Imaging of small airways disease, with emphasis on high resolution computed tomography

Bronchioles are the airways less than 2-3 mm in diameter. Normal bronchioles cannot be reliably detected by means of high resolution computed tomography (HRCT). Nevertheless, in pathological cases, bronchiolar lesions can be identified by taking into account direct and indirect signs. On radiological grounds, bronchiolar lesions can be classified into four groups, on the basis of HRCT findings: 1) prevailing nodular opacities and "tree in bud" pattern; 2) consolidations or ground-glass opacities; 3) mosaic oligosemia with expiratory air-trapping; and 4) mixed cases. In this review, we present the main radiological and HRCT findings in four different entities, representing the more typical cases of bronchiolar pathology; bronchiolitis obliterans; bronchiolitis obliterans with organizing pneumonia; diffuse panbronchiolitis; and respiratory bronchiolitis with associated interstitial lung disease. HRCT sometimes allows a precise diagnosis of bronchiolar pathology; however, more often, it permits only a range of hypotheses to be advanced. More importantly, it allows a precise localization for biopsy procedures and an exact follow-up after institution of therapy. In normal subjects, less than 1% of the whole bronchial tree is visible on the standard chest radiograph. HRCT offers a good insight and invaluable information. New techniques, such as volumetric HRCT with sliding-thin-slab maximum- and minimum-intensity projections (MIP and minip) could represent an important additional tool in the evaluation of small airways disease.