Jeannette Hiromi Spühler

Patient-specific flow simulation of the left ventricle from 4D echocardiography - feasibility and robustness evaluation

In recent years, computational fluid dynamics (CFD) simulations on in-silico models of the heart have provided a valuable insight into cardiac hemodynamic behaviour. However, so far most models have been either based on simplified geometries or on imaging acquisitions with relatively low temporal resolution. It has been suggested that models based entirely on subject-specific ultrasonic images should be used to capture transient flow changes. Therefore, the aim of this study is to present a pathway from routine 4D echocardiography to a patient-specific flow simulation of the left ventricle (LV), evaluating the model robustness and clinical feasibility. The created pathway consisted of initial LV segmentation and mitral/aortic valve positioning, being subsequently used as input for the CFD simulations (based on solving the Navier-Stokes equation using an Arbitrary Lagrangian-Eulerian approach). The output consisted of 4D blood flow velocities and relative pressures in the entire LV. On five subjects, the model robustness was evaluated with regards to variations in singular boundary conditions. The clinical feasibility of the output was compared to clinical PW Doppler measurements and, as a proof-of-concept, synthetic contrast enhanced ultrasound images were simulated on the flow field using the COLE-method. Results indicated a relatively robust model, with variations in regional flow of approximately 5.1/6.2% and 9.7/7.0% for healthy and pathological subject respectively (end diastole/end systole). Furthermore, showing similar behaviour to clinical Doppler measurements the technique serves as a promising tool for future clinical investigations. Additionally, the ability of simulating synthetic ultrasound images further underlines the applicability of the pathway, being potentially useful in studies on improved echocardiographic image analysis.

Multimodal validation of patient-specific intraventricular flow simulations from 4D echocardiography

The combination of refined medical imaging techniques and computational fluid dynamics (CFD) models has enabled the study of complex flow behavior on a highly regional level. Recently, we have developed a platform for patient-specific CFD modelling of blood flow in the left ventricle (LV), with input data and required boundary conditions acquired from 4D echocardiography. The platform robustness has been evaluated with respect to input variable variations, but for any clinical implementation model flow validation is essential. Therefore, the aim of this study is to evaluate the accuracy of the patient-specific CFD model against multimodal image-based flow measurements. For the validation, 4D echocardiography was acquired from two healthy subjects, from which LV velocity fields were simulated. In-vivo flows from the same two subjects were then acquired by pulsed wave (PW) Doppler imaging over both LV-valves, and by cine phase-contract magnetic resonance imaging (PC-MRI) at eight defined anatomical planes in the LV. By fusing PC-MRI and the ultrasound acquisitions using a three-chamber alignment algorithm, simulated and measured flows were quantitatively compared. General flow pattern correspondence was observed, with a mean error of 1.4 cm/s and root mean square deviation of 5.7 cm/s for all measured PC-MRI LV-planes. For the PW-Doppler comparison, a mean error of 3.6 cm/s was reported. Overall, the following work represents a validation of the proposed patient-specific CFD platform, and the agreement with clinical data highlight the potential for future clinical use of the models.

Patient-Specific Left Ventricular Flow Simulations From Transthoracic Echocardiography: Robustness Evaluation and Validation Against Ultrasound Doppler and Magnetic Resonance Imaging

The combination of medical imaging with computational fluid dynamics (CFD) has enabled the study of 3-D blood flow on a patient-specific level. However, with models based on gated high-resolution data, the study of transient flows, and any model implementation into routine cardiac care, is challenging. This paper presents a novel pathway for patient-specific CFD modelling of the left ventricle (LV), using 4-D transthoracic echocardiography (TTE) as input modality. To evaluate the clinical usability, two sub-studies were performed. First, a robustness evaluation was performed, where repeated models with alternating input variables were generated for six subjects and changes in simulated output quantified. Second, a validation study was carried out, where the pathway accuracy was evaluated against pulsed-wave Doppler (100 subjects), and 2-D through-plane phase-contrast magnetic resonance imaging measurements over seven intraventricular planes (6 subjects). The robustness evaluation indicated a model deviation of <12%, with highest regional and temporal deviations at apical segments and at peak systole, respectively. The validation study showed an error of <11% (velocities <10 cm/s) for all subjects, with no significant regional or temporal differences observed. With the patient-specific pathway shown to provide robust output with high accuracy, and with the pathway dependent only on 4-D TTE, the method has a high potential to be used within future clinical studies on 3-D intraventricular flow patterns. To this, future model developments in the form of e.g., anatomically accurate LV valves may further enhance the clinical value of the simulations.

