Time-averaged Wall Shear Stress Research Articles

Slice-based and time-specific hemodynamic measurements discriminate carotid artery vulnerable atherosclerotic plaques

Background and objectiveHemodynamic patterns play key roles in progression of carotid vulnerable plaques. However, most of previous studies utilized maximum or averaged value of hemodynamic measurements which is not an ideal representative of hemodynamic patterns. This study aimed to investigate the association of slice-based and time-specific hemodynamic measurements with carotid vulnerable plaque using magnetic resonance (MR) vessel wall imaging and histology. MethodsThirty-two patients (mean age: 63.9±8.1 years; 25 males) with carotid atherosclerotic stenosis (≥50% stenosis) referred to carotid endarterectomy were recruited and underwent MR vessel wall imaging. Carotid plaque burden was evaluated on MR images and vulnerable plaque features including calcification, lipid-rich necrotic core, and intra-plaque hemorrhage (IPH) were identified by histology. The slice-based and time-specific hemodynamic measurements were extracted from computational fluid dynamics simulation of 3D carotid arterial model. Correlation coefficients between hemodynamic measurements and carotid plaque features were calculated and the logistic regressions with generalized estimating equation (GEE) were conducted. The value in discriminating carotid vulnerable plaque features was determined by receiver-operating-characteristic analysis. ResultsOf 102 MR-histology matched slices from 32 patients, time-averaged wall shear stress (TAWSS) (r=0.263, p=0.008), oscillatory shear index (OSI) (r=-0.374, p<0.001), and peakWSS (r=0.232, p=0.019) were significantly associated with carotid IPH. The logistic regression with GEE revealed that peakWSS (OR, 1.206; 95% CI, 1.026-1.418; p, 0.023) and TAWSS (OR, 0.364, 95% CI, 0.138-0.959; p, 0.041) were significantly associated with presence of IPH after adjusting for age and BMI. In discriminating carotid IPH, the AUC of TAWSS, OSI, combined TAWSS with maximum wall thickness (MWT) and combined OSI with MWT was 0.656, 0.722, 0.761, and 0.764, respectively. ConclusionsSlice-based and time-specific hemodynamic characteristics could effectively discriminate carotid IPH. Combination of hemodynamic measurements with carotid plaque burden might be a stronger indicator for carotid vulnerable plaque features than each measurement alone.

Non-contrast enhanced silent MR angiography to evaluate hemodynamics and morphology of unruptured intracranial aneurysms: a comparative computational fluid dynamics study

BackgroundSilent MR angiography (silent MRA) is a new generation of non-contrast enhanced angiography with outstanding advantages in visualizing cerebrovascular lesions and the follow-up after endovascular treatment for intracranial aneurysms (IAs)....

O024 Prognostic value of haemodynamic parameters in predicting adverse clinical events in type B aortic dissection

Abstract Introduction Ascertain the prognostic role of in-vivo 4D-flow magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) aortic haemodynamic parameters as predictors of adverse dissection-related events in type B aortic dissection (TBAD). Methods A systematic literature search was conducted according to the PRISMA guidelines using electronic databases searched from 1946 to 2021 for studies reporting on the relationship between aortic haemodynamics and aortic dissection-related events defined as aortic growth, aneurysm development, false lumen thrombosis, need for surgery and aortic rupture. Results Fourteen studies were included; eleven studies used CFD and three studies used 4D-flow MRI. Two studies found increased FL ejection fraction (EF) significantly associated with aortic growth rate, and another study found increased FL stroke volume significantly associated with increased aortic expansion rate. Result for time-averaged wall shear stress (TAWSS) and relative residence time (RRT) are contradictory; one study found TAWSS significantly associated with aortic wall deformation, but another study found no significant difference. Five studies suggested possible associations between slow flowing regions or increased RRT and FL thrombosis, with another study finding significant associations between FL thrombosis and adverse aortic outcomes. However, one study revealed opposite findings where decreased RRT significantly correlated with aneurysm formation. Conclusion In-vivo aortic haemodynamic measurements such as FL EF, FL stroke volume, TAWSS and RRT show possible associations with FL and aortic expansion, as well as FL thrombosis. Larger prospective studies are now needed to determine the prognostic utility of in vivo aortic haemodynamic metrics in predicting TBAD clinical outcomes to guide management. Take-home message Aortic haemodynamic parameters measured by 4D-flow magnetic resonance imaging and computational fluid dynamics show promising potential as predictors of adverse dissection-related events in type B aortic dissection.

