Purpose: There is mounting evidence that the shear forces generated from abnormal blood flow drive platelet activation, potentially increasing stroke risk. This investigation utilizes velocity fields determined from X-Ray Particle Image Velocimetry (X-PIV) to evaluate shear stress on sub-millimeter radiopaque microspheres in-vitro.
Methods: Shear stress changes were evaluated with increasing percent vessel occlusion; X-PIV was performed in three 3D-printed carotid bifurcation models, where artificial stenoses were created in an external branch of each model equivalent to 33% and 66% occlusion. Particle motions were imaged at 1000 fps and tracked over 1-millisecond intervals throughout the FOV. Because evaluation of Lagrangian metrics such as shear stress history rely on neighboring velocity estimates, an outlier analysis was performed to remove spurious vectors. The spatial velocity gradient at each particle-centroid location was determined. Individual particle trajectories could then be selected to evaluate shear stress throughout the sequence or within a particular ROI. Stress accumulation (SA) was calculated along several trajectories by integrating shear stress over a specified time interval.
Results: The highest velocity gradients and thus shear stresses were found in the 66% stenosis region (Median = 17.1 mPa). Notably, a small recirculation region was present distal to the stenosis where several particles experienced strong deflections; these shear microgradients are known to contribute to plaque instability. This is in contrast to the non-stenosed branches (Median = 9.5 mPa), with only a slight increase in the 33% stenosis region (Median = 10.4 mPa). SA along particle trajectories reached upwards of 1000 mPa∙ms in the 66% stenosis model.
Conclusion: High-speed X-PIV enables a Lagrangian approach to velocity and shear stress estimation; in this case, substantial increases in both velocity and shear were found along particle trajectories at 66% vessel occlusion. This methodology can be applied to a wide range of vascular geometries and flow conditions in-vitro.
Funding Support, Disclosures, and Conflict of Interest: This research was supported by NIH grant R01EB030092, and in part by Canon Medical Systems Inc.