Pulmonary Hyperlucency in Adults

Transcription

1 Residents Section Pattern of the Month Nemec et al. Pulmonary Hyperlucency in dults Residents Section Pattern of the Month Residents inradiology Stefan F. Nemec 1 lexander. ankier Ronald L. Eisenberg Nemec SF, ankier, Eisenberg RL Keywords: CT, hyperlucency, hypoattenuation, lung, radiography DOI: /JR Received March 14, 2012; accepted after revision March 30, ll authors: Department of Radiology, eth Israel Deaconess Medical Center, Harvard Medical School, 330 rookline ve, oston, M ddress correspondence to R. L. Eisenberg (rleisenb@bidmc.harvard.edu). WE This is a Web exclusive article. JR 2013; 200:W101 W X/13/2002 W101 merican Roentgen Ray Society Pulmonary Hyperlucency in dults D ecreased lung density on images can be described as pulmonary hyperlucency on conventional chest radiographs and hypoattenuation on CT scans. For the purpose of this review, we call both findings hyperlucency. bnormal lucency of the lung can be unilateral or bilateral, focal or diffuse. Hyperlucency on images can result from an excess of air in the pulmonary parenchyma or a decrease in mass of the pulmonary parenchyma caused by a reduction in vasculature or blood flow, reduction or obliteration of airways, or a combination of these potential causes. Even in adults, pulmonary hyperlucency can be caused by congenital and acquired conditions. n understanding of the broad differential diagnosis of pulmonary hyperlucency is necessary to determine the underlying cause and provide appropriate patient care. The most common causes of pulmonary hyperlucency in adults are summarized in Figure 1. Technical Requirements and Diagnostic Pitfalls Inadequate patient position (patient rotation) resulting in one hemithorax being closer to the receiver plate is one technical cause of asymmetric radiodensity of the lungs (Fig. 2). similar technical pitfall is improper positioning of the lung apices or bases, which also results in asymmetric radiodensity between the lower and upper zones. From a practical perspective, in a patient with an apparently hyperlucent hemithorax, concomitant decreased density of the ipsilateral soft tissues and ribs suggests that the cause is likely a technical artifact (Fig. 2). Conversely, if the soft issues are of symmetric density and one lung is more lucent than the other, the patient probably has a real pulmonary abnormality. CT can produce a variety of artifacts caused by the imaging plate, plate reader, image-processing software, laser printer, or operator error. t CT, the diagnosis of lung hyperlucency requires full inspiration, which is best obtained by coaching the patient. The window settings must be set at a width of at least 1000 HU to avoid an artificial increase in image contrast with subsequent overemphasis of low-attenuation (dark) areas. Insufficient inspiration can result in air trapping, Focal Unilateral ronchial atresia ullae Endobronchial obstruction roncholith Endobronchial tumor Foreign body Pulmonary thromboembolism Pulmonary Hyperlucency Diffuse Chest wall abnormalities Mastectomy Poland syndrome Compensatory overinflation telectasis Pneumonectomy Pneumothorax Pneumatocele Swyer-James syndrome Focal ilateral ullae Emphysema (early stage) Multiple cysts Diffuse sthma ronchiolitis Cystic fibrosis Emphysema (advanced stage) Technical error Fig. 1 Flowchart shows differential diagnosis of unilateral and bilateral pulmonary hyperlucencies. JR:200, February 2013 W101