3D Fluid-Structure Interaction Simulation of Aortic Valves Using a Unified Continuum ALE FEM Model

Due to advances in medical imaging, computational fluid dynamics algorithms and high performance computing, computer simulation is developing into an important tool for understanding the relationship between cardiovascular diseases and intraventricular blood flow. The field of cardiac flow simulation is challenging and highly interdisciplinary. We apply a computational framework for automated solutions of partial differential equations using Finite Element Methods where any mathematical description directly can be translated to code. This allows us to develop a cardiac model where specific properties of the heart such as fluid-structure interaction of the aortic valve can be added in a modular way without extensive efforts. In previous work, we simulated the blood flow in the left ventricle of the heart. In this paper, we extend this model by placing prototypes of both a native and a mechanical aortic valve in the outflow region of the left ventricle. Numerical simulation of the blood flow in the vicinity of the valve offers the possibility to improve the treatment of aortic valve diseases as aortic stenosis (narrowing of the valve opening) or regurgitation (leaking) and to optimize the design of prosthetic heart valves in a controlled and specific way. The fluid-structure interaction and contact problem are formulated in a unified continuum model using the conservation laws for mass and momentum and a phase function. The discretization is based on an Arbitrary Lagrangian-Eulerian space-time finite element method with streamline diffusion stabilization, and it is implemented in the open source software Unicorn which shows near optimal scaling up to thousands of cores. Computational results are presented to demonstrate the capability of our framework.

Estimation of left ventricular blood flow parameters : Clinical application of patient-specific CFD simulations from 4D echocardiography

Echocardiography is the most commonly used image modality in cardiology, assessing several aspects of cardiac viability. The importance of cardiac hemodynamics and 4D blood flow motion has recently been highlighted, however such assessment is still difficult using routine echo-imaging. Instead, combining imaging with computational fluid dynamics (CFD)-simulations has proven valuable, but only a few models have been applied clinically. In the following, patient-specific CFD-simulations from transthoracic dobutamin stress echocardiography have been used to analyze the left ventricular 4D blood flow in three subjects: two with normal and one with reduced left ventricular function. At each stress level, 4D-images were acquired using a GE Vivid E9 (4VD, 1.7MHz/3.3MHz) and velocity fields simulated using a presented pathway involving endocardial segmentation, valve position identification, and solution of the incompressible Navier-Stokes equation. Flow components defined as direct flow, delayed ejection flow, retained inflow, and residual volume were calculated by particle tracing using 4th-order Runge-Kutta integration. Additionally, systolic and diastolic average velocity fields were generated. Results indicated no major changes in average velocity fields for any of the subjects. For the two subjects with normal left ventricular function, increased direct flow, decreased delayed ejection flow, constant retained inflow, and a considerable drop in residual volume was seen at increasing stress. Contrary, for the subject with reduced left ventricular function, the delayed ejection flow increased whilst the retained inflow decreased at increasing stress levels. This feasibility study represents one of the first clinical applications of an echo-based patient-specific CFD-model at elevated stress levels, and highlights the potential of using echo-based models to capture highly transient flow events, as well as the ability of using simulation tools to study clinically complex phenomena. With larger patient studies planned for the future, and with the possibility of adding more anatomical features into the model framework, the current work demonstrates the potential of patient-specific CFD-models as a tool for quantifying 4D blood flow in the heart.

FEniCS-HPC: Coupled Multiphysics in Computational Fluid Dynamics

We present a framework for coupled multiphysics in computational fluid dynamics, targeting massively parallel systems. Our strategy is based on general problem formulations in the form of partial d ...