Sang-Wook Lee

Geometry of the Carotid Bifurcation Predicts Its Exposure to Disturbed Flow

Background and Purpose— That certain vessels might be at so-called geometric risk of atherosclerosis rests on assumptions of wide interindividual variations in disturbed flow and of a direct relationship between disturbed flow and lumen geometry. In testing these often-implicit assumptions, the present study aimed to determine whether investigations of local risk factors in atherosclerosis can indeed rely on surrogate geometric markers of disturbed flow. Methods— Computational fluid dynamics simulations were performed on carotid bifurcation geometries derived from MRI of 25 young adults. Disturbed flow was quantified as the surface area exposed to low and oscillatory shear beyond objectively-defined thresholds. Interindividual variations in disturbed flow were contextualized with respect to effects of uncertainties in imaging and geometric reconstruction. Relationships between disturbed flow and various geometric factors were tested via multiple regression. Results— Relatively wide variations in disturbed flow were observed among the 50 vessels. Multiple regression revealed a significant (P<0.002) relationship between disturbed flow and both proximal area ratio (&bgr;≈0.5) and bifurcation tortuosity (&bgr;≈−0.4), but not bifurcation angle, planarity, or distal area ratio. These findings were shown to be insensitive to assumptions about the flow conditions and to the choice of disturbed flow indicator and threshold. Conclusions— Certain geometric features of the young adult carotid bifurcation are robust surrogate markers of its exposure to disturbed flow. It may therefore be reasonable to consider large-scale retrospective or prospective imaging studies of local risk factors for atherosclerosis without the need for time-consuming and expensive flow imaging or CFD studies.

Correlations among indicators of disturbed flow at the normal carotid bifurcation.

A variety of hemodynamic wall parameters (HWP) has been proposed over the years to quantify hemodynamic disturbances as potential predictors or indicators of vascular wall dysfunction. The aim of this study was to determine whether some of these might, for practical purposes, be considered redundant. Image-based computational fluid dynamics simulations were carried out for N=50 normal carotid bifurcations reconstructed from magnetic resonance imaging. Pairwise Spearman correlation analysis was performed for HWP quantifying wall shear stress magnitudes, spatial and temporal gradients, and harmonic contents. These were based on the spatial distributions of each HWP and, separately, the amount of the surface exposed to each HWP beyond an objectively-defined threshold. Strong and significant correlations were found among the related trio of time-averaged wall shear stress magnitude (TAWSS), oscillatory shear index (OSI), and relative residence time (RRT). Wall shear stress spatial gradient (WSSG) was strongly and positively correlated with TAWSS. Correlations with Himburg and Friedmans dominant harmonic (DH) parameter were found to depend on how the wall shear stress magnitude was defined in the presence of flow reversals. Many of the proposed HWP were found to provide essentially the same information about disturbed flow at the normal carotid bifurcation. RRT is recommended as a robust single metric of low and oscillating shear. On the other hand, gradient-based HWP may be of limited utility in light of possible redundancies with other HWP, and practical challenges in their measurement. Further investigations are encouraged before these findings should be extrapolated to other vascular territories.

On the Relative Importance of Rheology for Image-Based CFD Models of the Carotid Bifurcation

BACKGROUND Patient-specific computational fluid dynamics (CFD) models derived from medical images often require simplifying assumptions to render the simulations conceptually or computationally tractable. In this study, we investigated the sensitivity of image-based CFD models of the carotid bifurcation to assumptions regarding the blood rheology. METHOD OF APPROACH CFD simulations of three different patient-specific models were carried out assuming: a reference high-shear Newtonian viscosity, two different non-Newtonian (shear-thinning) rheology models, and Newtonian viscosities based on characteristic shear rates or, equivalently, assumed hematocrits. Sensitivity of wall shear stress (WSS) and oscillatory shear index (OSI) were contextualized with respect to the reproducibility of the reconstructed geometry, and to assumptions regarding the inlet boundary conditions. RESULTS Sensitivity of WSS to the various rheological assumptions was roughly 1.0 dyn/cm(2) or 8%, nearly seven times less than that due to geometric uncertainty (6.7 dyn/cm(2) or 47%), and on the order of that due to inlet boundary condition assumptions. Similar trends were observed regarding OSI sensitivity. Rescaling the Newtonian viscosity based on time-averaged inlet shear rate served to approximate reasonably, if overestimate slightly, non-Newtonian behavior. CONCLUSIONS For image-based CFD simulations of the normal carotid bifurcation, the assumption of constant viscosity at a nominal hematocrit is reasonable in light of currently available levels of geometric precision, thus serving to obviate the need to acquire patient-specific rheological data.

