Correlation of respiratory changes in lung density on dynamic chest radiographs with changes in the CT value: A computational phantom study

Pulmonary impairments are observed as decreased changes in lung density during respiration on dynamic chest radiographs (DCR). To facilitate pulmonary function evaluation based on DCR, the present study was conducted to correlate respiratory changes in pixel values (pixel values) on DCR images with those in computed tomography (CT) values (Delta CT values) using four-dimensional extended cardiac-torso (XCAT) phantoms. Twenty XCAT phantoms with forced breathing with/without chest wall motion were generated, and then, were projected using an X-ray simulator. Delta pixel values and Delta CT values on respiration were calculated on the simulated projection and CT images, respectively, to create a conversion table. The equation (regression line) of the relationship between the Delta pixel values and Delta CT values in the lung field was calculated, and statistical analysis was performed to test whether there was a significant difference in the inclination of the regression line (P < 0.05) in terms of physique, sex, and breathing manner. There was a significant difference in the inclination of the regression lines between males and females, with and without chest wall motion. We developed a conversion table from Delta pixel values to Delta CT values in the lung field. Our results demonstrated the feasibility of the quantitative evaluation of pulmonary function based on Delta pixel values on DCR. It would be helpful to provide a better understanding of the pulmonary function and conditions based on Delta pixel values while considering Delta CT values.