2 Nemec et al. which manifests itself as zones of low attenuation in the lung parenchyma. lthough often a sign of lung disease, air trapping can be a physiologic finding with increasing age. ir trapping is also seen in smokers with normal pulmonary function test results and correlates with the severity of the smoking history. The diagnosis of air trapping often requires expiratory scans that increase the natural contrast between healthy (lighter, or high attenuation) and trapped (darker, or low attenuation) zones of lung parenchyma. Unilateral Focal Findings ronchial tresia ronchial atresia is congenital airway obstruction that most commonly affects the apical posterior segment of the left upper lobe (less frequent on the right). It can be asymptomatic and found incidentally (50%) or cause recurrent pulmonary infections, wheezing, and dyspnea. On images, focal hyperlucency of the affected lung is due to hyperinflation of lung peripheral to the atretic bronchial segment related to collateral ventilation (Fig. 3). On radiographs and on CT scans, a characteristic Fig year-old woman whose rotation has caused asymmetric lung radiodensity. Posteroanterior chest radiograph shows apparent hyperlucency of right hemithorax, especially in upper and mid lung zones. Concomitant decreased density of ipsilateral ribs (arrows) indicates technical artifact. feature of bronchial atresia is a branched parahilar tubular opacity (Y or V shaped) (Fig. 3). This finger-in-glove sign represents a bronchocele, which results from bronchial dilation due to mucoid impaction. CT is the imaging procedure of choice for delineating airway anatomy and to show the typical findings of bronchial atresia, which include hyperlucency based on air trapping and a mucus-filled bronchocele (Fig. 3). CT is superior to chest radiography for differentiating among the other conditions associated with mucoid impaction, such as cystic fibrosis, allergic bronchopulmonary aspergillosis, and carcinoid tumor. The mucoid impaction in bronchial atresia typically has low attenuation, unlike the high attenuation due to calcium deposits that is often observed in allergic bronchopulmonary aspergillosis and bronchocentric granulomatosis. CT also can be of value in guiding endoscopic procedures in patients with symptomatic bronchial atresia. Endobronchial Obstruction ronchial obstruction is incomplete or complete obliteration of the bronchial lumen by an intraluminal structure, which can represent a foreign body, mucus plug, or solid lesion (most often neoplastic). Incomplete bronchial obstruction can cause a check-valve mechanism, leading to overinflation of a pulmonary lobe or an entire lung, which is most apparent during expiration. Complete obstruction causes atelectasis of a lobe or an entire lung, with resulting compensatory overinflation and hyperlucency of an adjacent normal lobe or lobes that is most apparent on inspiration. The finding of hyperlucency after incomplete obstruction is primarily ipsilateral, whereas hyperlucency after complete obstruction can be ipsilateral or contralateral. Hyperlucency related to bronchial obstruction is equally well seen on radiographs and CT scans, though the latter also may directly depict the cause and exact location. Theoretically, a hyperlucent lung in an adult can be caused by extrinsic bronchial compression related to hilar lymphadenopathy secondary to neoplastic disease, infection, or granulomatous disease. From a practical point of view, however, pulmonary hyperlucency due to lymph node compression is hardly ever found, on either chest radiographs or CT scans. roncholith broncholith is a calcified peribronchial lymph node (usually from a granulomatous infection with tuberculosis or histoplasmosis) that erodes into an adjacent bronchus. broncholith can cause partial or complete bronchial obstruction and manifests itself as a small calcified focus in or immediately adjacent to an airway (most commonly the middle lobe bronchus). lthough broncholiths are difficult to identify on radiographs, CT not only W102 JR:200, February 2013

3 Pulmonary Hyperlucency in dults clearly shows a broncholith but also precisely depicts its location with respect to the bronchial wall and lumen, helping in differentiation of a true broncholith from a calcified peribronchial lymph node (Fig. 4). CT also shows more detail of associated findings such as air trapping, pneumonia, bronchiectasis, and mucoid impaction. Endobronchial tumors The classic endobronchial tumor is a bronchial carcinoid, a neuroendocrine neoplasm that accounts for approximately 2% of all pulmonary tumors. Carcinoids tend to occur in the main, lobar, and segmental bronchi and are less common in peripheral locations. On radiographs, a carcinoid tumor typically appears as a well-defined intrabronchial mass with secondary changes of hyperinflation or atelectasis. On CT images, a carcinoid tumor gener- Fig year-old woman with bronchial atresia., Posteroanterior chest radiograph shows hyperlucency of left upper lobe and branched parahilar tubular opacities (finger-in-glove sign) (arrows). and C, Transverse () and coronal (C) CT images of chest show hyperlucency of apical posterior segment of left upper lobe and mucus-filled bronchoceles (arrows). Fig. 4 roncholiths., 76-year-old woman. Coronal CT image of chest shows calcified lesion (broncholith) in apical segment bronchus (arrow) of right upper lobe and concomitant air trapping (hyperlucency). ilateral air trapping due to small airways disease also is evident., 55-year-old woman. Coronal CT image shows several broncholiths (arrows) in apical posterior segment bronchus of left upper lobe causing slight atelectatic changes. C JR:200, February 2013 W103

