Structural and functional assessments of normal vs. asthmatic populations via image registration and CFD techniques

The aim of this study is to investigate the functional and structural differences between normal subjects and asthmatics via image registration and computational fluid dynamics (CFD), together with pulmonary function test's (PFT) and one-image-based variables. We analyzed three populations of CT images: 50 normal, 42 non-severe asthmatic and 52 severe asthmatic subjects at total lung capacity (TLC) and functional residual capacity (FRC). A mass preserving image registration technique was employed to match CT images at TLC and FRC for assessments of regional volume change and anisotropic deformation. Instead of existing threshold-based air-trapping measure, a fraction-based air-trapping measure was proposed to account for inter-site and inter-subject variations of CT density. We also analyzed structural alterations of asthmatic airways, including bifurcation angle, hydraulic diameter, luminal area and wall area. CFD and particle tracking simulations are employed with physiologically-consistent boundary condition. As compared with normal subjects, severe asthmatics exhibit reduced air volume change (consistent with air-trapping) and more isotropic deformation in the basal lung regions, but increased air volume change associated with increased anisotropic deformation in the apical lung regions. In the multi-center study, the traditional air-trapping measure showed the significant site-variability due to the differences of scanners and coaching methods. The proposed fraction-based air-trapping measure is able to overcome the inter-site and inter-subject variations, allowing analysis of large data sets collected from multiple centers. We further demonstrate alterations of bifurcation angle, constriction, wall thickness and non-circularity at local branch level in severe asthmatics. The bifurcation angle, non-circularity and especially reduced hydraulic diameter significantly affect the increase of particle deposition in severe asthmatics. In summary, the two-image registration-based deformation provides a tool for distinguishing differences in lung mechanics among populations. The new fraction-based air-trapping measure significantly improves the association of air-trapping with the presence and severity of asthma and the correlation with forced expiratory volume in 1 second over forced vital capacity (FEV1/FVC) than existing approaches. The altered functions and structures such as air-volume change, branching angles, non-circular shapes, wall thickness and hydraulic diameters that found in asthmatics are strongly associated with the flow structures and particle depositions.