Nicolas Aristokleous

Variability of Computational Fluid Dynamics Solutions for Pressure and Flow in a Giant Aneurysm: The ASME 2012 Summer Bioengineering Conference CFD Challenge

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 choose their solver, discretization, and solution strategies. Variability was assessed by having each group interpolate their results onto a standardized mesh and centerline. For phase II, a physical model of the geometry was constructed, from which pressure and flow rates were measured. Groups repeated their simulations using a geometry reconstructed from a micro-computed tomography (CT) scan of the physical model with the measured flow rates and fluid properties. Phase I results from 25 groups demonstrated remarkable consistency in the pressure patterns, with the majority predicting peak systolic pressure drops within 8% of each other. Aneurysm sac flow patterns were more variable with only a few groups reporting peak systolic flow instabilities owing to their use of high temporal resolutions. Variability for phase II was comparable, and the median predicted pressure drops were within a few millimeters of mercury of the measured values but only after accounting for submillimeter errors in the reconstruction of the life-sized flow model from micro-CT. In summary, pressure can be predicted with consistency by CFD across a wide range of solvers and solution strategies, but this may not hold true for specific flow patterns or derived quantities. Future challenges are needed and should focus on hemodynamic quantities thought to be of clinical interest.

Effect of Posture Change on the Geometric Features of the Healthy Carotid Bifurcation

Segmented cross-sectional MRI images were used to construct 3-D virtual models of the carotid bifurcation in ten healthy volunteers. Geometric features, such as bifurcation angle, internal carotid artery (ICA) angle, planarity angle, asymmetry angle, tortuosity, curvature, bifurcation area ratio, ICA/common carotid artery (CCA), external carotid artery (ECA)/CCA, and ECA/ICA diameter ratios, were calculated for both carotids in two head postures: 1) the supine neutral position; and 2) the prone sleeping position with head rotation to the right (~80°). The results obtained have shown that head rotation causes 1) significant variations in bifurcation angle [32% mean increase for the right carotid (RC) and 21% mean decrease for the left carotid (LC)] and internal carotid artery angle (97% mean increase for the RC, 43% mean decrease for the LC); 2) a slight increase in planarity and asymmetry angles for both RC and LC; 3) minor and variable curvature changes for the CCA and for the branches; 4) slight tortuosity changes for the braches but not for the CCA; and 5) unsubstantial alterations in area and diameter ratios (percentage changes <;10%). The significant geometric changes observed in most subjects with head posture may also cause significant changes in bifurcation hemodynamics and warrant future investigation of the hemodynamic parameters related to the development of atherosclerotic disease such as low oscillating wall shear stress and particle residence times.

Effect of head posture on the healthy human carotid bifurcation hemodynamics

Head and neck postures may cause morphology changes to the geometry of the carotid bifurcation (CB) that alter the low and oscillating wall shear stress (WSS) regions previously reported as important in the development of atherosclerosis. Here the right and left CB were imaged by MRI in two healthy subjects in the neutral head posture with the subject in the supine position and in two other head postures with the subject in the prone position: (1) rightward rotation up to 80°, and (2) leftward rotation up to 80°. Image-based computational models were constructed to investigate the effect of posture on arterial geometry and local hemodynamics. The area exposure to unfavorable hemodynamics, based on thresholds set for oscillatory shear index (OSI), WSS and relative residence time, was used to quantify the hemodynamic impact on the wall. Torsion of the head was found to: (1) cause notable changes in the bifurcation and internal carotid artery angles and, in most cases, on cross-sectional area ratios for common, internal and external carotid artery, (2) change the spatial distribution of wall regions exposed to unfavorable hemodynamics, and (3) cause a marked change in the hemodynamic burden on the wall when the OSI was considered. These findings suggest that head posture may be associated with the genesis and development of atherosclerotic disease as well as complications in stenotic and stented vessels.

Impact of Head Rotation on the Individualized Common Carotid Flow and Carotid Bifurcation Hemodynamics

This paper aims at evaluating the changes that head rotation poses on morphological and flow characteristics of the carotid bifurcation (CB) and on the distribution of parameters that are regarded as important in atherosclerosis development, such as relative particle residence time (RRT), normalized oscillatory shear index (nOSI), and helicity (HL). Using a subject-specific approach, six healthy volunteers were MR-scanned in two head postures: supine neutral and prone with rightward head rotation. Cross-sectional flow velocity distribution was obtained using phase-contrast MRI at the common carotid artery (CCA). Our results indicate that peak systolic flow rate is reduced at the prone position in most cases for both CCAs. Morphological MR images are used to segment and construct the CB models. Numerical simulations are performed and areas exposed to high helicity or unfavorable hemodynamics are calculated. Head rotation affects the instantaneous spatial extent of high helicity regions. Posture-related observed differences in the distribution of nOSI and RRT suggest that inlet flow waveform tends to moderate geometry-induced changes in the qualitative and quantitative distribution of atherosclerosis-susceptible wall regions. Overall, presented results indicate that an individualized approach is required to fully assess the postural role in atherosclerosis development and in complications arising in stenotic and stented vessels.

