Quantification of pulmonary emphysema using high-resolution computed tomography

Objective quantification of emphysema by high-resolution computed tomography (HRCT) correlates with emphysema determined microscopically. The usefulness of HRCT in characterising chronic obstructive pulmonary disease (COPD) remains to be determined. Aims: To relate lung morphology quantified by HRCT to physiological and functional indices. Methods: Patients with stable COPD were imaged using HRCT (0.5mm collimation, 15mm intervals, lung apex to base, full inspiration). With image analysis software, a CT emphysema index was calculated as percentage of lung areas in all images (density <-400HU) occupied by emphysema (density <-950HU). Results: 47 patients (12F, 35M) of mean (±SD) age 67(±10) years, 50(±31) pack years smoking and FEV. 1.3(±0.6)L were studied prospectively. Mean CT emphysema index was 16.2(±12.0)%, with range 2.1 to 48.7% (normal <6.8%). COPD was classified as emphysema/no bronchitis (34%), chronic bronchitis/no emphysema (6%), both (45%), neither (15%). CT emphysema index correlated with % predicted for DiCO (r= -0.58), maximal VOz/kg (r=-0.56), FEV, (r=-0.43), BMI (r=-0.47) and FRC (r=0.37) (all p<0.01). No correlation was found with age, pack years, PaOz, 6 minute walk or quality of life. Correlation was stronger in chronic bronchitis and no bronchodilator reversibility, whereas lung region analysis as upper/lower or thirds did not strengthen correlation. Conclusions: This is the first study to use 0.5mm HRCT images to define emphysema. Whilst HRCT measurement of alveolar destruction was related to reduced gas transfer and maximal exercise capacity, it was not highly predictive of other indices, suggesting that respiratory physiology and imaging of lung morphology are complementary. Quantifying emphysema in COPD has implications for early detection, prognosis and stratification for clinical research.