[Imaging techniques in the examination of the distal airways: asthma and COPD]

Imaging techniques are highly useful diagnostic tools to study small airway diseases. Despite their differences, from a clinical pathological perspective, these diseases show similar radiological manifestations. High-resolution computed tomography (CT) is the technique of choice to study diffuse diseases and those of the small airways; the slices obtained in expiratory high-resolution CT scan should form part of the study protocol of small airway diseases. Based on the findings of high-resolution CT, small airway diseases can be divided into two large groups: (a) those presenting direct morphological signs of bronchiolar involvement, and (b) those showing indirect signs of bronchiolar involvement (air trapping/mosaic pattern). High-resolution CT is highly useful to study the complications of asthma (allergic bronchopulmonary aspergillosis) and to evaluate clinically similar processes, such as hypersensitivity pneumonitis. In asthmatic patients, multi-detector CT (MDTC) allows clinical symptoms, thickening of the airway wall and the degree of airflow obstruction to be directly correlated. MDTC is also useful for quantitative evaluation of the degree of air trapping in patients with emphysema. Magnetic resonance imaging after inhalation of distinct gases, such as (3)He and xenon-129 and dual-energy CT are also useful imaging techniques in the direct or indirect evaluation of the degree of airflow obstruction (air trapping).