Tree-in-Bud Sign

Tree-in-bud sign refers to the condition in which small centrilobular nodules less than 10 mm in diameter are associated with centrilobular branching nodular structures [1] (Fig. 9.1). The small nodules represent lesions involving the small airways. However, vascular lesions involving the arterioles and capillaries may simulate the centrilobular small nodules and branching nodular structures (vascular tree-in-bud sign) [2–5] (Fig. 9.2).

Diseases Causing the Sign

The localized tree-in-bud sign is typically seen in infectious bronchiolitis, aspiration bronchiolitis, and bronchopneumonia including tuberculosis and nontuberculous mycobacterial pulmonary disease. Vascular tree-in-bud sign can be observed in foreign-body-induced necrotizing pulmonary vasculitis (cellulose and talc granulomatosis) [2–4] and in localized pulmonary lymphatic metastasis [5] (Table 9.1). For diseases related to this chapter, please refer to section “Small Nodules with Centrilobular Distribution” in Chap. 18 and Chap. 26.

Distribution

In infectious and aspiration bronchiolitis, the tree-in-bud sign shows middle and lower lung zone predominance. The upper lobes and the superior segment of the lower lobes are usually involved in tuberculosis, whereas the right middle lobe and lingular division of the left upper lobe are typical location of nodular bronchiectatic form of nontuberculous mycobacterial pulmonary disease.

In foreign-body-induced necrotizing vasculitis and in localized lymphatic metastasis, the vascular tree-in-bud sings demonstrate lower lung zone and subpleural predominance.

Tree-in-bud pattern with bronchial wall thickening is one (accounts for 31 % in one series) of the important CT findings of community-acquired respiratory viral infection in adults [6]. Predisposing factors for aspiration bronchiolitis include structural abnormalities of the pharynx, esophageal disorders, neurologic defects, and chronic illness [7]. Nontuberculous mycobacterial disease, nodular bronchiectatic form, is seen in an elderly lady who has lowered body mass index, and tuberculosis is a common infectious disease in immunocompromised hosts. Foreign-body-induced necrotizing vasculitis (cellulose and talc granulomatosis) is common among drug addicts who grind up and intravenously inject various drugs such as amphetamines, methylphenidate, or hydromorphone. These drugs are prepared as oral medication [3].

Key Points for Differential Diagnosis

1.Various diseases show this pattern of lung abnormality and thus distinction of one disease from others, the presence of associated imaging findings, along with patient history and clinical presentation, is often useful in suggesting the specific cause of this pattern (Table 9.1).

Aspiration Bronchiolitis

Pathology and Pathogenesis

Aspiration of gastric contents results in a chemical burn of the tracheobronchial tree and pulmonary parenchyma with an intense parenchymal inflammatory reaction. Once localized to the lung, neutrophils play a key role in the development of lung injury through the release of oxygen radicals and proteases. Acid aspiration pneumonitis reduces host

Fig. 9.1 Aspiration pneumonia in a 66-year-old man with Parkinson’s disease. (a–c) Lung window images of thin-section (1.0-mm section thickness) CT scans obtained at levels of the azygos arch (a), right middle lobar bronchus (b), and inferior pulmonary veins (c), respectively,

show multifocal areas of tree-in-bud signs (arrows) in both lungs and parenchymal consolidation in the right middle lobe. Also note large amount of secretion (open arrows) in tracheobronchial trees.

defenses against infection, increasing the risk of superinfection. In these circumstances, the pulmonary inflammatory response is likely to result from both bacterial infection and the inflammatory response of the gastric particulate matter [8].

acute onset of symptoms while sleeping. Occult aspiration in the elderly is usually associated with neurologic impairment, esophageal dysmotility, and gastroesophageal reflux.

Symptoms and Signs

Diffuse aspiration bronchiolitis is suspected in elderly patients with recurrent episodes of bronchorrhea, bronchospasm, and dyspnea [8]. It frequently presents with

CT Findings

HRCT manifestations include centrilobular nodules and unior bilateral foci of branching areas of increased attenuation with a tree-in-bud appearance in the dependent portions of both lungs [9] (Fig. 9.1).