Effect of Infrarenal Flow Waveform on Hemodynamics of Abdominal Aortic Aneurysms and Selection of Rheology Models

A numerical study is performed to investigate hemodynamic factors using Newtonian and non-Newtonian blood viscosity models under pulsatile blood flow condition. In this study, simulation is done on 90° bifurcating left coronary artery (LCA) by using Computational Fluid Dynamics (CFD). Comparative analysis is performed among one Newtonian and four non-Newtonian blood viscosity models. Wall shear stress (WSS), oscillatory shear index (OSI), global non-Newtonian importance factor (IG) and time-averaged wall shear stress (TAWSS) are shown at a specific point during the cardiac cycle. It is noticed that the pattern of WSS distribution is mostly consistent in all the models. However, the difference is only in the magnitude of WSS. For low inlet velocity, non-Newtonian power law predicts high WSS and Casson model predicts low WSS at all inlet velocity conditions which is indifferent from other non-Newtonian models. In moderate and high flow rates all the models are indistinguishable except in low flow rate. In case of increasing flow rate, Carreau and Herschel-Bulkley model demonstrate decreasing the value of IG thus acting as Newtonian fluid except for Casson and non-Newtonian power law. In conclusion, Carreau and Herschel-Bulkley models can be used for bifurcating LCA rather than Casson and non-Newtonian power law as they are very much sensitive to the non-Newtonian behavior of blood. As Herschel-Bulkley model predicts lower IG values than the Carreau model thus Carreau is more appropriate as blood viscosity model for bifurcating LCA.A numerical study is performed to investigate hemodynamic factors using Newtonian and non-Newtonian blood viscosity models under pulsatile blood flow condition. In this study, simulation is done on 90° bifurcating left coronary artery (LCA) by using Computational Fluid Dynamics (CFD). Comparative analysis is performed among one Newtonian and four non-Newtonian blood viscosity models. Wall shear stress (WSS), oscillatory shear index (OSI), global non-Newtonian importance factor (IG) and time-averaged wall shear stress (TAWSS) are shown at a specific point during the cardiac cycle. It is noticed that the pattern of WSS distribution is mostly consistent in all the models. However, the difference is only in the magnitude of WSS. For low inlet velocity, non-Newtonian power law predicts high WSS and Casson model predicts low WSS at all inlet velocity conditions which is indifferent from other non-Newtonian models. In moderate and high flow rates all the models are indistinguishable except in low flow rate. In c...

Carotid atherosclerotic vascular disease significantly contributes to strokes, presenting a heightened risk of early recurrent ischemia. Computational fluid dynamics (CFD) has shown potential in predicting subsequent stroke recurrence in patients with carotid stenosis. This study aims to investigate the differences in computational time and accuracy of four key hemodynamic indices-wall shear stress (WSS), time-averaged wall shear stress (TAWSS), Oscillatory Shear Index (OSI), and relative residence time (RRT)-across different viscosity models, thereby providing optimal model selection for clinical cases and offering guidance for clinicians' decision-making. A three-dimensional vessel model was established using computed tomography angiography (CTA), followed by CFD simulations to calculate WSS, TAWSS, OSI, and RRT. The accuracy of the simulations was validated by comparing the results with those from Razavi et al. (10.1016/j.jbiomech.2011.04.023). Numerical errors in different parameters under varying stenosis levels and viscosity models were analyzed. In the transient state, when degree of stenosis is 38%, 72%-84%, the performance difference between the two is less than 6%. For TAWSS, the difference is 0% when degree of stenosis is 12%, 18%, 26%, 54%, and 76%. For OSI, the difference is 0% when stenosis is 16%, 18%, 26%. For RRT, the difference between the two is 0% when degree of stenosis is 18% and 84%. WSS exhibited an increasing trend with higher degrees of stenosis. TAWSS demonstrated significant variation in moderate to severe stenosis, while OSI increased markedly in cases of moderate to severe stenosis. High RRT values in severely stenotic regions indicated a propensity for atherosclerotic lesion development. This study systematically quantified the discrepancies between Newtonian and non-Newtonian blood viscosity models in predicting hemodynamic parameters across different degrees of carotid artery stenosis. Statistical analyses revealed significant differences between the two models in WSS, TAWSS, OSI, and RRT (p<0.001 for all parameters). Newtonian models, while computationally efficient, overestimated TAWSS, OSI, and RRT while underestimating WSS, particularly in moderate to severe stenosis. In contrast, non-Newtonian models provided more physiologically accurate predictions, especially in regions with high shear stress variations. The results emphasize the importance of selecting appropriate viscosity models for CFD-based patient-specific risk assessment, particularly in stroke prediction, plaque evaluation, and surgical planning. Non-Newtonian models should be prioritized in high-risk patients where flow disturbances are more pronounced, whereas Newtonian models remain suitable for early screening and rapid assessments.

