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Signs in Imaging |
1 From the Department of Radiology, University of Vienna, Waehringer Guertel 18-20, A-1090 Vienna, Austria. Received November 11, 1999; revision requested December 10; revision received and accepted May 22, 2000. Address correspondence to the author (e-mail: edith.eisenhuber@univie.ac.at).
Index terms: Bronchi, abnormalities, 671.265 • Bronchiolitis, 671.2191 • Lung, infection, 671.203, 671.265, 671.23 • Signs in Imaging
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The appearance of the tree-in-bud sign is closely linked to the anatomy of the secondary pulmonary lobule. Each secondary lobule is supplied by a lobular bronchiole and a lobular artery that are located in the center of the lobule. Under normal circumstances, the intralobular bronchiole is less than 1 mm in diameter and is not normally visible on CT scans (8). However, diseased bronchioles with mucous plugging, wall thickening, or dilatation can be visualized on thin-section CT scans, often displaying the tree-in-bud phenomenon.
The tree-in-bud sign has primarily been used as a descriptive term for abnormalities found on CT scans of the lung in patients with endobronchial spread of Mycobacterium tuberculosis (6). In the past several years, however, it has become clear that the finding of a tree-in-bud sign on a CT scan is not specific for a single pulmonary disease entity, but it can be found with a large number of conditions, primarily those of infectious origin, but also with immunologic disorders, congenital disorders, neoplasms, aspiration of irritant substances, and disease entities with idiopathic causes (2,5).
Pulmonary infectious disorders involving the small airways are the most common causes of the tree-in-bud sign (2). Any infectious organism, including bacterial, mycobacterial, viral, parasitic, and fungal agents, can involve the small airways and cause a tree-in-bud pattern. In pulmonary infectious disorders, the tree-in-bud sign has most commonly been described in patients with endobronchial spread of M tuberculosis (6,9). In patients with pulmonary tuberculosis, the tree-in-bud pattern is the most characteristic, but not pathognomonic, CT feature of active endobronchial spread and can be found in 72% of patients with active disease (6).
At histopathologic examination, the tree-in-bud opacities seen on CT scans were correlated with caseous material in the small airways (6). The terminal tufts of the tree-in-bud pattern may represent inflammation with caseous material in the respiratory bronchioles and alveolar ducts, whereas the stalks may represent caseous material within the terminal bronchiole (6). Similarly, bronchogenic dissemination of atypical mycobacterial organisms or pyogenic bacteria can result in tree-in-bud opacities (10–12) (Fig 2). Less frequently, the tree-in-bud sign is seen with viral and fungal infections (eg, invasive aspergillosis of the airways) and Pneumocystis carinii pneumonia (2,5).
The tree-in-bud sign is also a common finding on thin-section CT scans in patients with diffuse panbronchiolitis (1,7). In histopathologic studies, the centrilobularly distributed nodular and branching linear opacities at CT correspond to thickened and dilated bronchiolar walls with intraluminal mucous plugs (1,7).
In addition, various congenital disorders can cause diseases of the small airways that demonstrate a tree-in-bud pattern. In patients with cystic fibrosis, thick-walled mucus- or pus-filled bronchioles are frequently seen as branching or nodular centrilobular opacities at CT, usually associated with central bronchiectasis (2). Similar findings can be seen in patients with chronic infections of the small airways due to dyskinetic cilia syndrome, yellow nail syndrome, or congenital immunodeficiency states (5,10). Primary pulmonary lymphoma has recently been reported as a rare but important differential diagnosis for the tree-in-bud pattern. Lymphomatous involvement of the lung can simulate the radiologic findings of bronchiolitis (13).
In summary, the tree-in-bud sign is a characteristic and easily detectable CT finding in patients with disease of the small airways. It is a useful sign, which, in the appropriate context of clinical findings and laboratory features, almost invariably points to inflammatory disease of the small airways.
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  D. M. Hansell, A. A. Bankier, H. MacMahon, T. C. McLoud, N. L. Muller, and J. Remy Fleischner Society: Glossary of Terms for Thoracic Imaging Radiology, March 1, 2008; 246(3): 697 - 722. [Abstract] [Full Text] [PDF]
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 S Bosmans, B Weynand, and E Coche Pulmonary metastatic microangiopathy of colon cancer presenting as a "tree in bud" pattern Br. J. Radiol., January 1, 2008; 81(961): e11 - e12. [Abstract] [Full Text] [PDF]
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 S. E. Rossi, T. Franquet, M. Volpacchio, A. Gimenez, and G. Aguilar Tree-in-Bud Pattern at Thin-Section CT of the Lungs: Radiologic-Pathologic Overview RadioGraphics, May 1, 2005; 25(3): 789 - 801. [Abstract] [Full Text] [PDF]
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