4 Nemec et al. C Fig year-old woman with endobronchial carcinoid., Transverse CT image of chest shows round tumor (arrow) in right main bronchus that has strong contrast enhancement., Transverse CT image shows intratumoral calcification (arrow). C, Transverse CT image shows moderate atelectatic changes in apical lower lobe segment (arrow) but distinct overinflation of right lung due to endobronchial obstruction. ally is a well-defined, round or ovoid, hypervascular lesion that typically has strong and homogeneous contrast enhancement (Fig. 5). Calcifications are present in approximately one third of cases (Fig. 5). The tumor may have a distinct extrabronchial component. ir trapping, mucus plugging, and postobstructive pneumonia may be present. t PET/CT, carcinoid tumors have less radionuclide uptake than squamous cell carcinomas and other endobronchial neoplasms, which tend to be more ill-defined and originate in a more peripheral location. Rare benign neoplasms (hamartomas, lipomas) also cause bronchial obstruction and can be distinguished on CT images by their characteristic popcorn calcifications and homogeneous fat attenuation. Foreign body aspiration spiration of a solid foreign body can occur in adults, especially those with impaired consciousness, swallowing disorders, and postsurgical structural abnormalities of the pharynx. Common examples of aspirated foreign bodies are food components and broken teeth. The acute presentation can be cough, shortness of breath, hemoptysis, or pneumonia. Chronic foreign body aspiration can result in recurrent infection and the development of bronchiectasis. Most aspirated foreign bodies are located in the right main bronchus because of its larger diameter and vertical course compared with the left side. Most are not radiopaque owing to their organic nature (e.g., food) and thus not directly visible on radiographs, though they can cause detectable overinflation secondary to partial bronchial obstruction. Complete bronchial obstruction results in segmental or complete lobar collapse. CT is far more sensitive than radiography for showing opaque and, in particular, nonopaque intrabronchial foreign bodies (Fig. 6). CT is also valuable for detecting abnormalities related to an aspirated foreign body, such as pneumonia and atelectasis, and for guiding its removal at bronchoscopy. Pulmonary Thromboembolism In both acute and chronic pulmonary embolism (PE), there may be a regional decrease in the number and diameter of identifiable pulmonary vessels owing to a regional reduction in W104 JR:200, February 2013

5 Pulmonary Hyperlucency in dults Fig year-old woman with foreign body aspiration., Coronal CT image of chest shows calcified foreign body (fish bone) in right lower lobe bronchus (arrow)., Transverse expiratory CT image shows air trapping (hyperlucency) due to partial bronchial obstruction (arrow). C Fig year-old woman with acute pulmonary embolism. and, Transverse () and coronal () contrastenhanced CT images of chest show multiple bilateral filling defects in pulmonary arteries (arrows). C, Transverse CT image also shows evidence of mosaic attenuation due to pulmonary perfusion disorder. Lung cyst (arrow) is unrelated finding. JR:200, February 2013 W105

6 Nemec et al. pulmonary blood volume secondary to obstruction of a lobar or segmental pulmonary artery. The resulting oligemia (Westermark sign) due to acute PE can present as hyperlucency on chest radiographs. Other radiographic indications of acute pulmonary embolism include central pulmonary artery enlargement (Fleischner sign), a pleurabased opacity (Hampton hump), pleural effusion, and an elevated hemidiaphragm. CT shows a focal reduction of the affected vasculature (local oligemia) in patients with acute or chronic PE. Many patients with acute disease have a local decrease in diameter of pulmonary arteries in the involved area. In chronic PE, the pulmonary vessels may completely disappear. The resulting hyperlucencies can mimic air trapping, but the regional paucity or absence of vessels is a helpful diagnostic clue (Fig. 7). dditional expiratory scans can aid in diagnosis, as can Fig year-old woman who has undergone mastectomy. Posteroanterior chest radiograph shows apparent hyperlucency of lower right lung (arrows) caused by lack of soft tissue. the presence of other signs suggesting PE, such as arterial filling defects, intraarterial webs, and peripheral wedge-shaped parenchymal opacities (Fig. 7). Unilateral Diffuse Findings Chest Wall bnormalities Mastectomy On chest radiographs, pulmonary hyperlucency can be caused by the lack of soft tissue after mastectomy (Fig. 8). history of breast cancer surgery can be confirmatory. Poland syndrome Poland syndrome is a congenital chest wall anomaly associated with unilateral partial or complete absence of the pectoralis major muscle. There may also be hypoplasia or absence of the intercostal and shoulder muscles, rib anomalies, and aplasia of mammary tissue. ssociated findings include ipsilateral syndactyly, brachydactyly, and carpal bone anomalies. On radiographs, the affected hemithorax appears hyperlucent owing to the lack of anterior musculature (Fig. 9). Rib anomalies may also be seen. CT findings usually confirm the presence of the muscular and soft-tissue abnormalities, and CT with MRI may be valuable for planning chest-wall and breast reconstruction (Fig. 9). Fig year-old man with Poland syndrome., Posteroanterior chest radiograph shows apparent hyperlucency of entire left hemithorax., Transverse contrast-enhanced CT image of chest shows absence of pectoral musculature and nipple on left side (arrow). W106 JR:200, February 2013

