1. A method for automatically segmenting an image of a patient's lung, comprising: inputting data from one or more computed tomography images; segmenting a thoracic airspace by intensity-thresholding image voxels; subdividing the thoracic airspace into axial 2D components; identifying a midline of the airspace; detecting a maximum intensity cost path in a region associated with the midline; and assigning 2D components to left and right regions of interest based on the detected maximum intensity cost path.