The reliability and utility of on-site CT-derived fractional flow reserve (FFR) based on fluid structure interactions: comparison with FFRCT based on computational fluid dynamics, invasive FFR, and resting full-cycle ratio

Fractional flow reserve (FFR) derived off-site by coronary computed tomography angiography (CCTA) (FFRCT) is obtained by applying the principles of computational fluid dynamics. This study aimed to validate the overall reliability of on-site CCTA-derived FFR based on fluid structure interactions (CT-FFR) and assess its clinical utility compared with FFRCT, invasive FFR, and resting full-cycle ratio (RFR). We calculated the CT-FFR for 924 coronary vessels in 308 patients who underwent CCTA for clinically suspected coronary artery disease. Of these patients, 35 patients with at least one obstructive stenosis (> 50%) detected on CCTA underwent both CT-FFR and FFRCT for further investigation. Furthermore, 24 and 20 patients underwent invasive FFR and RFR in addition to CT-FFR, respectively. The inter-observer correlation (r) of CT-FFR was 0.93 (95% confidence interval [CI] 0.85-0.97, P < 0.0001) with a mean absolute difference of - 0.0042 (limits of agreement - 0.073, 0.064); 97.3% of coronary arteries without obstructive lesions on CCTA had negative results for ischemia on CT-FFR (> 0.80). The correlation coefficient between CT-FFR and FFRCT for 105 coronary vessels was 0.87 (95% CI 0.82-0.91, P < 0.0001) with a mean absolute difference of - 0.012 (limits of agreement - 0.12, 0.10). CT-FFR correlated well with both invasive FFR (r = 0.66, 95% CI 0.36-0.84, P = 0.0003) and RFR (r = 0.78, 95% CI 0.51-0.91, P < 0.0001). These data suggest that CT-FFR can potentially substitute for FFRCT and correlates closely with invasive FFR and RFR with high reproducibility. Our findings should be proven by further clinical investigation in a larger cohort.

Riccati transfer equations for fluid structure interaction in liquid-filled piping systems

In this paper, based on the Riccati transfer matrix method (RTMM), the Riccati fluid structure interaction transfer equations (FSIRTE) are established to improve the numerical stability of the classical fluid structure interaction transfer matrix method (FSITMM). Combined with numerical algorithms for eliminating the singularity points of the Riccati equations, the spare root problem in the calculation process is solved. This method can be used for the natural frequency calculation of liquid-filled piping systems. Compared with finite element method (FEM), it has the characteristics of high calculation efficiency; meanwhile, good numerical stability, compared with FSITMM; and accurate calculation results, compared with method of characteristics (MOC). Numerical simulation results of typical classical examples are given.

High-fidelity fluid structure interaction simulations of turbulent-like aneurysm flows reveals high-frequency narrowband wall vibrations: A stimulus of mechanobiological relevance?

Recent high-fidelity/resolution computational fluid dynamics simulations of intracranial aneurysm hemodynamics have revealed turbulent-like flows. We hypothesized that the associated high-frequency pressure fluctuations could promote aneurysm wall vibrations. We performed fully coupled high-fidelity transient fluid structure interaction simulations between the blood flow and compliant aneurysm sac wall taking 5,000 time steps per second using a 3D patient-specific model previously shown to harbour turbulent-like flow. Our results show that the flow velocity contained fluctuations with a smooth and continuously decaying energy up to ∼160Hz, and fluctuating pressures with characteristic frequency peaks at approximately 30, 130 and 210Hz. There was a strong two-way coupling between the pressure and the wall deformation, for which the frequency spectrum showed similar characteristics, but with a narrow band peak at ∼120Hz with large regional differences in amplitude up to 80μm. The physics of the flow is broadly consistent with clinical reports of turbulent-like flows, while the physics of the wall is consistent with reports of spectral peaks in aneurysm patients. As many aneurysms are known to harbour turbulent-like flows, wall vibrations could be a widespread phenomenon. Finally, since aneurysms are vascular pathologies by definition and many/most aneurysms do not have endothelial cells but still display a focal remodeling, we hypothesize that vibrations and stresses within the wall itself might play a role in the mechanobiological processes of vessel wall pathology.