7 Pulmonary Hyperlucency in dults Fig year-old man with normal postoperative findings after left upper lobectomy. Transverse CT image of chest shows hyperlucency on left side caused by overinflation compensating for volume loss. Widely separated vessels are relatively decreased in number compared with normal right side. There is minimal mediastinal shift toward left side and slight posterior displacement of left hilum after upper lobe bronchus transection (arrow). Compensatory Overinflation If a part of the lung is collapsed because of atelectasis or has been removed in lobectomy, the remaining lung parenchyma occupies the entire available space in the thorax (Figs. 10 and 11). The result is a relative increase in the volume of air and a decrease in the amount of pulmonary parenchyma. These effects combine to produce the appearance of a hyperlucent lung. When the amount of parenchymal loss is small, compensatory overinflation is limited to the ipsilateral hemithorax. Greater amounts of parenchymal loss can lead to overinflation of both the ipsilateral and contralateral lungs with potential mediastinal displacement toward the side of volume loss (e.g., in complete atelectasis of more than one lobe). On chest radiographs and CT scans, the hyperlucency caused by overinflation is accompanied by relative oligemia, characterized by vessels being more widely separated and relatively fewer in number compared with the parenchyma that is not overinflated (Fig. 10). Compensatory overinflation can be an indirect sign of atelectasis when the parenchymal opacity caused by atelectasis is not readily visible. For example, the paraaortic crescent of hyperlucency (luftsichel sign) that is characteristic of left upper lobe atelectasis on frontal chest radiographs is often better seen than the collapsed lung itself (Fig. 11). This sign is caused by the hyperexpanded superior segment of the left lower lobe interposed between the aortic arch and the atelectatic left upper lobe. The direct diagnosis of atelectasis is easier with CT than with radiography, especially when atelectasis involves the middle lobe or lingula or the atelectatic area is small. Compensatory overinflation and the resulting hyperlucency are evident on CT scans, but these findings have less importance than the chest radiographic findings because CT can directly show the atelectatic lung. Fig year-old woman with luftsichel sign in left upper lobe atelectasis caused by central bronchial carcinoma., Posteroanterior chest radiograph shows paraaortic crescent of hyperlucency (white arrows) caused by interposition of hyperexpanded lower lobe between aortic arch and atelectatic upper lobe. lack arrow indicates elevation of left hemidiaphragm., Lateral radiograph shows collapsed left upper lobe (white arrows). There is distinct left hemidiaphragm elevation (black arrow) due to phrenic nerve involvement. JR:200, February 2013 W107

