X-ray measurement of regional blood flow distribution using radiopaque contrast medium: influence of gravity

X-ray CT measurement of regional blood flow distribution in the lungs is potentially biased because the contrast medium used to track the flow is denser than blood. To evaluate this gravity effect, cross-sectionally uniform boluses (Reno 60, density 1.3) were delivered at the upstream end of a horizontal tube connected to a downstream axisymmetric bifurcation. When the plane of the bifurcation was vertical and actual flow through the two branches was equal, the fraction of contrast medium passing through the downward-directed branch increased with decreasing Reynolds number, increasing length-to-diameter ratio of the horizontal tube, and decreasing bolus volume. In the lungs, Reynolds number decreases and pathway length increases with decreasing vessel diameter. Thus, the results suggest that the spatial resolution of CT flow measurement within the lungs may be limited by density differences between contrast medium and blood.