Reliability of using generic flow conditions to quantify aneurysmal haemodynamics: A comparison against simulations incorporating boundary conditions measured in vivo

Background and objectivesInitiation, growth, and rupture of intracranial aneurysms are believed to be closely related to their local haemodynamic environment. While haemodynamics can be characterised by use of computational fluid dynamics (CFD), its reliability depends heavily upon accurate assumption of the boundary conditions. Herein, we compared the simulated aneurysmal haemodynamics obtained by use of generic boundary conditions against those obtained under flow conditions measured in vivo. MethodsWe prospectively recruited 19 patients with intracranial aneurysms requiring 3-dimensional rotational angiography, during which blood pressure at the internal carotid artery was probed by catheter and flowrate measured by a dedicated software tool. Using these flow conditions measured in vivo, we quantified the aneurysmal haemodynamics for each patient by CFD, and then compared the results with those derived from a generic condition reported in the literature, in terms of the time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), relative residence time (RRT), and percentage of the intra-aneurysmal flow (PIAF). In addition, the effects on aneurysmal haemodynamics of different outflow strategies (splitting method vs. Murray's Law) and simulation schemes (transient vs. steady-state) relative to each flow condition were also assessed. ResultsDifferences in the simulated TAWSS (−6.08 ± 10.64 Pa, p = 0.001), OSI (0.06 ± 0.13, p = 0.001), and PIAF (−0.05 ± 0.20, p = 0.012) between the patient-specific and generic boundary conditions were found to be statistically significant, in contrast to that in the RRT (49 ± 307 Pa−1, p = 0.062). Outflow strategies did not yield statistically significant differences in any of the investigated parameters (all p > 0.05); rather, the resulting parameters were found to be in good correlations (all r > 0.71, p < 0.001). Difference between the aneurysmal TAWSS and the WSS derived from cycle-averaged flowrate condition was found to be minor (0.66 ± 1.36 Pa, p = 0.000), so was that between PIAFs obtained respectively from the transient and steady-state simulations (0.02 ± 0.05, p = 0.000). ConclusionsIncorporating into simulation the patient-specific boundary conditions is critical for CFD to characterise aneurysmal haemodynamics, while outflow strategies may not introduce significant uncertainties. Steady-state simulation incorporating the cycle-averaged flow condition may produce unbiased WSS and PIAF compared to the transient analysis.

The hemodynamic effect of eccentricity in visceral branched aneurysms with multilayer stents.

It is preliminarily acknowledged that multilayer stent (MS) is a promising alternative technology in the treatment of visceral branched aneurysms, but hemodynamic consequences of eccentricity in such aneurysms with MS are less examined. In this work, we performed a time-dependent simulation of branched aneurysms of various eccentricities with different stent layers, and thrombosis-related parameters, such as time-averaged wall shear stress (TAWSS), oscillating shear index (OSI), and relative residence time (RRT), were also analyzed. Our results revealed that MS can generally restore laminar flow inside the stent, and allow proper perfusion to vital organs while also fostering a relatively secluded hemodynamic environment for thrombosis formation. Particularly, a flow in the aneurysm sac communicating between the main artery and side branch forms at early systole. However, MS fails to completely eliminate detrimental flow impingement after peak systole, which may hinder aneurysm recovery, especially in the cases of eccentric aneurysms. Therefore, saccular aneurysms should be treated with more caution than fusiform aneurysms. And further therapeutic attempts to keep both perfusion in the proximal region of the aneurysm and isolation in the distal region of the aneurysm should be considered.

Helical Flow in Healthy and Diseased Patient-specific Coronary Bifurcations.

