先大津法计算全局图像的阈值，再用阈值分割找出肺实质区域，再填补肺实质空洞，实现用边界追踪检测肺实质边界的目的(The calculation of the overall image threshold Otsu threshold segmentation first, and then find out the lung parenchyma, and pulmonary parenchyma fill holes, trace detection boundary in the purpose of lung parenchyma)