8 Nemec et al. Fig year-old man with pneumothorax. nteroposterior chest radiograph shows abnormal air collection in right pleural space resulting in hyperlucency bounded by thin visceral pleural line (arrows). Distinct thoracic soft-tissue emphysema also is evident. Pneumothorax Pneumothorax is defined as abnormal air collection in the pleural space with variable collapse of the ipsilateral lung parenchyma, which results in hyperlucency on images. Pneumothorax can occur spontaneously (at times with evidence of apical bullae) or be iatrogenic or secondary to trauma, diffuse lung disease, or a neoplasm. Patients commonly present with sudden chest pain and dyspnea, though in rare cases pneumothorax is asymptomatic. On radiographs, pneumothorax is characterized by a thin visceral pleural line that parallels the chest wall and has no lung markings beyond it (Fig. 12). The line is most commonly in an apical or lateral-apical location. Radiographs are most helpful when acquired with the patient in an upright position. Supine studies are less sensitive, and the actual size of the pneumothorax can be underestimated. Expiratory radiographs are of value in detecting small pneumothoraces, because they increase the relative amount of pleural air with respect to the lungs. Lines caused by skin folds can mimic the pleural line of a pneumothorax (Fig. 13). However, unlike pneumothorax, skin folds often extend beyond the rib cage, and lung parenchyma is seen beyond them. Like radiographs, CT scans show pneumothorax as a plural air collection that separates the lung from the chest wall. However, CT is indicated only for patients with a questionable small pneumothorax who are unable to sit or stand for acquisition of an upright radiograph. Pneumatocele pneumatocele is a thin-walled, air-filled space in the lung parenchyma that develops as a result of pneumonia, trauma, barotrauma, or inhalation of hydrocarbon fluid, as by fire eaters. The mechanism of pneumatocele formation is Fig year-old man with skin folds. nteroposterior chest radiograph shows right-sided skin folds that produce edge (arrows) consisting of density and lucency. Fig year-old immune-compromised man with pneumatoceles of pneumococcal pneumonia. Transverse CT image of chest shows thick-walled airspaces (arrows) in right upper lobe and bilateral diffuse opacities with incipient cavities. W108 JR:200, February 2013

9 Pulmonary Hyperlucency in dults Fig year-old man with Swyer-James syndrome. Posteroanterior chest radiograph shows left-sided hyperlucency with decreased density of vascular structures that has upper zone predominance (large arrow). Central pulmonary arteries (small arrow) are smaller than those on right. thought to be a combination of parenchymal necrosis and check-valve airway obstruction that enables air to enter the parenchymal space during inspiration but prevents its egress during expiration. lthough pneumatoceles are almost invariably transient, persistent changes rarely require percutaneous drainage or surgical intervention. Postinfectious pneumatoceles (especially caused by Staphylococcus species) are relatively rare among adults, and most spontaneously disappear within weeks to months after treatment. In immune-compromised patients, Pneumocystis jiroveci pneumonia is an important cause of pneumatocele. On chest radiographs and CT scans, acute pneumatoceles may be thick-walled and contain air-fluid levels (Fig. 14). In the healing phase, pneumatoceles become thin-walled and the air-fluid levels often disappear. fter trauma, pneumatoceles are commonly associated with pulmonary laceration and contusion, resulting in air trapping in the parenchyma. Those changes are typically observed within hours after trauma and spontaneously resolve within weeks. CT can be valuable for differentiating a pneumatocele from a bulla or lung abscess. Large pneumatoceles can be complicated by rupture and subsequent pneumothorax. Swyer-James Syndrome Swyer-James syndrome, also known as Macleod syndrome, becomes evident as a unilateral hyperlucent lung, probably due to infectious bronchiolitis that most often follows a viral or Mycoplasma infection in infancy or childhood. Children with Swyer-James syndrome may experience chronic coughing and wheezing or recurrent pneumonia. Some cases of the syndrome remain asymptomatic, and the condition is diagnosed only later in adulthood. On radiographs, the affected lung parenchyma appears hyperlucent as a result of overinflation due to bronchiolar obstruction and collateral ventilation (Fig. 15). ecause the condition is usually acquired in childhood before the lungs have developed fully, the pulmonary arteries are generally hypoplastic and cause reduced lung volume. On CT scans, the affected lung is hyperlucent owing to overinflation, and there may be evidence of bronchiectasis (Fig. 16). lthough small, the segmental pulmonary arteries usually can be identified with CT angiography. ecause Fig year-old man with Swyer-James syndrome. and, Transverse CT images of chest show areas of hyperlucency (arrows) in left upper () and lower () lung zones with decreased vascular markings, resulting in mosaic attenuation pattern. JR:200, February 2013 W109

