Subvoxel Accurate Airway Wall Measurements using an Intensity Integration Approach: Comparative Study using the COPDGene Phantom

PURPOSE Accurate quantification of airway wall thickness in thoracic CT is a key technique for the extraction of imaging biomarkers in obstructive pulmonary disease. We present an intensity integration approach that allows measurements of airway wall thickness with an accuracy higher than the resolution of the CT data. METHOD AND MATERIALS For most airways visible on CT, wall thickness is less than the resolution of the image data and partial volume effects hamper accurate measurement. By making appropriate assumptions about the density of air, airway wall and lung parenchyma, it is possible to fit a model to an array of line profiles perpendicular to the airway wall and thus measure the wall thickness precisely. This intensity integration approach was compared with two established methods: Full-Width-at-Half-Maximum (FWHM) and Phase Congruency (PC). We used the improved COPDGene phantom with 7 polycarbonate tubes with varying sizes (lumen diameter: 2.5/3/3/6/6/6/6 mm, wall thickness: 0.4/0.6/0.6/0.9/1.2/1.2/1.5 mm) and orientations (tubes 3 and 6 at 30A,Adeg angle from scanner z-axis, all others in z-axis direction). The intensity integration and FWHM method were implemented by the authors, PC measurements were obtained using the open source software Airway Inspector. We measured each tube at five positions and calculated the average measurement. Accuracy of each method was determined by calculating the minimum, mean and maximum signed as well as mean unsigned measurement errors for each method. Additionally, linear regression was used to determine each methodAC/a,!a,,C/s ability to detect changes in wall thickness regardless of constant measurement bias. RESULTS The smallest minimum/mean/maximum signed errors were obtained for intensity integration: -0.07/-0.01/0.05 mm versus 0.39/0.74/1.04 mm (FWHM) and -0.26/0.13/0.57 mm (PC). Intensity integration also gave the lowest mean absolute errors: 0.05 mm versus 0.74mm (FWHM) and 0.27mm (PC). Correlation was very high for intensity integration and FWHM (r=0.997 and r=0.972, both p<0.001) and only high for PC (r=0.746, p=0.054). CONCLUSION Intensity integration yielded the superior results in terms of overall error and measurement consistency. FWHM significantly overestimates thin walls but provides consistent measurements. PC is unreliable for measuring small airways. CLINICAL RELEVANCE/APPLICATION Airway wall thickness measurements are best performed with an intensity integration quantification algorithm.