Wall Shear Stress Patterns Research Articles

Retinal and cerebral hemodynamics redistribute to favor thermoregulation in response to passive environmental heating and heated exercise in humans

ABSTRACT Core temperature (TC) changes, alongside exercise, affect hemodynamic responses across different conduit and microvascular beds. This study investigated impacts of ecologically valid environmental heat and exercise exposures on cerebral, skin and retinal vascular responses by combining physiological assessments alongside computational fluid dynamics (CFD) modeling. Young, healthy participants (n = 12) were exposed to environmental passive heating (PH), and heated exercise (HE) (ergometer cycling), in climate-controlled conditions (50 mins, 40°C, 50% relative humidity) while maintaining upright posture. Blood flow responses in the common carotid (CCA), internal carotid (ICA) and central retinal (CRA) arteries were assessed using Duplex ultrasound, while forearm skin microvascular blood flow responses were measured using optical coherence tomography angiography. Three-dimensional retinal hemodynamics (flow and pressure) were calculated via CFD simulation, enabling assessment of wall shear stress (WSS). TC rose following PH (+0.2°C, p = 0.004) and HE (+1.4°C, p < 0.001). PH increased skin microvascular blood flow (p < 0.001), whereas microvascular CRA flow decreased (p = 0.038), despite unchanged ICA flow. HE exacerbated these differences, with increased CCA flow (p = 0.007), unchanging ICA flow and decreased CRA flow (p < 0.001), and interactions between vascular (CCA vs. ICA p = 0.018; CCA vs. CRA p = 0.004) and microvascular (skin vs. retinal arteriolar p < 0.001) territories. Simulations revealed patterns of WSS and lumen pressure that uniformly decreased following HE. Under ecologically valid thermal challenge, different responses occur in distinct conduit and microvascular territories, with blood flow distribution favoring systemic thermoregulation, while flow may redistribute within the brain.

Some effects of limited wall-sensor availability on flow estimation with 3D-GANs

In this work we assess the impact of the limited availability of wall-embedded sensors on the full 3D estimation of the flow field in a turbulent channel with Reτ=200\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Re_{\ au }=200$$\\end{document}. The estimation technique is based on a 3D generative adversarial network (3D-GAN). We recently demonstrated that 3D-GANs are capable of estimating fields with good accuracy by employing fully-resolved wall quantities (pressure and streamwise/spanwise wall shear stress on a grid with DNS resolution). However, the practical implementation in an experimental setting is challenging due to the large number of sensors required. In this work, we aim to estimate the flow fields with substantially fewer sensors. The impact of the reduction of the number of sensors on the quality of the flow reconstruction is assessed in terms of accuracy degradation and spectral length-scales involved. It is found that the accuracy degradation is mainly due to the spatial undersampling of scales, rather than the reduction of the number of sensors per se. We explore the performance of the estimator in case only one wall quantity is available. When a large number of sensors is available, pressure measurements provide more accurate flow field estimations. Conversely, the elongated patterns of the streamwise wall shear stress make this quantity the most suitable when only few sensors are available. As a further step towards a real application, the effect of sensor noise is also quantified. It is shown that configurations with fewer sensors are less sensitive to measurement noise.

Development of idealized human aortic models for in vitro and in silico hemodynamic studies.

The aorta, a central component of the cardiovascular system, plays a pivotal role in ensuring blood circulation. Despite its importance, there is a notable lack of idealized models for experimental and computational studies. This study aims to develop computer-aided design (CAD) models for the idealized human aorta, intended for studying hemodynamics or solid mechanics in both in vitro and in silico settings. Various parameters were extracted from comprehensive literature sources to evaluate major anatomical characteristics of the aorta in healthy adults, including variations in aortic arch branches and corresponding dimensions. The idealized models were generated based on averages weighted by the cohort size of each study for several morphological parameters collected and compiled from image-based or cadaveric studies, as well as data from four recruited subjects. The models were used for hemodynamics assessment using particle image velocimetry (PIV) measurements and computational fluid dynamics (CFD) simulations. Two CAD models for the idealized human aorta were developed, focusing on the healthy population. The CFD simulations, which align closely with the PIV measurements, capture the main global flow features and wall shear stress patterns observed in patient-specific cases, demonstrating the capabilities of the designed models. The collected statistical data on the aorta and the two idealized aorta models, covering prevalent arch variants known as Normal and Bovine types, are shown to be useful for examining the hemodynamics of the aorta. They also hold promise for applications in designing medical devices where anatomical statistics are needed.

