Validation of lung density indices by cardiac CT for quantification of lung emphysema

Andreas Ronit,1 Thomas Kristensen,2 Yunus Çolak,3 Jørgen Tobias Kühl,4 Anna Kalhauge,2 Peter Lange,5,6 Børge G Nordestgaard,3,7 Jørgen Vestbo,8 Susanne D Nielsen,1 Klaus F Kofoed2,4 1Department of Infectious Diseases 8632, Rigshospitalet, Copenhagen University Hospital, Copenhagen, Denmark; 2Department of Radiology, Rigshospitalet, Copenhagen University Hospital, Copenhagen, Denmark; 3Department of Clinical Biochemistry and the Copenhagen General Population Study, Herlev and Gentofte Hospital, Copenhagen University Hospital, Herlev, Denmark; 4Department of Cardiology, Rigshospitalet, Copenhagen University Hospital, Copenhagen, Denmark; 5Department of Public Health, Section of Social Medicine, University of Copenhagen, Copenhagen, Denmark; 6Medical Unit, Respiratory Section, Hvidovre Hospital, Copenhagen University Hospital, Hvidovre, Denmark; 7Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark; 8Division of Infection, Immunity and Respiratory Medicine, University of Manchester, Manchester, UK Objectives: Cardiovascular disease is often associated with COPD. Lung density quantification of images obtained from cardiac computed tomography (CT) scans would allow simultaneous evaluation of emphysema and coronary artery calcification score and provide further mechanistic insight into the relationship between these syndromes.Patients and methods: We assessed the agreement between lung density indices obtained by cardiac and full-lung CT scans. Paired cardiac and chest CT scans were assessed in 156 individuals with and without airflow limitation. Quantitative threshold indices of low attenuation area (LAA) and 15th percentile density index (PD15) were compared in terms of precision using Spearman’s correlation coefficient, accuracy using concordance correlation coefficient (CCC), and relative accuracy using P15 and P30. We also assessed the relationship between visually and quantitatively determined emphysema and used receiver operating characteristic curves to evaluate the ability of lung density indices to discriminate airflow limitation.Results: Correlation coefficients between lung density indices obtained from cardiac and chest CT scans were 0.49 for percent LAA (%LAA)-950 and 0.71 for PD15. Corresponding values for CCC, P15, and P30 were 0.33, 3.2, and 5.1, respectively, for %LAA-950, and 0.34, 17.3, and 37.8, respectively, for PD15. For both cardiac and chest CT scans, visually determined emphysema was associated with higher %LAA-950 and lower PD15, and the ability of %LAA-950 and PD15 to discriminate airflow limitation were comparable.Conclusion: Although chest CT imaging is preferable, cardiac CT imaging may also be used for lung emphysema quantification where association measures are of primary interest. Keywords: agreement, cardiac CT, chest CT, chronic obstructive pulmonary disease, emphysema, lung density