Tree-in-bud sign (lung)

Tree-in-bud sign or pattern describes the CT appearance of multiple areas of centrilobular nodules with a linear branching pattern. Although initially described in patients with endobronchial tuberculosis, it is now recognized in a large number of conditions.

Pathology

Pathogenesis

Simply put, the tree-in-bud pattern can be seen with two main sites of disease :

  • distal airways
  • distal pulmonary vasculature

More specifically, the pattern can be manifest because of the following disease processes, often in combination:

Etiology

While the tree-in-bud appearance usually represents an endobronchial spread of infection, given the proximity of small pulmonary arteries and small airways (sharing branching morphology in the bronchovascular bundle), a rarer cause of the tree-in-bud sign is infiltration of the small pulmonary arteries/arterioles or axial interstitium .

Causes include:

Radiographic features

Tree-in-bud sign is not generally visible on plain radiographs . It is usually visible on standard CT, however, it is best seen on HRCT chest. Typically the centrilobular nodules are 2-4 mm in diameter and peripheral, within 5 mm of the pleural surface. The connection to opacified or thickened branching structures extends proximally (representing the dilated and opacified bronchioles or inflamed arterioles) .

Practical points