Hemodynamic Characteristics of a Tortuous Microvessel Using High-Fidelity Red Blood Cell Resolved Simulations.

Tortuous microvessels are characteristic of microvascular remodeling associated with numerous physiological and pathological scenarios. Three-dimensional (3D) hemodynamics in tortuous microvessels influenced by red blood cells (RBCs), however, are largely unknown, and important questions remain. Is blood viscosity influenced by vessel tortuosity? How do RBC dynamics affect wall shear stress (WSS) patterns and the near-wall cell-free layer (CFL) over a range of conditions? The objective of this work was to parameterize hemodynamic characteristics unique to a tortuous microvessel. RBC-resolved simulations were performed using an immersed boundary method-based 3D fluid dynamics solver. A representative tortuous microvessel was selected from a stimulated angiogenic network obtained from imaging of the rat mesentery and digitally reconstructed for the simulations. The representative microvessel was a venule with a diameter of approximately 20 μm. The model assumes a constant diameter along the vessel length and does not consider variations due to endothelial cell shapes or the endothelial surface layer. Microvessel tortuosity was observed to increase blood apparent viscosity compared to a straight tube by up to 26%. WSS spatial variations in high curvature regions reached 23.6 dyne/cm2 over the vessel cross-section. The magnitudes of WSS and CFL thickness variations due to tortuosity were strongly influenced by shear rate and negligibly influenced by tube hematocrit levels. New findings from this work reveal unique tortuosity-dependent hemodynamic characteristics over a range of conditions. The results provide new thought-provoking information to better understand the contribution of tortuous vessels in physiological and pathological processes and help improve reduced-order models.

The impact of choledochal cysts on bile fluid dynamics: A perspective using computational fluid dynamics and surface mapping technique

Choledochal cysts (CCs) are an important risk factor for cholangiocarcinoma, though their etiology remains debated. Given the vital role of bile fluid in digestive processes within the biliary system, examining such mechanisms from the perspective of bile fluid dynamics may offer additional insights for clinical use. This study utilized magnetic resonance imaging (MRI)-based patient-specific scans for detailed reconstruction and further employed the computational fluid dynamic method to assess the physiological functions of each system, including refilling and emptying processes. The impact of bile rheological property was also examined. Key biomechanical parameters—pressure and wall shear stress (WSS)—were displayed on a two-dimensional plane via surface mapping for enhanced visualization and comparative analysis. Outcomes demonstrated a significant reduction in bile flow velocity in CCs patients due to common bile duct's anatomical features and bile's shear-thinning, non-Newtonian nature, with a notable increase in pressure drop observed. In healthy biliary systems, WSS variations were minimal; however, in CCs patients, extreme WSS differences were found, with the highest WSS in the segmental bile duct and the lowest in the dilatation area, presenting a magnitude difference of approximately 1000. CCs one showed WSS levels 100–250 times higher than healthy ones in the common bile duct. Bile rheological properties substantially affect pressure and WSS patterns, particularly WSS, where pathological bile caused a tenfold increase in WSS compared to healthy bile. The findings aimed to enhance the understanding of biliary fluid mechanics in CCs and offer insights into selected fluidic variables for future microfluidic chip experiments.

Atheroprotective WSS effects on human aortic endothelial cells in a new generation bioreactor

