Magnified computed tomography images of airways in the right lower lobe from a–d) a healthy male subject and e–h) an asthmatic male subject. Magnified computed tomography images of airways in the right lower lobe from a–d) a healthy male subject and e–h) an asthmatic male subject. The arrows indicate airways cut in approximate cross-section, which can be seen to bifurcate caudally (from a–d and e–h). When the airways bifurcate to ∼2-mm diameter airways, the challenges of measuring the thin airway walls and small lumen diameters is apparent, whereas in comparison, regions of gas trapping as shown in figure 1 are much easier to measure. Determining localised wall thickening for a single airway is difficult. However, heterogeneity between sister airways (i.e. bifurcating from the same parent airway) and closure of airways have been shown to be greater in asthmatic compared with non-asthmatic airways [42, 75], albeit in medium- to small-sized airways. The functional consequences of heterogeneous narrowing and closure have been extensively examined in modelling [77] and inert gas washout studies [59, 61, 62], as well as the ventilation imaging studies described in this review. Scale bars=10 mm. Gregory G. King et al. Eur Respir Rev 2019;28:180111 ©2019 by European Respiratory Society