Kristian Valen-Sendstad

• PhD
• Senior Researcher at Simula Research Laboratory

Background and purpose: Computational fluid dynamics has become a popular tool for studying intracranial aneurysm hemodynamics, demonstrating success for retrospectively discriminating rupture status; however, recent highly refined simulations suggest potential deficiencies in solution strategies normally used in the aneurysm computational fluid d...

The carotid siphon is by nature a tortuous vessel segment with sharp bends and large area variations, and of relevance to the study of intracranial aneurysm initiation and rupture. The aim of this paper was to determine whether the siphon might harbour flow instabilities, if care is take taken to resolve them. This study focused on five consecutive...

Patient-specific flow rates are rarely available for image-based computational fluid dynamics models. Instead, flow rates are often assumed to scale according to the diameters of the arteries of interest. Our goal was to determine how choice of inlet location and scaling law affect such model-based estimation of inflow rates. We focused on 37 inter...

Recent high-resolution computational fluid dynamics studies have uncovered the presence of laminar flow instabilities and possible transitional or turbulent flow in some intracranial aneurysms. The purpose of this study was to elucidate requirements for computational fluid dynamics to detect these complex flows, and, in particular, to discriminate...

Purpose Image-based computational fluid dynamics (CFD) is widely used to predict intracranial aneurysm wall shear stress (WSS), particularly with the goal of improving rupture risk assessment. Nevertheless, concern has been expressed over the variability of predicted WSS and inconsistent associations with rupture. Previous challenges, and studies f...

Despite significant computational and experimental efforts and progress made over the past few decades, the mechanobiological mechanism underlying vascular stenosis formation in arteriovenous fistulae (AVF) for hemodialysis remains mostly unknown. To investigate potential mechanisms, we presented the case of a 72-years old end-stage kidney disease...

The mechanism behind hemodialysis arteriovenous fistula (AVF) failure remains poorly understood, despite previous efforts to correlate altered hemodynamics with vascular remodeling. We have recently demonstrated that transitional flow induces high-frequency vibrations in the AVF wall, albeit with a simplified model. This study addresses the key lim...

Recent high-fidelity fluid-structure interaction simulations have revealed turbulent-like flow-induced aneurysm wall vibrations, yet the sensitivity to parameter uncertainty remains largely unexplored. We varied flow rate and wall stiffness/thickness and found that flow rate had a profound influence on flow instability and 274% change of the vibrat...

Atrial fibrillation (AF) poses a significant risk of stroke due to thrombus formation, which primarily occurs in the left atrial appendage (LAA). Medical image-based computational fluid dynamics (CFD) simulations can provide valuable insight into patient-specific hemodynamics and could potentially enhance personalized assessment of thrombus risk. H...

Building credibility of computational models in cardiovascular medicine through verification and validation Cardiovascular diseases affect 15 million people in Europe and digital solutions are now seen as very useful tools in the search for new drugs and medical devices. SimCardioTest is a 4-year project funded by the European Commission that aims...

Approximately 3% of the population is estimated to have cerebral aneurysms, which are the leading cause of subarachnoid haemorrhage. Convincing evidences suggest that wall shear stresses (WSS) play a role in vessel remodeling and in the development of vascular diseases. SinceWSS cannot be directly measured, researchers have resorted to using medica...

Computational fluid dynamics (CFD) studies of left atrial flows have reached a sophisticated level, for example, revealing plausible relationships between hemodynamics and stresses with atrial fibrillation. However, little focus has been on fundamental fluid modeling of LA flows. The purpose of this study was to investigate the spatiotemporal conve...

Fluid structure interaction simulations (FSI) can be used to predict transitional (unstable) flow and vibrations in intracranial aneurysms, but FSI is computationally expensive. To approximate vibration frequency and amplitude predictions from FSI, we predicted the fundamental vibration frequencies with modal analysis and estimated the driving flui...

Hemodialysis is the lifeline for nearly three million end stage renal disease patients worldwide. Native arteriovenous fistula (AVF) is the preferred vascular access, but 40% fail within 1 year. We recently demonstrated that AVFs harbour transitional flows and the goal of the present study was to investigate whether the associated high-frequency pr...

Background Since the 1960s, the origins of intracranial aneurysm bruits and musical murmurs have been debated, with proposed mechanisms ranging from self-excitation (i.e., resonance) by stable pulsatile flow, to vibration caused by unstable (laminar vortex shedding or turbulent) flow. This knowledge gap has impeded the use of intracranial sounds a...

Computational fluid dynamics (CFD) in combination with patient-specific medical images has been used to correlate flow phenotypes with disease initiation, progression and outcome, in search of a prospective clinical tool. A large number of CFD software packages are available, but are typically based on rigid domains and low-order finite volume meth...

