Disorders of the small airways: high-resolution computed tomographic features.

Several infectious and noninfectious processes may affect predominantly or exclusively the small airways and result in reversible or irreversible abnormalities. Small-airway diseases can be considered as synonymous with bronchiolitis and can be classified into three main categories: (a) obliterative (constrictive) bronchiolitis, (b) cellular bronchiolitis, and (c) respiratory bronchiolitis. The introduction of high-resolution computed tomography (HRCT) has led to a considerable improvement in our ability to diagnose small-airway diseases. The characteristic HRCT findings of obliterative bronchiolitis consist of areas of decreased attenuation and vascularity with blood flow redistribution resulting in areas of increased lung attenuation and vascularity ("mosaic perfusion" pattern). In cellular bronchiolitis, the characteristic HRCT findings consist of centrilobular nodules and branching opacities ("tree-in-bud" pattern). Finally, bilateral areas of ground-glass attenuation and/or poorly defined centrilobular nodules are characteristic of respiratory bronchiolitis and respiratory bronchiolitis-associated interstitial lung disease (RB-ILD). This article reviews the clinical, pathological, and HRCT features of some of the most common small-airway diseases.