DIRECT NUMERICAL SIMULATION OF TRANSITIONAL FLOW IN A STENOSED CAROTID BIFURCATION

The blood flow dynamics of a stenosed, subject-specific, carotid bifurcation were numerically simulated using the spectral element method. Pulsatile inlet conditions were based on in vivo color Doppler ultrasound measurements of blood velocity. The results demonstrated the transitional or weakly turbulent state of the blood flow, which featured rapid velocity and pressure fluctuations in the post-stenotic region of the internal carotid artery (ICA) during systole and laminar flow during diastole. High-frequency vortex shedding was greatest downstream of the stenosis during the deceleration phase of systole. Velocity fluctuations had a frequency within the audible range of 100-300Hz. Instantaneous wall shear stress (WSS) within the stenosis was relatively high during systole ( approximately 25-45Pa) compared to that in a healthy carotid. In addition, high spatial gradients of WSS were present due to flow separation on the inner wall. Oscillatory flow reversal and low pressure were observed distal to the stenosis in the ICA. This study predicts the complex flow field, the turbulence levels and the distribution of the biomechanical stresses present in vivo within a stenosed carotid artery.

Parallel simulation of high reynolds number vascular flows

Publisher Summary The chapter provides an overview of the governing equations, time advancement scheme, and spectral element method. The chapter describes boundary condition treatment for simulating transition in bifurcation geometries. The chapter also presents parallel considerations and performance results, and provides results for transitional flow in an arteriovenous graft model. The simulation of turbulent vascular flows presents significant numerical challenges. Because such flows are weakly turbulent, they lack an inertial subrange that is amenable to subgrid-scale (SGS) modeling required for large-eddy or Reynolds-averaged Navier–Stokes simulations. The only reliable approach at present is to directly resolve all scales of motion. While the Reynolds number is not high, the physical dissipation is small. Weakly turbulent blood flow—such as the one that occurs in post-stenotic regions or subsequent to graft implantation—exhibits a much broader range of scales than does its laminar counterpart, and thus requires an order of magnitude increase in spatial and temporal resolution, making fast iterative solvers and parallel computing necessities.

Intravascular Turbulence in a Diseased Carotid Bifurcation: Numerical Study

Diseased vessels experience a significantly different biomechanical environment than healthy vessels due to the presence of transitional and turbulent flow, which may be responsible for cell damage or plaque disruption. With an objective of investigating turbulent characteristics in a diseased vessel, direct numerical simulations were conducted based on a patient-specific carotid bifurcation with a severe stenosis under pulsatile flow conditions. Computerized tomography images and color Doppler ultrasound measurements were acquired to obtain the anatomical geometry and in vivo flow waveform, respectively. The spectral element method, which is ideally suited for transitional and turbulent flow simulation, was employed in the numerical technique.

Influence of Inlet Secondary Curvature on Image-Based CFD Models of the Carotid Bifurcation

In image-based CFD modelling of carotid bifurcation hemodynamics, it is often not possible (or at least not convenient) to impose measured velocity profiles at the common carotid artery (CCA) inlet. Instead, fully-developed velocity profiles are usually imposed based on measured flow rates. Previous work from our group showed that this is reasonable [1], in the sense that errors made in doing so are substantially less than uncertainties inherent in the model construction process itself. In that study, long helical inlet sections were imposed to induce asymmetric (Dean-type) velocity flows profiles consistent with in vivo velocity profiles measured by others at the CCA [2, 3].Copyright

Hemodynamic Phenotypes of Anatomically Realistic Terminal Aneurysms

Intracranial aneurysms occur in approximately 4% of the population. While advances in medical imaging and surgical procedures have led to improved diagnosis and treatment, the decision of whether or not too treat an unruptured aneurysm is still largely subjective. The size of the aneurysm combined with its location and shape are the major determining factors, along with experience, when considering treatment. There is increasing recognition that hemodynamic forces play a key role in the life cycle of an aneurysm; however, it is difficult to provide this information in the clinic, owing to the need for time-consuming computational fluid dynamics (CFD) simulations. A more pragmatic solution, for now at least, may be to predict the gross flow patterns (“hemodynamic phenotype”) from simpler-to-measure geometric parameters.Copyright

Geometry Anticipates Hemodynamic Phenotype of Basilar Tip Aneurysms

The prevalence of unruptured cerebral aneurysms is estimated to be as high as 5% [1]. Basilar tip aneurysms account for 4–5% of these, but have a higher risk of rupture [2]. They are also difficult to treat surgically, and so endovascular therapy is often the only option. Hemodynamic forces have been implicated in the risk of rupture [3] and complications of endovascular therapy [4]; however, hemodynamic information is difficult to acquire clinically. Computational fluid dynamics (CFD), in combination with clinical imaging, can be used to accurately capture the intra-aneurysmal hemodynamics in a patient-specific manner [5]. Still, these techniques have not translated to routine clinical use, largely due to the time and effort required to construct, simulate, and interpret these models.Copyright

Variation in the Hemodynamics of Young Adult Carotid Bifurcations

Recent clinical studies have reinvigorated the search for purported “geometric risk factors” for atherosclerosis [1,2], due in part to the finding of wide interindividual variations in carotid bifurcation geometry. We have shown, however, that, relative to those study groups, the carotid bifurcations of young adults exhibit significantly less geometric variability [3,4]. In fact, these variations approached the threshold of reproducibility in the image-derived measurements themselves, from which we concluded that, if there is such a thing as geometric risk for atherosclerosis, its early detection might prove challenging. This, however, presupposed that interindividual variations in the local hemodynamics of young adult carotid bifurcations would be similarly modest. The purpose of this study was to test that implicit assumption.© 2007 ASME