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Hemodynamics and Transport in Patient-specific Abdominal Aortic Aneurysms

01 Jan 2016-
TL;DR: Arzani et al. as mentioned in this paper investigated the role of hemodynamics in AAA progression, complex vectorial wall shear stress (WSS) patterns, and near-wall transport in abdominal aorta.
Abstract: Author(s): Arzani, Amirhossein | Advisor(s): Shadden, shawn C. | Abstract: Abdominal aortic aneurysm (AAA) is a permanent local enlargement of the abdominal aorta. Complex anatomies, presence of side branches, and pulsatility of blood flow creates a complex chaotic flow field in AAAs. The progression of AAA can lead to rupture, which is one of the leading causes of death in the elderly. In this study, the flow topology in AAAs, role of hemodynamics in AAA progression, complex vectorial wall shear stress (WSS) patterns, and near-wall transport in AAAs were investigated.Patient-specific computational fluid dynamics (CFD) was used to obtain blood flow information. Lagrangian coherent structures (LCS) were computed to study the flow physics. The utility of these structures in studying chaotic mixing and transport, flow separation, and vortex wall interaction was demonstrated in different patients. The effect of exercise on flow topology and quantitative mixing was evaluated. The evolution of a systolic vortex formed in the proximal region, strongly influenced the flow topology in the aneurysms. Intraluminal thrombus (ILT) deposition and lumen progression were quantified in several patients using magnetic resonance imaging over a 2--3 year followup. Point-wise spatial correlation of hemodynamic parameters to ILT deposition, revealed a negative correlation between oscillatory shear stress and ILT deposition. This was attributed to persistence recirculation, which can lead to unidirectional backward WSS. Complex vectorial variations in WSS was studied. Namely, variations in WSS magnitude, direction, and vector in space and time were quantified and compared. Several new WSS measures were introduced to better quantify WSS vectorial variations. The concept of Lagrangian wall shear stress structures (WSS LCS) was introduced. WSS was scaled to obtain a first order representation of near-wall velocity. Tracers representing biochemicals in thin concentration boundary layers were tracked on the aneurysm surface based on the WSS vector field. Formation of coherent structures from WSS tracers were shown. The WSS LCS organize near-wall transport in high Schmidt number flows and could be used to predict regions of high near-wall stagnation and concentration. A wall shear stress exposure time (WSSET) measure was introduced to quantify near-wall stagnation and concentration. Excellent agreement between WSSET and surface concentration obtained from 3D continuum mass transport was obtained. Finally, the important roles that WSS fixed points play in cardiovascular flows was discussed.
Citations
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Journal Article
TL;DR: It is shown that chaotic advection is inherent to flow through all types of porous media, from granular and packed media to fractured and open networks, and has significant implications for the description of transport, mixing, chemical reaction and biological activity in porous media.
Abstract: We show that chaotic advection is inherent to flow through all types of porous media, from granular and packed media to fractured and open networks. The basic topological complexity inherent to all porous media gives rise to chaotic flow dynamics under steady flow conditions, where fluid deformation local to stagnation points imparts a 3D fluid mechanical analog of the baker's map. The ubiquitous nature of chaotic advection has significant implications for the description of transport, mixing, chemical reaction and biological activity in porous media.

17 citations

01 Jan 1995
TL;DR: In this article, heat and mass transfer in the recirculation region of a pipe under steady and pulsatile conditions were studied under uniform and parabolic entrance velocity profiles and the results demonstrate the complexity of separation flows and identify characteristic regions of high and low heat/mass transfer.
Abstract: Abstract Heat and mass transfer phenomena were studied in the sudden expansion region of a pipe under steady and pulsatile conditions. The Prandtl number was varied from 100 to 12 000 and the flow was characterized for both uniform and parabolic entrance velocity profiles. A uniform velocity profile was used for pulsatile flow. It was found that heat transfer in the recirculation region was maximal near the area where wall shear was minimal. Blunting of the inlet profile caused the point of maximum heat transfer to move upstream. There was a nonlinear effect of Prandtl number on heat transfer which plateaued for Pr > 10 3 . The wall shear rate in the separation zone varied markedly with pulsatile flows, but the wall heat transfer remained relatively constant. The time-averaged pulsatile heat transfer at the wall was approximately the same as with steady flow with the mean Reynolds number. However, the isotherms within the pulsatile flow were markedly different from steady flow. The results demonstrate the complexity of separation flows and identify characteristic regions of high and low heat/mass transfer for high Prandtl/Schmidt pulsatile flow.

4 citations

References
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Journal ArticleDOI
TL;DR: The methods and software engineering philosophy behind this new tool, ITK-SNAP, are described and the results of validation experiments performed in the context of an ongoing child autism neuroimaging study are provided, finding that SNAP is a highly reliable and efficient alternative to manual tracing.

6,669 citations


Additional excerpts

  • ...Segmentations were performed using ITK-Snap [294]....

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Journal ArticleDOI
TL;DR: In this article, a new finite element formulation for convection dominated flows is developed, based on the streamline upwind concept, which provides an accurate multidimensional generalization of optimal one-dimensional upwind schemes.