10 Nemec et al. of the vascular abnormalities, CT may show a mosaic attenuation pattern, and expiratory scans show air trapping. The most important differential diagnoses are agenesis and hypoplasia of a pulmonary artery. These conditions also cause hyperlucency of the lung, but the hilum is small and not associated with air trapping or bronchiectasis. ilateral Focal Findings ullae bulla is a well-defined, air-containing space in the subpleural lung or the visceral pleura. ullae can be unilateral or bilateral, solitary or multiple. Ranging in size from a few millimeters to several centimeters, bullae always have a thin wall ( 1 mm). Histologically, these lesions have a fibrous wall and may be trabeculated by remnants of alveolar Fig year-old man with giant bullous disease. Posteroanterior chest radiograph shows enormous bilateral upper lung bullae causing circumscribed hyperlucency (arrows). septa. ullae most frequently occur in individuals with connective tissue disorders such as Marfan syndrome and Ehlers-Danlos syndrome. They also can be associated with pulmonary emphysema, more commonly panlobular and paraseptal than the centrilobular type. bulla manifests itself radiographically as a focal rounded lucency in the subpleural area of the lungs (Fig. 17). On a CT scan, a bulla appears as a rounded area of decreased attenuation that is sharply marginated by a thin wall usually better seen on CT scans than on radiographs (Fig. 18). The presence of a wall is important in differentiating bullae from centrilobular emphysema. lthough bullae are commonly seen in the lung apices of patients with spontaneous pneumothorax, there is no definite association between their occurrence, number, and size with the development of pneumothorax. Indeed, pneumothorax is the most important entity in the differential diagnosis of larger bullae. In pneumothorax, the visceral pleura tends to parallel the chest wall, whereas in a bulla the angle between its outer margin and the chest wall is acute. In patients with recurrent or persistent pneumothorax, CT can be valuable for the detailed assessment of underlying bullous changes that plays an important role in decisions regarding follow-up and therapeutic intervention. Multiple Lung Cysts In adults, various pulmonary diseases are associated with multiple lung cysts. cyst appears as a round, well-defined collection of air surrounded by a thin (< 2 mm) epithelial or fibrous wall. On radiographs, lung cysts are often a subtle finding, appearing as focal hyperlucencies or, rarely, as linear opacities if only a part of the wall is visible. CT is substantially more sensitive than radiography for detecting a lung cyst, which appears as a ringlike hyperlucency surrounded by a thin wall. The most common causes of multiple lung cysts have been previously addressed in the JR Pattern of the Month series. Fig year-old man with apical bullous disease. Transverse CT image of chest shows large bilateral apical bullae with complete lack of normal lung tissue. W110 JR:200, February 2013

11 Pulmonary Hyperlucency in dults ilateral Diffuse Findings sthma sthma is characterized by chronic inflammatory bronchoconstriction. In an acute asthma attack or exacerbation, bronchoconstriction causes severe bilateral overinflation of the lung parenchyma. The manifestations on both radiographs and CT scans are a substantial increase in lung volume, flattening and depression of the hemidiaphragms, and sometimes a decrease in the diameter of the cardiac silhouette (Fig. 19). The sequelae of overinflation may be the only visible change on chest radiographs. Subtle pneumopericardium or pneumomediastinum can be caused by prolonged and forced coughing complicating an asthma attack (Fig. 20). CT may show bronchial wall thickening and irregularity due to bronchial remodeling. Expiratory scans may reveal air trapping and resulting mosaic attenuation. In both acute asthma attacks and exacerbations, the clinical presentation rather than the imaging appearance is likely to guide further diagnostic and therapeutic steps. C Fig year-old man with asthma attacks before and after corticosteroid therapy., Posteroanterior chest radiograph shows severe bilateral overinflation with low position of hemidiaphragms and dome flattening (black arrows) and hyperlucency that primarily involves lower lung zones (white arrows)., Lateral radiograph shows signs of overinflation as increased retrosternal space (black arrow) and hyperlucency of lower lung zones (white arrow). C, Posteroanterior chest radiograph after treatment shows normal position and curved shape of both hemidiaphragms (black arrows) and normal lucency of both lungs (white arrows). D, Lateral radiograph obtained treatment shows normal retrosternal space (black arrow) and regular lower lung density (white arrow) without signs of overinflation. D JR:200, February 2013 W111

