Detailed analysis of the density change on chest CT of COPD using non-rigid registration of inspiration/expiration CT scans.

OBJECTIVES: One objective was to evaluate the air trapping index (ATI), measured by inspiration/expiration CT, in COPD patients and nonsmokers. Another objective was to assess the association between the pulmonary function test (PFT) and CT parameters such as ATI or other indices, separately in the whole lung, in emphysema, and in hyperinflated and normal lung areas.
METHODS: One hundred and thirty-eight COPD patients and 29 nonsmokers were included in our study. The ATI, the emphysema index (EI), the gas trapping index (Exp -856) and expiration/inspiration ratio of mean lung density (E/Iratio of MLD) were measured on CT. The values of the whole lung, of emphysema, and of hyperinflated and normal lung areas were compared and then correlated with various PFT parameters.
RESULTS: Compared with nonsmokers, COPD patients showed a higher ATI in the whole lung and in each lung lesion (all P < 0.05). The ATI showed a higher correlation than EI with FEF25-75%, RV and RV/TLC, and was comparable to Exp -856 and the E/I ratio of MLD. The ATI of emphysema and hyperinflated areas on CT showed better correlation than the normal lung area with PFT parameters.
CONCLUSIONS: Detailed analysis of density change at inspiration and expiration CT of COPD can provide new insights into pulmonary functional impairment in each lung area. KEY POINTS: • COPD patients show significant air trapping in the lung. • The air trapping index is a comparable parameter to other CT indices. • Air trapping of emphysema and hyperinflated lung areas relates to functional loss. • The emphysema area changes more, with less air trapping than other areas.