Quantification of Emphysema and Small Airway Disease in COPD Patients from Lobar Analysis of Volumetric Inspiration and Expiration Thoracic CT Scans

PURPOSE In COPD patients, airflow obstruction can be caused by emphysema and/or small airways disease, leading to air trapping. We quantified both underlying causes of COPD from lobar analysis of paired inspiration-expiration CT scans. METHOD AND MATERIALS A database was collected of 231 patients and heavy smokers with GOLD stages 0 (at risk) to 4 who received volumetric inspiration CT (16x0.75mm, 120-140 kVp, 30-160 mAs), ultra low dose volumetric expiration CT (90 kVp, 20 mAs) and pulmonary function testing on the same day. In house developed software automatically extracted the five pulmonary lobes in all scans. Cases where segmentation failed in one scan in a pair were excluded. As a measure for emphysema, the percentages of lobar volume below -950 HU were computed in inspiration scans (IN-950). As a quantification of either emphysema or air-trapping, the percentages of lobar volume below -850 HU were computed in expiration scans (EX-850). A lobe was considered to be affected by emphysema if IN-950 was larger than the 95th percentile for that lobe in GOLD 0 subjects. Otherwise, the lobe was considered to be affected by air-trapping if EX-850 was larger than the 95th percentile for that lobe in GOLD 0 subjects. Otherwise, the lobe was considered normal. If four or five lobes were affected by emphysema, airtrapping or were normal, that subject was classified as emphysema dominant (ED), air trapping dominant (ATD) or normal; otherwise, the COPD phenotype was considered 'mixed'. RESULTS 213 pairs were available for analysis with GOLD stages 0-4: 49, 49, 50, 40, 25, respectively. The occurrence of normal, ED, ATD and mixed phenotypes for GOLD 0 subjects was 98%, 2% 0%, and 6%. For GOLD 1: 69%, 4%, 0%, 27%. For GOLD 2: 36%, 20%, 14%, 30%. For GOLD 3: 5%, 47%, 15%, 33%. For GOLD 4: 0%, 84%, 4%, 12%. CONCLUSION Lobar analysis of inspiration and expiration CT scan revealed different patterns in COPD patients. For the lower GOLD stages (1-3), a substantial subgroup of COPD patients have predominantly small airway disease, and a mixed pattern with emphysema and air trapping affecting different lobes is also a common finding. COPD patients in GOLD category 4 usually have four or all lobes affected by emphysema. CLINICAL RELEVANCE/APPLICATION This study indicates that the combination of inspiration and expiration CT can be used to identify COPD patients with predominantly small airway disease, for whom drug treatment could be effective.