Group-wise Construction of Reduced Models for Understanding and Characterization of Pulmonary Blood Flows from Medical Images

International audience3D computational fluid dynamics (CFD) in patient-specific geometries provides complementary insights to clinical imaging, to better understand how heart disease, and the side effects of treating heart disease, affect and are affected by hemodynamics. This information can be useful in treatment planning for designing artificial devices that are subject to stress and pressure from blood flow. Yet, these simulations remain relatively costly within a clinical context. The aim of this work is to reduce the complexity of patient-specific simulations by combining image analysis, computational fluid dynamics and model order reduction techniques. The proposed method makes use of a reference geometry estimated as an average of the population, within an efficient statistical framework based on the currents representation of shapes. Snapshots of blood flow simulations performed in the reference geometry are used to build a POD (Proper Orthogonal Decomposition) basis, which can then be mapped on new patients to perform reduced order blood flow simulations with patient specific boundary conditions. This approach is applied to a data-set of 17 tetralogy of Fallot patients to simulate blood flow through the pulmonary artery under normal (healthy or synthetic valves with almost no backflow) and pathological (leaky or absent valve with backflow) conditions to better understand the impact of regurgitated blood on pressure and velocity at the outflow tracts. The model reduction approach is further tested by performing patient simulations under exercise and varying degrees of pathophysiological conditions based on reduction of reference solutions (rest and medium backflow conditions respectively)

A reduced-order modeling for efficient design study of artificial valve in enlarged ventricular outflow tracts

A computational approach is proposed for efficient design study of a reducer stent to be percutaneously implanted in enlarged right ventricular outflow tracts (RVOT). The need for such a device is driven by the absence of bovine or artificial valves which could be implanted in these RVOT to replace the absent or incompetent native valve, as is often the case over time after Tetralogy of Fallot repair. Hemodynamics are simulated in the stented RVOT via a reduce order model based on proper orthogonal decomposition (POD), while the artificial valve is modeled as a thin resistive surface. The reduced order model is obtained from the numerical solution on a reference device configuration, then varying the geometrical parameters (diameter) for design purposes. To validate the approach, forces exerted on the valve and on the reducer are monitored, varying with geometrical parameters, and compared with the results of full CFD simulations. Such an approach could also be useful for uncertainty quantification

Efficient blood flow simulations for the design of stented valve reducer in enlarged ventricular outflow tracts

Tetralogy of Fallot is a congenital heart disease characterized over time, after the initial repair, by the absence of a functioning pulmonary valve, which causes regurgitation, and by progressive enlargement of the right ventricle and pulmonary arteries. Due to this pathological anatomy, available transcatheter valves are usually too small to be deployed in the enlarged right ventricular outflow tracts (RVOT). To avoid surgical valve replacement, an alternative consists in implanting a reducer prior to or in combination with a transcatheter valve. We describe a computational model to study the effect of a stented valve RVOT reducer on the hemodynamics in enlarged ventricular outflow tracts. To this aim, blood flow in the right ventricular outflow tract is modeled via the incompressible Navier--Stokes equations coupled to a simplified valve model, numerically solved with a standard finite element method and with a reduced order model based on Proper Orthogonal Decomposition (POD). Numerical simulations are based on a patient geometry obtained from medical imaging and boundary conditions tuned according to measurements of inlet flow rates and pressures. Different geometrical models of the reducer are built, varying its length and/or diameter, and compared with the initial device-free state. Simulations thus investigate multiple device configurations and describe the effect of geometry on hemodynamics. Forces exerted on the valve and on the reducer are monitored, varying with geometrical parameters. Results support the thesis that the reducer does not introduce significant pressure gradients, as was found in animal experiments. Finally, we demonstrate how computational complexity can be reduced with POD

Fluid Dynamic Structures in the Right Ventricle with Presence of Pulmonary Regurgitation after Tetralogy of Fallot Repair

