Intraluminal layering of iodinated contrast on a single axial computed tomography image is described as a possible sign of slow arterial flow and is quantified using a new region of interest-based metric, that is, gravitational gradient. The metric was measured in 4 patients who demonstrated this phenomenon and in the aorta of 10 patients with no clinical sign suggestive of slow flow. Future studies are needed to validate the relationship between gravitational gradient, slow flow, and patient outcome.