[Assessment of imaging techniques in the evaluation of small airways disease in asthma]

The imaging techniques that have been used in the exploration and evaluation of treatment in asthma include computed tomography (CT), helium-magnetic resonance imaging (MRI), single photon emission computerized tomoscintigraphy (SPECT) and positron emission tomography (PET). Only MRI does not involve radiation. However, in the case of CT, technical improvements and the advantages of the air-filled lung have resulted in a decreased radiation burden. High resolution examinations, using a low dose of about one-year of natural background irradiation, are possible even in infancy. CT is the best evaluated so far, the simplest to perform and the least expensive. In asthma several morphological changes related to small airway disease can be visualised on CT images: cylindrical bronchiolectasis, thickening of the bronchial walls and air trapping, an indirect marker of bronchiolar obstruction. Today there is a robust body of evidence that valid indices of small airways disease can be deduced from quantitative analysis of lung density, indices that correlate well with clinical severity and functional measurements. In addition, the sensitivity of the method is sufficient to demonstrate therapeutic effects.