Tree-in-bud sign

In immuno-compromised hosts a peribronchial distribution (resulting in a tree-in-bud sign) of small, sometimes cavitated ill-defined nodules can be obtained due to miliar distribution (Goo and Im 2002 Van Dyck et al. 2003). Ganghopulmonary (primary) forms, however, present with In homogenous consohdation and necrotic mediastinal/hilar lymphadenopathy (Van Dyck et al. 2003). 

Small airways disease on CT can be categorized into visible and indirect patterns of the disease. The tree-in-bud sign reflects the presence of dilated centrilobular bronchioles with lumina that are impacted with mucus, fluid, or pus it is often associated with peribronchiolar inflammation (Webb 2006). Cicatricial scarring of many bronchioles results in the indirect sign of patchy density differences of the lung parenchyma, reflecting areas of hypoventilation and air trapping, as well as subsequent hypoperfusion (mosaic perfusion). 

HRCT often discloses a complex admixture of lesions, termed rheumatoid lung. Bronchiectasis is present in 8% to 75% of patients, but the distinction between primary bronchiectasis and traction bronchiectasis due to pulmonary fibrosis is often difficult (139,140,149,151,152,155,166,177). Bronchiolitis is also frequent, manifesting as centrilobular nodules, tree-in-bud sign, mosaic perfusion, or air trapping (139,149,151,152,155,166,177). Emphysema is present in 5% to 43% of patients (13,139,140,149,151,152,155,166,174,177) and is widely viewed as a trait of RA-ILD, unlike findings in other CTDs. Emphysema is more frequent with an HRCT pattern of UIP, as opposed to NSIP (155). Other features include pulmonary rheumatoid nodules and pleural effusion or thickening can be observed. Pulmonary artery enlargement has been observed in nearly half of patients with RA-ILD, in spite of the fact that overt PH is rare in RA (155). 

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