Endovascular treatment of the femoro-popliteal artery has recently become a valuable therapeutic option for popliteal arterial aneurysms. However, its efficacy remains controversial due to the relatively high rate of complications, such as stent occlusion as result of intra-stent thrombosis. The elucidation of the interplay among vessel geometrical features, local hemodynamics, and leg bending seems crucial to understand onset and progression of popliteal intra-stent thrombosis. To this aim, patient-specific computational fluid dynamic simulations were performed in order to assess the intra-stent hemodynamics of two patients endovascularly treated for popliteal arterial aneurysm by stent-grafts and experiencing intra-stent thrombosis. Both Newtonian and non-Newtonian blood rheological models were considered. Results were presented in terms of tortuosity, luminal area exposed to low (< 0.4 Pa) and high (> 1.5 Pa) time-averaged wall shear stress (TAWSS), area exposed to high (> 0.3) oscillatory shear index (OSI), and flow helicity. Study outcomes demonstrated that leg bending induced significant hemodynamic differences (> 50% increase) in both patients for all the considered variables, except for OSI in one of the two considered patients. In both leg configurations, stent-graft overlapping induced a severe discontinuity of the lumen diameter where the proximal stented zone is characterized by low tortuosity, low velocity, low helicity, low TAWSS, and high OSI; while the distal part has higher tortuosity, velocity, helicity, TAWSS, and lower OSI. Sensitivity study on applied boundary conditions showed that the different inlet velocity profiles for a given inlet waveform affect slightly the numerical solution; conversely, the shape and magnitude of the prescribed inlet waveform is determinant. Focusing on the comparison between the Newtonian and non-Newtonian blood models, the area with low TAWSS is greater in the Newtonian model for both patients, while no significant difference occurs between the surfaces with high TAWSS.GraphicPatient-specific computational fluid dynamic simulations were performed in order to assess the intra-stent hemodynamics of two patients endovascularly treated for popliteal arterial aneurysm and experiencing intra-stent thrombosis. Both Newtonian and non-Newtonian blood rheological models were considered. In both straight and bent leg configurations, stent-graft overlapping induced a severe discontinuity of the lumen diameter where the proximal stented zone is characterized by low tortuosity, low velocity, low helicity, low time-averaged wall shear stress (TAWSS), and high oscillatory index (OSI); while the distal part has higher tortuosity, velocity, helicity, TAWSS, and lower OSI.

To compare the hemodynamic performance of three (Bottom Up non-ballet, Top-Down non-ballet, Top Down ballet) idealized stent graft configurations used during endovascular repair of abdominal aortic aneurysms, under the influence of various rheological models. Ten rheological models are assumed and a commercial finite volume solver is employed for the simulation of blood flow under realistic boundary conditions. An appropriate mesh convergence study is performed and five hemodynamic variables are computed: the time average wall shear stress (TAWSS), oscillatory shear index (OSI), relative residence time (RRT), endothelial cell activation potential and displacement force (DF) for all three configurations. The choice of blood flow model may affect results, but does not constitute a significant determinant on the overall performance of the assumed stent grafts. On the contrary, stent graft geometry has a major effect. Specifically, the Bottom Up non-ballet type is characterized by the least favorable performance presenting the lowest TAWSS and the highest OSI, RRT and ECAP values. On the other hand, the Top Down ballet type presents hemodynamic advantages yielding the highest TAWSS and lowest OSI, RRT and ECAP average values. Furthermore, the ballet type is characterized by the lowest DF, although differences observed are small and their clinical relevance uncertain. The effect of the assumed rheological model on the overall performance of the grafts is not significant. It is thus relatively safe to claim that it is the type of stent graft that determines its overall performance rather than the adopted blood flow model.

