Blood Flow Simulation in Stented Vessels and Flow Reversal Conditions
Hemodynamics is one of the major factors involved in the development of cardiovascular diseases and adverse events associated with endovascular stent implantation such as in-stent restenosis and thrombosis. The first part of this work is dedicated to the imaging of stent deployment in graft tubes and ex vivo arterial segments and the investigation of in-stent hemodynamics through computational hemodynamic simulations, i.e., CHD, based on realistic in vitro stent and wall geometries. The second part is concerned with the parameterization of flow rate waveforms resembling physiologic waveforms and the description of waveform properties that lead to the occurrence of wall flow reversal in straight, cylindrical pipes. Stent reconstruction by high-resolution micro-computed tomography (microCT) is compared with reconstructions obtained from clinical CT: multislice CT (MSCT), C-arm flat detector CT (C-arm CT), and flat panel CT (FP-CT). The spatial resolution of current clinical CT for stent imaging is insufficient to visualize fine geometrical details as stent struts appear over-sized. Deployment characteristics such as stent strut prolapse into the lumen, strut vertex misalignment, underdeployment, wall prolapse, and wall creases at strut vertices of intracranial and coronary stents with open- and closed-cell designs are demonstrated by microCT imaging. In-stent hemodynamics are significantly altered by non-uniform deployment characteristics (misaligned strut vertices, malapposed struts, or wall creases), important effects not realistically captured in previous, computer generated stent models. Periodic waveforms with positive net flow rates can exhibit flow reversal. The physiological flow waveform is divided into acceleration and deceleration phases described by sinusoids, and two non-dimensional parameters to quantify wall flow reversal conditions are proposed. For waveforms typical of arterial blood flow, the wall shear stress reversal during the second deceleration phase is strongly influenced by the amplitude ratio of the preceding acceleration and deceleration phases and less by the relative time period of the deceleration phase. The method presented here can identify the occurrence of wall flow reversal and indicate the possibility of oscillatory wall shear stress that may negatively affect endothelial cell function due to blood flow waveform characteristics.