Hemodialysis is the lifeline for nearly three million end stage renal disease patients worldwide. Native arteriovenous fistula (AVF) is the preferred vascular access, but 40% fail within 1 year. We recently demonstrated that AVFs harbour transitional flows and the goal of the present study was to investigate whether the associated high-frequency pr...

Computational fluid dynamics (CFD) in combination with patient-specific medical images has been used to correlate flow phenotypes with disease initiation, progression and outcome, in search of a prospective clinical tool. A large number of CFD software packages are available, but are typically based on rigid domains and low-order finite volume meth...

Clinical, experimental, and recent computational studies have demonstrated the presence of wall vibrations in cerebral aneurysms, thought to be induced by blood flow instability. These vibrations could induce irregular, high-rate deformation of the aneurysm wall, and potentially disrupt regular cell behavior and promote deleterious wall remodeling....

Computational fluid dynamics (CFD) studies of left atrial flows have reached a sophisticated level, e.g., revealing plausible relationships between hemodynamics and stresses with atrial fibrillation. However, little focus has been on fundamental fluid modelling of LA flows. The purpose of this study was to investigate the spatiotemporal convergence...

In-silico fluid simulations of the left atria (LA) in atrial fibrillation (AF) patients can help to describe and relate patient-specific morphologies and complex flow haemodynamics with the pathophysiological mechanisms behind thrombus formation. Even in AF patients, LA wall motion plays a non-negligible role in LA function and blood flow patterns....

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...

Automated tools for landmarking the internal carotid artery (ICA) bends have the potential for efficient and objective medical image-based morphometric analysis. The two existing algorithms rely on numerical approximations of curvature and torsion of the centerline. However, input parameters, original source code, comparability, and robustness of t...

High-fidelity computational fluid dynamics (HF-CFD) has revealed the potential for high-frequency flow instabilities (aka turbulent-like flow) in intracranial aneurysms, consistent with classic in vivo and in vitro reports of bruits and/or wall vibrations. However, HF-CFD has typically been performed on limited numbers of cases, often with unphysio...

Recent comparisons between experiments and computational fluid dynamics (CFD) simulations of flow in the Food and Drug Administration (FDA) standardized nozzle geometry have highlighted the potential sensitivity of axisymmetric CFD models to small perturbations induced by mesh and inlet velocity, particularly for Reynolds numbers (Re) in the transi...

Early detection of asymptomatic carotid stenosis is crucial for treatment planning in the prevention of ischemic stroke. Auscultation, the current first-line screening methodology, comes with severe limitations that create urge for novel and robust techniques. Laser Doppler Vibrometer (LDV) is a promising tool for inferring carotid stenosis by meas...

Patient‐specific medical image‐based computational fluid dynamics has been widely used to reveal fundamental insight into mechanisms of cardiovascular disease, for instance, correlating morphology to adverse vascular remodeling. However, segmentation of medical images is laborious, error‐prone, and a bottleneck in the development of large databases...

Perfusion is one of the most important processes maintaining organ health. From a computational perspective, however, perfusion is amongst the least studied physiological processes of the heart. The recent development of novel nanoparticle-based targeted cardiac therapy calls for novel simulation methods that can provide insights into the distribut...

Abnormal hemodynamic stresses are thought to correlate with aneurysm initiation, growth, and rupture. We have previously investigated the role of wall shear stress (WSS) and WSS gradients (WSSG) in search for a mechanistic link to formation of sidewall aneurysms using an automated and objective tool for aneurysm removal and arterial reconstruction...

Purpose: Screening of asymptomatic carotid stenoses is performed by auscultation of the carotid bruit, but the sensitivity is poor. Instead, it has been suggested to detect carotid bruit as neck's skin vibrations. We here take a first step towards a computational fluid dynamics proof-of-concept study, and investigate the robustness of our numerica...

Purpose Assessing the rupture probability of intracranial aneurysms (IAs) remains challenging. Therefore, hemodynamic simulations are increasingly applied toward supporting physicians during treatment planning. However, due to several assumptions, the clinical acceptance of these methods remains limited. Methods To provide an overview of state-of-...

Steady inflow through a non-axisymmetric stenotic model at Re = 500-1000 for Newtonian and shear-thinning non-Newtonian rheologies was studied numerically to investigate the experimental evidence of stabilizing effect of shear-thinning fluids. A minimally-dissipative and energy-preserving finite-element based code was used, and results were verifie...

Purpose Advanced morphology analysis and image-based hemodynamic simulations are increasingly used to assess the rupture risk of intracranial aneurysms (IAs). However, the accuracy of those results strongly depends on the quality of the vessel wall segmentation. Methods To evaluate state-of-the-art segmentation approaches, the Multiple Aneurysms A...

