Influence of gravity on radiographic contrast material-based measurements of regional blood flow distribution

Rationale and Objectives. Radiographic measurement of regional blood flow distribution in the lungs is potentially biased because the contrast material used to track flow is denser than blood. The authors performed this study to evaluate I the effect of gravity on flow estimates by using an experimental test phantom and numeric simulations. Materials and Methods. Cross-sectionally uniform boluses of radiopaque contrast material were delivered at the upstream end of a horizontal inlet tube connected to a downstream axisymmetric bifurcation attached to collecting tubing spirals. The phantom was imaged by using both planar angiography and dynamic multi-detector row computed tomography (CT) during the passage of the bolus through the phantom. The images were analyzed to determine the relative amounts of contrast material traveling through the top and bottom branches of the bifurcation by using varying Reynolds numbers and ratios of inlet tube volume to bolus volume. Numeric simulations of flow within a straight channel, with use of a dispersion operator intended to simulate settling of the bolus due to gravity, were performed under conditions representative of those in the experiments. Results. When the plane of the bifurcation was vertical and actual flow through the two branches was equal, the fraction of contrast material passing through the downward-directed branch increased with decreasing Reynolds number and increasing inlet tube-bolus volume ratio. This occurred in both the experiments and the simulations. Conclusion. Because in the circulation Reynolds number decreases and pathway length increases with decreasing vessel diameter, the accuracy of regional flow measurements obtained with angiography or CT within the lungs may be limited by density differences between contrast material and blood.