Hemodynamics and bio-mechanics of morphologically distinct saccular intracranial aneurysms at bifurcations: Idealised vs Patient-specific geometries

Understanding the factors that influence the rupture of aneurysms is of primary concern to the clinicians, who are grappled with patient management. It is important to know how the relation between morphological features of the cerebral aneurysm, and the mechanical stresses on the containing arterial walls are influenced by the hemodynamic forces. Present study investigates three different shapes, which have been identified correspondingly in patient-specific scenarios as well. The primary objective is to categorize the bifurcation aneurysms into standard shapes such as, spherical, beehive and pear-shaped, based on patient-specific clinical studies and further compare and contrast the model aneurysms with the patient specific configurations, for their hemodynamic factors as well as the attendant stresses on the wall. MethodsComputational fluid dynamic simulations are performed accounting for the fluid-structure interaction (FSI) effects between the flowing fluid and the containing vessel wall. Blood is assumed to be Newtonian, while the arterial walls are assumed to be linearly elastic. A commercial solver is used for performing detailed calculations. Hemodynamic and bio-mechanical rupture predictions are carried out for the three different shapes. Observations derived from the idealised simulations are compared and contrasted against their patient-specific counterparts. ResultsFrom detailed numerical simulations, it was observed that pear-shaped aneurysms exhibit large re-circulation bubble and flow stagnation zone, with higher residence time for the particles, which may lead to atherosclerotic lesions. Beehive shape allows for maximum flow into the aneurysmal sac with concentrated jet impinging on the dome, leading to high values of maximum WSS (MWSS) resulting in great propensity to form a secondary bleb. However, flow field inside a spherical aneurysm is found to be stable with fewer vortices, and nearly uniform distribution of wall stresses are observed though-out the sac, which perhaps signifies hemodynamically and bio-mechanically stable condition. ConclusionCategorizing patient-specific intracranial aneurysms into standard shapes viz, spherical, beehive and pear could generalize the process of prediction of hemodynamic and bio-mechanical rupture indicators. Comparative assessment of the flow field and stresses reported from the simulations on idealised models, with corresponding patient-specific simulations reveal that, these studies could aid in understanding the generalised shape dependence of hemodynamic and bio-mechanical behaviour of aneurysms.

Fluid structure computational model of simulating mitral valve motion in a contracting left ventricle

BACKGROUND

Fluid structure interaction simulations h hold promise in studying normal and abnormal cardiac function, including the effect of fluid dynamics on mitral valve (MV) leaflet motion. The goal of this study was to develop a 3D fluid structure interaction computational model to simulate bileaflet MV when interacting with blood motion in left ventricle (LV).

METHODS

The model consists of ideal geometric-shaped MV leaflets and the LV, with MV dimensions based on human anatomical measurements. An experimentally-based hyperelastic isotropic material was used to model the mechanical behaviour of the MV leaflets, with chordae tendineae and papillary muscle tips also incorporated. LV myocardial tissue was prescribed using a transverse isotropic hyperelastic formulation. Incompressible Navier-Stokes fluid formulations were used to govern the blood motion, and the Arbitrary Lagrangian Eulerian (ALE) method was employed to determine the mesh deformation of the fluid and solid domains due to trans-valvular pressure on MV boundaries and the resulting leaflet movement.

RESULTS

The LV-MV generic model was able to reproduce physiological MV leaflet opening and closing profiles resulting from the time-varying atrial and ventricular pressures, as well as simulating normal and prolapsed MV states. Additionally, the model was able to simulate blood flow patterns after insertion of a prosthetic MV with and without left ventricular outflow tract flow obstruction. In the MV-LV normal model, the regurgitant blood flow fraction was 10.1 %, with no abnormality in cardiac function according to the mitral regurgitation severity grades reported by the American Society of Echocardiography.

CONCLUSION

Our simulation approach provides insights into intraventricular fluid dynamics in a contracting LV with normal and prolapsed MV function, as well as aiding in the understanding of possible complications after transcatheter MV implantation prior to clinical trials.

Hemodynamic simulation of abdominal aortic aneurysm on idealised models: Investigation of stress parameters during disease progression

BACKGROUND AND OBJECTIVE

Analysis and prediction of rupture risk of abdominal aortic aneurysms (AAA) facilitates planning for surgical interventions and assessment of plausible treatment modalities. Present approach of using maximum diameter criterion, is giving way to hemodynamic and bio-mechanical based predictors in conjunction with Computational fluid dynamic (CFD) simulations. Detailed studies on hemodynamic and bio-mechanical parameters at the stage of maximum growth/rupture is of practical importance to the clinical community. However, understanding the changes in these parameters at different stages of growth, will be useful for clinicians, in planning routine monitoring to reduce the risk of sudden rupture. This is particularly useful in medical resource starved nations. Present study investigates the hemodynamic and bio-mechanical changes occurring during the growth stages of aortic aneurysms using fluid structure interaction (FSI) studies.

