Estimation of Lung Volume Superimposed on the Mediastinum and Diaphragm on Chest Radiographs Using Upright Multidetector Row CT.

Purpose To quantify lung and lobar volumes superimposed on the mediastinum and diaphragm on upright chest radiographs using upright multidetector row CT and to compare them with supine measurements. Materials and Methods In this prospective study, asymptomatic Asian participants underwent both upright and supine multidetector row CT within 2 hours. Lung and lobar volumes were measured in each position. Ray-sum images were reconstructed to delineate mediastinal and diaphragmatic boundaries, and superimposed volumes in the posteroanterior direction were calculated. These CT-derived volumes were considered to approximate lung fields projected over the mediastinum and diaphragm on radiographs. Percentages of superimposed volumes relative to total lung and lobar volumes were determined. Mixed-effects models and multiple linear regression analyses were used. Results The study included 132 participants (mean age ± SD, 47.4 years ± 11.4; 85 female; mean body mass index ± SD, 22.2 ± 3.4 [calculated by dividing weight in kilograms by height in meters squared]). The proportion of lung volume superimposed on the mediastinum and diaphragm was 23.9% ± 3.1 in the upright position, significantly lower than 25.7% ± 3.3 in the supine position (P < .001), despite a 9.8% increase in total lung volume upright. Superimposed percentages of the right upper, middle, and lower lobes and the left upper and lower lobes in the upright position were 10.4% ± 2.5, 7.5% ± 3.2, 28.8% ± 7.9, 12.3% ± 1.9, and 46.0% ± 8.3, respectively. All lobes except the left lower lobe showed significantly lower superimposition upright than supine (all P < .03). Multiple linear regression identified older age, higher body mass index, and male sex as independent predictors of greater superimposition in the upright position (all P < .03). Conclusion Lung volume superimposed on the mediastinum and diaphragm was lower on upright than supine chest radiographs despite greater lung volume in the upright position. University Hospital Medical Information Network Clinical Trials Registry: UMIN000026586 Keywords: CT, Thorax, Lung, Mediastinum, Diaphragm, Anatomy, Comparative Studies, Technology Assessment Supplemental material is available for this article. © The Author(s) 2026. Published by the Radiological Society of North America under a CC BY 4.0 license.