Helical flow (HF) exists in healthy and diseased coronary bifurcations and was found to have a protective atherosclerotic vascular effect in other vessels. However, the role of HF in patient-specific human coronary arteries still needs further study, and is therefore the objective of this study in both healthy and diseased bifurcations. Computational studies were conducted on 16 patient-specific coronary bifurcations, including eight healthy and eight identical cases with idealized narrowing to represent disease. In general, higher HF intensity may have a favorable effect as it corelated to the reduction of the percentage vessel area exposed to adverse time averaged wall shear stress (TAWSS%) in both healthy and diseased models. The HF intensity and distribution of each model varies due to the complex shape of patient-specific models. The presence of disease appears to have an important impact on the downstream HF patterns and the TAWSS distributions. Clinical Relevance- By understanding the relationship between HF and hemodynamics, HF may be used as a predictor for the formation and progression of atherosclerotic plaque in coronary arteries instead of near-wall WSS measures, which can be determined with higher accuracy in vivo.

Aneurysmal wall enhancement and hemodynamics: pixel-level correlation between spatial distribution.

Inflammation and hemodynamics are interrelated risk factors for intracranial aneurysm rupture. This study aimed to identify the relationship between these risk factors from an individual-patient perspective using biomarkers of aneurysm wall enhancement (AWE) derived from high-resolution magnetic resonance imaging (HR-MRI) and hemodynamic parameters by four-dimensional flow MRI (4D-flow MRI). A total of 29 patients with 29 unruptured intracranial aneurysms larger than 4 mm were included in this prospective cross-sectional study. A total of 24 aneurysms had AWE and 5 did not have AWE. A three-dimensional (3D) vessel model of each individual aneurysm was generated with 3D time-of-flight magnetic resonance angiography (3D TOF-MRA). Quantification of AWE was sampled with HR-MRI. Time-averaged wall shear stress (WSS) and oscillatory shear index (OSI) were calculated from the 4D-flow MRI. The correlation between spatial distribution of AWE and hemodynamic parameters measured at pixel-level was evaluated for each aneurysm. In aneurysms with AWE, the spatial distribution of WSS was negatively correlated with AWE in 100% (24/24) of aneurysms, though 2 had an absolute value of the correlation coefficient <0.1. The OSI was positively correlated with AWE in 91.7% (22/24) of aneurysms; the other 2 aneurysms showed a negative correlation with AWE. In aneurysms with no AWE, there was no correlation between WSS (100%, 5/5), OSI (80%, 4/5), and wall inflammation. The spatial distribution of WSS was negatively correlated with AWE in aneurysms with AWE, and OSI was positively correlated with AWE in most aneurysms with AWE. While aneurysms that did not contain AWE showed no correlation between hemodynamics and wall inflammation.

Fluid-Structure Interaction Simulations of Repaired Type A Aortic Dissection: a Comprehensive Comparison With Rigid Wall Models.

This study aimed to evaluate the effect of aortic wall compliance on intraluminal hemodynamics within surgically repaired type A aortic dissection (TAAD). Fully coupled two-way fluid-structure interaction (FSI) simulations were performed on two patient-specific post-surgery TAAD models reconstructed from computed tomography angiography images. Our FSI model incorporated prestress and different material properties for the aorta and graft. Computational results, including velocity, wall shear stress (WSS) and pressure difference between the true and false lumen, were compared between the FSI and rigid wall simulations. It was found that the FSI model predicted lower blood velocities and WSS along the dissected aorta. In particular, the area exposed to low time-averaged WSS () was increased from 21 cm2 (rigid) to 38 cm2 (FSI) in patient 1 and from 35 cm2 (rigid) to 144 cm2 (FSI) in patient 2. FSI models also produced more disturbed flow where much larger regions presented with higher turbulence intensity as compared to the rigid wall models. The effect of wall compliance on pressure difference between the true and false lumen was insignificant, with the maximum difference between FSI and rigid models being less than 0.25 mmHg for the two patient-specific models. Comparisons of simulation results for models with different Young’s moduli revealed that a more compliant wall resulted in further reduction in velocity and WSS magnitudes because of increased displacements. This study demonstrated the importance of FSI simulation for accurate prediction of low WSS regions in surgically repaired TAAD, but a rigid wall computational fluid dynamics simulation would be sufficient for prediction of luminal pressure difference.

