LAUSR

We previously reported that apparent lung mass varies across the phases of 4D computed tomography (4DCT) images. We hypothesize that these variations correspond to the physiologic changes in pulmonary perfusion induced during normal tidal breathing, and should therefore be present in every breathing patient. In this study, we characterize and quantify the respiratory induced variation in pulmonary blood mass (▵PBM) on 89 patients treated with stereotactic body radiotherapy. ▵PBM was computed from the treatment planning helical 4DCT images of each patient. Conversion from Hounsfield Units (HU) to density and mass per voxel was made using the density calibration curve, applied to the lung parenchyma volume within each phase. A difference in the lung mass with breathing was found for all cases, as was a substantial individual variation in lung volume. We found that the ▵PBM increased during inhalation, and decreased during exhalation. A significant correlation between the individual ▵PBM and tidal volume was observed; ▵PBM increased with tidal volume. We further evaluated the anatomic distribution of ▵PBM variation comparing the central versus peripheral lung, cranial versus caudal, dependent versus non-dependent lung. Our observations regarding spatial distribution of the ▵PBM agree with previously reported differences among similar regions for the supine patient. These results show that a variation in pulmonary mass during respiration is apparent on 4DCT and suggest that these variations reflect respiratory induced changes in the pulmonary perfusion. Therefore, the 4DCT derived respiratory induced ▵PBM signal can provide further insight into the pulmonary circulation and advance the overall understanding and diagnosis of human health and disease.