Thin-Section CT of the Secondary Pulmonary Lobule
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Note: This copy is for your personal non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, contact us at www.rsna.org/rsnarights. REVIEW Thin-Section CT of the Ⅲ Secondary Pulmonary Lobule: Anatomy and the Image— The 2004 Fleischner Lecture1 REVIEWS AND COMMENTARY W. Richard Webb, MD The secondary pulmonary lobule is a fundamental unit of lung structure, and it reproduces the lung in miniature. Airways, pulmonary arteries, veins, lymphatics, and the lung interstitium are all represented at the level of the secondary lobule. Several of these components of the sec- ondary lobule are normally visible on thin-section com- puted tomographic (CT) scans of the lung. The recognition of lung abnormalities relative to the structures of the sec- ondary lobule is fundamental to the interpretation of thin- section CT scans. Pathologic alterations in secondary lob- ular anatomy visible on thin-section CT scans include in- terlobular septal thickening and diseases with peripheral lobular distribution, centrilobular abnormalities, and pan- lobular abnormalities. The differential diagnosis of lobular abnormalities is based on comparisons between lobular anatomy and lung pathology. RSNA, 2006 1 From the Department of Radiology, University of Califor- nia San Francisco, 505 Parnassus Ave, San Francisco, CA 94143-0628. Received November 19, 2004; revision re- quested January 10, 2005; revision received February 16; accepted March 9; final review by the author March 18. Address correspondence to the author (e-mail: [email protected]). RSNA, 2006 322 Radiology: Volume 239: Number 2—May 2006 REVIEW: Thin-Section CT of Secondary Pulmonary Lobule Webb he secondary pulmonary lobule is size, measuring from 1 to 2.5 cm in injecting mercury and other fluids into a fundamental unit of lung struc- diameter in most locations (8,11–14). the bronchi and pulmonary vessels. He Tture, and an understanding of In one study (14), the average diameter found that “little lobes” (ie, the lobules) lobular anatomy is essential to the inter- of secondary lobules measured in sev- arose from small branches of the tra- pretation of thin-section computed to- eral adults ranged from 11 to 17 mm. chea and were separated from each mographic (CT) scans of the lung. Thin- Airways, pulmonary arteries and other by a “membrane” (Fig 3). Bron- section CT can show many features of veins, lymphatics, and the various com- chioles entering the little lobes were de- the secondary pulmonary lobule in both ponents of the pulmonary interstitium scribed as dividing into a large number normal and abnormal lungs, and many are all represented at the level of the of fine branches, which led to minute lung diseases produce characteristic ab- secondary lobule (Figs 1, 2). Each sec- “bladders” or “vesicles.” normalities of lobular anatomy (1–7). ondary lobule is supplied by a small bron- Georg Rindfleisch (in 1875) first chiole and pulmonary artery branch and used the term acinus to indicate a sub- is variably marginated in different lung lobular lung unit. He described the sec- The Secondary Pulmonary Lobule and regions by connective tissue, the interlob- ondary lobule as supplied by a bronchi- Lung Acinus ular septa, that contains pulmonary veins ole, which divided into progressively The lung is made up of numerous ana- and lymphatics (15). Secondary lobular smaller bronchiolar branches, finally tomic units smaller than a lobe or seg- anatomy is easily visible on the surface of giving rise to arborizing alveolengange ment. The secondary pulmonary lobule the lung because of these interlobular (alveolar passages), which collectively and lung acinus are widely regarded to septa (8,11). formed a “lung acinus” (Fig 4). The aci- be the most important of these subseg- The pulmonary acinus is smaller nus, according to Rindfleisch, was a mental lung units. than the secondary lobule. It is defined much more consistent unit of lung struc- The secondary pulmonary lobule, as as the portion of lung distal to a terminal ture than was the secondary lobule be- defined by Miller, refers to the smallest bronchiole (the last purely conducting cause of variation in the size of lobules. unit of lung structure marginated by airway) and is supplied by a first-order On the other hand, he regarded the sec- connective tissue septa (8,9) (Figs 1, 2). respiratory bronchiole or bronchioles ondary lobule to be more important Secondary pulmonary lobules are irreg- (16). Since respiratory bronchioles are than the acinus pathologically, in that ularly polyhedral in shape and vary in the largest airways that have alveoli in disease processes tended to be limited their walls, an acinus is the largest lung by the connective tissue septa that