12 Nemec et al. ronchiolitis Obliterans In bronchiolitis obliterans (O), concentric luminal narrowing of the membranous and respiratory bronchioles secondary to submucosal and peribronchial fibrosis leads to chronic airflow obstruction. Rarely idiopathic, this condition can result from infection, inhalation of noxious fumes, graft-versus-host disease, lung transplant, and rheumatoid arthritis. Viral and Mycoplasma pneumonias are the most common infections resulting in O. In lung transplant recipients, O is the most common cause of chronic rejection. The clinical course of O can resemble chronic obstructive pulmonary disease, though O progresses more rapidly. The radiographic findings in O are nonspecific and are present only in the most severe cases. They include overinflation, bronchial wall thickening, and decreased pulmonary vasculature. CT is much more sensitive than radiography in the detection and characterization of this small airways disease (Fig. 21). ronchial wall thickening and dilatation of the bronchioles are common, and there may be centrilobular nodules and branching opacities from luminal impaction (tree-in-bud pattern) (Fig. 21). In addition to showing air trapping, expiratory CT scans can help differentiate among causes of mosaic perfusion, such as infiltrative lung disease, pulmonary vasculature disease, and small airways disease. CT may also be important for differentiating O from asthma (mosaic attenuation but no micronodules) and lower-lobe-predominant panlobular emphysema (no mosaic attenuation). Cystic Fibrosis Cystic fibrosis is an autosomal recessive hereditary disease characterized by exocrine pancreatic insufficiency, intestinal derangement, sweat gland dysfunction, and chronic airway infections due to decreased mucus Fig year-old man with complication of pneumomediastinum in asthma., Posteroanterior chest radiograph shows slight paraaortic and paracardiac hyperlucent lines (arrows)., Transverse CT image of chest confirms distinct air collections along upper mediastinal structures (arrows) and shows mosaic attenuation based on small airways disease. Fig year-old man with bronchiolitis obliterans. Transverse CT image of chest shows moderate bronchiectasis and bronchial wall thickening (arrows). Multiple areas of different attenuation indicate air trapping caused by small airways disease. W112 JR:200, February 2013

13 Pulmonary Hyperlucency in dults Fig year-old man with cystic fibrosis., Transverse CT image at upper lobe level shows bronchiectasis with mucous plugging (finger-in-glove sign) (arrow)., Transverse CT image at lung base shows diffuse bronchiectasis and bronchial wall thickening (arrows). Mosaic attenuation indicates air trapping caused by small airways disease. clearance. Most cases are diagnosed by the age of 3 years, though mild cases occasionally may not be recognized until adulthood. Clinically, the diagnosis of cystic fibrosis is confirmed by a positive result of a sweat chloride test. The role of imaging is to follow the course of patients with known disease. Initially, radiographs show hyperinflation of both lungs secondary to small airways obstruction. s the disease progresses, bronchial and peribronchial wall thickening, bronchiectasis, and mucus impaction develop, usually in the upper lobes. Lobar atelectasis and recurrent pneumonia may be seen in advanced disease. lthough radiography is widely used to monitor cystic fibrosis, Fig year-old man with severe generalized emphysema., Posteroanterior chest radiograph shows upper lobe predominant hyperlucency (arrows) due to focal absence and reduced caliber of pulmonary vessels and decrease in lung tissue. Increased intercostal spaces indicate overinflation., Lateral chest radiograph shows distinct flattening of hemidiaphragms (arrow) as sign of overinflation. JR:200, February 2013 W113