Abstract Funding Acknowledgements None. Background Luminal arterial endothelial cells are mainly aligned along the blood flow direction, whereas the in vitro endothelial cells (EC) showed heterogeneous distribution. In atherosclerosis the altered blood flow, acting on mechano-transduction, contributes to endothelial dysfunction and barrier disorganization. A low wall shear stress (WSS) has a pro-atherosclerotic effect, whilst elevated WSS is atheroprotective. Purposes Aim of this work is to study the primary human aortic EC cells (HAOEC) capability to acquire a distribution reminiscent of the luminal alignment under atheroprotective WSS using a prototypal new generation bioreactor. Methods We have developed a novel device compatible with culture inside standard sterile incubator, allowing to apply complex WSS patterns in controlled conditions. The bioreactor includes a sample housing and a parallel plate flow chamber and is controlled by a custom electronic interface actuating motor and pump. The cells were settled over PMMA supports and coating conditions were setup. Atheroprotective WSS patterns were tested. We compared HAOEC submitted to high flow (1.2 Pa for 24h) vs. ramps with different slopes (3h, 8h, 24h) followed by steady-state flow (1.2Pa for 24h, 48h). Viability and distribution of Acridine Orange-labelled HAOEC were assessed over bioreactor supports at both beginning and end of the experiments using fluorescence microscope. Hematoxylin/Eosin and Coomassie Blue staining were tested after the experiment to evaluate the shape and cell orientation (directionality FiJi plugin). Confocal microscopy on HAOEC labelled with Phalloidin–TRITC, anti-CD31/PECAM1-AlexaFluor488 and DAPI displayed the flow-related changes. Results HAOEC settled over naked sterile PMMA supports mainly detached under WSS, independently from the cell number. Coating with extracellular matrix components (i.e., human laminin, bovine fibronectin, chicken collagen type II, human collagen type I) showed the superiority of collagen type I to obtain homogeneously distributed, flow-resistant cell monolayers. In static condition, HAOEC on coated supports demonstrated ≥50% detachment within 24h, those submitted to a direct atheroprotective WSS of 1.2Pa almost completely detached. Consistently, to ramps with high slope (range 0.2-1,2Pa in 3h or 8h) followed by1.2Pa WSS corresponded 60%-90% cell detachment. Conversely, a 24h ramp (range 0.2-1,2Pa) followed by 24h or 48h of 1.2Pa WSS preserved the HAOEC monolayer adherence, and aligned cells were found. Microscopy comparative analysis of selected areas before/after WSS showed a partial spatial reorganization, with changes in the F-actin cytoskeleton and CD31 distribution. Conclusion We have optimized a protocol for studying HAOEC under atheroprotective WSS. In perspective further increasing the duration of the stimulus under constant WSS will achieve full alignment and provide a pseudo-physiologic model for multidirectional atherogenic WSS studies.

Hemodynamic Characteristics of Tortuous Microvessels Using High-Fidelity Red Blood Cell Resolved Simulations

Introduction: Tortuous microvessels are characteristic of microvascular remodeling associated with numerous physiological and pathological scenarios. These vessels have unique morphology compared to straight vessels, the latter being the subject of extensive flow modeling studies over the past few decades. Three-dimensional hemodynamics in tortuous microvessels influenced by red blood cell (RBCs), however, remain largely unknown and important questions remain. Is blood viscosity influenced by vessel tortuosity? Do RBC flow dynamics cause localized hematocrit variations in tortuous vessels? How do these dynamics affect wall shear stress patterns and the near-wall cell-free layer? Previous work has connected physiological function to tortuosity1, while other recent work has shown how RBC dynamics and vessel complexity can influence hemodynamics2. Such findings drive our hypothesis that unique hemodynamic characteristics exist in tortuous microvessels compared to straight vessels. Revealing such differences and answering the above questions requires 3D RBC-resolved simulations. The objective of this work is to investigate the hemodynamic characteristics of tortuous microvessels using high-fidelity red blood cell (RBC) resolved simulations and real image data. Methods: High fidelity RBC resolved simulations were performed using a 3D immersed boundary method (IBM) based fluid dynamic solver3. A tortuous microvessel is constructed in 3D based on imaging of a rat mesenteric microvascular network post angiogenic remodeling. The diameter and length of the vessel are 20 μm and 930 μm, respectively. Simulations were performed spanning a range of physiological shear rates (SR) and overall hematocrit levels (Ht). Output simulation data is used to parametrically quantify characteristics of apparent viscosity, local hematocrit variations, time-averaged wall shear stress (TAWSS) and cell free layer (CFL) patterns due to vessel tortuosity. Data and Results: The findings show how curvature can increase apparent viscosity to different degrees based on overall Ht &amp; SR (max 236% increase), with local variations occurring over the vessel length at sites of curvature changes (max 2.25cP change). Because of RBC flow patterns, local variations in tube hematocrit are found to occur which result in local curvature-dependent variations in the Fahraeus effect. We further characterize dependencies of the CFL and TAWSS on Ht &amp; SR and resultant lengthwise 3D spatial variations along the vessel, and demonstrate correlation patterns between these two quantities as influenced by tortuosity. Conclusions: New findings from this work reveal significant dependency of hemodynamic parameters and spatial variations on vessel tortuosity. The results provide new information to better understand the role of vessel tortuosity in physiological and pathological processes, as well as help improve reduced-order models. NSF CBET 2309559, NIH R21HL159501, and Expanse at the San Diego Supercomputer Center per NSF Accelerate Award BIO230073. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Haemodynamics of stent-mounted neural interfaces in tapered and deformed blood vessels

