preface
Studying interstitial lung disease requires mastery of normal anatomy of secondary lobular pulmonary lobules. Secondary lobules (SPL) are the smallest independent units of lung structure, margined by connective tissue septum, approximately 1 to 2.5 cm in size, innervated by lobular bronchiolulus and pulmonary arteries, and are key anatomical structures in the presentation of HRCT.
General pathology of secondary lobules, square or polygonal, lobular septum is black
Guide part: main bronchi leaf bronchial segment bronchi small bronchi bronchi bronchi, secondary lobules are composed of bronchi and branches, acinar vesicles
Respiratory Department: Bronchi Terminal Bronchiole Respiratory Bronchiole Tube Alveolar Sac Alveoli, Alveoli: The basic unit of lung ventilation function, consisting of functional lung parenchyma such as respiratory bronchi, alveolar duct, alveolar sac, alveolar and alveolar wall capillary bed
Anatomical pattern diagram of secondary lobules, square or polygonal: lobular arteries and bronchioles are located in the center of the secondary lobules, and lobular veins and lymphatic vessels are located within the lobular septum. Two sets of lymphatic systems (yellow), the central lymphatic network along the tracheal vascular bundle to the center of the lobules, the peripheral lymphatic network located within the lobular septum and along the pleura; Note: The pulmonary arteries are hypoxic blood (blue) and the pulmonary veins are oxygenated blood (red).
The center of the secondary lobular lobules is composed of bronchiol (B) and lobular center artery (A); the lobular septum (S) contains the pulmonary veins (V) and lymphatic vessels
It is best to divide the secondary lobules into 3 parts: 1. Lobular septum and connected subpleural interstitium (yellow); Central structure of the leaflets (blue); Lobular parenchyma and acinar vesicles (black)
Normal HRCT of secondary lobular pulmonary lobules
HRCT manifestations of lobular septa: secondary lobules are bounded by the lobular septum of connective tissue that extends from the pleura into the lungs. The lobular septum is part of the peripheral interstitial fibrous system and contains pulmonary veins and lymphatic vessels. Secondary lobules around the lungs are larger, more uniform, often cubed or tapered, and the lobules that serve as margins are thicker and more clearly spaced than other sites. The secondary lobules of the central lung are smaller and irregular than the surrounding parts, and the marginal lobules are thinner and less clearly spaced. When lobules of the lung in the central part of the lungs are visible, they are hexagonal or polygonal. In normal HRCT, lobular septa is uncommon, and normal people often see a little lobular septum around the lungs, but tends to be inconspicuous; Normal lobular septa is most commonly found in the most developed areas (i.e., apex, anterior part, along the mediastinal pleural surface), and when visible, they often extend to the pleural surface. The spacing in the central part of the lung is thinner than the surrounding part and is rare in normal people, and the lobular septum that can be clearly displayed in these areas is often abnormally thickened. When the lobular septa is not clearly visible, their position can be judged by branches of pulmonary veins located within the lobular septum with a diameter of about 5 mm. The veins are sometimes linear, arched, or branched around the lobular center artery 5 to 10 mm apart. Pulmonary veins can also be identified by their branching morphology, and the venilets are often nearly right-angled from the larger main branch.