A comparative study of Newtonian and non-Newtonian blood flow through Bi-Leaflet Mechanical Heart Valve

Objective: The aim of this study is to perform specific hemodynamic simulations of idealized abdominal aortic aneurysm (AAA) models with different diameters, curvatures and eccentricities and evaluate the risk of thrombosis and aneurysm rupture. Methods: Nine idealized AAA models with different diameters (3cm or 5cm), curvatures (0° or 30°) and eccentricities (centered on or tangent to the aorta), as well as a normal model, were constructed using commercial software (Solidworks; Dassault Systemes S.A, Suresnes, France). Hemodynamic simulations were conducted with the same time-varying volumetric flow rate extracted from the literature and 3-element Windkessel model (3 EWM) boundary conditions were applied at the aortic outlet. Several hemodynamic parameters such as time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), relative residence time (RRT), endothelial cell activation potential (ECAP) and energy loss (EL) were obtained to evaluate the risk of thrombosis and aneurysm rupture under different conditions. Results: Simulation results showed that the proportion of low TAWSS region and high OSI region increases with the rising of aneurysm diameter, whereas decreases in the curvature and eccentric models of the corresponding diameters, with the 5cm normal model having the largest low TAWSS region (68.5%) and high OSI region (40%). Similar to the results of TAWSS and OSI, the high ECAP and high RRT areas were largest in the 5cm normal model, with the highest wall-averaged value (RRT: 5.18 s, ECAP: 4.36 Pa-1). Differently, the increase of aneurysm diameter, curvature, and eccentricity all lead to the increase of mean flow EL and turbulent EL, such that the highest mean flow EL (0.82W · 10-3) and turbulent EL (1.72W · 10-3) were observed in the eccentric 5cm model with the bending angle of 30°. Conclusion: Collectively, increases in aneurysm diameter, curvature, and eccentricity all raise mean flow EL and turbulent flow EL, which may aggravate the damage and disturbance of flow in aneurysm. In addition, it can be inferred by conventional parameters (TAWSS, OSI, RRT and ECAP) that the increase of aneurysm diameter may raise the risk of thrombosis, whereas the curvature and eccentricity appeared to have a protective effect against thrombosis.

Effect of Sac Asymmetry, Neck and Iliac Angle on the Hemodynamic Behavior of Idealized Abdominal Aortic Aneurysm Geometries

Abdominal aortic aneurysm (AAA) is an irreversible dilation of the abdominal aorta that carries a significant risk of rupture if not adequately screened and treated. This condition poses a severe threat, with a mortality rate exceeding 80% in certain age groups. The enlargement of the abdominal aorta leads to notable hemodynamic alterations in AAAs, characterized by flow separation and vortical structures. Current understanding acknowledges a correlation between the growth and rupture mechanisms of AAA and the disturbed hemodynamics, emphasizing metrics such as time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), endothelial cell activation potential (ECAP), and relative residence time (RRT). In this study, we utilized a quantitative velocity measurement technique, particle image velocimetry (PIV), to characterize the flow structure and wall shear stress in a patient-specific aneurysmal abdominal aorta phantom. AAA phantoms generated from patient computed tomography (CT) images were used. Phase-averaged flow fields for 12 phases of physiological flow were investigated, and velocity contours, streamline patterns, and swirling strength contours were constructed in the AAA at three different PIV planes. A method previously developed and validated to extract wall shear stress from PIV measurements is applied to obtain shear stress indexes, including TAWSS, OSI, ECAP, and RRT. In addition, to link our findings with the clinical rupture risk, actual rupture location in the CT images of the aneurysm sac for the studied case was compared with the flow structure and shear stress distributions obtained from PIV measurements. The progression of vortex structures in the bulge along with the flow separation and reattachment zones in relation to the shear stress indexes are presented and discussed in detail. When flow dynamics in actual rupture location is analyzed, there is a high level of flow disturbance characterized by flow circulation, low TAWSS, and high OSI, ECAP, and RRT, consistent with previous studies. Here, we present a PIV-based flow examination through patient-specific phantom, which will contribute to experimental investigations for understanding the influence of disturbed hemodynamics on AAA biomechanics.

The relationship between hemodynamics and thrombus deposition in abdominal aortic aneurysm is investigated for three patients (A, B and C), each with mature fusiform aneurysms. Our methodology utilises initial and follow-up computerised tomography scans for each patient to identify regions of mural thrombus growth and to provide patient-specific models for hemodynamic analysis using computational fluid dynamics. The intervals between scans for patients A, B and C were 17, 15 and 3 months, respectively. The simulations were performed using physiologically realistic boundary conditions. The hemodynamic features of the flow considered include the velocity field, the shear strain rate field, the time averaged wall shear stress and the oscillatory shear index. The parameter that showed best correlation with the location of thrombus growth was the oscillatory shear index. In particular, in the case of patient C where the interval between scans was the shortest, thrombus growth was observed at regions of low oscillatory shear index (OSI < 0.1).

