Outflow Conditions for Image-Based Hemodynamic Models of the Carotid Bifurcation: Implications for Indicators of Abnormal Flow

Abstract

Computational fluid dynamics (CFD) models have become very effective tools for predicting the flow field within the carotid bifurcation, and for understanding the relationship between local hemodynamics, and the initiation and progression of vascular wall pathologies. As prescribing proper boundary conditions can affect the solutions of the equations governing blood flow, in this study, we investigated the influence to assumptions regarding the outflow boundary conditions in an image-based CFD model of human carotid bifurcation. Four simulations were conducted with identical geometry, inlet flow rate, and fluid parameters. In the first case, a physiological time-varying flow rate partition at branches along the cardiac cycle was obtained by coupling the 3D model of the carotid bifurcation at outlets with a lumped-parameter model of the downstream vascular network. Results from the coupled model were compared with those obtained by imposing three fixed flow rate divisions (50/50, 60/40, and 70/30) between the two branches of the isolated 3D model of the carotid bifurcation. Three hemodynamic wall parameters were considered as indicators of vascular wall dysfunction. Our findings underscore that the overall effect of the assumptions done in order to simulate blood flow within the carotid bifurcation is mainly in the hot-spot modulation of the hemodynamic descriptors of atherosusceptible areas, rather than in their distribution. In particular, the more physiological, time-varying flow rate division deriving from the coupled simulation has the effect of damping wall shear stress (WSS) oscillations (differences among the coupled and the three fixed flow partition models are up to 37.3% for the oscillating shear index). In conclusion, we recommend to adopt more realistic constraints, for example, by coupling models at different scales, as in this study, when the objective is the outcome prediction of alternate therapeutic interventions for individual patients, or to test hypotheses related to the role of local fluid dynamics and other biomechanical factors in vascular diseases.

Physiological Significance of Helical Flow in the Arterial System and its Potential Clinical Applications

“…3 On the other hand, hemodynamic indicator such as helical flow index was proposed to reveal the global organization of blood flow, which was calculated from a Lagrangian-based method for helical flow quantification based on the velocity data from either computational fluid dynamics or phase contrast magnetic resonance imaging. 33,34,68,69,71,72 Through simulating the blood flow in an idealized model of aortocoronary bypass, Morbiducci et al found a linear inverse relationship between the oscillating shear index (a well-known risk index for atherogenesis) and the HFI. 51,71 Furthermore, based on hemodynamic simulations data of 50 carotid bifurcations, Gallo et al 33 demonstrated that the exposure of each carotid bifurcation to disturbed flow measured by time-averaged wall shear stress, oscillatory shear index and relative residence time decreased as the timeaveraged value of the absolute value of helicity increased.…”

Quantitative Analysis of Bulk Flow in Image-Based Hemodynamic Models of the Carotid Bifurcation: The Influence of Outflow Conditions as Test Case

“…To achieve this goal, we applied the consolidated approach of coupling a three-dimensional vascular model, where local information is needed, to a reduced-order model representing the downstream vasculature. 4,15,16,36,41,48,52,60 In this way, more realistic adaptive boundary conditions can be derived because through-scale models work together in order to accommodate the complex interaction between the phenomena modeled by each subcomponent of the whole system. For the sake of comparison, we also imposed, in the same isolated model of carotid bifurcation, three different fixed flow rate redistributions of the inlet common carotid artery (CCA) flow rate between the internal and external carotid arteries (ICA and ECA, respectively).…”

“…Several authors assessed the sensitivity of image-based CFD studies within the carotid bifurcation. 14,31,41,46,47,59,61 All of the authors focused their analyses on the sensitivity of WSS-based metrics.…”

Simulated hemodynamics in human carotid bifurcation based on Doppler ultrasound data

“…For each bifurcation analysis, subject-specific velocity boundary conditions were prescribed at CCA inlet and ICA outlet using Womersley velocity profiles derived from pulsatile spectral waveforms obtained by pulsed Doppler and traction-free boundary condition was defined at ECA outlet [9,[26][27][28]. Figure 3 presents the volume hexahedral mesh (left) and the distal CCA velocity waveform (right) used for solving the hemodynamics of subject P1.…”