Biorheology - Volume 39, issue 3-4

Authors: Lu, Yiling | Lu, Xiyun | Zhuang, Lixian | Wang, Wen

Article Type: Research Article

Abstract: Non‐planarity in blood vessels is known to influence arterial flows and wall shear stress. To gain insight, computational fluid dynamics (CFD) has been used to investigate effects of curvature and out‐of‐plane geometry on the distribution of fluid flows and wall shear stresses in a hypothetical non‐planar bifurcation. Three‐dimensional Navier–Stokes equations for a steady state Newtonian fluid were solved numerically using a finite element method. Non‐planarity in one of the two daughter vessels is found to deflect flow from the inner wall of the vessel to the outer wall and to cause changes in the distribution of wall shear stresses. Results …from this study agree to experimental observations and CFD simulations in the literature, and support the view that non‐planarity in blood vessels is a factor with important haemodynamic significance and may play a key role in vascular biology and pathophysiology. Show more

Keywords: Wall shear stress, non‐planar bifurcation, CFD modeling

Citation: Biorheology, vol. 39, no. 3-4, pp. 431-436, 2002

Authors: Mallinger, Francois | Drikakis, Dimitris

Article Type: Research Article

Abstract: Laminar‐to‐turbulent transition in pulsatile flow through a stenosis is studied by means of three‐dimensional numerical simulations. The flow transition is associated with the occurrence of a flow instability initiating in the stenosis region. The instability is manifested by a three‐dimensional symmetry‐breaking and leads to asymmetric separation and intense swirling motion downstream of the stenosis. The above have profound effects on the wall shear stress (WSS). The simulations reveal that the asymmetric separation is extended several radii downstream of the stenosis with substantial WSS fluctuations, in both space and time, occurring in the poststenotic region.

Keywords: Stenotic vessels, atherosclerosis, blood flow, instability, simulation

Citation: Biorheology, vol. 39, no. 3-4, pp. 437-441, 2002

Authors: Thomas, Jonathan B. | Milner, Jaques S. | Steinman, David A.

Article Type: Research Article

Abstract: The human carotid artery bifurcation is a complex, three‐dimensional structure exhibiting non‐planarity and both in‐ and out‐of‐plane curvature. The aim of this study was to determine the relative importance of vessel planarity, a potential geometric risk factor for atherogenesis, in determining the local hemodynamics. A combination of computational fluid dynamics and magnetic resonance imaging was used to reconstruct the subject‐specific hemodynamics for three subjects. Planar models were then constructed by translating the centroids of the lumen contours onto a plane defined by the centroids of the vessel branches near the bifurcation apex. A novel “patching” technique was used to convert …the continuous arterial surfaces into contiguous but discrete patches according to an objective scheme, making it possible to compare the original and planar models without the need for registration and warping. Results suggest that the planarity of the vessel has a relatively minor effect on the spatial distribution of mean and oscillatory wall shear stress. Out‐of‐plane curvature was, however, found to have a marked influence on the extent and magnitude of these hemodynamic variables. We conclude that vessel curvature – whether in‐ or out‐of‐plane – rather than planarity may deserve further scrutiny as a potential geometric risk for atherogenesis. Show more

Keywords: Computational fluid dynamics, magnetic resonance imaging, atherosclerosis, geometric risk

Citation: Biorheology, vol. 39, no. 3-4, pp. 443-448, 2002

Authors: Hoskins, P.R.

Article Type: Research Article

Abstract: This article will review the ability of ultrasound techniques to provide 3D information on arterial geometry, blood flow and tissue motion. 3D systems. 3D datasets can be obtained by sequential acquisition of 2D slices. Ideally a transducer is required in which there is full control of beam steering within a 3D volume. This requires a 2D array consisting of several thousand elements. Prototype 2D arrays have been built which provide several 3D datasets per second. Blood velocity measurement. Current Doppler systems estimate only the component of velocity in the direction of the Doppler beam. Lack of knowledge of …the direction of blood motion and also other effects associated with ‘spectral broadening’ limit the accuracy of velocity measurement. Improved accuracy can be obtained using vector Doppler systems using 2 or 3 beam directions; this approach is referred to as ‘vector Doppler’. Tissue motion. Doppler techniques can also be used to detect tissue motion (Tissue Doppler Imaging or TDI). Motion of the artery wall can be calculated from the TDI images. It is possible to estimate simultaneously motion for adjacent diameters within the longitudinal plane, and to visualise the relative motion at different parts of the wall. Show more

Keywords: Doppler ultrasound, velocity measurement, vector Doppler, 2D arrays, 3D

Citation: Biorheology, vol. 39, no. 3-4, pp. 451-459, 2002

Authors: How, T.V. | Fisher, R.K. | Hoedt, M.T.C. | Brennan, J. | Harris, P.L.

