The secondary pulmonary lobule: a practical concept for interpretation of chest radiographs. I. Roentgen anatomy of the normal secondary pulmonary lobule.

Abstract

A thorough understanding of normal roentgen anatomy forms the basis for the interpretation of any radiograph. The radiologist commonly visualizes the anatomy of the lung in terms of lobes, segments, or subsegments; rarely does he consider pulmonary structure in terms of smaller anatomic units. This is true despite the fact that the work of William Snow Miller (9) in the early part of this century established the secondary pulmonary lobule as the basic unit of lung structure and function. Although the anatomic study of the secondary pulmonary lobule has continued for over half a century, relatively little consideration has been given to its roentgen appearance and significance. The purpose of this paper is to unify past concepts of the anatomy of the secondary pulmonary lobule and to clarify the role which the secondary pulmonary lobule plays in the normal roentgen appearance of the lung. Some controversy has arisen regarding the basic unit of lung structure, due to the use of such terms as the primary pulmonary lobule and the pulmonary acinus. Miller (9) has defined the primary pulmonary lobule as “The ensemble of ductulous alveolaris, atria, sacculi alveolares and alveoli, together with their accompanying blood vessels, lymph vessels and nerves.” The number of primary lobules forming a secondary pulmonary lobule is variable. According to Wyatt et al. (14), the number is between 30 and 50. Other observers feel that the number is somewhat smaller. Since the primary pulmonary lobule cannot be demonstrated by roentgenographic methods in either normal or abnormal states, this unit is of no practical radiologic significance. The term pulmonary acinus has also come into common usage. Somewhat differing definitions of this unit can also be found in the literature. Recavarren, Benton, and Gall (10) state “The term acinus has been used to designate a variety of terminal units of the bronchial tree stemming from either terminal bronchioles, respiratory bronchioles or alveolar ducts.” The size of the unit appears to vary from 1 mm according to the definition of Berdal (1) to 5 mm according to the definition of Recavarren et al. (10). With the abandonment of the term acinar nodule (4) the term pulmonary acinus no longer seems to have radiologic relevance. The secondary pulmonary lobule remains the basic unit of lung structure and function as originally emphasized by Miller (9). The secondary pulmonary lobule can be defined in two ways: (a) a unit of lung structure composed of a variable number of primary pulmonary lobules and separated from other secondary pulmonary lobules by connective tissue septa, and septa, and (b) a unit of lung structure supplied by a cluster of three to five terminal bronchioles. This alternate definition has been applied by Reid and Simon (12) to those portions of lung having poorly developed connective tissue septa. Pulmonary segments and subsegments usually cannot be identified in chest radiographs of normal subjects.