A fully automated artificial intelligence-driven software for planning of transcatheter aortic valve replacement.

BACKGROUND: Transcatheter aortic valve replacement (TAVR) is increasingly performed for the treatment of aortic stenosis. Computed tomography (CT) analysis is essential for pre-procedural planning. Currently available software packages for TAVR planning require substantial human interaction. We describe development and validation of an artificial intelligence (AI) powered software to automatically rend anatomical measurements and other information required for TAVR planning and implantation. METHODS: Automated measurements from 100 CTs were compared to measurements from three expert clinicians and TAVR operators using commercially available software packages. Correlation coefficients and mean differences were calculated to assess precision and accuracy. RESULTS: AI-generated annular measurements had excellent agreements with manual measurements by expert operators yielding correlation coefficients of 0.97 for both perimeter and area. There was no relevant bias with a mean difference of -0.07 mm and - 1.4 mm2 for perimeter and area, respectively. For the ascending aorta measured 5 cm above the annular plane, correlation coefficient was 0.95 and mean difference was 1.4 mm. Instruction for use-based sizing yielded agreement with the effective implant size in 87-88 % of patients for self-expanding valves (perimeter-based sizing) and in 88 % for balloon-expandable valves (area-based sizing). CONCLUSIONS: A fully automated software enables accurate and precise anatomical segmentation and measurements required for TAVR planning without human interaction and with high reliability.

In vitro investigation of the blood flow downstream of a 3D-printed aortic valve.

The hemodynamics in the aorta as well as the durability of aortic valve prostheses vary greatly between different types of devices. Although placement and sizing of surgical aortic valve prostheses are excellent, the valve geometry of common devices cannot be customized to fit the patient's anatomy perfectly. Similarly, transcatheter aortic valve implantation (TAVI) devices are not customizable and may be orientated unfavorably during implantation. Imperfect fit of an aortic valve prosthesis may result in suboptimal performance and in some cases the need for additional surgery. Leveraging the advent of precision, multi-material 3D-printing, a bioinspired silicone aortic valve was developed. The manufacturing technique makes it fully customizable and significantly cheaper to develop and produce than common prostheses. In this study, we assess the hemodynamic performance of such a 3D-printed aortic valve and compare it to two TAVI devices as well as to a severely stenosed valve. We investigate the blood flow distal to the valve in an anatomically accurate, compliant aorta model via three-dimensional particle tracking velocimetry measurements. Our results demonstrate that the 3D-printed aortic valve induces flow patterns and topology compatible with the TAVI valves and showing similarity to healthy aortic blood flow. Compared to the stenosis, the 3D-printed aortic valve reduces turbulent kinetic energy levels and irreversible energy losses by over 75%, reaching values compatible with healthy subjects and conventional TAVIs. Our study substantiates that the 3D-printed heart valve displays a hemodynamic performance similar to established devices and underscores its potential for driving innovation towards patient specific valve prostheses.

A comparative study on the analysis of hemodynamics in the athlete's heart.

The pathophysiological mechanisms underlying the development of the athlete's heart are still poorly understood. To characterize the intracavitary blood flows in the right ventricle (RV) and right-ventricular outflow tract (RVOT) in 2 healthy probands, patients with arrhythmogenic right ventricular cardiomyopathy (ARVC) and 2 endurance athletes, we performed 4D-MRI flow measurements to assess differences in kinetic energy and shear stresses. Time evolution of velocity magnitude, mean kinetic energy (MKE), turbulent kinetic energy (TKE) and viscous shear stress (VSS) were measured both along the whole RV and in the RVOT. RVOT regions had higher kinetic energy values and higher shear stresses levels compared to the global averaging over RV among all subjects. Endurance athletes had relatively lower kinetic energy and shear stresses in the RVOT regions compared to both healthy probands and ARVC patients. The athlete's heart is characterized by lower kinetic energy and shear stresses in the RVOT, which might be explained by a higher diastolic compliance of the RV.

Turbulent Kinetic Energy Loss and Shear Stresses Before and After Transcatheter Aortic Valve Replacement.