METHOD

Six idealized fusiform aneurysm models spanning high (shorter) and low (longer) values of the shape index (DHr), have been analysed at three different stages of growth viz, a D_max of 3.5cm, 4.25cm, 5cm. Pulsatile Newtonian blood flow, passing through an elastic arterial vessel wall with uniform thickness is assumed. Two-way coupled fluid structure interaction have been employed for the numerical simulation of blood flow dynamics and arterial wall mechanics.

RESULTS

Wall shear stress (WSS) parameters and vonmises stress indicators, co-relating rupture and thrombus formation, have been extracted and reported, at each growth stage. When the aneurysm progresses in diameter, the areas recording abnormally low TAWSS, as well as areas of high/low OSI were found to increase at different rates for shorter and longer aneurysms. Moreover, drastic increase in the maximum wall stresses (MWS) and wall displacement were observed as the aneurysm approached the critical diameter.

CONCLUSION

Hemodynamic predictors were found to be highly dependent on the shape index (DHr), when the aneurysm was small, whereas significant influence of DHr on the wall stresses happens, as the aneurysm approaches the critical diameter. Inconsistent variation of these indicators exhibited by shorter aneurysms (high DHr) at different growth stages, demands routine monitoring (using scans), of such aneurysms, to prevent unexpected rupture.

Assessing mechanical vibration-altered wall shear stress in digital arteries

The aim of this study is to implement and validate a method for assessing acute vibration-altered Wall Shear Stress (WSS) in the proper volar digital artery of the non-exposed left forefinger when subjecting the right hand to mechanical vibration. These changes of WSS may be involved in Vibration White Finger. Hence, an experimental device was set-up to link a vibration shaker and an ultra-high frequency ultrasound scanner. The Womersley-based WSS was computed by picking up the maximum velocity from pulse Wave Doppler measurements and extracting the artery diameter from B-mode images through an in-house image processing technique. The parameters of the former method were optimised on numerical ultrasound phantoms of cylindrical and lifelike arteries. These phantoms were computed with the FIELD II and FOCUS platforms which mimicked our true ultrasound device. The Womersley-based WSS were compared to full Fluid Structure Interaction (FSI) and rigid wall models built from resonance magnetic images of a volunteer-specific forefinger artery. Our FSI model took into account the artery's surrounding tissues. The diameter computing procedure led to a bias of 4%. The Womersley-based WSS resulted in misestimating the FSI model by roughly 10% to 20%. No difference was found between the rigid wall computational model and FSI simulations. Regarding the WSS measured on a group of 20 volunteers, the group-averaged basal value was 3 Pa, while the vibration-altered WSS was reduced to 1 Pa, possibly triggering intimal hyperplasia mechanisms and leading to the arterial stenoses encountered in patients suffering from vibration-induced Raynaud's syndrome.

The state-of-art of design and research for Pelton turbine casing, weight estimation, counterpressure operation and scientific challenges

The Pelton turbine is the most widespread and efficient impulse hydropower turbine. The Pelton casing is a static, but key component: the internal hydrodynamic phenomena affect the performance of the hydropower plant, the vibration of the equipment and water quality (dissolved oxygen downstream). However, the literature information is very fragmented and not well organized, so that the design is generally based on empirical rules and on proper know-how of hydropower companies. In this paper, the state-of-the-art of the Pelton casing is reviewed and organized under three macro areas: hydraulics, mechanics (vibrations and weight) and aeration. The preliminary design procedure is described and discussed in light of recent scientific results, and the open questions and research challenges are highlighted. Innovative case studies are described (including counterpressure operation) and a dataset of installed casings (not available in literature) is elaborated to derive an empirical equation to estimate the casing weight. The efficiency can be improved by 3% by an optimal fluid dynamic design and a better understanding of the internal hydrodynamics. Proper inserts can improve the hydraulic efficiency by 2%, reduce the weight (by about 12%) and better bear the vibrations. Several scientific questions are still open, and a better understanding of the fluid structure interaction is needed to improve efficiency, operation and water quality.

Ocular biomechanics due to ground blast reinforcement

BACKGROUND AND OBJECTIVE

Bomb blast injuries exerts a shearing force on the air-tissue interfaces, causing devastating ocular injury from the blast wave. Improvised explosive devices (IEDs) are usually placed at different heights from the ground to induce more severe injury through ground blast reinforcement (GBR). However, there is still a lack of knowledge of the role of GBR and IED height from the ground on ocular biomechanics, and how they can affect the intraocular pressure (IOP) in the eye. This study aimed to estimate the IOP due to frontal IED explosion at different heights from the ground using a fluid-structure interaction model with and without GBR effects.