5,157 citations


"Hemodynamics and Transport in Patie..." refers methods in this paper

  • ...A finite element method was used to solve the equation using streamline upwind/Petrov-Galerkin (SUPG) formulation [36, 24], implemented in the FEniCS package [158]....

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  • ...The Navier-Stokes equations were discretized using a streamline-upwind Petrov-Galerkin finite element formulation [36] with same order interpolation for velocity and pressure....

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Journal ArticleDOI
TL;DR: The transmission of hemodynamic forces throughout the endothelium and the mechanotransduction mechanisms that lead to biophysical, biochemical, and gene regulatory responses of endothelial cells to hemodynamic shear stresses are reviewed.
Abstract: Mechanical forces associated with blood flow play important roles in the acute control of vascular tone, the regulation of arterial structure and remodeling, and the localization of atherosclerotic lesions. Major regulation of the blood vessel responses occurs by the action of hemodynamic shear stresses on the endothelium. The transmission of hemodynamic forces throughout the endothelium and the mechanotransduction mechanisms that lead to biophysical, biochemical, and gene regulatory responses of endothelial cells to hemodynamic shear stresses are reviewed.

2,719 citations


"Hemodynamics and Transport in Patie..." refers background in this paper

  • ...In the indirect mechanism, agonists in the blood flow interact with ECs to activate various responses [72]....

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Journal ArticleDOI
TL;DR: These studies confirm earlier findings under steady flow conditions that plaques tend to form in areas of low, rather than high, shear stress, but indicate in addition that marked oscillations in the direction of wall shear may enhance atherogenesis.
Abstract: Fluid velocities were measured by laser Doppler velocimetry under conditions of pulsatile flow in a scale model of the human carotid bifurcation. Flow velocity and wall shear stress at five axial and four circumferential positions were compared with intimal plaque thickness at corresponding locations in carotid bifurcations obtained from cadavers. Velocities and wall shear stresses during diastole were similar to those found previously under steady flow conditions, but these quantities oscillated in both magnitude and direction during the systolic phase. At the inner wall of the internal carotid sinus, in the region of the flow divider, wall shear stress was highest (systole = 41 dynes/cm2, diastole = 10 dynes/cm2, mean = 17 dynes/cm2) and remained unidirectional during systole. Intimal thickening in this location was minimal. At the outer wall of the carotid sinus where intimal plaques were thickest, mean shear stress was low (-0.5 dynes/cm2) but the instantaneous shear stress oscillated between -7 and +4 dynes/cm2. Along the side walls of the sinus, intimal plaque thickness was greater than in the region of the flow divider and circumferential oscillations of shear stress were prominent. With all 20 axial and circumferential measurement locations considered, strong correlations were found between intimal thickness and the reciprocal of maximum shear stress (r = 0.90, p less than 0.0005) or the reciprocal of mean shear stress (r = 0.82, p less than 0.001). An index which takes into account oscillations of wall shear also correlated strongly with intimal thickness (r = 0.82, p less than 0.001). When only the inner wall and outer wall positions were taken into account, correlations of lesion thickness with the inverse of maximum wall shear and mean wall shear were 0.94 (p less than 0.001) and 0.95 (p less than 0.001), respectively, and with the oscillatory shear index, 0.93 (p less than 0.001). These studies confirm earlier findings under steady flow conditions that plaques tend to form in areas of low, rather than high, shear stress, but indicate in addition that marked oscillations in the direction of wall shear may enhance atherogenesis.

2,623 citations


"Hemodynamics and Transport in Patie..." refers background or methods in this paper

  • ...OSI [135] is a leading WSS measure that has been widely used to characterize oscillations in the WSS vector field....

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  • ...In addition, reduced retrograde flow, higher mean wall shear stress, lower oscillatory shear index (OSI), and a decreased diastolic length resulting from exercise [53, 252] may be beneficial in preventing or slowing AAA....

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  • ...Les et al. [151] compared WSS, OSI, and turbulent kinetic energy between different patient specific AAA models during rest and exercise....

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  • ...Oscillatory shear index (OSI) has been correlated with plaque formation [135, 133]....

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Book
01 May 1989
TL;DR: In this article, the authors present a list of frequently used symbols for chaotic flows and their application in different types of chaotic flows, such as mixing and chaos in two-dimensional time-periodic flows, three-dimensional and open flows, and Hamiltonian systems.
Abstract: Preface Acknowledgments 1. Introduction 2. Flow, trajectories and deformation 3. Conservation equations, change of frame, and vorticity 4. Computation of stretching and efficiency 5. Chaos in dynamical systems 6. Chaos in Hamiltonian systems 7. Mixing and chaos in two-dimensional time-periodic flows 8. Mixing and chaos in three-dimensional and open flows 9. Epilogue: diffusion and reaction in lamellar structures and microstructures in chaotic flows Appendix List of frequently used symbols References Author index Subject index.

2,107 citations


"Hemodynamics and Transport in Patie..." refers background in this paper

  • ...While defining when a flow is chaotic is a delicate task [37], chaotic flows are generally governed by homoclinic and heteroclinic tangles of invariant manifolds between hyperbolic sets [190]....

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