Systemic risk factors are known to correlate with cardiovascular diseases, but e.g., atherosclerotic plaques are focally distributed and highlight the role of hemodynamically induced forces on vascular remodeling. Computational fluid dynamics (CFD) shows great promise for revealing mechanisms of atherosclerotic plaque progression, but the utility o...

Turbulent-like flows without cycle-to-cycle variations are more frequently being reported in studies of cardiovascular flows. The associated stimuli might be of mechanobiological relevance, but how to quantify them objectively is not obvious. Classical Reynolds decomposition, where the flow is separated into mean and fluctuating velocity components...

Reduced-order modelling offers the possibility to study global flow features in cardiovascular networks. In order to validate these models, previous studies have been conducted in which they compared 3D computational fluid dynamics simulations with reduced-order simulations. Discrepancies have been reported between the two methods. The loss of ener...

Computational fluid dynamics (CFD) shows promise for informing treatment planning and rupture risk assessment for intracranial aneurysms. Much attention has been paid to the impact on predicted hemodynamics of various modelling assumptions and uncertainties, including the need for modelling the non-Newtonian, shear-thinning rheology of blood, with...

Oasis is a high-level/high-performance finite element Navier-Stokes solver written from scratch in Python using building blocks from the FEniCS project (fenicsproject.org). The solver is unstructured and targets large-scale applications in complex geometries on massively parallel clusters. Oasis utilizes MPI and interfaces, through FEniCS, to the l...

Background and purpose: A variety of approaches exists where numerical methods that underlie numerous assumptions are used in order to predict rupture in intracranial aneurysms. Since many hemodynamic parameters have been derived from these computations but no persistent explanation for rupture has been found, the acceptance of CFD among physician...

Oasis is an open-source Computational Fluid Dynamics solver designed to handle both laminar, transitional, and fully developed turbulent flows. Oasis has been developed by combining FEniCS' (http://fenicsproject.org/) user-friendly Python interface with highly optimized energy conserving numerical schemes, where the incompress-ible Navier-Stokes eq...

In reduced order (0D/1D) blood or respiratory flow models, pressure losses at junctions are usually neglected. However, these may become important where velocities are high and significant flow redirection occurs. Current methods for estimating losses rely on relatively complex empirical equations that are only valid for specific junction geometrie...

Oasis is a high-level/high-performance finite element Navier–Stokes solver written from scratch in Python using building blocks from the FEniCS project (fenicsproject.org). The solver is unstructured and targets large-scale applications in complex geometries on massively parallel clusters. Oasis utilizes MPI and interfaces, through FEniCS, to the l...

Cardiovascular diseases are burdening the healthcare systems and the costs are expected to rise in the years to come. It is therefore of great importance and interest to better understand arterial remodelling in order to prevent diseases to develop and possibly also assist with model-based surgical planning. Computational Fluid Dynamics (CFD) has a...

Stimulated by a recent controversy regarding pressure drops predicted in a giant aneurysm with a proximal stenosis, the present study sought to assess variability in the prediction of pressures and flow by a wide variety of research groups. In phase I, lumen geometry, flow rates, and fluid properties were specified, leaving each research group to c...

Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal

FEniCS is a finite element library for ‘automated, efficient solution of differential equations’ (www.fenicsproject.org). The focus has been to develop a high-level software interface that allows the user to type in the equations nearly as they are read. Recent development has involved extensions for parallel computing and building an efficient pip...

Numerical algorithms for the computation of fluid flow have been an active area of research for 12062 several decades and still remain an important field to study. As a result, there exists a large literature 12063 on discretization schemes for the incompressible Navier–Stokes equations, and it can be hard to 12064 judge which method works best for...

Our purpose was to compare quantitatively velocity fields in and around experimental canine aneurysms as measured using an accelerated 4D PC-MR angiography (MRA) method and calculated based on animal-specific CFD simulations. Two animals with a surgically created bifurcation aneurysm were imaged using an accelerated 4D PC-MRA method. Meshes were cr...

The etiology and progression of intracranial aneurysms (IAs) are closely associated with complex patterns of disturbed blood flow. [1] Consequently, blood flow imaging [2] and CFD simulations in realistic geometries [3, 4] are of clinical interests because flow information obtained by such techniques can provide insight not only into the developmen...

Background: Subarachnoid hemorrhage (SAH) is a serious condition, occurring more frequently in females than in males. SAH is mainly caused by rupture of an intracranial aneurysm, which is formed by localized dilation of the intracranial arterial vessel wall, usually at the apex of the arterial bifurcation. The female preponderance is usually expla...

Computational fluid dynamics (CFD) is a tool with great potential in medicine. Using traditional engineering techniques, one may compute, e.g., the blood flow in arteries and the resulting stress on the vessel wall to understand, treat, and prevent various cardiovascular diseases. This chapter is devoted to the computation of blood flow in large ce...