Computational Fluid Dynamics of the Right Atrium: A Comparison of Modeling Approaches in a Range of Flow Conditions

Abstract The right atrium (RA) combines flows from the inferior (IVC) and superior vena cava (SVC). Here RA mixing is simulated using computational fluid dynamics, comparing four modeling approaches. A patient-averaged model (11 M cells) was created from four volunteers. We compared: (1) unsteady k–ω Reynolds-averaged Navier–Stokes (URANS) (2) implicit large eddy simulation with second-order upwind convection scheme (iLES-SOU) (3) iLES with bounded-central difference convection scheme (iLES-BCD) and (4) LES with wall-adapting local eddy-viscosity (LES-WALE). A constant inlet flow rate of 6 L/min was applied with both IVC/SVC contributions ranging from 30–70%. A higher density mesh (37 M cells) was also simulated for models 2 and 4 (equal IVC/SVC flow) to assess the accuracy of models 1–4. Results from the 11 M cell LES-WALE model showed good agreement with the 37 M cell meshes. All four 11 M cell models captured the same large-scale flow structures. There were local differences in velocity, time-averaged wall shear stress, and IVC/SVC mixing when compared to LES-WALE, particularly at high SVC flow. Energy spectra and velocity animations from the LES-WALE model suggest the presence of transitional flow. For the general flow structures, all four methods provide similar results, though local quantities can vary greatly. On coarse meshes, the convection scheme and subgrid-scale (SGS) model have a significant impact on results. For RA flows, URANS should be avoided and iLES models are sensitive to convection scheme unless used on a highly resolved grid.

A Hemodynamic Analysis of the Thrombosis Within Occluded Coronary Arterial Fistulas With Terminal Aneurysms Using a Blood Stasis Model.

Objective: The aim of this study is to numerically evaluate thrombosis risk within occluded coronary arterial fistulas (CAF) with terminal aneurysms, and provide guidance in choosing occlusion positions, with clinical observations as reference. Method: Four patients with CAF were studied, with different occlusion positions in actual treatments. Hemodynamics simulations were conducted, with blood residue predicted using the blood stasis model. Three types of models (untreated model, aneurysm-reserved model and aneurysm-removed model) were studeid for each patient. Four metrics, i.e., proportion of high oscillatory shear index (OSI), area of high OSI, old blood volume fraction (OBVF)) and old blood volume (OBV) was obtained to distinguish the thrombosis risk of different treatments (proximal or distal occlusion), comparing with the follow-up CTA. Results: For all the postopertive models, the high OBVF, high OSI(>0.3) and low time-averaged wall shear stress (TAWSS) regions were mainly at the distal fistula, indicating these regions were prone to thrombosis. The regions where blood residue remains are roughly regions of high OSI, corresponding well with clinical observations. In contrast, TAWSS failed to distinguish the difference in thrombosis risk. Absolute values (area of high OSI, OBV) can better reflect the degree of thrombosis risk between treatment types compared with percentage values (proportion of high OSI, OBVF). By comparing with the actual clinical treatments and observations, the OBV is superior to the area of high OSI in determining treatment type. Conclusion: The OBV, a volumetric parameter for blood stasis, can better account for the CAF thrombosis and reflect the degree of blood stasis compared with OSI or TAWSS, is a more appropriate metric for thrombosis in the fistula. Together with morphological parameters, the OBV could guide clinicians to formulate more appropriate surgical plans, which is of great significance for the preoperative evaluation and treatment prognosis of CAF patients.

