Figure 2. Lobar and gravitational segmentation of sagittal lung .

A, for each subject and lung volume a vascular structure image optimized to highlight the vascular structure in the lung was acquired, and used to delineate three lobes in the right lung. B, the lung was manually segmented into three lobes based on fissures apparent on the vascular structure images (ASL difference image, not shown). There are three lobes in the right lung; the oblique fissure separates the lower lobe from the rest of the lung and the horizontal fissure divides the upper and middle lobes (right lower lobe, RLL; right upper lobe, RUL; and right middle lobe, RML). C, the lung was also divided into three equidistant vertical gravitational regions, starting from the most posterior part of the lung and proceeding anteriorly giving dependent, middle and non‐dependent regions, respectively.