Blood flow and shear stresses were quantified using 4-dimensional flow cardiac magnetic resonance and 3-dimensional particle velocimetry before and after transcatheter aortic valve replacement (TAVR). TAVR reduced turbulent kinetic energy by 47% and shear stresses by 33%, illustrating that the benefit of TAVR extends beyond a simple reduction in transvalvular gradients. (Level of Difficulty: Advanced.).

Toward an accurate estimation of wall shear stress from 4D flow magnetic resonance downstream of a severe stenosis.

PURPOSE: First, to investigate the agreement between velocity, velocity gradient, and Reynolds stress obtained from four-dimensional flow magnetic resonance (4D flow MRI) measurements and direct numerical simulation (DNS). Second, to propose and optimize based on DNS, 2 alternative methods for the accurate estimation of wall shear stress (WSS) when the resolution of the flow measurements is limited. Thirdly, to validate the 2 methods based on 4D flow MRI data. METHODS: In vitro 4D MRI has been conducted in a realistic rigid stenosed aorta model under a constant flow rate of 12 L/min. A DNS of transitional stenotic flow has been performed using the same geometry and boundary conditions. RESULTS: Time-averaged velocity and Reynolds stresses are in good agreement between in vitro 4D MRI data and DNS (errors between 2% and 8% of the reference downsampled data). WSS estimation based on the 2 proposed methods applied to MRI data provide good agreement with DNS for slice-averaged values (maximum error is less than 15% of the mean reference WSS for the first method and 25% for the second method). The performance of both models is not strongly sensitive to spatial resolution up to 1.5 mm voxel size. While the performance of model 1 deteriorates appreciably at low signal-to-noise ratios, model 2 remains robust. CONCLUSIONS: The 2 methods for WSS magnitude give an overall better agreement than the standard approach used in the literature based on direct calculation of the velocity gradient close to the wall (relative error of 84%).

A Novel Estimation Approach of Pressure Gradient and Haemodynamic Stresses as Indicators of Pathological Aortic Flow Using Subvoxel Modelling.

OBJECTIVE: The flow downstream from aortic stenoses is characterised by the onset of shear-induced turbulence that leads to irreversible pressure losses. These extra losses represent an increased resistance that impacts cardiac efficiency. A novel approach is suggested in this study to accurately evaluate the pressure gradient profile along the aorta centreline using modelling of haemodynamic stress at scales that are smaller than the typical resolution achieved in experiments. METHODS: We use benchmark data obtained from direct numerical simulation (DNS) along with results from in silico and in vitro three-dimensional particle tracking velocimetry (3D-PTV) at three voxel sizes, namely 750  µm, 1 mm and 1.5 mm. A differential equation is derived for the pressure gradient, and the subvoxel-scale (SVS) stresses are closed using the Smagorinsky and a new refined model. Model constants are optimised using DNS and in silico PTV data and validated based on pulsatile in vitro 3D-PTV data and pressure catheter measurements. RESULTS: The Smagorinsky-based model was found to be more accurate for SVS stress estimation but also more sensitive to errors especially at lower resolution, whereas the new model was found to more accurately estimate the projected pressure gradient even for larger voxel size of 1.5 mm albeit at the cost of increased sensitivity at this voxel size. A comparison with other methods in the literature shows that the new approach applied to in vitro PTV measurements estimates the irreversible pressure drop by decreasing the errors by at least 20%. CONCLUSION: Our novel approach based on the modelling of subvoxel stress offers a validated and more accurate way to estimate pressure gradient, irreversible pressure loss and SVS stress. SIGNIFICANCE: We anticipate that the approach may potentially be applied to image-based in vivo, in vitro 4D flow data or in silico data with limited spatial resolution to assess pressure loss and SVS stresses in disturbed aortic blood flow.

Assessment of the Flow Field in the HeartMate 3 Using Three-Dimensional Particle Tracking Velocimetry and Comparison to Computational Fluid Dynamics.

