Tree-in-bud pattern

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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 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)
- e.g. breast cancer, stomach cancer
bronchovascular interstitial infiltration
- e.g. sarcoidosis, lymphoma, leukaemia ^4-5,5

Aetiology

While the tree-in-bud appearance usually represents endobronchial spread of infection, given the closeness of small pulmonary arteries and small airways (sharing branching morphology-bronchovascular bundle), a rarer cause of the tree-in-bud sign is infiltration of the small pulmonary arteries or axial interstitium ^3,6-7,7.

Causes include:

infective bronchiolitis
- pulmonary tuberculosis, e.g. Mycobacterium tuberculosis infection
- atypical mycobacterial infections, e.g. Mycobacterium avium (MAIC)
- viral pneumonia
- fungal pneumonia, e.g. aspergillus
- allergic bronchopulmonary aspergillosis (ABPA)
- pneumocystis pneumonia
congenital
connective tissue disorders
- rheumatoid arthritis (RA)
- Sjögren syndrome
bronchial
neoplastic (i.e. carcinomatous endarteritis ^3,6-7,7or bronchovascular interstitial infiltration ^4-5,5)
- bronchioloalveolar cell carcinoma
- distant metastatic disease (e.g. breast, liver, ovary, prostate, kidney)
- primary pulmonary lymphoma⁵
- chronic lymphocytic leukemia ⁴

Radiographic features

Tree-in-bud sign is not visible on plain radiographs ². 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) ^1-3,6.

Practical points

using maximum intensity projection (MIP) can facilitate detection of particularly the centrilobular nodules ⁶
identification of the tree-in-bud sign should urge you to
- look for further imaging findings e.g. thickening of the bronchial wall, narrowing of bronchi, bronchiectasis, consolidation, cavitation, necrotic lymphadenopathy
- determine the location (with gravitational or lower lobe predominance ~~favoring~~favouring aspiration) ^6-7,7
- ~~scrutinize~~scrutinise patient history, including appropriate exposure history, as this may aid in determining the most likely diagnosis ^6-7,7

-<a title="Bronchioles" href="/articles/bronchioles">bronchioles </a>filled with pus or inflammatory exudate<ul><li>e.g. <a href="/articles/tuberculosis-pulmonary-manifestations-1">pulmonary tuberculosis</a>, <a href="/articles/aspiration-bronchopneumonia">aspiration bronchopneumonia</a>
+<a href="/articles/bronchioles">bronchioles </a>filled with pus or inflammatory exudate<ul><li>e.g. <a href="/articles/tuberculosis-pulmonary-manifestations-1">pulmonary tuberculosis</a>, <a href="/articles/aspiration-bronchopneumonia">aspiration bronchopneumonia</a>
-<a href="/articles/bronchiectasis">bronchiectasis</a> with <a title="Mucus plugging" href="/articles/mucoid-impaction-lung-1">mucus plugging</a><ul><li>e.g. <a href="/articles/cystic-fibrosis">cystic fibrosis</a>
+<a href="/articles/bronchiectasis">bronchiectasis</a> with <a href="/articles/mucoid-impaction-lung-1">mucus plugging</a><ul><li>e.g. <a href="/articles/cystic-fibrosis">cystic fibrosis</a>
~~-<li>bronchovascular interstitial infiltration<ul><li>e.g. <a href="/articles/sarcoidosis-1">sarcoidosis</a>, <a title="Lymphoma" href="/articles/lymphoma">lymphoma</a>, leukaemia 4-5~~
+<li>bronchovascular interstitial infiltration<ul><li>e.g. <a href="/articles/sarcoidosis-1">sarcoidosis</a>, <a href="/articles/lymphoma">lymphoma</a>, leukaemia 4,5
-</ul><h5>Aetiology</h5>While the tree-in-bud appearance usually represents endobronchial spread of infection, given the closeness of small pulmonary arteries and small airways (sharing branching morphology-bronchovascular bundle), a rarer cause of the tree-in-bud sign is infiltration of the small pulmonary arteries or axial interstitium 3,6-7.Causes include:<ul>
+</ul><h5>Aetiology</h5>While the tree-in-bud appearance usually represents endobronchial spread of infection, given the closeness of small pulmonary arteries and small airways (sharing branching morphology-bronchovascular bundle), a rarer cause of the tree-in-bud sign is infiltration of the small pulmonary arteries or axial interstitium 3,6,7.Causes include:<ul>
~~-neoplastic (i.e. carcinomatous endarteritis 3,6-7 or bronchovascular interstitial infiltration 4-5)<ul>~~
+neoplastic (i.e. carcinomatous endarteritis 3,6,7 or bronchovascular interstitial infiltration 4,5)<ul>
-<li>determine the location (with gravitational or <a href="/articles/conditions-with-lower-lobe-predominance-mnemonic">lower lobe predominance</a> favoring <a href="/articles/aspiration-pneumonia">aspiration</a>) 6-7
+<li>determine the location (with gravitational or <a href="/articles/conditions-with-lower-lobe-predominance-mnemonic">lower lobe predominance</a> favouring <a href="/articles/aspiration-pneumonia">aspiration</a>) 6,7
~~-<li>scrutinize patient history, including appropriate exposure history, as this may aid in determining the most likely diagnosis 6-7~~
+<li>scrutinise patient history, including appropriate exposure history, as this may aid in determining the most likely diagnosis 6,7

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