Normal human upper lobe HRCT of the left lung, A- posterior part can see a faint and thin lobular septum (black arrow), outlining the normal secondary lobules, can clearly see the central lobular artery (white arrow); B- left upper lobe normal pulmonary vein (black arrow) can be used as a lobular septum, outlining the normal secondary lobules, can clearly see the central lobular artery
HRCT manifestations of the lobular center structure: the central lobular structure contains the pulmonary arteries and bronchiol branches that supply the lobules, lymphatic vessels, and some supportive connective tissue, and ITCT is difficult to accurately determine the relationship between the lobular bronchiols or arterial trees. Lobular bronchiol and arterial branches are irregularly double-branched, and when branching, they are generally divided into 2 branches, and more commonly, two branches of varying sizes are separated (one branch is almost the same size as the one from which it originated, and the other branch is slightly smaller). The imaging and visibility of HRCT of the lobular center structure is mainly determined by their size, the secondary lobules are composed of arteries and bronchiols with a diameter of about 1 mm, and linear, branched, and comma-like dense shadows are often seen in the center of the lobular lobules or 1 cm from within the pleural surface, representing the branches of the internal lobular arteries or their branches. While the terminal bronchiole and artery in the lobules are about 0.7 mm in diameter and the acinar bronchiol and artery diameter range from 0.3-0.5 mm, arteries of this size are easily distinguishable with HRCT technology. In the absence of atelectasis, the lobular center artery seen on HRCT does not extend towards the pleural surface. The visibility of bronchioles in normal people depends on the thickness of the bronchiole wall, with a bronchiole wall thickness of approximately 0.15 mm for secondary lobules, which is the lowest limit of HRCT resolution. The wall thickness of the terminal bronchiole is only 0.1 mm, and the wall thickness of the acinar bronchiole is only 0.05 mm, both of which are lower than the resolution of the tubular structure in HRCT technology. Importantly, in HRCT, the bronchi in the secondary lobules is not normally seen and can only be shown when the disease is developed.
Normal upper lobe HRCT, A and B- can clearly see the lobular center artery (white arrow)
Parenchymal and acinar vesicles: Secondary lobular parenchyma and acinar vesicles surround the central structure of the lobular lobes and are contained in the lobular septum, including alveoli and pulmonary capillary beds called functional parenchyma, supplied by branches of the small airways and pulmonary arteries and pulmonary veins, supported by a fine network of connective tissue matrix composed of very fine fibers within the alveolar septum, called the parietal parietta, which is not normally seen in HRCT and is also not visible on HRCT.
Diagnosis of SECONDARY LOBULAR ANATOMY-BASED HRCT DISEASE
Schematic of three adjacent voxels in an HRCT slice. A. In normal conditions, most of the voxels are occupied by air. B. In the case of significant thickening of the interstitium and/or the presence of some cells or fluid in the alveoli, the density increases, which appears as ground glass on HRCT. C. Pulmonary fibrosis: fibrosis occupies most of the voxels, so pulmonary fibrosis appears as a reticular shadow in HRCT
Thickening of smooth lobular septals: interstitial pulmonary edema, cancerous lymphangitis, alveolar proteinosis
Smooth lobular septum thickens in pulmonary edema, outlining many secondary lobules, and the visible secondary lobules vary in size, at least in part because of the position of the lobules relative to the scanning plane. Pulmonary veins (large arrows) are small dots or linear or branched, lobular septs are well developed at the apex, and spacing thickening is usually evident
Smooth lobular septal spacing thickens in alveolar proteinosis
Thickening of lobular septal septum in cancerous lymphangitis, (a) HRCT of right lung cancer shows smooth thickening of the upper lobular septum septum of the right lung (small arrow), thickening of the interstitial around bronchial vessels leads to a marked increase in the thickness of the bronchi in the right lung (large arrow), and a right pleural effusion. The left lung presents normally. (b) General pathology shows smooth thickening of the lobular septum (small arrow) and peri-bronchial interstitial (large arrow).
Thickening of nodular lobular septum: cancerous lymphangitis, sarcoidosis, silyosis
Thickening of the lobular septum nodules, (a) thickening of the septal septal septum of the right lobular septum in patients with sarcoidosis (arrow). (b) Histological specimens of patients with cancerous lymphangitis showing thickening of the secondary lobular septum with tumor nodules (large arrows), tumors may also be seen in the peri-bronchial area of the lobular center (small arrows)
Cancerous lymphangitis - septal nodular thickening of the right lobules
Irregular leaflet septal thickening: interstitial fibrosis
Irregular lobular septal thickening, (a) irregular lobular septal thickening in idiopathic pulmonary fibrosis, and right upper lobe HRCT showing an irregular reticular shadow (arrow) of the subpleural region. (b) Histological specimen of open lung biopsy in another patient with idiopathic pulmonary fibrosis showing secondary irregular fibrosis around lobularized lung (arrow)
Interstitial fibrosis, HRCT presents as a typical honeycomb lung, and the pathologically cell is composed of small cystic spaces arranged in the epithelium of bronchiole, thickened walls of bronchoepithelial epithelium, and consisting of dense fibrous tissue
Increased structural density of lobular centers: bronchiolitis (nodular, bud-like, ground glass density leaflet center nodules), allergic pneumonia (ground glass density leaflet center nodules), diseases involving central lymphatic vessels (sarcoidosis, cancerous lymphangitis, silicosis)
According to the location of the nodule distribution, it can be divided into random type, lobular central type, and lymphatic vessel walking type
In the central nodules of the leaflets, the identification of the "tree bud sign" is valuable in narrowing the differential diagnosis, an irregular and often nodular branching structure most easily found around the lungs, which represents dilated and blocked (mucus) the central bronchi of the leaflets. May be seen in infected intrabronchial transmission: tuberculosis, MAC, or any bacterial bronchiopneumonia; infection-associated respiratory diseases: cystic fibrosis, bronchiectasis; and less commonly: a respiratory disorder primarily associated with mucus retention, such as allergic bronchopulmonary aspergillosis and asthma.