Flow fields in rotary blood pumps (RBPs) have a significant influence on hemocompatibility. Because flow characteristics vary with flow rate, different operating conditions play a role. Furthermore, turbulence is crucial in the evaluation of blood damage potential, but the level of turbulence in implantable RBPs is still unknown. In this study, we addressed both research aspects and for the first time measured turbulent flow fields in the HeartMate 3 (HM3) at different operating flows. The averaged, three-dimensional velocity field including fluctuating velocity components in a HM3 with a transparent lower housing was measured using three-dimensional particle tracking velocimetry (3D-PTV). In vitro results were compared with computational fluid dynamic (CFD) simulations for two flow cases, representing the lower and upper physiologic flow range (2.7 and 5.7 L/min), using two different turbulence models that account for fluctuating velocity fields: the k-ω shear stress transport and the Reynolds stress model (RSM). The measurements revealed higher mean and turbulent kinetic energies (TKEs) for the low-flow condition especially within the gap beneath the impeller. Computed mean fields agree well with 3D-PTV for both models, but the RSM predicts the TKE levels better than the k-ω model. Computational fluid dynamic results further show wall shear stresses higher than 150 Pa, a commonly used damage threshold, in the bottom gap for the lower flow condition. In conclusion, the low-flow condition was found to be more prone to blood damage. Furthermore, CFD predictions for turbulence must be carefully experimentally validated.

Performance analysis of the transcatheter aortic valve implantation on blood flow hemodynamics: An optical imaging-based in vitro study.

Cardiac implants may have a strong influence on the hemodynamics of the circulatory system. In this study, we aimed at investigating the impact of transcatheter aortic valve implantation (TAVI) devices on blood flow patterns that develop in the ascending aorta under physiological flow conditions in vitro. For this purpose, a noninvasive optical measurement tool, three-dimensional particle tracking velocimetry (3D-PTV), was used in a realistic compliant silicone aortic model. The performance and the influence of two TAVIs and one surgical valve on the aortic flow were investigated. Our results showed that valve design and materials may have a distinct influence on relevant hemodynamic properties, namely kinetic energy, production of turbulence, and shear stresses in the ascending aorta. All properties varied considerably between the different valve models. We found that the total aortic regurgitation composed of the closing volume, transvalvular and paravalvular leakages varied for the three valves investigated. Furthermore, peak mean kinetic energy (MKE) ranged from 61 to 116 J/m3 , whereas peak turbulent kinetic energy (TKE) ranged from 23 to 36 J/m3 . The analysis of shear showed that all the three studied devices had minimal overall risk for thrombus formation. We conclude that the characteristics and material designs of TAVI devices have strong influences on the hemodynamics in the ascending aorta.

Comprehensive In Vitro Study of the Flow Past Two Transcatheter Aortic Valves: Comparison with a Severe Stenotic Case.

We investigate the flow past two transcatheter aortic valves (TAVs) and one severely calcified valve in an anatomically realistic aorta geometry to evaluate the ability of the TAVs to establish a healthier aortic flow compared to a diseased case. Velocity measurements of pulsatile flow are carried out using the 3D-particle tracking velocimetry technique. We present a novel approach based on the Smagorinsky model to assess the important subvoxel-scale (here smaller than 750 [Formula: see text]m) shear stress contribution that is usually unavailable in experiments. Both TAV models feature a small retrograde flow of about 5% of the stroke volume and a lower number of coherent vortical structures. Turbulence past the TAVs is strongly suppressed as evidenced by the lower levels of turbulent kinetic energy even though the newer generation TAV performs better than the old one. Also lysis indices are substantially reduced in both models. The new generation TAV displays a slightly higher risk for thrombogenicity due to longer exposure times. We anticipate that our new approach to include turbulence and shear stress related quantities may help to validate the design of cardiovascular devices.

A Novel Approach for 3D-Structural Identification through Video Recording: Magnified Tracking.

