Figure 1. Thoracic computed tomography (CT) sections of two patients representing normally aerated and nonaerated lung parenchyma along with their corresponding density histograms. The CT section on the left side shows normally aerated left lung of patient 11. The corresponding histogram located under the CT section demonstrates that the majority of the lung volume has a density < -500 HU. The CT section on the right side shows nonaerated lower lobes along with normally aerated upper lobes in patient 6. Under the CT section, the corresponding histogram demonstrates tomographic densities concentrated at either end of the density spectrum representing the normally aerated and nonaerated lung areas. On CT sections, the limits of lung parenchyma and main fissura manually traced by the radiologist using the roller ball of the CT scan are represented. Dashed areas represent pleural effusion.