 13-35. Aspiration. CT of a 77-year-old man with recurrent aspiration shows bronchiectasis and tree-in-bud opacities in both lower lobes.
FIGURE 13-36. Aspiration. CT of a 57-year-old man with Parkinson disease and recurrent aspiration shows diffuse tree-in-bud opacities.
FIGURE 13-37. Alpha-1-antitrypsin deficiency. A: PA chest radiograph of a 43-year-old man shows hyperinflation and hyperlucency in the lower lungs. B: Lateral view shows increased retrosternal lucency and flattening of the diaphragm. C: CT shows bullous emphysema in the lower lungs. D: CT at a more superior level shows less severe emphysema. Compared with smoking-related centrilobular emphysema, emphysema caused by α-1-antitrypsin deficiency, although diffuse, is more severe in the lower lungs.
   Involves central portion of secondary pulmonary lobule
   Usually a result of cigarette smoking
   Upper lung–predominant distribution
   "Swiss cheese" appearance on CT (early on)
   Involves entire secondary pulmonary lobule
   Seen in α-1-antitrypsin deficiency
   Lower lung–predominant distribution
   "Lung simplification" appearance on CT
   Bullae or air cysts in a subpleural location
   Associated with spontaneous pneumothorax
   Most common in lung apices
CT, computed tomography.
FIGURE 13-38. Methylphenidate (Ritalin) lung. PA chest radiograph of a woman who intravenously injected crushed Ritalin tablets shows emphysema in the lower lungs, which is similar in appearance to the findings of α-1-antitrypsin deficiency.
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:
  • Depression and flattening of the diaphragm on the posteroanterior (PA) chest radiograph and blunting of costophrenic angles, with the actual level of the diaphragm not as significant as the contour (this can be determined from a straight line connecting the costophrenic junction to the vertebrophrenic junction on each side; if the highest level of the diaphragm contour is less than 1.5 cm above this line, the diaphragm can be recorded as flat).
  • Irregular radiolucency of the lung, as a result of irregularity in distribution of the emphysematous tissue destruction.
  • Increased retrosternal radiolucency, as seen on the lateral view, measuring 2.5 cm or more from the sternum to the most anterior margin of the ascending aorta.
  • Flattening or even concavity of the diaphragm contour on the lateral chest radiograph, as determined by the presence of a sternodiaphragmatic angle of 90 degrees or larger (100) (Fig. 13-39).
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.
FIGURE 13-39. Emphysema. PA (A) and lateral (B) chest radiographs of a long-time cigarette smoker show flattening of the diaphragm with blunting of the costophrenic angles, increased retrosternal lucency, increased AP diameter of the chest, and prominent central pulmonary arteries. Note that the sternodiaphragmatic angle is greater than 90 degrees, indicative of extreme flattening and even minimal concavity of the diaphragmatic contour, as seen on the lateral view.

FIGURE 13-40. Centrilobular emphysema. CT of a 50-year-old woman with a long history of cigarette smoking shows focal areas of low attenuation creating a "Swiss cheese" appearance. Note a central nodular opacity within several of the areas of lucency (arrows); these opacities represent the lobular arteries. This finding helps to distinguish emphysema from cystic lung diseases.
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).
FIGURE 13-41. Giant bullous emphysema. PA (A) and lateral (B) chest radiographs of a 47-year-old man with progressive severe shortness of breath shows marked hyperinflation and hyperlucency of the lungs. The vascular markings are sparse (so-called vanishing lung sign), with the majority of residual perfusion going to the right medial base. The crowding of vascular markings at the right lung base, from compressive emphysema, should not be mistaken for focal pneumonia. Pneumothorax can be confused with this appearance; in some cases, CT is the only way to exclude a pneumothorax. C: CT shows a huge bulla in the left upper lobe that displaces the major fissure posteriorly and medially (arrows) and a prominent bulla in the right lower lobe. Note abnormal lucent emphysematous spaces within the lungs (L).
FIGURE 13-42. Infected bulla. A: PA chest radiograph of a 68-year-old man shows an air–fluid level in the right medial base (arrow). B: Lateral view shows a thin-walled cystic structure with an air–fluid level (arrow) in the right middle lobe. C: CT confirms a thin-walled bulla with an air–fluid level in the right middle lobe (arrow). Additional bullae were seen at several other levels on CT.
FIGURE 13-43. Infected bulla. CT of a 39-year-old man with α-1-antitrypsin deficiency shows a bulla with an air–fluid level (arrow) in the left lower lobe.
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