The endovascular neural interface provides an appealing minimally invasive alternative to invasive brain electrodes for recording and stimulation. However, stents placed in blood vessels have long been known to affect blood flow (haemodynamics) and lead to neointimal growth within the blood vessel. Both the stent elements (struts and electrodes) and blood vessel wall geometries can affect the mechanical environment on the blood vessel wall, which could lead to unfavourable vascular remodelling after stent placement. With increasing applications of stents and stent-like neural interfaces in venous blood vessels in the brain, it is necessary to understand how stents affect blood flow and tissue growth in veins. We explored the haemodynamics of a stent-mounted neural interface in a blood vessel model. Results indicated that blood vessel deformation and tapering caused a substantial change to the lumen geometry and the haemodynamics. The neointimal proliferation was evaluated in sheep implanted with an endovascular neural interface. Analysis showed a negative correlation with the mean Wall Shear Stress pattern. The results presented here indicate that the optimal stent oversizing ratio must be considered to minimise the haemodynamic impact of stenting.

Kiosk 4R-TA-12 - Abnormal Patterns of Wall Shear Stress in Aortic Dilation Revealed by Permutation Tests

Kiosk 4R-TA-12 - Abnormal Patterns of Wall Shear Stress in Aortic Dilation Revealed by Permutation Tests

Computer simulation of two phase power-law nanofluid of blood flow through a curved overlapping stenosed artery with induced magnetic field: entropy generation optimization

PurposeThe purpose of this study is to determine the impact of two-phase power law nanofluid on a curved arterial blood flow under the presence of ovelapped stenosis. Over the past couple of decades, the percentage of deaths associated with blood vessel diseases has risen sharply to nearly one third of all fatalities. For vascular disease to be stopped in its tracks, it is essential to understand the vascular geometry and blood flow within the artery. In recent scenarios, because of higher thermal properties and the ability to move across stenosis and tumor cells, nanoparticles are becoming a more common and effective approach in treating cardiovascular diseases and cancer cells.Design/methodology/approachThe present mathematical study investigates the blood flow behavior in the overlapped stenosed curved artery with cylinder shape catheter. The induced magnetic field and entropy generation for blood flow in the presence of a heat source, magnetic field and nanoparticle (Fe3O4) have been analyzed numerically. Blood is considered in artery as two-phases: core and plasma region. Power-law fluid has been considered for core region fluid, whereas Newtonian fluid is considered in the plasma region. Strongly implicit Stone’s method has been considered to solve the system of nonlinear partial differential equations (PDE’s) with 10–6 tolerance error.FindingsThe influence of various parameters has been discussed graphically. This study concludes that arterial curvature increases the probability of atherosclerosis deposition, while using an external heating source flow temperature and entropy production. In addition, if the thermal treatment procedure is carried out inside a magnetic field, it will aid in controlling blood flow velocity.Originality/valueThe findings of this computational analysis hold great significance for clinical researchers and biologists, as they offer the ability to anticipate the occurrence of endothelial cell injury and plaque accumulation in curved arteries with specific wall shear stress patterns. Consequently, these insights may contribute to the potential alleviation of the severity of these illnesses. Furthermore, the application of nanoparticles and external heat sources in the discipline of blood circulation has potential in the medically healing of illness conditions such as stenosis, cancer cells and muscular discomfort through the usage of beneficial effects.