METHODS

A 2 kg IED was placed within 5 m of the victim at 5 different heights from the ground, including 0, 0.42, 0.85, 1.27, and 1.70 m. Two different blast formulations were used to simulate the IED explosion: (a) spherical airburst, with no amplification of the initial shock wave due to interaction with the ground-surface, and (b) hemispherical surface-burst, where the initial blast wave is immediately reflected and reinforced by the ground (GBR).

RESULTS

Results revealed that the blast wave due to GBR reaches to the skull prior to the IED blast itself. The GBR also reached to the skull ∼ 0.6 ms earlier when the IED was on the ground compared to the height of 1.70 m. The highest and lowest IOPs of ∼ 17,000 and ∼ 15,000 mmHg were observed at the IED heights of 1.70 and 0 m from the ground considering GBR. However, when the role of the GBR is ignored, IOP of ∼ 9,000 mmHg was observed regardless of the IED height from the ground. The deformation in the apex of the cornea was higher when considering the GBR (∼ 0.75 cm) versus no GBR (∼ 0.65 cm). Considering GBR led to higher stresses and strains in the sclera.

CONCLUSIONS

When the role of GBR was ignored, the results showed similar patterns and magnitudes of stresses and deformations in the skull and eye regardless of the height of the IED from the ground, which was not the case when GBR was considered. The findings of this study suggest the critical role of GBR in ocular blast simulations.

Aortic hemodynamics assessment prior and after valve sparing reconstruction: A patient-specific 4D flow-based FSI model

INTRODUCTION

Valve-sparing root replacement (VSRR) of the ascending aorta is a life-saving procedure for the treatment of aortic aneurysms, but patients remain at risk for post-operative events involving the downstream native aorta, the mechanism for which is uncertain. It is possible that proximal graft replacement of the ascending aorta induces hemodynamics alterations in the descending aorta, which could trigger adverse events. Herein, we present a fluid-structure interaction (FSI) protocol, based on patient-specific geometry and boundary conditions, to assess impact of proximal aortic grafts on downstream aortic hemodynamics and distensibility.

METHODS

Cardiac magnetic resonance (CMR), including MRA, cine-CMR and 4D flow sequences, was performed prior and after VSRR on one subject. Central blood pressure was non-invasively acquired at the time of the CMR: data were used to reconstruct the pre- and post-VSRR model and derive patient-specific boundary conditions for the FSI and a computational fluid dynamic (CFD) analysis with the same settings. Results were validated comparing the predicted velocity field against 4D flow dataset, over four landmarks along the aorta, and the predicted distensibility against the cine-CMR derived value.

RESULTS

Instantaneous velocity magnitudes extracted from 4D flow and FSI were similar (p > 0.05), while CFD-predicted velocity was significantly higher (p < 0.001), especially in the descending aorta of the pre-VSRR model (v_max was 73 cm/s, 76 cm/s and 99 cm/s, respectively). As measured in cine-CMR, FSI predicted an increase in descending aorta distensibility after grafting (i.e., 4.02 to 5.79 10^-3 mmHg^-1). In the descending aorta, the post-VSRR model showed increased velocity, aortic distensibility, stress and strain and wall shear stress.

CONCLUSIONS

Our Results indicate that i) the distensibility of the wall cannot be neglected, and hence the FSI method is necessary to obtain reliable results; ii) graft implantation induces alterations in the hemodynamics and biomechanics along the thoracic aorta, that may trigger adverse vessel remodeling.

Comparison of diagnostic performance in on-site based CT-derived fractional flow reserve measurements

Background

Computed tomography fractional flow reserve (CT-FFR), which can be acquired on-site workstation using fluid structure interaction during the multiple optimal diastolic phase, has an incremental diagnostic value over conventional coronary computed tomography angiography (CCTA). However, the appropriate location for CT-FFR measurement remains to be clarified.

Method

A total of 115 consecutive patients with 149 vessels who underwent CCTA showing 30-90% stenosis with invasive FFR within 90 days were retrospectively analyzed. CT-FFR values were measured at three points: 1 and 2 cm distal to the target lesion (CT-FFR_{1cm, 2cm}) and the vessel terminus (CT-FFR_lowest). The diagnostic accuracies of CT-FFR ≤ 0.80 for detecting hemodynamically significant stenosis, defined as invasive FFR ≤ 0.80, were compered.

Result

Fifty-five vessels (36.9%) had invasive FFR ≤ 0.80. The accuracy and AUC for CT-FFR_1cm and _2cm were comparable, while the AUC for CT-FFR_lowest was significantly lower than CT-FFR_1cm and _2cm. (lowest/1cm, 2 cm = 0.68 (95 %CI 0.63-0.73) vs 0.79 (0.72-0.86, p = 0.006), 0.80 (0.73-0.87, p = 0.002)) The sensitivity and negative predictive value of CT-FFR_lowest were 100%. The reclassification rates from positive CT-FFR_lowest to negative CT-FFR_1cm and _2cm were 55.7% and 54.2%, respectively.

