To segment low density lung regions in chest CT images, most of methods use the difference in gray-level value of
pixels. However, radiodense pulmonary vessels and pleural nodules that contact with the surrounding anatomy are often
excluded from the segmentation result. To smooth lung boundary segmented by gray-level processing in chest CT
images, we propose a new method using scan line search. Our method consists of three main steps. First, lung boundary
is extracted by our automatic segmentation method. Second, segmented lung contour is smoothed in each axial CT slice.
We propose a scan line search to track the points on lung contour and find rapidly changing curvature efficiently. Finally,
to provide consistent appearance between lung contours in adjacent axial slices, 2D closing in coronal plane is applied
within pre-defined subvolume. Our method has been applied for performance evaluation with the aspects of visual
inspection, accuracy and processing time. The results of our method show that the smoothness of lung contour was
considerably increased by compensating for pulmonary vessels and pleural nodules.