Influence of intracoronary hemodynamic forces on atherosclerotic plaque phenotypes

Background and aimsCoronary hemodynamics impact coronary plaque progression and destabilization. The aim of the present study was to establish the association between focal vs. diffuse intracoronary pressure gradients and wall shear stress (WSS) patterns with atherosclerotic plaque composition. MethodsProspective, international, single-arm study of patients with chronic coronary syndromes and hemodynamic significant lesions (fractional flow reserve [FFR] ≤ 0.80). Motorized FFR pullback pressure gradient (PPG), optical coherence tomography (OCT), and time-average WSS (TAWSS) and topological shear variation index (TSVI) derived from three-dimensional angiography were obtained. ResultsOne hundred five vessels (median FFR 0.70 [Interquartile range (IQR) 0.56–0.77]) had combined PPG and WSS analyses. TSVI was correlated with PPG (r = 0.47, [95% Confidence Interval (95% CI) 0.30–0.65], p < 0.001). Vessels with a focal CAD (PPG above the median value of 0.67) had significantly higher TAWSS (14.8 [IQR 8.6–24.3] vs. 7.03 [4.8–11.7] Pa, p < 0.001) and TSVI (163.9 [117.6–249.2] vs. 76.8 [23.1–140.9] m−1, p < 0.001). In the 51 vessels with baseline OCT, TSVI was associated with plaque rupture (OR 1.01 [1.00–1.02], p = 0.024), PPG with the extension of lipids (OR 7.78 [6.19–9.77], p = 0.003), with the presence of thin-cap fibroatheroma (OR 2.85 [1.11–7.83], p = 0.024) and plaque rupture (OR 4.94 [1.82 to 13.47], p = 0.002). ConclusionsFocal and diffuse coronary artery disease, defined using coronary physiology, are associated with differential WSS profiles. Pullback pressure gradients and WSS profiles are associated with atherosclerotic plaque phenotypes. Focal disease (as identified by high PPG) and high TSVI are associated with high-risk plaque features. Clinical trial registrationhttps://clinicaltrials,gov/ct2/show/NCT03782688

Abstract 18625: Mechano-Transcriptomic Interaction Mediates Spatial Variations in Early Vascular Fibrosis

Background: The spatiotemporal variations of wall shear stress (WSS) modulate vascular genomics and function. Despite the advances in imaging modalities and computational modeling, the link between local hemodynamics and genetic pathways underlying atherosclerosis development remains elusive. We hypothesized that differential WSS patterns in the aorta modulate fibroblast gene expression in response to high fat diet (HFD) leading to vascular stiffness. Methods: We performed ultrasound-based 4-D+time moving boundary computational fluid dynamics (CFD) on the aorta of Ldlr -/- mice fed a normal diet (ND) or HFD during 4 weeks. B-mode images (Vevo 3100) of the aortic arch were used to reconstruct a 3-D model of the lumen structure by revolving the vessel wall around the centerline in the aorta and the branches. Computational domain reconstruction was followed by image registration to generate a moving boundary model. WSS contours and velocity profiles were acquired by solving Navier-Stokes equations using svFSI code. The longitudinal strains along the outer curvature of the aortic arch over several cardiac cycles were measured, and the changes were compared with the effects of HFD. Mouse aortas were processed for single cell RNA sequencing and analyzed using the Scanpy package. Results and Conclusion: Our analysis of transcriptomics profiles revealed increased Acta2 and Postn expression in the myofibroblast subclusters, suggesting myofibroblast differentiation in response to HFD in the aortic arch. This was confirmed and localized to the lesser curvature of the aortic arch through Periostin immunostaining. We found that the increase in Postn and Acta2 is associated with reduced arterial strain or increased arterial stiffness measured in the aortic arch. In parallel, CFD analysis demonstrated that an elevated WSS along the outer curvature vs. low WSS and disturbed flow along the inner curvature in early diastole results in differential gene expression to regulate myofibroblast differentiation and modulates vascular stiffness.

Oscillatory shear stress is elevated in patients with bicuspid aortic valve and aortic regurgitation: a 4D flow cardiovascular magnetic resonance cross-sectional study.

