What are centrilobular nodules and tree in bud opacities?
Centrilobular nodules are small (less than 1 cm) focal opacities located in the centers of secondary pulmonary lobules on CT, which spare pleural and fissural surfaces and are often ill-defined. These are due to pathologies that affect the bronchioles, pulmonary arterioles, and lymphatic vessels found in the central portions of secondary pulmonary lobules. Although seen in various disease conditions, their presence usually indicates infectious or inflammatory small airways disease such as by hypersensitivity pneumonitis, respiratory (smoking-related) bronchiolitis, and endobronchial spread of infection.
Tree-in-bud opacities appear as tiny centrilobular branching structures on CT, most often in the lung periphery, which resemble budding trees ( Figure 18-4 ). These are due to filling of the distal bronchioles and involvement of the adjacent alveoli, most often caused by infectious bronchiolitis, bronchitis, and aspiration.