Prediction of Obstructive Pulmonary Function by Quantitative Lobar Evaluation of Density Change between Inspiration and Expiration on Thoracic CT

PURPOSE To investigate density change between inspiration and expiration in thoracic CT at a lobar level and establish its relationship with pulmonary function. METHOD AND MATERIALS Sixty-seven male former or current heavy smokers underwent low-dose CT (16x0.75mm, 30 mAs, 120-140 kVp) at full inspiration, ultra low-dose CT (20 mAs, 90 kVp) at full expiration, and pulmonary function testing on the same day as part of a lung cancer screening trial. With in-house software, automatic non-rigid registration was performed to align the expiration scans precisely with their inspiration counterparts and the lobes were automatically segmented in the inspiration scans. Both scans were then blurred slightly to reduce the effect of noise and vessels were excluded by means of thresholding. The ratio of the Hounsfield values at expiration (H_exp) and at inspiration (H_insp) was calculated for each voxel in the registered lung volumes. The median, H, of the H_exp/H_insp ratios was calculated per lobe and for the entire lung volume and these medians were plotted against the FEV1/FVC ratios. Correlation coefficients, r, were measured to determine the presence of linear relationships between H and FEV1/FVC. Lines were fitted to the data and the line slopes, m, calculated in order to determine the predictive power of H in determining pulmonary function impairment. RESULTS The FEV1/FVC ratios varied from 0.48 to 0.92 with a mean value of 0.71. A stronger correlation between between H and FEV1/FVC was revealed in the lower lobes (r from -0.66 to -0.61) than in the upper and middle lobes (r from -0.47 to -0.38). Additionally the H ratios were more strongly predictive of FEV1/FVC for the lower lobes (m from -0.38 to -0.30) than for the upper and middle lobes (m from -0.23 to -0.16). For the entire lung volume the correlation and slope values were r=-0.58, m=-0.27. CONCLUSION Automatic regional density analysis on inspiration and expiration CT allows for the calculation of the ratio H which correlates well with measures of obstructive pulmonary impairment. The lower lobes appear to contribute more to functional impairment than the upper or middle lobes. CLINICAL RELEVANCE/APPLICATION Quantitative analysis of regional density changes in the lung may allow for new insights into the mechanisms of COPD and the determinants of obstructive lung function in smokers.