Patients with bicuspid aortic valve (BAV) and aortic regurgitation have higher rate of aortic complications compared with patients with BAV and stenosis, as well as BAV without valvular disease. Aortic regurgitation alters blood haemodynamics not only in systole but also during diastole. We therefore sought to investigate wall shear stress (WSS) during the whole cardiac cycle in BAV with aortic regurgitation. Fifty-seven subjects that underwent 4D flow cardiovascular magnetic resonance imaging were included: 13 patients with BAVs without valve disease, 14 BAVs with aortic regurgitation, 15 BAVs with aortic stenosis, and 22 normal controls with tricuspid aortic valve. Peak and time averaged WSS in systole and diastole and the oscillatory shear index (OSI) in the ascending aorta were computed. Student's t-tests were used to compare values between the four groups where the data were normally distributed, and the non-parametric Wilcoxon rank sum tests were used otherwise. BAVs with regurgitation had similar peak and time averaged WSS compared with the patients with BAV without valve disease and with stenosis, and no regions of elevated WSS were found. BAV with aortic regurgitation had twice as high OSI as the other groups (P ≤ 0.001), and mainly in the outer mid-to-distal ascending aorta. OSI uniquely characterizes altered WSS patterns in BAVs with aortic regurgitation, and thus could be a haemodynamic marker specific for this specific group that is at higher risk of aortic complications. Future longitudinal studies are needed to verify this hypothesis.

Wall shear stress and pressure patterns in aortic stenosis patients with and without aortic dilation captured by high-performance image-based computational fluid dynamics.

Spatial patterns of elevated wall shear stress and pressure due to blood flow past aortic stenosis (AS) are studied using GPU-accelerated patient-specific computational fluid dynamics. Three cases of moderate to severe AS, one with a dilated ascending aorta and two within the normal range (root diameter less than 4cm) are simulated for physiological waveforms obtained from echocardiography. The computational framework is built based on sharp-interface Immersed Boundary Method, where aortic geometries segmented from CT angiograms are integrated into a high-order incompressible Navier-Stokes solver. The key question addressed here is, given the presence of turbulence due to AS which increases wall shear stress (WSS) levels, why some AS patients undergo much less aortic dilation. Recent case studies of AS have linked the existence of an elevated WSS hotspot (due to impingement of AS on the aortic wall) to the dilation process. Herein we further investigate the WSS distribution for cases with and without dilation to understand the possible hemodynamics which may impact the dilation process. We show that the spatial distribution of elevated WSS is significantly more focused for the case with dilation than those without dilation. We further show that this focal area accommodates a persistent pocket of high pressure, which may have contributed to the dilation process through an increased wall-normal forcing. The cases without dilation, on the contrary, showed a rather oscillatory pressure behaviour, with no persistent pressure "buildup" effect. We further argue that a more proximal branching of the aortic arch could explain the lack of a focal area of elevated WSS and pressure, because it interferes with the impingement process due to fluid suction effects. These phenomena are further illustrated using an idealized aortic geometry. We finally show that a restored inflow eliminates the focal area of elevated WSS and pressure zone from the ascending aorta.

Angiogenic Microvascular Wall Shear Stress Patterns Revealed Through Three-dimensional Red Blood Cell Resolved Modeling.

The wall shear stress (WSS) exerted by blood flowing through microvascular capillaries is an established driver of new blood vessel growth, or angiogenesis. Such adaptations are central to many physiological processes in both health and disease, yet three-dimensional (3D) WSS characteristics in real angiogenic microvascular networks are largely unknown. This marks a major knowledge gap because angiogenesis, naturally, is a 3D process. To advance current understanding, we model 3D red blood cells (RBCs) flowing through rat angiogenic microvascular networks using state-of-the-art simulation. The high-resolution fluid dynamics reveal 3D WSS patterns occurring at sub-endothelial cell (EC) scales that derive from distinct angiogenic morphologies, including microvascular loops and vessel tortuosity. We identify the existence of WSS hot and cold spots caused by angiogenic surface shapes and RBCs, and notably enhancement of low WSS regions by RBCs. Spatiotemporal characteristics further reveal how fluctuations follow timescales of RBC "footprints." Altogether, this work provides a new conceptual framework for understanding how shear stress might regulate EC dynamics in vivo.