Wall shear stress indicators influence the regular hemodynamic conditions in coronary main arterial diseases: cardiovascular abnormalities

Computational hemodynamic (CH) characteristics play a central role in the onset and expansion of atherosclerotic plaques in the coronary main arteries. This study has explored the effects of hemodynamic properties especially coronary arterial wall tangential stresses on various healthy and diseased patient-based coronary artery models based on coronary computed tomography angiography (CCTA) imaging. The key components of the work are the CCTA image acquisition, accurate three-dimensional (3 D) model segmentation, reconstruction, appropriate grid generation, CH simulations, and analysis of the results by using open-source techniques. The CH simulation results have produced hemodynamic variables, including velocity magnitude (VM), mean arterial pressure difference, wall shear stress (WSS), time-averaged WSS (TAWSS), oscillatory shear index (OSI), relative residence time (RRT), and finally, computational fractional flow reserve (cFFR), that allow the pathophysiological conditions in patient-based coronary models. The VM, mean pressure difference, and WSS indices have yielded consistent simulation results for predicting the severity conditions of coronary diseases. We have compared our cFFR results with the published results and observed that the WSS indices were a good alternative approach for measuring the severity of coronary lesions. The CH results allow a medical expert to estimate the severity of a lumen area and stenosis physiological blood flow conditions in a non-invasive way.

Risk evaluation of type B aortic dissection based on WSS-based indicators distribution in different types of aortic arch

Background and ObjectiveThe underlying mechanism of aortic dissection (AD) remains unclear and the onset of AD is still unpredictable. Although clinical study with statistical analysis has reported that type III aortic arch may have strong correlation with type B AD (TBD), the effects of different arch types on the wall shear stress (WSS) have not been clarified. MethodsAs a complementary work, this study numerically investigated the distribution of five WSS-based indicators in thirty aortic arches without AD, which were classified into three groups based on the arch types. ResultsThe distribution of most WSS indicators, such as time averaged WSS (TAWSS), oscillatory shear index (OSI) and relative residence time (RRT) had no significant difference among different types of aortic arches (P>0.05). However, a multidirectional WSS index, namely CFI, was found its maximum value was positively correlated with type III aortic arch in proximal descending aorta (p<0.001, r = 0.65). ConclusionsIt can be concluded that the enhancement or oscillation of WSS may not be the main reason of TBD is prevalence in type III arches, while the multidirectional WSS distribution may be an important factor. It can be further referred that the CFI may have a potential to predict the onset of TBD.

Patient-specific computational haemodynamics associated with the surgical creation of an arteriovenous fistula

Despite arteriovenous fistulae (AVF) being the preferred vascular access for haemodialysis, high primary failure rates (30-70%) and low one-year patency rates (40-70%) hamper their use. Furthermore, AVF creation has been associated with haemodynamic changes causing maladaptive cardiac remodelling leading to cardiovascular (CV) complications. In this study, we present a new workflow for characterising the haemodynamic profile prior to and following surgical creation of a successful left radiocephalic AVF in a 20-year-old end-stage kidney disease patient. The reconstructed vasculature was generated using multiple ferumoxytol-enhanced magnetic resonance angiography (FeMRA) datasets. Computational fluid dynamics (CFD) simulations utilising a scale-resolving turbulence model were completed to investigate the changes in the proximal haemodynamics following AVF creation, in addition to the post-AVF juxta-anastomosis flow patterns, which is impractical to obtain in-vivo. Following AVF creation, a significant 2-3-fold increase in blood flow rate was induced downstream of the left subclavian artery. This was validated through comparison with post-AVF patient-specific phase-contrast data. Proximal to the anastomosis, the increased flow rate yielded an increase in time-averaged wall shear stress (WSS), which is a key marker of adaptive vascular remodelling. In the juxta-anastomosis region, the success of the AVF was discussed with respect to the National Kidney Foundation's vascular access guidelines, where the patient-specific AVF met the flow rate and geometry criterion. The AVF venous diameter exceeded 6mm and the venous flow rate surpassed 600mL/min. This workflow may potentially be significant clinically when applied to multi-patient cohorts, with population-wide patient-specific conclusions being ascertained for the haemodynamic assessment of AVFs and improved surgical planning.

