What causes the tree-in-bud sign in lungs on CT?
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Answer:
Centrilobular branching nodules due to impaction of bronchioles by mucus, pus, or fluid from endobronchial infection or aspiration, seen in conditions such as bacterial bronchopneumonia, mycobacterial infections (tuberculosis, nontuberculous mycobacteria), fungal infections, chronic aspiration pneumonia, cystic fibrosis, and less commonly autoimmune diseases or malignancy.
Why is it called so?:
Named for its resemblance to the branching structures of a budding tree, with small centrilobular nodules representing buds and linear opacities depicting twigs or branches.
Pathophysiology:
Develops from bronchiolar luminal impaction with mucus, pus, fluid, or inflammatory debris, accompanied by bronchiolar dilatation, wall thickening, and peribronchiolar inflammation, rendering normally invisible peripheral airways visible on high-resolution CT as centrilobular nodules 2-4 mm in diameter connected to branching linear structures.
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