14 Nemec et al. CT allows reliable confirmation of small airways disease and shows diffuse bronchiectasis with mucus plugging, both of which have an upper lobe predominance (Fig. 22). Characteristic CT findings include bronchial wall and peribronchial thickening, airspace nodules with ground-glass appearance, and the tree-in-bud pattern caused by secretions in bronchioles. ir-trapping is seen on expiratory scans. The differential diagnosis includes Kartagener syndrome (involves additional situs anomalies) and postinfectious bronchiectasis (usually unilateral and predominant in the lower lobes). Fig year-old woman with centrilobular emphysema. Transverse CT image at upper lobe level shows destruction of lung tissue with multiple areas of low attenuation without actual walls (arrows) and surrounded by normal lung parenchyma. Emphysema Emphysema is permanent abnormal enlargement of airspaces distal to terminal bronchioles that is accompanied by destruction of alveolar walls without fibrosis. There are two major subtypes of emphysema. Centrilobular emphysema, which predominantly involves the upper lobes, results from dilation or destruction of the respiratory bronchioles and is associated with cigarette smoking. Panlobular emphysema, which is associated with α 1 -antitrypsin deficiency, destroys the entire acinus and has a lower lobe predominance. Early emphysema is characterized by focal destruction of lung parenchyma and becomes more diffuse with advanced disease. Classic radiographic signs are distortion of the vascular structures (focal absence or reduced caliber of pulmonary vessels), focal destruction of lung tissue, and increased lucency of the lung (Fig. 23). If emphysema is accompanied by overinflation, flattening of the hemidiaphragms and an increased retrosternal space may be present but are not pathognomonic of emphysema (Fig. 23). Radiography can be used in the diagnosis of moderate or severe emphysema but is much less sensitive than CT in detecting and evaluating the extent of mild disease. On CT scans, emphysema is characterized by areas of low attenuation in surrounding normal lung parenchyma. Mild to moderate centrilobular emphysema produces multiple small, round areas of low attenuation (several millimeters in diameter), which Fig year-old man with severe panlobular emphysema., Transverse CT image of chest shows distortion of vascular structures and distinct destruction of lung tissue that result in multiple areas of low attenuation (arrows) with only small regions of normal parenchyma., Sagittal CT image of chest shows emphysematous involvement of entire lung and flattening of diaphragm (arrow) indicating overinflation. W114 JR:200, February 2013

15 Pulmonary Hyperlucency in dults usually have an upper lobe predominance (Fig. 24). The lesions have no walls and can be grouped around the center of secondary pulmonary lobules. In contrast, panlobular emphysema is characterized by uniform destruction of the secondary pulmonary lobule, which results in a widespread and homogeneous pattern of low attenuation (Fig. 25). Panlobular emphysema can involve the entire lung or have lower lobe predominance. On radiographs, advanced cases of multicystic disorders such as lymphangiomyomatosis and Langerhans cell histiocytosis can mimic emphysema. However, CT readily shows the walls of the lesions, which differentiate these processes from emphysema. Suggested Reading 1. ankier, O Donnell CR, oiselle PM. Quality initiatives: respiratory instructions for CT examinations of the lungs a hands-on guide. RadioGraphics 2008; 28: ankier, Van Muylem, Knoop C, et al. ronchiolitis obliterans syndrome in heart-lung transplant recipients: diagnosis with expiratory CT. Radiology 2001; 218: Cantin L, ankier, Eisenberg RL. Multiple cystlike lung lesions in the adult. JR 2010; 194:3; [web]w1 W11 4. Eisenberg RL. Chest patterns. In: Eisenberg RL, Clinical imaging: an atlas of differential diagnosis, 5th ed. Philadelphia, P: Lippincott Williams & Wilkins, Gipson MG, Cummings KW, Hurth KM. ronchial atresia. RadioGraphics 2009; 29: Hansell DM, ankier, MacMahon TC, et al. Fleischner Society: glossary of terms for thoracic imaging. Radiology 2008; 246: Kang J, Litmanovich D, ankier, et al. Manifestations of systemic diseases on thoracic imaging. Curr Probl Diagn Radiol 2010; 39: Lee KW, Chung SY, Yang I, et al. Correlation of aging and smoking with air trapping at thin-section CT of the lung in asymptomatic subjects. Radiology 2000; 214: Litmanovich D, oiselle PM, ankier. CT of pulmonary emphysema current status, challenges, and future directions. Eur Radiol 2009; 19: Park CM, Goo JM, Lee HJ, et al. Tumors in the tracheobronchial tree: CT and FDG PET features. RadioGraphics 2009; 29: Sharma, Fidias P, Hayman L, et al. Patterns of lymphadenopathy in thoracic malignancies. Radio- Graphics 2004; 24: Shetty CM, arthur, Kambadakone, et al. Computed radiography image artifacts revisited. JR 2011; 196:157; [web]w37 W Worsley DF, lavi, ronchick JM, et al. Chest radiographic findings in patients with acute pulmonary embolism: observations from the PI- OPED Study. Radiology 1993; 189: Worthy S, Müller NL, Hartman TE, et al. Mosaic attenuation pattern on thin-section CT scans of the lung: differentiation among infiltrative lung, airway, and vascular diseases as a cause. Radiology 1997; 205: JR:200, February 2013 W115