1. A method for identifying lung fields within a chest region based on posteroanterior chest radiographic images, comprising: generating first image data representative of a posteroanterior chest image inclusive of lung fields; performing global threshold analysis of the posteroanterior chest image by processing said first image data to generate a processed image of the chest region inclusive of the lung fields; identifying the lung fields in said processed image of the chest region; and constructing, based on said lung fields identified in said processed image, first initial lung segmentation contours for said posteroanterior chest image.