Conclusion

The diagnostic performance of CT-FFR was comparable when measured at 1-to-2 cm distal to the target lesion, but significantly higher than CT-FFR_lowest. The lesion-specific CT-FFR could reclassify false positive cases in patients with positive CT-FFR_lowest, while all patients with negative CT-FFR_lowest were diagnosed as negative by invasive FFR.

Effect of stenosis and dilatation on the hemodynamic parameters associated with left coronary artery

BACKGROUND AND OBJECTIVE

The main objective of the work is to examine the curvature effects of stenosis/dilatation region pertaining to left coronary artery. The hemodymamic features during the cardiac cycle is thoroughly examined.

METHODS

A numerical fluid structure interaction model incorporating multi- layered elastic artery wall, non-Newtonian blood viscosity and pulsating boundary conditions is developed. The composite arterial wall consists of a thin layer tunica intima, atheroma and a thick wall. Higher stiffness of atheroma is captured by using higher Young's modulus. The CFD and FSI models are validated with available experimental and analytical data. Computations are done with five different non-Newtonian models and arterial wall with various elasticity levels. The local and time averaged WSS, velocity contours downstream of stenosis, wall pressure and pressure drop during various phases of cardiac cycle are provided in detail.

RESULTS

The influence of non-Newtonian effects of blood viscosity is found to be significant especially at stenosis regions. The flexible wall caused wall deformation and the associated flow and pressure wave propagation affecting WSS and pressure drop compared to the rigid wall. Flow recirculation is noticed at stenosis downstream locations and its strength increases with increased severity of the stenosis. A stenosis is characterised by a sudden drop in wall pressure and a slower two stage recovery during peak velocity periods of the cardiac cycle.

CONCLUSIONS

The pressure drop, local WSS at stenosis centre, and radial velocity increase are significantly higher for stenosis cases and the effect is severe during peak diastole. The variation in hemodynamic parameters is found to be less significant for dilatation. Significantly lower WSS is noticed for the recirculation regions downstream of stenosis which can enhance the tendency for monocytes to attach to the endothelium. The radius of curvature of the stenosis is found to be the most sensitive parameter affecting the hemodynamic characteristics rather than the detailed geometry of the stenosis. The main effect of variation of artery wall stiffness is noted at recirculation regions present downstream of stenosis. The results from the study may be useful for predicting wall shear stress signatures associated with stenosis/dilatation changes and the management of specific cases.

Computational models of cancer cell transport through the microcirculation

The transport of cancerous cells through the microcirculation during metastatic spread encompasses several interdependent steps that are not fully understood. Computational models which resolve the cellular-scale dynamics of complex microcirculatory flows offer considerable potential to yield needed insights into the spread of cancer as a result of the level of detail that can be captured. In recent years, in silico methods have been developed that can accurately and efficiently model the circulatory flows of cancer and other biological cells. These computational methods are capable of resolving detailed fluid flow fields which transport cells through tortuous physiological geometries, as well as the deformation and interactions between cells, cell-to-endothelium interactions, and tumor cell aggregates, all of which play important roles in metastatic spread. Such models can provide a powerful complement to experimental works, and a promising approach to recapitulating the endogenous setting while maintaining control over parameters such as shear rate, cell deformability, and the strength of adhesive binding to better understand tumor cell transport. In this review, we present an overview of computational models that have been developed for modeling cancer cells in the microcirculation, including insights they have provided into cell transport phenomena.

A computational study of the hemodynamics of bioprosthetic aortic valves with reduced leaflet motion

We employ a reduced degree-of-freedom aortic valve model to investigate the flow physics associated with early-stage reduced leaflet motion in bioprosthetic aortic valves. The model is coupled with a sharp-interface immersed boundary based incompressible flow solver to efficiently simulate the fluid-structure interaction. A total of 19 cases of flow through aortic valves with varying degrees of reduced leaflet motion (RLM) are considered. The characteristics of the aortic jet and the consequent aorta wall loading patterns are analyzed. Our results show that asymmetric RLM tilts the aortic jet and leads to large reverse and recirculating flow regions downstream from leaflets with restricted mobility. The changes in flow patterns increase wall pressure and shear stress fluctuations, and result in asymmetric oscillating shear on the aorta wall. These findings have implications for auscultation based diagnosis of this condition as well as the health of the aorta.

