Early Identification of Small Airways Disease on Lung Cancer Screening CT: Comparison of Current Air Trapping Measures

Background Lung cancer screening CT scans might provide valuable information about air trapping as an early indicator of smoking-related lung disease. We studied which of the currently suggested measures is most suitable for detecting functionally relevant air trapping on low-dose computed tomography (CT) in a population of subjects with early-stage disease. Methods This study was ethically approved and informed consent was obtained. Three quantitative CT air trapping measures were compared against a functional reference standard in 427 male lung cancer screening participants. This reference standard for air trapping was derived from the residual volume over total lung capacity ratio (RV/TLC) beyond the 95th percentile of predicted. The following CT air trapping measures were compared: expiratory to inspiratory relative volume change of voxels with attenuation values between −860 and −950 Hounsfield Units (RVC −860 to −950 ), expiratory to inspiratory ratio of mean lung density (E/I-ratio MLD ) and percentage of voxels below −856 HU in expiration (EXP −856 ). Receiver operating characteristic (ROC) analysis was performed and area under the ROC curve compared. Results Functionally relevant air trapping was present in 38 (8.9 %) participants. E/I-ratio MLD showed the largest area under the curve (0.85, 95 % CI 0.813–0.883), which was significantly larger than RVC −860 to −950 (0.703, 0.657–0.746; p