Advancements in optical imaging devices and computer vision algorithms allow the exploration of novel diagnostic techniques for use within engineering systems. A recent field of application lies in the adoption of such devices for non-contact vibrational response recordings of structures, allowing high spatial density measurements without the burden of heavy cabling associated with conventional technologies. This, however, is not a straightforward task due to the typically low-amplitude displacement response of structures under ambient operational conditions. A novel framework, namely Magnified Tracking (MT), is proposed herein to overcome this limitation through the synergistic use of two computer vision techniques. The recently proposed phase-based motion magnification (PBMM) framework, for amplifying motion in a video within a defined frequency band, is coupled with motion tracking by means of particle tracking velocimetry (PTV). An experimental campaign was conducted to validate a proof-of-concept, where the dynamic response of a shear frame was measured both by conventional sensors as well as a video camera setup, and cross-compared to prove the feasibility of the proposed non-contact approach. The methodology was explored both in 2D and 3D configurations, with PTV revealing a powerful tool for the measurement of perceptible motion. When MT is utilized for tracking "imperceptible" structural responses (i.e., below PTV sensitivity), via the use of PBMM around the resonant frequencies of the structure, the amplified motion reveals the operational deflection shapes, which are otherwise intractable. The modal results extracted from the magnified videos, using PTV, demonstrate MT to be a viable non-contact alternative for 3D modal identification with the benefit of a spatially dense measurement grid.

Advances in Biological Liquid Crystals.

Biological liquid crystals, a rich set of soft materials with rod-like structures widely existing in nature, possess typical lyotropic liquid crystalline phase properties both in vitro (e.g., cellulose, peptides, and protein assemblies) and in vivo (e.g., cellular lipid membrane, packed DNA in bacteria, and aligned fibroblasts). Given the ability to undergo phase transition in response to various stimuli, numerous practices are exercised to spatially arrange biological liquid crystals. Here, a fundamental understanding of interactions between rod-shaped biological building blocks and their orientational ordering across multiple length scales is addressed. Discussions are made with regard to the dependence of physical properties of nonmotile objects on the first-order phase transition and the coexistence of multi-phases in passive liquid crystalline systems. This work also focuses on how the applied physical stimuli drives the reorganization of constituent passive particles for a new steady-state alignment. A number of recent progresses in the dynamics behaviors of active liquid crystals are presented, and particular attention is given to those self-propelled animate elements, like the formation of motile topological defects, active turbulence, correlation of orientational ordering, and cellular functions. Finally, future implications and potential applications of the biological liquid crystalline materials are discussed.

Hemodynamic Changes in the Right Ventricle Induced by Variations of Cardiac Output: A Possible Mechanism for Arrhythmia Occurrence in the Outflow Tract.

The rationale of this paper is to investigate right ventricular (RV) hemodynamics in relation to changes in cardiac output, and in particular to study exercise-induced stresses at the RV outflow tract (RVOT), which is a common site of ventricular arrhythmias in the athlete's heart. We hypothesize that the thin-walled RVOT is exposed to high wall shear stresses (WSS) during physiological states associated with high cardiac output such as exercise, and therefore, may be particularly prone to substrate formation leading to ventricular tachyarrhythmias. 3D Particle Tracking Velocimetry (3D-PTV), an optical imaging method, has been performed in a novel anatomically accurate compliant silicone right heart model derived from a high resolution MRI heart scan of a healthy male proband. RV and RVOT flow patterns at resting conditions were obtained from two healthy athletic male proband's hearts and two patients with arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) via phase contrast magnetic resonance imaging (PC-MRI). The healthy case was used as a reference for validating the in vitro flow patterns of the silicone model, while the diseased cases were used to generalize our findings and investigate possible changes in hemodynamic stresses with RV morphological remodelling. Our results showed that both healthy and diseased geometries consistently displayed an increased WSS in the RVOT relative to the rest of the RV. We found that increases in cardiac output may lead to increases of mean kinetic energy (MKE), laminar viscous dissipation and WSS at the RVOT. Furthermore, higher peak WSS magnitudes were found for the diseased cases. The identified high WSS regions may correlate with the common site of RVOT ventricular tachycardia in athletes and patients with ARVC/D. Our results imply that exercise, as well as anatomical and functional remodeling might alter RV wall shear stress both in magnitude and spatial distribution, leading to increased hemodynamic stresses in the RVOT.

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.

Shear-scaling-based approach for irreversible energy loss estimation in stenotic aortic flow - An in vitro study.