A Pilot Study Characterizing Flow Patterns in the Thoracic Aorta of Patients With Connective Tissue Disease: Comparison to Age- and Gender-Matched Controls via Fluid Structure Interaction

Prior computational and imaging studies described changes in flow patterns for patients with Marfan syndrome, but studies are lacking for related populations. This pilot study addresses this void by characterizing wall shear stress (WSS) indices for patients with Loeys-Dietz and undifferentiated connective tissue diseases. Using aortic valve-based velocity profiles from magnetic resonance imaging as input to patient-specific fluid structure interaction (FSI) models, we determined local flow patterns throughout the aorta for four patients with various connective tissue diseases (Loeys-Dietz with the native aorta, connective tissue disease of unclear etiology with native aorta in female and male patients, and an untreated patient with Marfan syndrome, as well as twin patients with Marfan syndrome who underwent valve-sparing root replacement). FSI simulations used physiological boundary conditions and material properties to replicate available measurements. Time-averaged WSS (TAWSS) and oscillatory shear index (OSI) results are presented with localized comparison to age- and gender-matched control participants. Ascending aortic dimensions were greater in almost all patients with connective tissue diseases relative to their respective control. Differences in TAWSS and OSI were driven by local morphological differences and cardiac output. For example, the model for one twin had a more pronounced proximal descending aorta in the vicinity of the ductus ligamentum that impacted WSS indices relative to the other. We are optimistic that the results of this study can serve as a foundation for larger future studies on the connective tissue disorders presented in this article.

Computational fluid dynamic simulations informed by CT and 4D flow MRI for post-surgery aortic dissection – A case study

Aortic dissection is a serious pathological condition where tears in the aortic wall allow blood to flow between layers of the aortic wall, separating tissue and creating a false lumen, with type A dissection affecting the ascending aorta. The considerable change in post-surgical type A dissection morphology, with residual dissection within the descending aorta, significantly alters blood flow dynamics. Knowledge of haemodynamics in this patient population may provide greater insight into disease progression. Patient-specific computational fluid dynamics (CFD) can provide this haemodynamic information to assist clinicians in assessing timing and appropriateness of intervention to prevent long-term complications. In this work, an approach integrating anatomic data from CT and personalised haemodynamics derived from 4D flow magnetic resonance imaging (4D flow MRI) with CFD in qualitative and quantitative analyses of intra-aortic flow is presented for a patient with residual aortic dissection after ascending aortic repair for type A dissection. Results show good agreement between simulated flow and 4D flow MRI measurements in the aorta, but with discrepancies observed in the false lumen. Multiple recirculation zones are observed in the thoracic false lumen, which is likely to beassociated with thrombus deposition. Regions of lower time-averaged wall shear stress (WSS) and high pressure differences between true and false lumens were found, identified as risk factors for false lumen dilation in the literature. Regions of high time-averaged WSS are detected in the aorta adjacent to entrances and exits of the false lumen. WSS is regarded as a critical biomechanical factor in disease progression, and detailed analysis of computer derived WSS could in future provide evidence-based recommendations for clinical management.

