Pipavath, Sudhakar N. J. MD*; Schmidt, Rodney A. MD†; Takasugi, Julie E.‡; Godwin, J. David MD§
The introduction of high-resolution computed tomography (HRCT) of the lung in the early 1980s1–3 opened a new era in radiologic-pathologic correlation. Before CT and HRCT, the detection of the structural abnormalities of COPD (ie, emphysema) by ordinary chest radiograph was not possible until disease had reached an advanced stage.
An HRCT image can be compared with a gray-scale macroscopic low-field histologic view. It is able to diagnose early and even preclinical emphysema with a high degree of pathologic correlation and locate the exact site of irreversible structural change in its centrilobular,4 panlobular, paraseptal, or paracicatricial location. This review will discuss the relevant anatomy of the secondary pulmonary lobule, the subtypes of emphysema, and their imaging appearances and corresponding pathologic findings.
ANATOMY OF THE SECONDARY PULMONARY LOBULE
The secondary pulmonary lobule (Fig. 1) is the smallest unit of lung marginated by connective tissue.5 It is polyhedral and it contains pulmonary arteries, veins, lymphatics, airways, alveoli, and interstitium. It is supplied by a small bronchiole and a pulmonary arterial branch and is marginated by connective tissue—the interlobular septa, containing pulmonary venules and lymphatics. The airway supplying the secondary pulmonary lobule is the preterminal or simply “lobular bronchiole,” which gives rise to several terminal bronchioles. The terminal bronchioles end in respiratory bronchioles. Respiratory bronchioles end in alveolar ducts, sacs, and alveoli in succession. The respiratory bronchiole serves both for conduction and for gas exchange. The acinus is defined as the unit of lung that is distal to the terminal bronchiole, which is succeeded by 3 orders of respiratory bronchioles. The acinus typically measures about 7 mm in diameter.
All the acini arising from a terminal bronchiole comprise a primary lobule; a secondary lobule usually contains about 6 primary lobules with the center of each primary lobule being located about halfway between the center and periphery of a secondary lobule. The connective tissue septations that surround secondary lobules are not well defined everywhere in the human lung.
Chronic obstructive pulmonary disease (COPD) is defined as a preventable and treatable disease state characterized by airflow limitation that is not fully reversible. The airflow limitation is usually progressive and is associated with an abnormal inflammatory response of the lungs to noxious particles or gases, primarily caused by cigarette smoking.6 Emphysema is one of its components, along with asthma and chronic bronchitis. Emphysema is defined pathologically as permanent enlargement of the airspaces distal to the terminal bronchioles, accompanied by destruction of their walls and without obvious fibrosis.7 The essential feature is that alveolar septal walls are lost, resulting in residual airspaces that are larger than in normal lung tissue. Emphysema is classified according to the anatomic site of septal loss as centrilobular (proximal acinar), panlobular (panacinar), paraseptal (distal acinar), and irregular.8
The normal alveolus (0.1 to 0.2 mm diameter) is smaller than the resolving power of the unaided eye, chest radiography, and HRCT. In addition, the x-ray attenuation due to any individual alveolar septum is quite small. Destruction of multiple alveolar septa is required to recognize early emphysema qualitatively at HRCT.
Chest radiography provides the initial imaging tool for assessing COPD. Findings include hyperinflation of the lungs, flattening of the domes of the hemidiaphragms, attenuation or absence of pulmonary vasculature, loss of the regular vascular branching pattern, widened retrosternal space (Fig. 2), large focal lucencies indicating bullae, and bronchial wall thickening. According to the combined American Thoracic Society/European Respiratory Society statement on COPD diagnosis and management, the chest radiograph helps in differential diagnosis. More specifically it helps exclude other diagnoses, such as pneumonia, cancer, congestive heart failure, pleural effusion, and pneumothorax.6 Chest radiography is neither sensitive nor specific for diagnosing COPD, although it can help diagnose bullae.
