Simon described a technique to calculate the change in fractional content of air within pulmonary tissue between anatomically matched CT regions based on a simple model that assumes the density changes were solely due to air content. The investigators successfully applied that model to inhale and exhale breath-hold CT image pairs as well as 4DCT images to create ventilation images. However, the amount of blood in the thorax and lungs varies with the respiratory cycle, thus violating the assumption of this model. The investigators found a cyclic variation in the apparent weight of the lung on 4DCT and others have reported respiratory induced variations in pulmonary perfusion of the lung on MRI. The pulmonary density changes found on 4DCT thus result from both changes in air and blood content.