Automatic Estimation of Lobar and Segmental Variation of Emphysema Distribution in Chronic Smokers

PURPOSE To investigate the lobar and segmental variation in distribution of emphysema detected on volumetric high-resolution CT in a group of chronic smokers. METHOD AND MATERIALS A set of 350 volumetric CT scans was randomly selected from the NELSON study, a lung cancer screening program with low-dose CT (16x 0.75 mm collimation, 30 mAs). The pulmonary segments were segmented with an automatic algorithm based on automatic lung and fissure segmentations and a k-nearest neighbor classification. For each segment an emphysema score was computed as the percentage of attenuation values below a fixed threshold. Two thresholds T were applied: T = -910 HU and -950 HU. In addition, lobar and total emphysema scores were calculated. To assess whether distribution changed with larger amount of emphysema, we performed a separate analysis of patients in whom the total emphysema score was above 2% for A-A?A 1/2 950 HU (153 scans) and above 10% for A-A?A 1/2 910 HU (272 scans). Differences were evaluated using paired t-tests. RESULTS The upper and middle lobe/ lingula of either lung showed significantly higher emphysema scores than the lower lobe (p<0.05), independent of threshold or extent of emphysema. No significant difference was found for scores in the upper lobes, middle lobe or lingula. Segment R2 and segments 6, 9, and 10 in both lungs were significantly less affected than the other segments, independent of threshold or extent of emphysema (p<0.05). For T = -950 the mean emphysema score for segments 6, 9, 10 and R2 was below 2.9% (1.7% to 2.8%), while the mean score for all other segments was above 3.1% (3.2% to 5.2%). In the subgroup with total scores > 2% these limits were 5.6% and 6.2%, respectively. For T = -910 the mean emphysema score for segments R2, 6, 9 and 10 was below 16.3% (11.2% to 16.2% ), while the mean score for all other segments was above 18.4% (18.5% to 30.3%). In the subgroup with total scores > 10% these limits were 20.0% and 22.1%. CONCLUSION There is not only lobar variability in distribution of emphysema in chronic smokers but also segmental with segment R2, and segments 6, 9 and 10 of both lungs being least affected. CLINICAL RELEVANCE/APPLICATION The results provide new insights about the distribution of smoking-related emphysema.