Assessing the risk of rupture is extremely important to reduce the mortality of the abdominal aortic aneurysm (AAA). Current clinical guidelines suggest considering the maximum diameter as a criterion for planning and surgical intervention; however, this approach is too simplistic and overlooks other morphological parameters that are associated with the risk of rupture. The aim of this paper is to study the thrombogenicity and to predict the risk of AAA rupture by taking into consideration the geometrical asymmetry of the aneurysm, studying its effect on blood flow behaviour and vortical structure, spatiotemporal distribution of wall shear stresses (WSS), and their related parameters. To show the effect of asymmetry on blood flow dynamics and hemodynamic forces, five virtual models were constructed using five values of geometrical asymmetry ratio β ranging from β=0.2 (asymmetric model; AM) to β=1 (symmetric model; SM). Simulations were run for each geometry under transient physiological flow conditions using finite volume discretization. Resting flow rate was investigated in these models and our results demonstrate that the asymmetry of the aneurysm has a clear effect on the flow behaviour, and consequently on WSS distribution, oscillatory shear index (OSI) and time averaged wall shear stress (TAWSS). Furthermore, these preliminary findings suggest that thrombus formation and rupture risk are more probable in an asymmetric abdominal aortic aneurysm.

Prediction of iliac limb occlusion after endovascular aneurysm repair for abdominal aortic aneurysm by anatomical and near-wall hemodynamic characteristics combining numerical simulation and in vitro experiment.

The aim of this paper was to develop a scoring system to grade the risk of rupture of an abdominal aortic aneurysm (AAA) in individual patients. Computed tomography angiography of an AAA were coupled with computational fluid dynamics (CFD) evaluation performed using open source software (ElmerSolver, Institute of Technology, Espoo, Finland). CFD criteria studied were: Oscillatory Shear Index (OSI), time averaged wall shear stress (TAWSS) and residence relative time (RRT) on both two-dimensional (2D) and three-dimensional (3D) models. AAA rupture predictors were analyzed and a scoring system was generated using Arabic numerals for all significant variables in order to grade the individual patient risk of rupture. There were 143 patients examined. Ninety-one AAAs (18 ruptured AAAs), and 52 had a non-aneurysmal aorta. The 2D OSI was the best CFD criterion following multivariate analysis and ROC curves evaluation. An AAA was deemed respectively at low, moderate, or high risk of rupture, according to whether the risk score was defined as AAA I (total score <2.3), AAA II (2.3-6.5) or AAA III (>6.5). The only protective factor was found in diabetes (OR=0.775; CI: 0.665-0.902). The Florence Risk Score for AAA rupture based on this report may be a useful tool to predict AAA rupture. A prospective multicenter registry will need to confirm its validity.

Study of Physiological Flow Through an Abdominal Aortic Aneurysm (AAA)

The intraluminal thrombus (ILT) has been shown to negatively impact the progression of the abdominal aortic aneurysms (AAAs). The formation of this thrombus layer has been connected to the local flow environment within AAAs, but the specific mechanisms leading to thrombus formation are still not fully understood. Our study investigated the association between vortical structures, near-wall hemodynamic metrics (e.g., time averaged wall shear stress (TAWSS) and oscillatory shear index (OSI)), and ILT accumulation in a longitudinal cohort of 14 AAAs (53 scans total). Vortices and hemodynamic parameters were estimated using hemodynamic simulations performed to each scan of each patient and compared to local 3D changes of ILT thickness between two consecutive scans (ΔILT). Results showed that vortices formed and remained strong and close to the lumen surface in AAAs without an ILT, while in AAAs with ILTs these detached from the lumen surface and dissipated nearby wall region where an increase in ILT thickness was observed. Although low TAWSS was observed in regions with and without ILT accumulation, an inverse correlation between and TAWSS was observed within the regions that experienced a thrombus growth. Our results support the idea that vortical structures might be playing a role modulating ILT accumulation into specific wall regions. Also, it submits the idea that the low TAWSS will be modulating the growth of thrombus within these preferred ILT accumulated regions.

Hemodynamic transport in the stenosed blood vessel has been numerically investigated with five non-Newtonian rheological models. The results from Casson viscosity model, Carreau model, Cross model, power law viscosity model, and Quemada viscosity model as well as Newtonian model were compared to capture the physics at the arterial wall. At the inlet of a long artery, three different pulsating profiles are considered. Detailed flow statistics of the flow field were calculated by solving transient form of Navier-Stokes equation in an axi-symmetric domain. From the simulation data, wall shear stress and oscillatory shear index were calculated. The results show that among different rheological models, the predictions from Carreau model as well as Newtonian model differ significantly with other four models showing similar behaviour. The results demonstrate that the degree of stenoses have a significant effect on oscillatory shear index and recirculation length.