Tetralogy of Fallot (TOF), which causes 8-10% of all congenital defects, is a concern for 1 in 2518 parents. Although the effects of this disease can be remedied by a surgery at birth, regurgitation from the pulmonary valve seems to occur two to three decades after this repair in 50% of the operated cases. Since little is known regarding the cause of this regurgitation or simply the natural flows in the right ventricle (RV), this research, which is the first of its kind, aims to shed light on fluid flow in the right ventricle during normal function as well as the effects on the fluid flow due to the introduction of regurgitation from the pulmonary valve. An in-house cardiovascular simulator was used to simulate the pumping action of the right ventricle through the use of a linear motor, which gave rise to the E-wave, while a servo motor gave rise to the A-wave. Various severities of regurgitation, defined by ratio of the effective regurgitant orifice area over the total valve orifice area (ROA/A), (0 ROA/A, 0.012 ROA/A, 0.063 ROA/A and 0.174 ROA/A) were simulated through the restriction of the valve closure while the fluid used was a 60%-40% water-glycerol mixture in order to mimic human blood viscosity. Planar time-resolved particle image velocimetry measurements have been performed on a custom-made double activation simulator reproducing flow conditions in a model of a right ventricle. Changes in flow characteristics in the right ventricle have been evaluated in terms of velocity fields and profiles, tricuspid inflow jet orientation and viscous energy dissipation. Our results show that pulmonary valve regurgitation significantly alters the flow in the right ventricle mostly by impairing the diastolic inflow through the tricuspid valve and by increasing viscous energy loss. This fundamental work should allow for a better understanding of such changes in the RV flow dynamics. It may also help in developing new strategies allowing for a better follow-up of patients with repaired TOF and for decision-making in terms of pulmonary valve replacement

Blood flow simulations in the pulmonary bifurcation in relation to adult patients with repaired tetralogy of Fallot

Understanding the haemodynamic environment of the pulmonary bifurcation is important in adults with repaired conotruncal congenital heart disease. In these patients, dysfunction of the pulmonary valve and narrowing of the branch pulmonary arteries are common and can have serious clinical consequences. The aim of this study was to numerically investigate the underlying blood flow characteristics in the pulmonary trunk under a range of simplified conditions. For that, an in-depth analysis was conducted in idealised two-dimensional geometries that facilitate parametric investigation of healthy and abnormal conditions. Subtle variations in morphology influenced the haemodynamic environment and wall shear stress distribution. The pressure in the left pulmonary artery was generally higher than that in the right and main arteries, but was markedly reduced in the presence of a local stenosis. Different downstream pressure conditions altered the branch flow ratio, from 50:50% to more realistic 60:40% ratios in the right and left pulmonary artery, respectively. Despite some simplifications, this study highlights some previously undocumented aspects of the flow in bifurcating geometries, by clarifying the role of the stagnation point location on wall shear stress and differential branch pressures. In addition, measurements of the mean pressure ratios in the pulmonary bifurcation are discussed in the context of a new haemodynamic index which could potentially contribute to the assessment of left pulmonary artery stenosis in tetralogy of Fallot patients. Further studies are required to confirm the results in patient-specific models with personalised physiological flow conditions

