Relating Automatic Quantification of Airway Wall Thickness, Emphysema and Air Trapping in Inspiration and Expiration Chest CT Scans to COPD GOLD Stages

PURPOSE Previous studies investigated the relation of emphysema and air trapping measurements to COPD GOLD stages. This study investigates the additional value of airway wall thickness measurements in inspiration chest CT scans. METHOD AND MATERIALS A database of low-dose chest CT scans of control subjects (GOLD 0, n=44) and COPD subjects (GOLD I-IV, n=45/44/41/34) was used. In house developed software (Fraunhofer MEVIS, Bremen, Germany; Diagnostic Image Analysis Group, Nijmegen, the Netherlands) automatically segmented the lungs and airways in both the inspiration and expiration scans. Airway wall thickness was assessed in orthogonal cross-sections every 1mm throughout the entire airway tree (inspiration only) using an intensity-integration technique which accounts for partial volume effects. Wall thickness was averaged across all cross-sections with a lumen diameter of 2.75-3.25mm (WT3). Emphysema was determined as the percentage of lung volume below -950 HU in inspiration scans (IN-950), air trapping was determined as the percentage of lung volume below -850 HU in expiration scans (EX-850). Multiple linear regression models for different combinations of CT measurements (IN-950, EX-850, WT3, IN-950 & EX-850, IN-950 & WT3, IN-950 & EX-850 & WT3) as input variables and numeric GOLD stage (integer number 0-4) as output variable were established and compared to each other in terms of adjusted squared correlation coefficient R2 between output of the regression model and true GOLD stages. RESULTS The overall highest correlation was observed for the combination of all three measurements as input variables with adjusted R2=0.72, closely followed by R2=0.69 for the combination of inspiration only measurements IN-950 and WT3. Lower values were observed for all remaining models (IN-950 & EX-850: 0.64 / EX-850: 0.64 / IN-950: 0.53 / WT3: 0.23). CONCLUSION The combination of airway wall thickness and emphysema measurements from only inspiration CT scans provides more information for the prediction of lung function as determined by GOLD stages than combination of emphysema and air trapping measurements in inspiration and expiration CT scans. CLINICAL RELEVANCE/APPLICATION Airway wall thickness measurements contribute valuable information for the prediction of lung function from chest CT scans and may provide additional insight into development of COPD.