CT is better than chest radiography in qualitative assessment of emphysema,9 demonstrating its extent, type, and spatial distribution. HRCT is even better than conventional CT in assessment of emphysema.10 These days, with the common use of 64-detector-row CT scanners, routine chest CT scans acquired with 1.25-mm or 0.625-mm collimation are in effect contiguous HRCT scans, whether ordered as such or not. Contiguous thin sections are very helpful in detecting early centrilobular emphysema (CLE), when the lucencies are still small. Thus, the identification of structural alterations in COPD has become easier and subclinical emphysema is easily detected. Furthermore, the quality of postprocessed images has improved on modern multidetector CT scanners; one postprocessing technique of particular interest is the minimum intensity projection, which helps bring out the morphology of emphysema. Beyond demonstrating the structural alterations of emphysema, CT has also been validated in its quantification.11,12
CLE is defined by preferential loss of septa at the center of primary lobules; that is, around respiratory bronchioles. Destruction of respiratory bronchioles progresses distally and also involves adjacent units. Early in the course of the disease there is relative sparing of the distal alveolar ducts, alveolar sacs, and alveoli (Fig. 3), resulting in observable sparing at the periphery of the lobule (Fig. 4). The process affects the upper lungs more than lower and posterior segments more than the anterior. Cigarette smoking is the most common cause of CLE.
CLE can seldom be distinguished from other forms of emphysema by chest radiography, but it can occasionally be brought out by filling of surrounding airspaces by edema, hemorrhage, or pneumonia; the small centrilobular emphysematous spaces appear as small lucencies within the consolidation. Sometimes these features give the impression of reticulation (Fig. 5).
HRCT is the best technique for diagnosing CLE, with sensitivity, specificity, and accuracy of 88%, 90%, and 89%, respectively.13 A window width of 1500 HU and window level range of −700 to −550 HU are optimal.13 Centrilobular low-attenuation spaces with imperceptible walls, in a nonuniform distribution, are the principal feature of CLE.14 Upper lungs, especially the posterior lobes are more affected in cigarette smokers (Fig. 6). Postprocessing of images can bring out the distribution of emphysema (Figs. 6B, 7). Vascular architecture in the low-attenuation regions is usually preserved.
Panlobular emphysema (PLE) is defined by uniform loss of alveolar septa throughout the primary and secondary lobules, including the respiratory bronchioles, alveolar ducts, and alveolar sacs (Figs. 3, 8). Owing to uniformity, PLE changes are subtle and difficult to recognize in any given region pathologically and radiographically. PLE typically involves the lower lungs predominantly, with relative sparing of the upper lungs, especially in nonsmokers. Alpha-1-antitrypsin (AAT) deficiency is the most common cause of PLE, but it also occurs from intravenous injection of crushed methylphenidate (Ritalin) tablets,15 Swyer-James syndrome, old age, and rarely from cigarette smoking (without AAT deficiency).
The prototype disease in this category is AAT deficiency. AAT binds and inactivates neutrophil elastase, which is a product of inflammation. This inactivation limits the tissue destruction that would otherwise accompany the inflammatory response. In nonsmokers, there is limited if any neutrophil accumulation in the lungs. In smokers, however, there is persistent inflammation with accumulation of neutrophils. In persons with normal AAT levels, neutrophil elastase is neutralized. Low levels or absence of AAT leads to unrestricted activity of the neutrophil elastase. Symptoms appear early compared with CLE, possibly from the larger surface area being affected. In patients who abuse Ritalin, the pathogenesis of emphysema is not clearly elucidated. Increased inflammation and elastase activity have been proposed.15
On chest radiographs, the findings are lower-lung translucency, hyperinflation, and flattening of the diaphragm. There are no distinguishing features of PLE other than the characteristic lower-lung predominance (Fig. 9). Swyer-James syndrome and advanced smoking-related CLE are some times difficult to distinguish from AAT deficiency-related PLE.
In PLE, CT shows panlobular decrease in attenuation and loss of vessel caliber (Fig. 10). It occasionally can be difficult to distinguish PLE from obliterative bronchiolitis. In addition, patients with AAT deficiency may have associated bronchiectasis or bronchial wall thickening.16 Even with CT, it can be difficult to distinguish PLE from CLE. The study by Copley et al13 showed low sensitivity (48%) for detection of PLE; it was often confused with CLE. The specificity and accuracy were high, at 97% and 89%, respectively. HRCT is better than conventional CT at detection of PLE.17 Ritalin lung at CT shows PLE, with features and distribution otherwise indistinguishable from AAT deficiency (Fig. 11).18 However, histopathologic features are characteristic, with talc or other excipient material showing birefringence under polarized light (Fig. 12).
