Redistribution of Pulmonary Vascular Volume at Upright versus Supine CT

Background Gravity redistributes pulmonary blood flow in the upright position; however, how total pulmonary vascular volume (TPVV) and its arterial and venous components depend on position remains unclear. Purpose To quantify position-dependent changes in TPVV and the corresponding arterial and venous components across lung lobes and vertical lung regions using upright chest CT. Materials and Methods This secondary analysis of a prospective, single-center study enrolled healthy adult volunteers who underwent same-day noncontrast chest CT in supine and upright positions using a 320-detector row CT system between March 2018 and December 2018. Participants were excluded for incidental lung abnormalities or inadequate vessel separation. Pulmonary arteries and veins were segmented semiautomatically, and pulmonary vascular volumes were quantified for whole lung, lobes, and upper, middle, and lower lung regions. Paired two-sided t tests with Bonferroni correction were used for comparisons. Results Fifty participants (mean age ± SD, 47 years ± 9.7; 28 men) were evaluated. Standing reduced TPVV by 18.8% compared with the supine position (mean, 99.8 mL ± 18.2 vs 81.0 mL ± 14.3; P < .001). When standing, pulmonary venous volume decreased more than pulmonary arterial volume across the whole lung (25.4% reduction [mean, 50.7 mL ± 9.1 supine vs 37.5 mL ± 5.7 standing] vs 11.1% reduction [mean, 49.1 mL ± 9.9 supine vs 43.6 mL ± 9.4 standing]; P < .001). Volume reductions when standing followed a craniocaudal gradient, with greater reductions in the upper lung than in the lower lung (P < .001). Dependent lower lung regions also showed decreases in TPVV (mean reduction, -12.0%; mean, 40.4 mL ± 9.0 supine vs 35.4 mL ± 7.7 standing), pulmonary arterial volume (mean reduction, -6.0%; mean, 18.2 mL ± 3.9 supine vs 17.1 mL ± 4.5 standing), and pulmonary venous volume (mean reduction, -16.7%; mean, 22.2 mL ± 4.6 supine vs 18.3 mL ± 3.7 standing; all P < .001). Conclusion Upright chest CT showed that, compared with the supine position, standing reduced TPVV, predominantly venous, with a vertical gradient, where dependent lower lung regions exhibited modest volume reduction. © RSNA, 2026 Supplemental material is available for this article. See also the editorial by Suh in this issue.