DIFFERENCES IN CT DENSITY BETWEEN DEPENDENT AND NONDEPENDENT PORTIONSOF THE LUNG - INFLUENCE OF LUNG-VOLUME

OBJECTIVE. Lung tissue, blood, and air determine the physical density of the lung and hence the attenuation measured on CT scans. These comp onents are not homogeneously distributed throughout the lungs, and the ir relative proportion changes continuously during respiration. The ob jective of this study was to measure densities in various areas of the normal lung with CT and to examine the influences of gravity and of t he degree of lung inflation on these densities. SUBJECTS AND METHODS. The subjects in the study were six healthy male volunteers. CT scans w ere obtained at the level of the aortic arch and 2 cm above the expira tory level of the diaphragm with the subjects supine and then with the m prone. Scans were obtained at lung volumes of 10%, 50%, and 90% vita l capacity by using a respiratory gating device connected to the CT sc anner. The overall density of the lungs at these two levels and the de nsities in the dependent (posterior in supine, and anterior in prone b ody position) and nondependent (anterior in supine and posterior in pr one body position) areas were measured by using a semiautomatic evalua tion algorithm. RESULTS. Changes in lung volume caused the same change s in lung density in the right and left lungs and in the upper and low er parts of the lungs. For both body positions (supine and prone), the difference between lung density at 10% vital capacity and that at 90% vital capacity was significantly larger (p < .05) in the dependent pa rts of the lung than in the nondependent parts. In both positions, dif ferences in density between dependent and nondependent regions were si gnificantly (p < .05) greater at 10% vital capacity than at 90% vital capacity, and the differences became small when the volume of the lung was near total lung capacity. CONCLUSION. Changes in lung volume have different effects on changes in lung density in dependent and nondepe ndent parts of the lung. The largest changes in lung density occur in the dependent regions. The difference in density between dependent and nondependent lung regions is smallest for lung volumes near total lun g capacity. These findings may be useful as a baseline for interpretin g CT measurements of regional lung density in suspected cases of lung disease, which would alter the physical density of lung tissue. Our re sults also suggest that measurements near total lung capacity should b e included.