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CT Sign of Tree-In-Bud Appearance

The tree-in-bud sign is seen on thin-section CT images of the lung . Peripheral "within approximately 3–5 mm of the pleural surface", small "from 2–4 mm in diameter", centrilobular, and well-defined nodules of soft-tissue attenuation are connected to linear, branching opacities that have more than one contiguous branching site, thus resembling a tree in bud.

This thin-section computed tomography scan obtained in a 29y old man with acute myeloid leukemia after bone marrow transplantation. The patient had a history of fever and cough. Image shows multiple, small, centrilobular nodules of soft-tissue attenuation connected to linear branching opacities (arrows). Note the morphologic similarities to the photograph of the tree in bud . At serologic examination, an infection with Mycoplasma pneumoniae was confirmed.

Explanation of the tree-in-bud sign
The tree-in-bud pattern represents bronchiolar luminal impaction with mucus, pus, or fluid, which demarcates the normally invisible branching course of the peripheral airways . In addition, dilated and thickened walls of the peripheral airways and peribronchiolar inflammation can contribute to the visibility of affected bronchioles . In histopathologic studies, the tree-in-bud appearance correlates well with the presence of plugging of the small airways with mucus, pus, or fluid; dilated bronchioles; bronchiolar wall thickening; and peribronchiolar inflammation .