Today, the functional and risk assessment of stenosed arteries is mostly based on ultrasound Doppler blood flow velocity measurements or catheter pressure measurements, which rely on several assumptions. Alternatively, blood velocity including turbulent kinetic energy (TKE) may be measured using MRI. The aim of the present study is to validate a TKE-based approach that relies on the fact that turbulence production is dominated by the flow's shear to determine the total irreversible energy loss from MRI scans. Three-dimensional particle tracking velocimetry (3D-PTV) and phase-contrast magnetic resonance imaging (PC-MRI) simulations were performed in an anatomically accurate, compliant, silicon aortic phantom. We found that measuring only the laminar viscous losses does not reflect the true losses of stenotic flows since the contribution of the turbulent losses to the total loss become more dominant for more severe stenosis types (for example, the laminar loss is 0.0094±0.0015W and the turbulent loss is 0.0361±0.0015W for the Remax=13,800 case, where Remax is the Reynolds number based on the velocity in the vena-contracta). We show that the commonly used simplified and modified Bernoulli's approaches overestimate the total loss, while the new TKE-based method proposed here, referred to as "shear scaling" approach, results in a good agreement between 3D-PTV and simulated PC-MRI (mean error is around 10%). In addition, we validated the shear scaling approach on a geometry with post-stenotic dilatation using numerical data by Casas et al. (2016). The shear scaling-based method may hence be an interesting alternative for irreversible energy loss estimation to replace traditional approaches for clinical use. We expect that our results will evoke further research, in particular patient studies for clinical implementation of the new method.

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

PURPOSE: 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. THEORY AND METHODS: High-resolution particle tracking velocimetry data obtained in a realistic aortic phantom with stenotic flow were used to simulate PC-MRI measurements at different resolutions and noise levels. Laminar viscous losses were computed using the spatial gradients of the mean velocity vector field, and TKE levels were derived based on the intravoxel phase dispersion of flow-sensitized PC-MRI measurements. RESULTS: Increasing the voxel size from 0.625 to 2.5 mm resulted in an underestimation of viscous losses of up to 83%, whereas total TKE values showed errors of <15% and reduced sensitivity to voxel size. Relative errors in viscous loss quantification were found to be less dependent on noise levels when compared with TKE values. In general, a SNR of 20-30 is required for both methods. CONCLUSION: At spatial resolutions feasible in clinical three-dimensional PC-MRI measurements, viscous losses of stenotic flows are significantly underestimated, whereas TKE shows smaller errors and reduced sensitivity to spatial resolution. Magn Reson Med 76:191-196, 2016. © 2015 Wiley Periodicals, Inc.

An in vitro investigation of the influence of stenosis severity on the flow in the ascending aorta.

Cardiovascular diseases can lead to abnormal blood flows, some of which are linked to hemolysis and thrombus formation. Abnormal turbulent flows of blood in the vessels with stenosis create strong shear stresses on blood elements and may cause blood cell destruction or platelet activation. We implemented a Lagrangian (following the fluid elements) measurement technique of three dimensional particle tracking velocimetry that provides insight on the evolution of viscous and turbulent stresses along blood element trajectories. We apply this method to study a pulsatile flow in a compliant phantom of an aorta and compare the results in three cases: the reference case (called "healthy" case), and two cases of abnormal flows due to mild and severe stenosis, respectively. The chosen conditions can mimic a clinical application of an abnormal flow due to a calcific valve. We estimate the effect of aortic stenosis on the kinetic energy of the mean flow and the turbulent kinetic energy, which increases about two orders of magnitude as compared with the healthy flow case. Measuring the total flow stress acting on a moving fluid element that incorporates viscous stresses and the apparent turbulent-induced stresses (the so-called Reynolds stresses) we find out similar increase of the stresses with the increased severity of the stenosis. Furthermore, these unique Lagrangian measurements provide full acceleration and, consequently, the forces acting on the blood elements that are estimated to reach the level that can considerably deform red blood cells. These forces are strong and abrupt due to the contribution of the turbulent fluctuations which is much stronger than the typically measured phase-averaged values.

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.

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.