Sensitivity of hostile hemodynamics to aneurysm geometry via unsupervised shape interpolation

Background and ObjectiveVessel geometry and hemodynamics are intrinsically linked, whereby geometry determines hemodynamics, and hemodynamics influence vascular remodeling. Both have been used for testing clinical outcomes, but geometry/morphology generally has less uncertainty than hemodynamics derived from medical image-based computational fluid dynamics (CFD). To provide clinical utility, CFD-based hemodynamic parameters must be robust to modeling errors and/or uncertainties, but must also provide useful information not more-easily extracted from shape alone. The objective of this study was to methodically assess the response of hemodynamic parameters to gradual changes in shape created using an unsupervised 3D shape interpolation method. MethodsWe trained the neural network NeuroMorph on 3 patient-derived intracranial aneurysm surfaces (labelled A, B, C), and then generated 3 distinct morph sequences (A→B, B→C, C→A) each containing 10 interpolated surfaces. From high-fidelity CFD simulation of these, we calculated a variety of common reduced hemodynamic parameters, including many previously associated with aneurysm rupture, and analyzed their responses to changes in shape, and their correlations. ResultsThe interpolated surfaces demonstrate complex, gradual changes in branch angles, vessel diameters, and aneurysm morphology. CFD simulation showed gradual changes in aneurysm jetting characteristics and wall-shear stress (WSS) patterns, but demonstrated a range of responses from the reduced hemodynamic parameters. Spatially and temporally averaged parameters including time-averaged WSS, time-averaged velocity, and low-shear area (LSA) showed low variation across all morph sequences, while parameters of flow complexity such as oscillatory shear, spectral broadening, and spectral bandedness indices showed high variation between slightly-altered neighboring surfaces. Correlation analysis revealed a great deal of mutual information with easier-to-measure shape-based parameters. ConclusionsIn the absence of large clinical datasets, unsupervised shape interpolation provides an ideal laboratory for exploring the delicate balance between robustness and sensitivity of nominal hemodynamic predictors of aneurysm rupture. Parameters like time-averaged WSS and LSA that are highly “robust” may, as a result, be effectively redundant to morphological predictors, whereas more sensitive parameters may be too uncertain for practical clinical use. Understanding these sensitivities may help identify parameters that are capable of providing added value to rupture risk assessment.

A Computational Fluid Dynamics-Based Model for Assessing Rupture Risk in Cerebral Arteries with Varying Aneurysm Sizes

A cerebral aneurysm is a medical condition where a cerebral artery can burst under adverse pressure conditions. A 20% mortality rate and additional 30 to 40% morbidity rate have been reported for patients suffering from the rupture of aneurysms. In addition to wall shear stress, input jets, induced pressure, and complicated and unstable flow patterns are other important parameters associated with a clinical history of aneurysm ruptures. In this study, the anterior cerebral artery (ACA) was modeled using image segmentation and then rebuilt with aneurysms at locations vulnerable to aneurysm growth. To simulate various aneurysm growth stages, five aneurysm sizes and two wall thicknesses were taken into consideration. In order to simulate realistic pressure loading conditions for the anterior cerebral arteries, inlet velocity and outlet pressure were used. The pressure, wall shear stress, and flow velocity distributions were then evaluated in order to predict the risk of rupture. A low-wall shear stress-based rupture scenario was created using a smaller aneurysm and thinner walls, which enhanced pressure, shear stress, and flow velocity. Additionally, aneurysms with a 4 mm diameter and a thin wall had increased rupture risks, particularly at specific boundary conditions. It is believed that the findings of this study will help physicians predict rupture risk according to aneurysm diameters and make early treatment decisions.

Abnormal flow pattern of low wall shear stress and high oscillatory shear index in spontaneous vertebral artery dissection with vertebral artery hypoplasia.