Article Type: Research Article

Abstract: Clinical evidence suggests that the development of myointimal hyperplasia in prosthetic femorodistal bypass grafts may be reduced by the interposition of a cuff of autologous vein between the graft and the recipient artery. Previous experimental work has shown that some of the benefits may be attributed to the geometry of the cuffed anastomosis. Since the distal anastomosis in vivo is often non‐planar we have carried out a preliminary study in a model where the graft is at an angle of 45° to the anterior–posterior plane of the anastomosis. This out‐of‐plane angulation produces highly asymmetric flow patterns in the anastomosis with …significant flow separation on the ipsilateral side of the cuff. In the proximal and distal outflow, however, the velocity vectors show significant helical motion with temporal instability in the distal outflow. Show more

Keywords: Miller cuff, haemodynamics, intimal hyperplasia, prosthetic grafts

Citation: Biorheology, vol. 39, no. 3-4, pp. 461-465, 2002

Authors: Long, Quan | Xu, X. Yun | Köhler, Uwe | Robertson, Malcolm B. | Marshall, Ian | Hoskins, Peter

Article Type: Research Article

Abstract: Steady flow of a blood mimicking fluid in a physiologically realistic model of the human carotid bifurcation was studied using both magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) modelling techniques. Quantitative comparisons of the 3D velocity field in the bifurcation phantom were made between phase contrast MRI measurements and CFD predictions. The geometry for the CFD model was reconstructed from T1 weighted MR imaging of the test phantom. It was found that the predicted velocity fields were in fair agreement with MR measured velocities. In both the internal and external carotid arteries, the agreement between CFD predictions …and MRI measurements was better along the inner–outer wall axis with a correlation factor C>0.897 (average 0.939) where the velocity profiles were skewed, than along the anterior–posterior axis (average correlation factor 0.876) where the velocity profiles were in M‐shape. Show more

Keywords: Carotid bifurcation, magnetic resonance imaging, computational fluid dynamics, quantitative comparison, velocity field

Citation: Biorheology, vol. 39, no. 3-4, pp. 467-474, 2002

Authors: Walker, P.G. | Alshorman, A.A. | Westwood, S. | David, T.

Article Type: Research Article

Abstract: Leukocyte recruitment from blood to the endothelium plays an important role in atherosclerotic plaque formation. Cells show a primary and secondary adhesive process with primary bonds responsible for capture and rolling and secondary bonds for arrest. Our objective was to investigate the role played by this process on the adhesion of leukocytes in complex flow. Cells were modelled as rigid spheres with spring like adhesion molecules which formed bonds with endothelial receptors. Models of bond kinetics and Newton's laws of motion were solved numerically to determine cell motion. Fluid force was obtained from the local shear rate obtained from a …CFD simulation of the flow over a backward facing step. In stagnation point flow the shear rate near the stagnation point has a large gradient such that adherent cells in this region roll to a high shear region preventing permanent adhesion. This is enhanced if a small time dependent perturbation is imposed upon the stagnation point. For lower shear rates the cell rolling velocity may be such that secondary bonds have time to form. These bonds resist the lower fluid forces and consequently there is a relatively large permanent adhesion region. Show more

Keywords: Atherosclerosis, hemodynamics, mathematical modelling, rolling, CFD

Citation: Biorheology, vol. 39, no. 3-4, pp. 475-481, 2002

Authors: Affeld, K. | Kertzscher, U. | Goubergrits, L.

Article Type: Research Article

Abstract: The blood flow immediately adjacent to the wall of a blood vessel or an artificial surface is of great interest. This flow defines the shear stress at the wall and is known to have a great physiological importance. The use of models is a viable method to investigate this flow. However, even in models the shear stress at the wall is difficult to assess. A new optical method is based on transparent models and uses particles in the model fluid, which are only visible near the wall. This is achieved with a model fluid having a defined opacity. This fluid …obscures particles in the center of the models, but permits the observation and recording of particles close to the wall. The method has been applied for Hagen–Poiseuille flow and for the likewise well researched flow in a tube with a sudden expansion. Show more

Keywords: Wall shear stress, wall shear rate, blood wall interaction, Virchow's triad, PIV

Citation: Biorheology, vol. 39, no. 3-4, pp. 485-489, 2002

Authors: Angelini, Gianni D. | Jeremy, Jamie Y.

Article Type: Research Article

Abstract: Coronary artery bypass graft surgery (CABG) is widely used for the treatment of atheromatous stenosis of coronary arteries. However, as many as 50% of grafts fail within 10 years after CABG due to neointima (NI) formation, a process involving the proliferation and migration of vascular smooth muscle cells (VSMCs). Superimposed on neointima formation is accelerated atherogenesis which ultimately results in late vein graft failure. To date no therapeutic intervention has proved successful in treating late vein graft failure and as such is a matter of some urgency. However, in recent years, several diverse approaches aimed at preventing neointimal formation have …been devised which have yielded promising results. These include the use of external stents, gene therapy as well as conventional pharmacological interventions. The objective of this article, therefore, is to review these recent approaches and their potential clinical applications in the treatment of vein graft disease. Show more

Keywords: Vein graft failure, treatment

Citation: Biorheology, vol. 39, no. 3-4, pp. 491-499, 2002

Authors: Atherton, M.A. | Tesch, K. | Collins, M.W.

Article Type: Research Article

Abstract: Patient‐to‐patient variations in artery geometry may determine their susceptibility to stenosis formation. These geometrical variations can be linked to variations in flow characteristics such as wall shear stress through stents, which increases the risk of restenosis. This paper considers computer models of stents in non‐symmetric flows and their effects on flow characteristics at the wall. This is a fresh approach from the point of view of identifying a stent design whose performance is insensitive to asymmetric flow. Measures of dissipated energy and power are introduced in order to discriminate between competing designs of stents.

Citation: Biorheology, vol. 39, no. 3-4, pp. 501-506, 2002