Fluid structure interaction modelling of aortic valve stenosis: Effects of valve calcification on coronary artery flow and aortic root hemodynamics

BACKGROUND AND OBJECTIVE

Coronary artery diseases and aortic valve stenosis are two of the main causes of mortality and morbidity worldwide. Stenosis of the aortic valve develops due to calcium deposition on the aortic valve leaflets during the cardiac cycle. Clinical investigations have demonstrated that aortic valve stenosis not only affects hemodynamic parameters inside the aortic root but also has a significant influence on the coronary artery hemodynamics and leads to the initiation of coronary artery disease. The aim of this study is to investigate the effect of calcification of the aortic valve on the variation of hemodynamic parameters in the aortic root and coronary arteries in order to find potential locations for initiation of the coronary stenoses.

METHODS

Fluid structure interaction modelling methodology was used to simulate aortic valve hemodynamics in the presence of coronary artery flow. A 2-D model of the aortic valve leaflets was developed in ANSYS Fluent based on the available echocardiography images in literature. The k-ω SST turbulence model was utilised to model the turbulent flow downstream of the leaflets.

RESULTS

The effects of calcification of the aortic valve on aortic root hemodynamics including transvalvular pressure gradient, valve orifice dimeter, vorticity magnitude in the sinuses and wall shear stress on the ventricularis and fibrosa layers of the leaflets were studied. Results revealed that the transvalvular pressure gradient increases from 792 Pa (∼ 6 mmHg) for a healthy aortic valve to 2885 Pa (∼ 22 mmHg) for a severely calcified one. Furthermore, the influence of the calcification of the aortic valve leaflets on the velocity profile and the wall shear stress in the coronary arteries was investigated and used for identification of potential locations of initiation of the coronary stenoses. Obtained results show that the maximum velocity inside the coronary arteries at early diastole decreases from 1 m/s for the healthy valve to 0.45 m/s for the severely calcified case.

CONCLUSIONS

Calcification significantly decreases the wall shear stress of the coronary arteries. This reduction in the wall shear stress can be a main reason for initiation of the coronary atherosclerosis process and eventually results in coronary stenoses.

The predictive factors affecting false positive in on-site operated CT-fractional flow reserve based on fluid and structural interaction

Background

A novel algorithm has been developed for the on-site analysis of CT-fractional flow reserve (CT-FFR) using fluid structural interactions. There have been no reports on the factors affecting the diagnostic performance of CT-FFR using this algorithm. We evaluated the factors predictive of false-positive CT-FFR findings compared to invasive FFR as a reference standard.

Methods

The subjects were 66 consecutive cases (81 vessels) who underwent invasive FFR assessment within 90 days of the detection of 30–90% stenosis of one vessel of the major coronary artery, from among patients with suspected coronary arterial disease who underwent one-rotation scanning by 320-row coronary CT angiography (CCTA). The prospective CCTA mode was used for all patients, with the X-ray exposure set in a range of 70–99% of the RR interval. The FFR was calculated on-site from multiple cardiac phases. Factors associated with a false-positive finding of functional stenosis on CT-FFR, defined as an invasive FFR of ≤0.80, were evaluated using logistic regression analysis.

Results

Thirty-nine vessels (48.1%) had an invasive FFR of ≤0.80. CT-FFR and invasive FFR values disagreed in 13 vessels in 13 patients. The values were false positive in 12 of the vessels. In an analysis of patient characteristics, the body mass index (odds ratio, 1.33; 95%CI, 1.06–1.67; p = 0.01) and Image noise (odds ratio, 1.18; 95%CI, 1.01–1.40; p = 0.04) were predictive of false-positive findings. The presence of calcified plaque (odds ratio, 5.16; 95%CI, 1.06–20.85; p = 0.01) was the only significant predictive factor in a vessel-based analysis of lesion characteristics.

Conclusions

The presence of calcified plaque exerted a significant effect on the diagnostic performance of CT-FFR, and did so independently of the degree of calcification indicated by the Agatston score.

4D Flow MRI Estimation of Boundary Conditions for Patient Specific Cardiovascular Simulation

Accurate image based cardiovascular simulations require patient specific boundary conditions (BCs) for inlets, outlets and vessel wall mechanical properties. While inlet BCs are typically determined non-invasively, invasive pressure catheterization is often used to determine patient specific outlet BCs and vessel wall mechanical properties. A method using 4D Flow MRI to non-invasively determine both patient specific outlet BCs and vessel wall mechanical properties is presented and results for both in vitro validation with a latex tube and an in vivo pulmonary artery stenosis (PAS) stent intervention are presented. For in vitro validation, acceptable agreement is found between simulation using BCs from 4D Flow MRI and benchtop measurements. For the PAS virtual intervention, simulation correctly predicts flow distribution with 9% error compared to MRI. Using 4D Flow MRI to noninvasively determine patient specific BCs increases the ability to use image based simulations as pressure catheterization is not always performed.

