Abstract
Background: Small airways disease (SAD) is an important cause of airflow limitation in smokers. In vivo quantification of SAD in smokers may aid in automated identification and phenotyping of COPD. The optimal CT method of air trapping quantification, a measure of small airways disease, has not been established yet.
Purpose: To test three quantitative CT air trapping measures that correct for emphysema extent against the residual volume over total lung capacity ratio (RV/TLC) as reference standard for air trapping.
Methods: In 447 current or former heavy smokers volumetric inspiratory-expiratory CT scans, spirometry and body plethysmography were obtained on the same day. The expiratory to inspiratory ratio of mean lung density (E/I-ratioMLD), the relative inspiratory to expiratory volume change between Hounsfield units (HU) -860 and -950 (RVC-860to-950) and the expiratory percentage of voxels between -780HU and -910HU (EXP-780to-910) were calculated. ROC analysis was performed with RV/TLC as reference test, and the areas under the ROC curves (AUC) were compared to each other. The optimal cut-off and sensitivity and specificity was calculated for the best method.
Results: The E/I-ratioMLD showed an AUC (95% CI) of 0.84 (0.81-0.88), and performed significantly better than the RVC-860to-950 (AUC=0.70, p<0.001) and EXP-780to-910 (AUC=0.81, p=0.03). An E/I-ratioMLD >87% had a sensitivity of 0.82 and a specificity of 0.80 for the detection of abnormal RV/TLC.
Conclusion: The expiratory to inspiratory ratio of mean lung density on CT is the optimal quantitative CT measure for air trapping detection in current or former heavy smokers. This measure may prove important in future COPD research.
- © 2011 ERS