Spontaneous vertebral artery dissection (sVAD) might tend to develop in vertebral artery hypoplasia (VAH) with hemodynamic dysfunction and it is crucial to assess hemodynamics in sVAD with VAH to investigate this hypothesis. This retrospective study aimed to quantify hemodynamic parameters in patients with sVAD with VAH. Patients who had suffered ischemic stroke due to an sVAD of VAH were enrolled in this retrospective study. The geometries of 14 patients (28 vessels) were reconstructed using Mimics and Geomagic Studio software from CT angiography (CTA). ANSYS ICEM and ANSYS FLUENT were utilized for mesh generation, set boundary conditions, solve governing equations, and perform numerical simulations. Slices were obtained at the upstream area, dissection or midstream area and downstream area of each VA. The blood flow patterns were visualized through instantaneous streamline and pressure at peak systole and late diastole. The hemodynamic parameters included pressure, velocity, time-averaged blood flow, time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), endothelial cell action potential (ECAP), relative residence time (RRT) and time-averaged nitric oxide production rate (TARNO). Significant focal increased velocity was present in the dissection area of steno-occlusive sVAD with VAH compared to other nondissected areas (0.910 m/s vs. 0.449 vs. 0.566, p < 0.001), while focal slow flow velocity was observed in the dissection area of aneurysmal dilatative sVAD with VAH according to velocity streamlines. Steno-occlusive sVAD with VAH arteries had a lower time-averaged blood flow (0.499 cm3/s vs. 2.268, p < 0.001), lower TAWSS (1.115 Pa vs. 2.437, p = 0.001), higher OSI (0.248 vs. 0.173, p = 0.006), higher ECAP (0.328 Pa-1 vs. 0.094, p = 0.002), higher RRT (3.519 Pa-1 vs. 1.044, p = 0.001) and deceased TARNO (104.014 nM/s vs. 158.195, p < 0.001) than the contralateral VAs. Steno-occlusive sVAD with VAH patients had abnormal blood flow patterns of focal increased velocity, low time-averaged blood flow, low TAWSS, high OSI, high ECAP, high RRT and decreased TARNO. These results provide a good basis for further investigation of sVAD hemodynamics and support the applicability of the CFD method in testing the hemodynamic hypothesis of sVAD. More detailed hemodynamic conditions with different stages of sVAD are warranted in the future.

Simulation of dynamic changes in fluid parameters and flow patterns during condensation

The three-dimensional numerical simulation of the R32 flow condensation is conducted in a circular microchannel with a hydraulic diameter of 1 mm, and the setting of two phase inlets is applied to the transient simulation. The temporal process of flow patterns, velocity contour and temperature contour are obtained for inlet vapor quality of 0.5 and mass flow rate of 100, 150 and 200 kg/(m2·s), respectively. The corresponding relationship between each flow pattern and its internal parameters along the flow direction during condensation are studied in detail. The fluid in the microchannel undergoes annular, slug, bubbly and shrinking bubbly flow in sequence. The fluctuations of wall shear stress (WSS) and turbulent intensity (TI) caused by the oscillation of velocity gradient are the most intense near the transition point of flow regimes. Revealing the relationship between WSS, TI and flow patterns is an excellent supplement to the existing condensation flow. The local heat flux reaches its extreme value at the head of downstream of the annular flow, injection flow and slug flow. Similarly, the fluid temperature fluctuates violently at the same location. The time-average heat transfer coefficient of annular flow is higher than that of other flow patterns, and the peak value of heat transfer coefficient is located at the head of annular flow downstream under different mass flow rates.

Fluid–structure interaction simulation for studying hemodynamics and rupture risk of patient-specific intracranial aneurysms

Purpose The role of regional hemodynamics in intracranial aneurysms (IAs) hemodynamics and rupture risk has been widely discussed based on numerical models over the past decades. The aim of this paper is to investigate hemodynamics and rupture risk with a complicated IA model. Methods Fluid-structure interaction (FSI) simulations were performed to quantify the hemodynamic characteristics of the established IA models. Hemodynamic parameters, including wall shear stress (WSS), flow velocity, and flow pattern, were calculated and analyzed. Results In this paper, the risk assessment of intracranial aneurysms focuses on the mechanical properties of blood flow and blood vessel walls. Vortex flow and concentrated impact field during blood flow play a decisive role in the rupture and development of aneurysms. The uneven distribution of wall shear stress on the vessel wall has a great influence on growth and rupture. By observing the simulation results of rigid walls, risks can be predicted efficiently and accurately. Conclusions This paper focuses on the relationship between hemodynamics and ruptures risk with a double intracranial aneurysms disease mode and provides a new perspective on the treatment of intracranial aneurysms.