Ground glassloid center nodules in respiratory bronchiolitis, (a) HRCT shows central lobular nodules, and partially dilated lobular center bronchioles (arrows) are visible. (b) Histological specimens of lung biopsy of a patient with respiratory bronchiolitis show peri-bronchoin infiltrates (large arrows), lobular central bronchiodisis, and small arrows outlining a secondary lobule
Ground glass lobular center nodules of allergic pneumonia, (a) HRCT showing slightly denser central lobular nodules, a few millimeters away from the pleural surface and interloidal fissures, the formation of small nodules and the thickening of the walls of the bronchioles in the center of secondary lobules (arrows), (b) histological specimens showing periphery of bronchi and alveolar infiltration (large arrows), mainly distributed in the center of secondary lobules, with the interlobular septum (small arrows) outlining the partial contours of the three lobes
Tree bud sign associated with bronchiole infection, (a) a patient with bronchiotic infection associated with acquired immunodeficiency syndrome shows an increased center density of multiple lobules with an "arbore sign" (arrow). (b) Lung slices in patients with bronchiopneumonia, in which the entire lung is seen with mucus (arrows) in the affected bronchioles, which is a pathological examination equivalent to a tree bud sign
Decreased density of central plantar structures in lobular molobes: lobular central emphysema
lobular central emphysema, (a) upper lobular HRCT showing multiple low-density lobular central arteries (arrows), (b) histological specimens showing alveolar destruction areas around lobular center arteries (arrows)
Increased density of full leaflets: solid shadows and ground glass shadows
If the degree of cloudiness of the lungs increases vaguely, but the underlying blood vessels are not obscured, it is called a terrarium shadow, and if the increase in the degree of cloudiness of the lungs blurs the blood vessels, it is called consolidation. Whether ground glass or solid, an increase in lung density is the result of the air in the alveoli being replaced by fluids, cells, or fibrosis.
Increased total lobular density - ground glass shadow: (a) HRCT shows lobular pneumonia (bronchopneumonia), left upper lobe ground glass shadow (arrow), and bronchiol and arteries in the center of the lobules are visible. (b) In patients with pulmonary edema, there is also a pleural effusion
Increased total lobular density - solid shadowing: HRCT shows machined pneumonia in multiple divergent solid shadows of both lungs
Decreased total lobular density: total lobular emphysema, restrictive bronchiolitis (air retention)
Allergic pneumonia – (air retention), HRCT shows decreased total lobular density (arrows), which reflects mosaic perfusion due to air retention, and the presence of full lobular type-ground glass shadows.
Author | Imageshine
Content planning | Little Snowball, Peng Long
Title image source | Figureworm creative
Illustration source | Courtesy of the author
Submission and reprint | [email protected]
References | (Swipe down)
Webb W Richard,Thin-section CT of the secondary pulmonary lobule: anatomy and the image--the 2004 Fleischner lecture. [J] . Radiology, 2006, 239: 322-38
Im Jung-Gi,Itoh Harumi,Tree-in-Bud Pattern of Pulmonary Tuberculosis on Thin-Section CT: Pathological Implications. [J] . Korean J Radiol, 2018, 19: 859-865
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