In catheter-based angiography contrast is injected through a catheter into flowing arterial blood to attenuate the X-ray beam such that physicians can view vascular luminal morphology for diagnosis of vascular pathology [1]. Since real time angiography became available the transport of contrast could be visualized and numerous attempts have been made to relate the visualized transport of contrast to blood flow or blood velocity. Since the dispersion of contrast is a complex process and thorough understanding of contrast mixing with the flowing blood is missing, many physicians and researchers alike implicitly assume that mixing of contrast and blood is either instantaneously completed or that mixing does not take place at all during the short visualization period. In the former case many erroneously assume that the visualization of contrast transport directly reflect blood flow characteristics. Yet others attributed homogeneous mixing to turbulence even when the Reynolds number was as low as 77 [2]. In the latter case some reports suggested that since the two do not mix, the contrast, which is denser than blood, will tend to accumulate at low parts of the arterial system due to the gravity. The stereotypical concept is that under laminar flow conditions homogeneous mixing will take a long time and consequently a long distance. The fact that the angiographic contrast is a liquid tracer injected forcefully into the blood, generating an ejector effect is ignored. The velocity mismatch between the injected contrast and the flowing blood plays a major role in laminar mixing of the contrast. In this paper, we report our experimental investigation that was designed to find out how far distal to the catheter tip will the injected contrast reach homogeneous mixing with a blood analog fluid as evident by no further changes in the contrast density profiles measured downstream of the catheter tip.

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