Primary outcome
atelectasis
POSTOPERATIVE ATELECTASIS, ATELECTASIS

Within 30 min of extubation, the chest CT was performed by a trained and experienced technician who was unaware of the group assignment. All CT images were assessed by an experienced radiologist. The primary outcome was the postoperative atelectasis volume, expressed as a percentage of the total lung volume. The calculation of the percentage of atelectasis volume consisted of three steps. The first step was to measure the total lung area by accurately depicting the contour of the lung image on each CT image with a thickness of 5 mm. The pulmonary hilus vessels were manually excluded from the lung region of interest. The second step was to delineate the volume of the atelectasis region in each CT image. When drawing the atelectasis area, it should be outlined as close to the pleura as possible, and vascular structures with diameters larger than 3 mm should be manually excluded. Lastly, we used the histogram functional view using ITK-SNAP software (version 3.6.0) to identify the atelectasis region (Fig. 
Examples of CT scans. Original chest CT image: (