CT detection of layering of i.v. contrast material in the abdominal aorta

We sought to determine whether IV contrast material would layer dependently within the abdominal aorta if administered from a location and at a rate routinely used for CT angiography.

In 22 patients, three equal-sized regions of interest (ROIs) were symmetrically spaced across the abdominal aortic lumen at the level of the celiac axis. These positions corresponded to a dependent, a central, and a nondependent position. Time-enhancement curves were generated for each ROI after IV injection of 10 ml of nonionic contrast material, which was administered at a rate of 3.5 ml/sec, through an antecubital vein. The magnitude of and the time to peak enhancement for each ROI were determined. The 22 peak enhancement values obtained for each of the three ROIs were compared using the Newman-Keuls test.

A statistically significant increase in peak enhancement was identified when traversing the abdominal aortic lumen from the nondependent to the central ROI (p < .05) and from the central to the dependent ROI (p < .01). Levels of mean peak enhancement were 48.3+/-19.0, 38.9+/-18.2, and 33.0+/-19.7 for the dependent, central, and nondependent ROIs, respectively. The amount of time needed to reach peak enhancement of each of the three ROIs in an individual patient was similar.

A small dose of contrast material administered IV through an antecubital vein will layer dependently in the abdominal aorta--thus creating a gradient in enhancement--despite mixing in the heart. If layering of contrast material is subsequently detected during routine abdominal CT angiography, then patient positioning may need to be adjusted to maximize enhancement of targeted vessels.