Utku Gülan

Mapping mean and fluctuating velocities by Bayesian multipoint MR velocity encoding-validation against 3D particle tracking velocimetry

To validate Bayesian multipoint MR velocity encoding against particle tracking velocimetry for measuring velocity vector fields and fluctuating velocities in a realistic aortic model.

On the accuracy of viscous and turbulent loss quantification in stenotic aortic flow using phase-contrast MRI.

To investigate the limits of phase contrast MRI (PC‐MRI)–based measurements of viscous losses and turbulent kinetic energy (TKE) pertaining to spatial resolution, signal‐to‐noise ratio (SNR), and non‐Gaussian intravoxel velocity distributions.

Experimental Investigation of the Influence of the Aortic Stiffness on Hemodynamics in the Ascending Aorta

A three-dimensional (3-D) pulsatile aortic flow in a human ascending aorta is studied to investigate the effect of the aortic stiffness on the flow field and turbulent fluctuating velocities in the ascending aorta. A nonintrusive optical measurement technique, 3-D particle tracking velocimetry (3D-PTV), has been applied to anatomically accurate phantoms under clinically realistic conditions. A compliant silicon phantom was used to mimic the healthy aorta, and a rigid model was used to imitate the pathological case that appears in aortas for example as a result of aging. The realistic models are transparent which allows optical access to the investigation domain, and the index of refraction was matched to avoid optical distortions. Our results revealed that the aortic stiffness leads to an increase in systolic velocity and a decrease in the Windkessel effect, which is associated with the diastolic blood pressure. Furthermore, we found that the turbulent kinetic energy is about an order of magnitude higher for the rigid aorta, that is, an increase in aortic stiffness increases the magnitude of turbulent fluctuating velocities. The spatial distribution of the flow velocity showed that the flow is more organized and coherent spiraling patterns develop for the compliant aorta which helps to dampen the influence of disturbed flow. Finally, we observed higher Lagrangian acceleration and hence higher instantaneous forces acting on blood particles in the stiff case which implies that aging and hence arterial stiffening provokes distinctive alterations in blood flow, and these alterations may cause pathological symptoms in the cardiovascular system.

Analysis of thoracic aorta hemodynamics using 3D particle tracking velocimetry and computational fluid dynamics

Parallel to the massive use of image-based computational hemodynamics to study the complex flow establishing in the human aorta, the need for suitable experimental techniques and ad hoc cases for the validation and benchmarking of numerical codes has grown more and more. Here we present a study where the 3D pulsatile flow in an anatomically realistic phantom of human ascending aorta is investigated both experimentally and computationally. The experimental study uses 3D particle tracking velocimetry (PTV) to characterize the flow field in vitro, while finite volume method is applied to numerically solve the governing equations of motion in the same domain, under the same conditions. Our findings show that there is an excellent agreement between computational and measured flow fields during the forward flow phase, while the agreement is poorer during the reverse flow phase. In conclusion, here we demonstrate that 3D PTV is very suitable for a detailed study of complex unsteady flows as in aorta and for validating computational models of aortic hemodynamics. In a future step, it will be possible to take advantage from the ability of 3D PTV to evaluate velocity fluctuations and, for this reason, to gain further knowledge on the process of transition to turbulence occurring in the thoracic aorta.

Assessment of 3D velocity vector fields and turbulent kinetic energy in a realistic aortic phantom using multi-point variable-density velocity encoding

A multi-point velocity encoding approach for the assessment of velocity vector fields and TKE is shown in this work. The method is applied in an aortic arch phantom under different flow conditions.

An in vitro analysis of the influence of the arterial stiffness on the aortic flow using three-dimensional particle tracking velocimetry

A three-dimensional pulsatile aortic flow in a human ascending aorta is investigated in-vitro in this paper. A non-intrusive measurement technique, 3D Particle Tracking Velocimetry (3D-PTV), has been applied to the anatomically accurate silicon replicas. A compliant and a stiff aortic model were analyzed to better understand the influence of the arterial stiffness. The realistic models are transparent which allows optical access to the investigation domain. Our results showed that increasing the arterial stiffness considerably increases the systolic velocity and hencemean kinetic energy. Quite strikingly, the turbulent kinetic energy is about one order of magnitude higher in the stiffer model during the deceleration phase which manifests that a blood element is exposed to higher shear stresses in the stiffer model. Moreover, we found that the compliant model introduces pressure oscillations during the diastolic phase which are associated with the Windkessel effect.