The Effect of Head Rotation on the Geometry and Hemodynamics of Healthy Vertebral Arteries

AbstractThe geometric and hemodynamic characteristics of the left and right vertebral arteries (LVA, RVA) of six healthy volunteers were investigated for the supine (S) and the prone position (P) a common sleeping posture with head rotation. MRI images were used to reconstruct the subject specific three-dimensional solid models of the LVA and RVA from the level of the carotid bifurcation to the vertebrobasilar junction (VJ). Geometric parameters such as cross sectional area ratio, curvature, tortuosity and branch angle were estimated. MR-PCA was used to obtain the blood flow waveforms for the two positions and computational fluid dynamics (CFD) were used to assess the flow field in terms of wall shear stress (WSS) relative residence times (RRT) and localized normalized helicity (LNH). Significant geometric changes but moderate flow changes were observed for both vertebral arteries with head rotation. The CFD results at the VJ show that head rotation causes changes in the WSS distribution, RRT and LNH. Further studies are warranted to assess the clinical significance of the results in terms of atherosclerosis development at the VJ and how the observed geometric changes may affect blood flow to the brain in healthy subjects and vertebral artery stenosis patients, and in terms of increased rapture susceptibility in vertebrobasilar aneurysm patients.

Effect of posture change on the geometric features of the healthy carotid bifurcation

Segmented cross sectional MRI images were used to construct 3D virtual models of the carotid bifurcation in 5 healthy volunteers. Geometric features such as bifurcation angle, planarity angle, asymmetry angle tortuosity and curvature were calculated for the normal head posture and were compared to the equivalent values acquired with the head rotated clockwise by up to 80 degrees. The results obtained have shown that head rotation causes: 1) significant variations in bifurcation angle, planarity angle, asymmetry angle and internal carotid artery angle 2) tortuosity changes for the braches but not for the common carotid and 3) significant curvature changes for the common carotid artery (CCA) but not for the branches. The significant geometric changes observed in most subjects with head posture, may cause significant changes in hemodynamics and warrants future investigation of the hemodynamic parameters related to the development of atherosclerotic disease such as low oscillating wall shear stress and particle residence times.

Head Rotation Effects on the Flow and Hemodynamics of the Human Carotid Bifurcation

The use of realistic anatomic human carotid artery bifurcation (CB) models with a realistic blood waveform leads to physiologically relevant numerical simulations. To study the effects of head posture on the geometry and hemodynamics of the CB, Magnetic resonance imaging (MRI) was used on six healthy volunteers in two different head postures: 1) the supine neutral (N) and 2) the prone with rightward head rotation (P) up to 80°. Geometric differences with posture change in both the left (LCA) and right (RCA) carotid arteries were reported before [1]. The blood velocity waveform for each individual was obtained using phase-contrast MRI (PCMRI) at five diameters upstream of CB. Results have shown that peak systolic blood flow rate is reduced, in the prone position for both RCA and LCA in all six volunteers. To investigate the effects of the reduced peak systolic flow on the hemodynamics of the CB, numerical simulations were performed for a volunteer that exhibited the most geometric changes for the prone position in comparison to the other five based on specific geometric parameters [1, 2]. For the two investigated head postures the observed measured input waveforms were used.Copyright

Head posture influences the geometric and hemodynamic features on the healthy human carotid bifurcation

Atherosclerosis is the third leading cause of morbidity and mortality in the Western world. Low and oscillating wall shear stress (WSS) regions have been previously reported as parameters that correlate with the development of atherosclerosis. In this study we investigated geometric and hemodynamic changes in the carotid bifurcation as a result of posture change. Data from magnetic resonance imaging (MRI) were used to construct three dimensional (3D) surface models and computational fluid dynamic (CFD) fields. Two healthy volunteers were imaged by MRI in three different head postures: a) the supine neutral (N) head position, b) the prone position with leftward head rotation (LR) up to 80° and c) the prone position with rightward head rotation (RR) up to 80o. The area exposure to unfavorable hemodynamics, based on thresholds set for oscillatory shear index (OSI), WSS and relative residence times (RRT), was used to quantify the hemodynamic impact on the wall. Significant change in the hemodynamic burden on the wall was found for the OSI. The velocity profile at the common carotid artery (CCA) upstream of the carotid bifurcation (CB) was investigated at the supine and RR prone position for six healthy volunteers. The results indicated that blood flow rate decreased at peak systole, for the prone position for both the right and left CCAs.

CFD Challenge: Solutions Using the Commercial Finite Volume Solver, Fluent

This work is a collaborative effort between the Biomechanics and Living System Analysis Laboratory (BIOLISYS) in Cyprus and the Biomechanics Laboratory of IACM/FORTH in Greece. Both labs combine interdisciplinary skills from engineering, medicine and biology to provide solutions to clinical problems associated with cardiovascular and other diseases. For this study, numerical flow simulations were performed using: a) open source software VMTK and commercial software ICEM CFD as pre-processors, b) the finite volume based solver Fluent and c) Tecplot 360 (Amtec Inc.) for post-processing.Copyright

Head Rotation Influences the Geometric Features of the Stenotic Carotid Bifurcation

Previous investigators have reported that posture changes may influence the geometry and hemodynamics of the carotid bifurcation [1,2]. As a result, head rotation may cause geometric changes that alter the hemodynamic variables previously cited to correlate with the development of atherosclerosis. Glor et al. [1] had reported changes in the right carotid bifurcation geometry with leftward rotation of the head. Aristokleous et al. [2] have reported that geometric differences in the right and left carotid bifurcation occur with a rightward rotation of the head in 10 healthy volunteers [2]. Also, a similar study on bilateral head rotation on the left and right carotid bifurcation of two volunteers has shown similar results [3]. In this study a group of four patients with atherosclerotic disease in the carotid arteries was investigated at two head postures, a) the supine neutral and b) the prone sleeping with head rotation leftwards up to 80° to investigate the level of stenosis and the changes in geometric parameters with head rotation.© 2012 ASME