This form of emphysema is less well described than CLE, and its etiology is less well understood. Other names for this condition are distal acinar emphysema, superficial or mantle emphysema, and linear emphysema. Paraseptal emphysema (PSE) affects the most distal parts of the acinus, the alveolar sacs and ducts, and spares the respiratory bronchioles, hence the name distal acinar emphysema (Figs. 3, 13). It occurs most commonly in the upper lungs, especially the posterior upper lobes and anterior upper lobes, in a subpleural location, and it can also involve the posterior lower lobes.19 PSE has been implicated as a cause of spontaneous pneumothorax, typically in tall, thin men in the third or fourth decade.20,21 PSE may also occur in association with CLE.
PSE is difficult to diagnose at chest radiography. At CT, however, it has a characteristic appearance. It is usually in the periphery of the upper lungs, and the dilated distal airspaces are rectangular and they share walls (Fig. 14). PSE may progress to bullous emphysema. Another condition that can resemble PSE is honeycombing. However, honeycomb cysts are round, as opposed to rectangular. In addition, the walls of honeycomb cysts are usually thicker than those of PSE and the cysts are usually smaller. Furthermore, PSE occurs mostly in the upper lungs and is always subpleural, whereas honeycombing occurs mostly in the bases in the setting of pulmonary fibrosis and can extend deep into the lung beyond the immediately subpleural region.
PARACICATRICIAL OR IRREGULAR EMPHYSEMA
Paracicatricial emphysema (PCE) occurs around a scar, and its causes include tuberculosis, silicosis, sarcoidosis, paracoccidiodomycosis, and bronchioloalveolar carcinoma. PCE is secondary to airspace distortion by scarring rather than primary destruction of alveolar septa. Any part of the acinus may be affected (Fig. 3).
At imaging, this form of emphysema generally surrounds the scar. It has been well described in advanced stages of sarcoidosis and progressive massive fibrosis from silicosis and coal workers pneumoconiosis (Fig. 15). Confluence of lung nodules increases the incidence of PCE in silicosis,22 and a similar mechanism probably operates in advanced sarcoidosis. PCE may contribute to airflow obstruction in the setting of progressive massive fibrosis.
Chronic bronchitis, usually caused by cigarette smoking, is defined as the presence of chronic productive cough for at least 3 months in each of 2 successive years in a patient in whom other causes of productive chronic cough have been excluded.7 This clinical definition does not require abnormal pulmonary function tests or radiographic findings. Bronchial gland hypertrophy, goblet cell metaplasia, and excess mucus production are some of the pathologic findings of chronic bronchitis. In the airways, there may be squamous metaplasia of the epithelium, loss of cilia and ciliary dysfunction, and increased smooth muscle and connective tissue.
Chest radiographs are normal in a substantial number of patients with chronic bronchitis. Terms such as “increased lung markings” or “dirty lung” have been applied to describe the bronchial wall thickening (Fig. 16). HRCT shows bronchial wall thickening better than chest radiographs, but this finding is not specific for chronic bronchitis. Occasionally, the dominant CT feature in patients diagnosed to have chronic bronchitis is CLE, which often coexists with chronic bronchitis.23 Other findings include centrilobular opacities reflecting bronchiolar inflammation or thickening.
BULLA VERSUS BLEB
Strictly defined, a bulla is any emphysematous space that is more than 1 cm in diameter (Fig. 17) whereas a bleb is a collection of air trapped between the layers of the visceral pleura.24 A bleb is thus a variant of interstitial emphysema, which is distinct from the types of emphysema discussed above. It is reported by surgeons in cases of spontaneous pneumothorax and may result from rupture of peripheral alveoli.25 Bullae occur in emphysematous regions of the lung, whereas blebs occur typically in the lung apices. Complicating the clean dichotomy above is the fact that young thin spontaneous pneumothorax patients frequently have bulla-like subpleural separations of lung tissue from the pleura, but in the absence of emphysema elsewhere. As both true blebs and these lesions are associated with spontaneous pneumothorax and because CT does not have sufficient resolution to determine whether the origin of the abnormal airspace is intrapleural or subpleural, the common practice is to call both lesions blebs.
CT is the best modality available to detect a bulla (Fig. 18) or a bleb (Fig. 19), but they can be visible on chest radiographs when large enough. Distinguishing the two is based mostly on location, given that blebs are usually located at the apices, whereas bullae can be located anywhere.
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