The influence of bileaflet prosthetic aortic valve orientation on the blood flow patterns in the ascending aorta

We investigate three-dimensional pulsatile aortic flow in the ascending aorta with mechanical prosthetic aortic valve implanted at two different orientations under physiological flow conditions using an anatomically accurate aorta. We perform 3D Particle Tracking Velocimetry measurements to assess the phase averaged and fluctuating velocity patterns as well as the shear stresses. A St Jude Medical prosthetic heart valve is implanted in an anatomically accurate silicone model of an aorta obtained from high resolution magnetic resonance imaging of a healthy proband at two different orientations. Our results show that the mechanical prosthetic valve orientation has considerable impact on the local kinetic energy and shear stress distributions but minor effects on the spatially averaged kinetic energy (10%) and shear stresses (15%). We show that the valve orientation plays a distinct role in spatial distribution of wall shear stresses and vortical structures. We show that our results, which show good agreement with the in silico and in vitro studies in the literature, provide full 3D kinetic energy and shear stress information over the entire cardiac cycle for different bileaflet prosthetic valve orientations under physiological flow conditions.

Blood flow patterns and pressure loss in the ascending aorta: A comparative study on physiological and aneurysmal conditions

An aortic aneurysm is defined as a balloon-shaped bulging of all three histologic components of the aortic vessel walls (intima, media and adventitia). This dilation results from vessel weakening owing to remodeling, i.e. due to cystic degeneration of the Tunica media (Marfan), progression of atherosclerosis or presence of a bicuspid aortic valve. The growth rate of the aortic diameter varies from patient to patient and may progress until the aneurysm ultimately ruptures. The role of hemodynamics, i.e. blood flow patterns, and shear stresses that are supposed to intensify during aneurysm growth are not yet fully understood, but thought to play a key role in the enlargement process. The aim of this study is to characterize the aortic blood flow in a silicone model of a pathological aorta with ascending aneurysm, to analyze the differences in the blood flow pattern compared to a healthy aortic model, and to single out possible blood flow characteristics measurable using phase contrast magnetic resonance imaging (MRI) that could serve as indicators for aneurysm severity. MRI simulations were performed under physiological, pulsatile flow conditions using data obtained from optical three dimensional particle tracking measurements. In comparison to the healthy geometry, elevated turbulence intensity and pressure loss are measured in the diseased aorta, which we propose as a complimentary indicator for assessing the aneurysmal severity. Our results shed a light on the interplay between the blood flow dynamics and their contribution to the pathophysiology and possible role for future risk assessment of ascending aortic aneurysms.

Investigation of Atrial Vortices Using a Novel Right Heart Model and Possible Implications for Atrial Thrombus Formation

The main aim of this paper is to characterize vortical flow structures in the healthy human right atrium, their impact on wall shear stresses and possible implications for atrial thrombus formation. 3D Particle Tracking Velocimetry is applied to a novel anatomically accurate compliant silicone right heart model to study the phase averaged and fluctuating flow velocity within the right atrium, inferior vena cava and superior vena cava under physiological conditions. We identify the development of two vortex rings in the bulk of the right atrium during the atrial filling phase leading to a rinsing effect at the atrial wall which break down during ventricular filling. We show that the vortex ring formation affects the hemodynamics of the atrial flow by a strong correlation (ρ = 0.7) between the vortical structures and local wall shear stresses. Low wall shear stress regions are associated with absence of the coherent vortical structures which might be potential risk regions for atrial thrombus formation. We discuss possible implications for atrial thrombus formation in different regions of the right atrium.

Experimental and Numerical Study of Ascending Aorta Hemodynamics Through 3D Particle Tracking Velocimetry and Computational Fluid Dynamics

The complex hemodynamics observed in the human aorta make this district a site of election for an in depth investigation of the relationship between fluid structures, transport and pathophysiology. In recent years, the coupling of imaging techniques and computational fluid dynamics (CFD) has been applied to study aortic hemodynamics, because of the possibility to obtain highly resolved blood flow patterns in more and more realistic and fully personalized flow simulations [1]. However, the combination of imaging techniques and computational methods requires some assumptions that might influence the predicted hemodynamic scenario. Thus, computational modeling requires experimental cross-validation. Recently, 4D phase contrast MRI (PCMRI) has been applied in vivo and in vitro to access the velocity field in aorta [2] and to validate numerical results [3]. However, PCMRI usually requires long acquisition times and suffers from low spatial and temporal resolution and a low signal-to-noise ratio. Anemometric techniques have been also applied for in vitro characterization of the fluid dynamics in aortic phantoms. Among them, 3D Particle Tracking Velocimetry (PTV), an optical technique based on imaging of flow tracers successfully used to obtain Lagrangian velocity fields in a wide range of complex and turbulent flows [4], has been very recently applied to characterize fluid structures in the ascending aorta [5].Copyright