Physiological associations of computerized tomography lung density: a factor analysis

BACKGROUND Objective quantification of emphysema using computerized tomography (CT) density measurements is rapidly gaining wide acceptance as an in vivo measurement tool. However, some studies have suggested that abnormal lung function in the absence of emphysema can affect lung density, and the role of such measurements in identifying and monitoring the progression of emphysema is not clear. OBJECTIVE To clarify the relationship between lung density measurements and pulmonary function. METHODS CT measurements of the proportion of lung occupied by low density tissue (as percentage of lung area below predetermined Hounsfield unit [HU] thresholds) were obtained in a large random population (n = 739) and the association with detailed pulmonary function tests studied using factor analysis. RESULTS Density measurements showed a greater association with measures of hyperinflation and airflow obstruction than measures of gas transfer (correlation coefficient, high resolution scan, -950HU threshold vs FEV1/FVC, RV, and DLCO/VA of -0.39, 0.22, and -0.15 respectively). The strongest lung density factor coefficients of 0.51 (standard resolution scan, - 950 HU threshold) and 0.46 (high resolution scan, - 910 HU threshold) were seen with factors predominantly consisting of measures of airflow obstruction and hyperinflation. Most variation in lung density was not accounted for by lung function measurements (communality 0.21-0.34). CONCLUSION Lung density measurements associate most strongly with measures of airway disease that are not specific to emphysema.