Tree in bud opacification refers to a sign on chest CT where small centrilobular nodules and corresponding small branches simulate the appearance of the end of a branch belonging to a tree that is in bud.
Originally and still often thought to be specific to endobronchial Tb, the sign is actually non-specific and is the manifestation of pus, mucus, fluid or other material within the terminal bronchioles, respiratory bronchioles and alveolar ducts. See figure 2. It can therefore be seen in Tb; bacterial, fungal and viral infections; and diseases causing bronchiectasis, bronchiolectasis and bronchiolitis. The last of these, bronchiolitis, may be caused by genetic disorders including cystic fibrosis, primary ciliary dyskinesia (including Kartagener’s syndrome), aspiration, inhalation of toxic substances, rheumatological/connective tissue disorders and Asian/Japanese/diffuse panbronchiolitis. As you can see the possible causes of a tree in bud appearance are legion.
Where there is small airways disease and tree in bud is present, this can be termed an exudative bronchiolitis. E.g. diffuse panbronchiolitis is an exudative bronchiolitis. This is distinct from the small airways disease with obliterative bronchiolitis although the two may share a common cause and coexist.
Figure 1 (below): A branch of a walnut tree in bud.
Figure 2 (below): A diagrammatic representation of tree in bud opacification within the secondary pulmonary lobule.
Figure 3 and 4 (below): Tree in bud opacification in presumed diffuse panbronchiolitis.
Figure 5 (below): Tb – tree in bud opacification is present, but tending more to nodularity so is difficult to discern.