Fluid-structure interaction simulation of artificial textile reinforced aortic heart valve: Validation with an in-vitro test

Prosthetic heart valves deployed in the left heart (aortic and mitral) are subjected to harsh hemodynamical conditions. Most of the tissue engineered heart valves have been developed for the low pressure pulmonary position because of the difficulties in fabricating a mechanically strong valve, able to withstand the systemic circulation. This necessitates the use of reinforcing scaffolds, resulting in a tissue-engineered textile reinforced tubular aortic heart valve. Therefore, to better design these implants, material behaviour of the composite, valve kinematics and its hemodynamical response need to be evaluated. Experimental assessment can be immensely time consuming and expensive, paving way for numerical studies. In this work, the material properties obtained using the previously proposed multi-scale numerical method for textile composites was evaluated for its accuracy. An in silico immersed boundary (IB) fluid structure interaction (FSI) simulation emulating the in vitro experiment was set-up to evaluate and compare the geometric orifice area and flow rate for one beat cycle. Results from the in silico FSI simulation were found to be in good coherence with the in vitro test during the systolic phase, while mean deviation of approximately 9% was observed during the diastolic phase of a beat cycle. Merits and demerits of the in silico IB-FSI method for the presented case study has been discussed with the advantages outweighing the drawbacks, indicating the potential towards an effective use of this framework in the development and analysis of heart valves.

Blast wave dynamics: The influence of the shape of the explosive

A numerical model is developed to analyse the influence of the shape of a high-explosive on the dynamics of the generated pressure wave. A Multi-Material Arbitrary Lagrangian Eulerian (MM-ALE) technique is used as the CONWEP approach is not adequate to model such situations. Validation and verification of the proposed numerical model is achieved based on experimental data obtained from the bibliography. The numerical model provides relevant information that cannot be obtained from the experimental results. The influence of the mass and shape of the high-explosive is studied and correlated to the dynamics of the generated blast wave through the analysis of peak pressures, time of arrival and impulse. Tests are done with constant mass hemispherical, cylindrical and flat-shaped Formex F4HV samples. A detailed analysis of the generated blast wave is done, along with a thorough comparison between incident and reflected waves. It is concluded that the dynamic effects of the reflected pressure pulses should always be considered in structural design, most relevantly when analysing closed structures where the number of reflections can be significant. The model is proved reliable, concluding that the frontal area of the high-explosive is a determinant driving parameter for the impulse generated by the blast.

Numerical simulation of mitral valve prolapse considering the effect of left ventricle

Heart failure is one of the most important issues that has been investigated in recent research studies. Variations that occur in apparatus of mitral valve, such as chordae tendineaea rupture, can affect the valve function during ventricular contraction and lead to regurgitation from the left ventricle into the left atrium. One method for understanding mitral valve function in such conditions is computational analysis. In this paper, we develop a finite element model of mitral valve prolapse, considering the direct effect of left ventricular motion on blood flow interacting with the mitral valve. Ventricular wall motion is used as a constraint for fluid domain. Arbitrary Lagrangian-Eulerian finite element method formulation is used for numerical solution of transient dynamic equations of the fluid domain. Leaflets' stresses and chordal forces during prolapse are determined and compared to previous healthy results, as well as flow characteristics in the computational domain. Results show considerable increases in the stress magnitudes of interior and posterior leaflets in prolapse condition in comparison with previous healthy studies. In addition, chordae tendineae forces are distributed non-uniformly with higher maximum value here, as a result of other chordae tendineae rupture.

Effects of Leaflet Design on Transvalvular Gradients of Bioprosthetic Heart Valves

Bioprosthetic aortic valves (BAVs) are becoming the prostheses of choice in heart valve replacement. The objective of this paper is to assess the effects of leaflet geometry on the mechanics and hemodynamics of BAVs in a fluid structure interaction model. The curvature and angle of leaflets were varied in 10 case studies whereby the following design parameters were altered: a circular arch, a line, and a parabola for the radial curvature, and a circular arch, a spline, and a parabola for the circumferential curvature. Six different leaflet angles (representative of the inclination of the leaflets toward the surrounding aortic wall) were analyzed. The 3-dimensional geometry of the models were created using SolidWorks, Pointwise was used for meshing, and Comsol Multiphysics was used for implicit finite element calculations. Realistic loading was enforced by considering the time-dependent strongly-coupled interaction between blood flow and leaflets. Higher mean pressure gradients as well as von Mises stresses were obtained with a parabolic or circular curvature for radial curvature or a parabolic or spline curvature for the circumferential curvature. A smaller leaflet angle was associated with a lower pressure gradient, and, a lower von Mises stress. The leaflet curvature and angle noticeably affected the speed of valve opening, and closing. When a parabola was used for circumferential or radial curvature, leaflets displacements were asymmetric, and they opened and closed more slowly. A circular circumferential leaflet curvature, a linear leaflet radial curvature, and leaflet inclination toward the surrounding aortic wall were associated with superior BAVs mechanics.

