12B-4 Real-Time Multi-Component Hemodynamic Measurement in Vascular Aneurysms Using Echo Particle Image Velocimetry: Comparison of In Vitro and Computational Results

Abstract

Abdominal aortic aneurysms (AAAs) are localized balloon-shaped expansions commonly found in the infrarenal segment of the abdominal aorta, between the renal arteries and the iliac bifurcation. From a biomechanical standpoint, AAA rupture risk is related to mechanical and hemodynamic factors such as localized flow fields and velocity patterns, and flow- induced stresses within the fluid and in the aneurysm structure. Thus, localized hemodynamics proximal, within and distal to AAA formations play an important role in modulating the disease process, and non-invasive and easy-to-implement methods to characterize and quantify these complex hemodynamics would be tremendously useful. Based on the synthesis of two existing technologies, particle image velocimetry (PIV) and brightness-mode (B-mode) contrast ultrasound echo imaging, we have recently developed an ultrasound based velocimetry technique, termed echo particle image velocimetry (Echo PIV), to perform accurate non-invasive measurement of velocity profiles, multiple velocity vectors and local shear rate in arteries and hearts by identifying and tracking flow tracers (ultrasound contrast microbubbles) within the flow fields. Here, we examine the utility of Echo PIV for quantifying the complex hemodynamics found within aneurysms. Three-dimensional computational fluid dynamics (CFD) simulations were used for comparison. The Echo PIV system was used to measure velocity vectors within in vitro fusiform AAA models. Ultrasound contrast microbubbles (Optisonreg, Amersham, UK) were seeded into the flows. To verify Echo PIV results, 3D computational fluid dynamics (CFD) was used on an identical aneurysm model. The computational solid model was imported into ICEM-CFD (ANSYS Inc., PA) and meshed using 35,000 hexahedral elements. The same flow conditions were used for both in vitro and CFD studies. Comparison results show that Echo PIV measures the flow field accurately and provides clear delineation of the complex flow patterns within the AAA model. To verify the performance of the Echo PIV system on a point-by-point basis, we compared velocity profiles at the bulge center from both CFD and Echo PIV results. Difference in velocity magnitudes was < 5 %, verifying the accuracy of Echo PIV in this model. Echo PIV was also able to resolve the multiple velocity scales seen within the vascular aneurysm (< 2 cm/sec to > 20 cm/sec). These results show that our custom-designed Echo PIV system is capable of accurately providing detailed multi-component, angle- independent velocity vectors within vascular aneurysms.