mar- unit in which all airways participate in ginate the lobules. Essentials gas exchange. Acini are usually de- In 1881 Rudolph Kolliker, using the Ⅲ The secondary pulmonary lobule scribed as ranging from 6 to 10 mm in lung of an executed criminal, provided a is a fundamental unit of lung diameter (14,17) (Fig 1). more detailed analysis of the finer divi- structure, and an understanding Secondary pulmonary lobules are sions of the bronchial tree and de- of lobular anatomy is essential to usually made up of a dozen or fewer scribed respiratory bronchioles as air- the interpretation of thin-section acini, although the number varies con- ways that have both bronchiolar epithe- CT of the lung. siderably in different reports (18,19). In lium and alveoli in their walls. He Ⅲ Pulmonary disease occurring pre- a study by Itoh et al (10), the number of distinguished respiratory bronchioles dominantly in relation to interlob- acini counted in lobules of varying sizes from proximal airways that do not have ular septa and the periphery of ranged from three to 24. alveoli in their walls (ie, terminal bron- lobules is termed “perilobular”; chioles) and distal airways that have nu- this distribution of disease may merous alveoli in their walls (ie, alveo- reflect abnormalities of the inter- Historical Considerations lengange, subsequently termed alveolar lobular septa or peripheral alve- Concepts regarding the importance of ducts), thus providing the basis for de- oli. the secondary pulmonary lobule, aci- fining the lung acinus relative to airway Ⅲ Centrilobular abnormalities visi- nus, and smaller lung units have evolved anatomy. ble on thin-section CT scans may during the past 300 years in conjunction 1947, in his book entitled The Lung consist of nodular opacities; the with continued progress in the under- (21), William Snow Miller reviewed tree-in-bud appearance, which standing of lung anatomy, pathology, lung anatomy in detail. His definitions of usually indicates the presence of a and physiology. An excellent perspective small-airways abnormality; in- on the sequence of events and incremen- creased visibility of centrilobular tal discoveries made during this period Published online before print structures due to thickening or has been provided by Miller (20). 10.1148/radiol.2392041968 infiltration of the surrounding in- The earliest detailed description terstitium; or abnormal low-atten- (from 1676) of the secondary pulmo- Radiology 2006; 239:322–338 uation areas related to bronchio- nary lobule was provided by Thomas Originally presented at the 34th Annual Fleischner Society lar dilatation or emphysema. Willis, who studied lung structure by Conference on Chest Disease, Orlando, Fla, May 20, 2004. Radiology: Volume 239: Number 2—May 2006 323 REVIEW: Thin-Section CT of Secondary Pulmonary Lobule Webb the secondary pulmonary lobule and ac- tailed in his book entitled The Lung: which extend inward from the pleural inus are still in use today (see above). Radiologic-Pathologic Correlations (22). surface. The interlobular septa are part However, he also considered the pri- Heitzman described the radiographic of the peripheral interstitial fiber sys- mary pulmonary lobule to be a funda- appearances of various lobular abnor- tem described by Weibel (12). The pe- mental unit of lung structure. He de- malities, as carefully correlated with in- ripheral interstitium extends over the fined the primary pulmonary lobule as flated and fixed lung specimens. In the surface of the lung beneath the visceral all the alveolar ducts, alveolar sacs, and initial articles (11,15), Heitzman et al pleura and envelopes the lung in a fi- alveoli distal to the last respiratory described the appearance of septal brous sac from which the connective- bronchiole, along with their associated thickening associated with fibrosis or tissue septa penetrate the lung paren- blood vessels, nerves, and connective lymphatic and pulmonary venous abnor- chyma. Pulmonary veins and lymphatics tissues (Fig 5). However, since the term malities, as well as panlobular consoli- lie within the connective-tissue interlob- “primary pulmonary lobule” is not in dation in pulmonary infarction and ular septa (Figs 1, 2). common use today, “secondary pulmo- bronchopneumonia. In the later and Secondary pulmonary lobules in the nary lobule”, “secondary lobule”, and more detailed descriptions, Heitzman lung periphery are relatively large and “lobule” are often used interchangeably; (22) further emphasized the radio- are marginated by interlobular septa in general, they should be considered as graphic appearances of the lobular that are thicker and better defined than synonymous. “core” structures and demonstrated the lobules in other parts of the lung In 1958, Reid suggested an alternate radiographic and pathologic