Case report: the effect of contrast density on computed tomographic arterial portography

Contrast settling in cerebral aneurysm angiography

During angiography, blood flow is visualized with a radiopaque contrast agent, which is denser than blood. In complex vasculature, such as cerebral saccular aneurysms, the density difference may produce an appreciable gravity effect, where the contrast material separates from blood and settles along the gravity direction. Although contrast settling has been occasionally reported before, the fluid mechanics behind it have not been explored. Furthermore, the severity of contrast settling in cerebral aneurysms varies significantly from case to case. Therefore, a better understanding of the physical principles behind this phenomenon is needed to evaluate contrast settling in clinical angiography. In this study, flow in two identical groups of sidewall aneurysm models with varying parent-vessel curvature was examined by angiography. Intravascular stents were deployed into one group of the models. To detect contrast settling, we used lateral view angiography. Time-intensity curves were analysed from the angiographic data, and a computational fluid dynamic analysis was conducted. Results showed that contrast settling was strongly related to the local flow dynamics. We used the Froude number, a ratio of flow inertia to gravity force, to characterize the significance of gravity force. An aneurysm with a larger vessel curvature experienced higher flow, which resulted in a larger Froude number and, thus, less gravitational settling. Addition of a stent reduced the aneurysmal flow, thereby increasing the contrast settling. We found that contrast settling resulted in an elevated washout tail in the time-intensity curve. However, this signature is not unique to contrast settling. To determine whether contrast settling is present, a lateral view should be obtained in addition to the anteroposterior (AP) view routinely used clinically so as to rule out contrast settling and hence to enable a valid time-intensity curve analysis of blood flow in the aneurysm.

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 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 bifuration 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.