Fluid‐Structure Interaction Study of Coronary Artery Disease Biomechanics

Coronary artery disease (CAD) kills 1 in 7 in the United States. Atherosclerosis and thrombosis are both disease states that are impacted by biomechanical conditions in the blood vessels. Platelets and vascular endothelial cells are known to respond to their mechanical environments, which can be altered around a coronary stenosis. Certain pertinent mechanical parameters, such as vascular wall shear stress and strain, can be difficult or impossible to measure directly in vivo.Computational fluid dynamics (CFD) models and Fluid‐Structure Interaction (FSI) models are often used to assess coronary artery biomechanics. In this study, we developed three FSI models based on the computed tomography data from a patient: a normal model, a severe stenosis model with a 71% diameter occlusion (which would be a candidate for intervention), and a mild stenosis model (50% diameter occlusion, which would not be considered clinically severe). Our hypothesis was that the high‐resolution FSI model could reveal small changes in coronary biomechanics at the cellular level (microscopic scale), which may be used as a biomarker/indicator for endothelial cell and platelet activation at the early stage of disease development. FSI simulations were conducted in COMSOL Multiphysics. Time‐dependent velocity and pressure boundary conditions were applied at the inlet and outlet of the fluid domain, respectively. A five‐parameter Mooney‐Rivlin hyperelastic material model was used to model the material properties of the artery. A time‐dependent pressure boundary condition was applied to the outside of the vessel, mimicking squeezing of the cardiac muscle in areas of myocardial bridging. In the region of interest, a spatial resolution of less than 40 µm was achieved. This is comparable to the size of vascular endothelial cells (which can range in size from 20‐100 µm), allowing us to capture hemodynamic and solid mechanical behavior at the cellular level. In the normal geometry, wall shear stress (WSS) values peaked at 1.51 Pa. In the 71% stenosis throat, transient WSS could reach 200 Pa, while in the 50% stenosis throat, the maximum WSS was less than 5 Pa. In the recirculation zone of the 71% stenosis, WSS peaked at 4.66 Pa, while in the recirculation zone of the 50% stenosis WSS reduced to 0.83 Pa, which is pathologically low. Time‐averaged wall shear stress (TAWSS) was 0.62+0.002 Pa (average + standard error) in the normal model, 1.60+0.004 Pa in the 50% stenosis model, and 7.47+0.017 Pa in the 71% stenosis model. Time‐averaged wall shear stress gradient (TAWSSG) was 0.018+6.25*10‐5 N/m in the normal model, 0.150+3.4*10‐4 N/m in the 50% stenosis model, and 0.970+0.002 N/m in the 71% stenosis model. Radial strain in the normal, 71% stenosis throat, 50% stenosis throat, 71% recirculation zone, and 50% recirculation zone peaked at 7.5%, 4.0%, 6.3%, 0.5%, and 5.1% respectively, and the circumferential strain peaked at 8.9%, 4.4%, 6.6%, 0.6%, and 4.2% respectively. In summary, we were able to develop mesoscopic FSI models that can provide microscopic hemodynamic information at the cellular level. Such information can be used in the in vitro models to study biomechanical pathways associated with coronary artery atherosclerosis and thrombosis; it may also be used as biomarkers at the early stage of atherosclerosis development.

Computational Fluid Dynamics for the Prediction of Endograft Thrombosis in the Superficial Femoral Artery

Purpose: Contemporary diagnostic modalities, including contrast-enhanced computed tomography (CTA) and duplex ultrasound, have been insufficiently able to predict endograft thrombosis. This study introduces an implementation of image-based computational fluid dynamics (CFD), by exemplification with 4 patients treated with an endograft for occlusive disease of the superficial femoral artery (SFA). The potential of personalized CFD for predicting endograft thrombosis is investigated. Materials and Methods: Four patients treated with endografts for an occluded SFA were retrospectively included. CFD simulations, based on CTA and duplex ultrasound, were compared for patients with and without endograft thrombosis to investigate potential flow-related causes of endograft thrombosis. Time-averaged wall shear stress (TAWSS) was computed, which highlights areas of prolonged residence times of coagulation factors in the graft. Results: CFD simulations demonstrated normal TAWSS (>0.4 Pa) in the SFA for cases 1 and 2, but low levels of TAWSS (<0.4 Pa) in cases 3 and 4, respectively. Primary patency was achieved in cases 1 and 2 for over 2 year follow-up. Cases 3 and 4 were complicated by recurrent endograft thrombosis. Conclusion: The presence of a low TAWSS was associated with recurrent endograft thrombosis in subjects with otherwise normal anatomic and ultrasound assessment and a good distal run-off.

Association Between Aneurysmal Hemodynamics and Rupture Risk of Unruptured Intracranial Aneurysms.

