Tree-in-bud sign

The tree-in-bud sign 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 recognised in a large number of conditions.

Pathology

Pathogenesis

The tree-in-bud sign occurs as a result of a number of processes, although often they co-exist in the same condition:

• bronchioles filled with pus or inflammatory exudate
  • e.g. pulmonary tuberculosis, aspiration bronchopneumonia
• bronchiolitis - thickening of bronchiolar walls and bronchovascular bundle
  • e.g. cytomegalovirus pneumonitis, obliterative bronchiolitis
• bronchiectasis with mucus plugging
  • e.g. cystic fibrosis
• tumour emboli to centrilobular arteries (or carcinomatous endarteritis) ³
  • e.g. breast cancer, stomach cancer
• bronchovascular interstitial infiltration
  • e.g. lymphoma, leukaemia ^4-5

Aetiology

The tree-in-bud appearance represents endobronchial spread of infection. Causes include

• infective bronchiolitis
  • pulmonary tuberculosis, e.g. Mycobacterium tuberculosis infection 
  • atypical mycobacterial infections, e.g Mycobacterium avium (MAIC)
  • viral pneunomia
  • fungal pneumonia, e.g. aspergillus
  • allergic bronchopulmonary aspergillosis (ABPA)
  • pneumocystis pneumonia
• congenital
  • cystic fibrosis
  • immotile cilia syndrome, e.g. Kartagener syndrome
  • yellow nail syndrome
• connective tissue disorders
  • rheumatoid arthritis (RA)
  • Sjogren syndrome
• bronchial
  • obliterative (constrictive) bronchiolitis 
  • diffuse panbronchiolitis
  • follicular bronchiolitis
• neoplastic
  • bronchioloalveolar cell carcinoma
  • primary pulmonary lymphoma ⁵
  • chronic lymphocytic leukaemia ⁴

Radiographic features

Tree-in-bud sign is not visible on plain film ² and is best seen on HRCT. Typically they are composed of centrilobular nodules (which are usually 2 - 4mm in diameter and peripheral, within 5 mm of pleural surface) connected by opacified or thickened branching structres extending proximally (representing the dilated and opacified bronchioles) ^1-3.