Blood flow dynamics in surviving patients with repaired Tetralogy of Fallot

Tetralogy of Fallot (TOF) is a congenital heart disease that causes structural abnormalities in the pulmonary arteries, which in turn disrupt the blood flow. Surgical repair is necessary early in childhood, but chronic complications are common in the adult surviving patients. Pulmonary valve replacement is an operation performed in the repaired TOF (rTOF) patients to overcome the right ventricular overload, but the optimal timing remains a challenge. The main research question is whether the haemodynamic environment of the pulmonary junction can clarify the interplay between the upstream and downstream pulmonary vasculature. Therefore, an extensive analysis of the effect of morphological and flow characteristics in healthy and rTOF models was performed, under various boundary conditions (BCs). The effects of branch angle and origin, branch stenosis, flow splits and pulmonary resistance were investigated in idealised two-dimensional geometries, representative of healthy and rTOF cases, explaining the elevated pressure in the LPA, and clearly showing that downstream pressure and peripheral resistance alter the flow development and the flow split between the two daughter branches. Various modelling parameters were also tested, demonstrating the importance of the valve, and how it disturbs the flow patterns along the MPA. The elasticity of arterial wall had a minimal effect on the flow development while the WSS deviated based on the rheological model assumed. Finally, anatomically realistic three-dimensional models of rTOF patients and healthy volunteers were reconstructed and morphological and flow features were analysed. Higher curvature and tortuosity were correlated with more complex secondary flow patterns, and higher Reynolds and Dean numbers, with increased regions of time-averaged wall shear stress. More importantly, the importance of patient-specificity in the rTOF models, and the variability of the geometric and flow characteristics within the population was highlighted, contrary to the observations in the healthy models. The results of this work could help clinicians evaluate the haemodynamic environment in the rTOF population and potentially predict patients at higher risk, prior to the appearance of severe complications.Tetralogy of Fallot (TOF) is a congenital heart disease that causes structural abnormalities in the pulmonary arteries, which in turn disrupt the blood flow. Surgical repair is necessary early in childhood, but chronic complications are common in the adult surviving patients. Pulmonary valve replacement is an operation performed in the repaired TOF (rTOF) patients to overcome the right ventricular overload, but the optimal timing remains a challenge. The main research question is whether the haemodynamic environment of the pulmonary junction can clarify the interplay between the upstream and downstream pulmonary vasculature. Therefore, an extensive analysis of the effect of morphological and flow characteristics in healthy and rTOF models was performed, under various boundary conditions (BCs). The effects of branch angle and origin, branch stenosis, flow splits and pulmonary resistance were investigated in idealised two-dimensional geometries, representative of healthy and rTOF cases, explaining the elevated pressure in the LPA, and clearly showing that downstream pressure and peripheral resistance alter the flow development and the flow split between the two daughter branches. Various modelling parameters were also tested, demonstrating the importance of the valve, and how it disturbs the flow patterns along the MPA. The elasticity of arterial wall had a minimal effect on the flow development while the WSS deviated based on the rheological model assumed. Finally, anatomically realistic three-dimensional models of rTOF patients and healthy volunteers were reconstructed and morphological and flow features were analysed. Higher curvature and tortuosity were correlated with more complex secondary flow patterns, and higher Reynolds and Dean numbers, with increased regions of time-averaged wall shear stress. More importantly, the importance of patient-specificity in the rTOF models, and the variability of the geometric and flow characteristics within the population was highlighted, contrary to the observations in the healthy models. The results of this work could help clinicians evaluate the haemodynamic environment in the rTOF population and potentially predict patients at higher risk, prior to the appearance of severe complications

Shape and function in congenital heart disease: a translational study using image, statistical and computational analyses

While medical image analysis techniques are becoming technically more advanced, analysis of shape and structure in clinical practice is often limited to two-dimensional morphometry, neglecting potentially crucial three-dimensional (3D) anatomical information provided by the original images. This thesis aims at closing this gap by combining state-of-the-art medical image analysis, engineering and data analysis tools to elucidate relationships between 3D shape features and clinically relevant functional outcomes. In particular, patient cohorts affected by congenital heart disease were studied since shape and structure of the heart and its components are crucial for diagnosis, therapy and management of those patients. At first, a statistical shape model was coupled with partial least squares regression to extract anatomical 3D shape biomarkers related to clinical parameters from cardiovascular magnetic resonance image data. After establishing a step-by-step protocol to guide the user with respect to parameter selection, results were shown to be in accordance with traditional morphometry as well as with clinical expert opinion. Novel aortic arch shape biomarkers relating to cardiac functional parameters were found in a cohort of patients post aortic coarctation repair (CoA). By combining statistical shape modelling results with computational fluid dynamics simulations, a mechanistic basis for the observed results was provided. Methods were then extended towards a hierarchical shape clustering framework, which achieved good unsupervised classification performance in a population of healthy and pathological aortic arch shapes. Applied to a cohort of CoA patients, previously unknown anatomical patterns were discovered. This thesis demonstrates that combining medical image analysis and engineering tools with data mining and statistics provides a powerful platform to detect novel shape biomarkers and patient sub-groups. Results may ultimately improve risk-stratification, treatment-planning and medical device development, thereby promoting translation of advanced computational analysis techniques into clinical practice