BackgroundAssessing rupture risk in patients with unruptured intracranial aneurysms (UIAs) remains challenging. Hemodynamics plays an important role in the natural history of intracranial aneurysms. This study aimed to compare aneurysmal hemodynamic features between patients with different rupture risk as determined by PHASES score.MethodsWe retrospectively examined 238 patients who harbored a solitary saccular UIA. Patients were stratified by rupture risk into low-, intermediate-, and high-risk groups according to PHASES score. Flow simulations were performed to compare differences in hemodynamics among the groups.ResultsAneurysmal time-averaged wall shear stress (WSSa) and normalized WSS (WSSn) decreased progressively as PHASES score increased. WSSa and WSSn significantly differed among the low-, intermediate-, and high-risk groups (p < 0.001). WSSa was significantly lower in the high-risk group than the low-risk group (p < 0.001) and the intermediate-risk group (p = 0.004). WSSn was also significantly lower in the high-risk group than the low-risk group (p < 0.001) and the intermediate-risk group (p = 0.001).ConclusionsLow WSS was significantly associated with higher risk of intracranial aneurysm rupture as determined by PHASES score, indicating that hemodynamics may play an important role in aneurysmal rupture. In the future, a multidimensional rupture risk prediction model that includes hemodynamic parameters should be investigated.

A New Method for Quantifying Abdominal Aortic Wall Shear Stress Using Phase Contrast Magnetic Resonance Imaging and the Womersley Solution.

Wall shear stress (WSS) is an important mediator of cardiovascular pathologies and there is a need for its reliable evaluation as a potential prognostic indicator. The purpose of this work was to develop a method that quantifies WSS from two-dimensional (2D) phase contrast magnetic resonance (PCMR) imaging derived flow waveforms, apply this method to PCMR data acquired in the abdominal aorta of healthy volunteers, and to compare PCMR-derived WSS values to values predicted from a computational fluid dynamics (CFD) simulation. The method uses PCMR-derived flow versus time waveforms constrained by the Womersley solution for pulsatile flow in a cylindrical tube. The method was evaluated for sensitivity to input parameters, intrastudy repeatability and was compared with results from a patient-specific CFD simulation. 2D-PCMR data were acquired in the aortas of healthy men (n = 12) and women (n = 15) and time-averaged WSS (TAWSS) was compared. Agreement was observed when comparing TAWSS between CFD and the PCMR flow-based method with a correlation coefficient of 0.88 (CFD: 15.0 ± 1.9 versus MRI: 13.5 ± 2.4 dyn/cm2) though comparison of WSS values between the PCMR-based method and CFD predictions indicate that the PCMR method underestimated instantaneous WSS by 3.7 ± 7.6 dyn/cm2. We found no significant difference in TAWSS magnitude between the sexes; 8.19 ± 2.25 versus 8.07 ± 1.71 dyn/cm2, p = 0.16 for men and women, respectively.

Computational Investigation of Outflow Graft Variation Impact on Hemocompatibility Profile in LVADs

<h3>Purpose</h3> Hemocompatibility related adverse events (HRAE) such as pump thrombosis, stroke, and gastrointestinal (GI) bleeding occur commonly in patients with left ventricular assist devices (LVADs). It is unclear whether the outflow graft orientation can impact flow conditions leading to HRAE. This study, used a simulation-based approach to investigate the influence that the cannula orientation has in modulating flow conditions leading to HRAEs. <h3>Methods</h3> A 3D model of a proximal aorta and outflow graft was reconstructed from a computer tomography (CT) scan of an LVAD patient and virtually modified to account for multiple cannula orientations (n=10) by varying polar (n=5) and off-set (n=2) angles. Time-dependent computer flow simulations were then performed and flow metrics as time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), endothelial cell activation potential (ECAP), flow shear (S), particle residence time (PRT), and platelet activation potential (PLAP) estimated. <h3>Results</h3> Flow shear was highest and lowest at the proximal aorta closer to the anastomosis and aortic sinus, respectively, in all configurations. Among all angles, time and space-averaged flow shear values were lowest at polar angles of 65° and 75° for 0° and 12° off-set angles, respectively. Flow metrics associated to thrombosis such as low TAWSS and high PRT, OSI, and ECAP were high at the aorta proximal to the sinus. <h3>Conclusion</h3> Acuter polar angles are commonly associated to flow conditions leading to fewer HRAEs. Using a time-depend flow simulation, this study suggests that an "ideal acute" rather than an acuter polar angle might lead to such flow conditions.