Biomechanical Behavior of Bioprosthetic Heart Valve Heterograft Tissues: Characterization, Simulation, and Performance

The use of replacement heart valves continues to grow due to the increased prevalence of valvular heart disease resulting from an ageing population. Since bioprosthetic heart valves (BHVs) continue to be the preferred replacement valve, there continues to be a strong need to develop better and more reliable BHVs through and improved the general understanding of BHV failure mechanisms. The major technological hurdle for the lifespan of the BHV implant continues to be the durability of the constituent leaflet biomaterials, which if improved can lead to substantial clinical impact. In order to develop improved solutions for BHV biomaterials, it is critical to have a better understanding of the inherent biomechanical behaviors of the leaflet biomaterials, including chemical treatment technologies, the impact of repetitive mechanical loading, and the inherent failure modes. This review seeks to provide a comprehensive overview of these issues, with a focus on developing insight on the mechanisms of BHV function and failure. Additionally, this review provides a detailed summary of the computational biomechanical simulations that have been used to inform and develop a higher level of understanding of BHV tissues and their failure modes. Collectively, this information should serve as a tool not only to infer reliable and dependable prosthesis function, but also to instigate and facilitate the design of future bioprosthetic valves and clinically impact cardiology.

Computational modeling with fluid-structure interaction of the severe m1 stenosis before and after stenting

PURPOSE

Image-based computational models with fluid-structure interaction (FSI) can be used to perform plaque mechanical analysis in intracranial artery stenosis. We described a process in FSI study applied to symptomatic severe intracranial (M1) stenosis before and after stenting.

MATERIALS AND METHODS

Reconstructed 3D angiography in STL format was transferred to Magics for smoothing of vessel surface and trimming of branch vessels and to HyperMesh for generating tetra volume mesh from triangular surface-meshed 3D angiogram. Computational analysis of blood flow in the blood vessels was performed using the commercial finite element software ADINA Ver 8.5. The distribution of wall shear stress (WSS), peak velocity and pressure was analyzed before and after intracranial stenting.

RESULTS

The wall shear stress distributions from Computational fluid dynamics (CFD) simulation with rigid wall assumption as well as FSI simulation before and after stenting could be compared. The difference of WSS between rigid wall and compliant wall model both in pre- and post-stent case is only minor except at the stenosis region. These WSS values were greatly reduced after stenting to 15~20 Pa at systole and 3~5 Pa at end-diastole in CFD simulation, which are similar in FSI simulations.

CONCLUSION

Our study revealed that FSI simulation before and after intracranial stenting was feasible despite of limited vessel wall dimension and could reveal change of WSS as well as flow velocity and wall pressure.

Computational flow dynamics of the severe m1 stenosis before and after stenting

Purpose

Computational flow dynamic (CFD) study has not been widely applied in intracranial artery stenosis due to requirement of high resolution in identifying the small intracranial artery. We described a process in CFD study applied to symptomatic severe intracranial (M1) stenosis before and after stenting.

Materials and Methods

Reconstructed 3D angiography in STL format was transferred to Magics (Materialise NV, Leuven, Belgium) for smoothing of vessel surface and trimming of branch vessels and to HyperMesh (Altair Engineering Inc., Auckland, New Zealand) for generating tetra volume mesh from triangular surface-meshed 3D angiogram. Computational analysis of blood flow in the blood vessels was performed using the commercial finite element software ADINA Ver 8.5 (ADINA R & D, Inc., Lebanon, MA). The distribution of wall shear stress (WSS), peak velocity and pressure in a patient was analyzed before and after intracranial stenting.

Results

Computer simulation of wall shear stress, flow velocity and wall pressure before and after stenting could be demonstrated three dimensionally by video mode according to flow vs. time dimension. Such flow model was well correlated with angiographic finding related to maximum degree of stenosis. Change of WSS, peak velocity and pressure at the severe stenosis was demonstrated before and after stenting. There was no WSS after stenting in case without residual stenosis.

Conclusion

Our study revealed that CFD analysis before and after intracranial stenting was feasible despite of limited vessel wall dimension and could reveal change of WSS as well as flow velocity and wall pressure.