Finite element computation of magneto-hemodynamic flow and heat transfer in a bifurcated artery with saccular aneurysm using the Carreau-Yasuda biorheological model.

"Existing computational fluid dynamics studies of blood flows have demonstrated that the lower wall stress and higher oscillatory shear index might be the cause of acceleration in atherogenesis of vascular walls in hemodynamics. To prevent the chances of aneurysm wall rupture in the saccular aneurysm at distal aortic bifurcation, clinical biomagnetic studies have shown that extra-corporeal magnetic fields can be deployed to regulate the blood flow. Motivated by these developments, in the current study a finite element computational fluid dynamics simulation has been conducted of unsteady two-dimensional non-Newtonian magneto-hemodynamic heat transfer in electrically conducting blood flow in a bifurcated artery featuring a saccular aneurysm. The fluid flow is assumed to be pulsatile, non-Newtonian and incompressible. The Carreau-Yasuda model is adopted for blood to mimic non-Newtonian characteristics. The transformed equations with appropriate boundary conditions are solved numerically by employing the finite element method with the variational approach in the FreeFEM++ code. Hydrodynamic and thermal characteristics are elucidated in detail for the effects of key non-dimensional parameters i.e. Reynolds number (Re = 14, 21, 100, 200), Prandtl number (Pr = 14, 21) and magnetic body force parameter (Hartmann number) (M = 0.6, 1.2, 1.5) at the aneurysm and throughout the arterial domain. The influence of vessel geometry on blood flow characteristics i.e. velocity, pressure and temperature fields are also visualized through instantaneous contour patterns. It is found that an increase in the magnetic parameter reduces the pressure but increases the skin-friction coefficient in the domain. The temperature decreases at the parent artery (inlet) and both the distant and prior artery with the increment in the Prandtl number. A higher Reynolds number also causes a reduction in velocity as well as in pressure. The blood flow shows different characteristic contours with time variation at the aneurysm as well as in the arterial segment. The novelty of the current research is therefore to present a combined approach amalgamating the Carreau-Yasuda model, heat transfer and magnetohydrodynamics with complex geometric features in realistic arterial hemodynamics with extensive visualization and interpretation, in order to generalize and extend previous studies. In previous studies these features have been considered separately and not simultaneously as in the current study. The present simulations reveal some novel features of biomagnetic hemodynamics in bifurcated arterial transport featuring a saccular aneurysm which are envisaged to be of relevance in furnishing improved characterization of the rheological biomagnetic hemodynamics of realistic aneurysmic bifurcations in clinical assessment, diagnosis and magnetic-assisted treatment of cardiovascular disease."

Hemodynamic Effects on Particle Targeting in the Arterial Bifurcation for Different Magnet Positions.

The present study investigated the possibilities and feasibility of drug targeting for an arterial bifurcation lesion to influence the host healing response. A micrometer sized iron particle was used only to model the magnetic carrier in the experimental investigation (not intended for clinical use), to demonstrate the feasibility of the particle targeting at the lesion site and facilitate the new experimental investigations using coated superparamagnetic iron oxide nanoparticles. Magnetic fields were generated by a single permanent external magnet (ferrite magnet). Artery bifurcation exerts severe impacts on drug distribution, both in the main vessel and the branches, practically inducing an uneven drug concentration distribution in the bifurcation lesion area. There are permanently positioned magnets in the vicinity of the bifurcation near the diseased area. The generated magnetic field induced deviation of the injected ferromagnetic particles and were captured onto the vessel wall of the test section. To increase the particle accumulation in the targeted region and consequently avoid the polypharmacology (interaction of the injected drug particles with multiple target sites), it is critical to understand flow hemodynamics and the correlation between flow structure, magnetic field gradient, and spatial position.

Computational study of the effects of arterial bifurcation on the temperature distribution during cryosurgery.

BACKGROUND: Thermally significant blood flows into locally cooled diseased tissues and warm them during cryosurgery so that the iceball is often hard to cover the whole diseased volume. This paper is aimed at investigating the effects of large arterial bifurcation on the temperature distribution during cryosurgery through simulation method. METHODS: A parametric geometry model is introduced to construct a close-to-real arterial bifurcation. The three-dimensional transient conjugate heat transfer between bifurcated artery and solid tissues with phase change during cryosurgery is performed by finite volume method. RESULTS: The discussion was then made on the effects of the relative position between cryoprobe and artery bifurcation, the inlet velocity of root artery and the layout of multiple cryoprobes on the temperature distribution and iceball evolution. The results show that the thermal interaction between blood flow and iceball growth near bifurcation is considerable complex. The thermal effects of bifurcation could modulate the iceball morphology, severely weaken its freezing volume and prevent the blood vessel from being frozen. CONCLUSION: The present work is expected to be valuable in optimizing cryosurgery scheme of the situation that the bifurcated artery is embedded into the disease tissue.

Distribution of iliac veins posterior to the common iliac artery bifurcation related to pelvic lymphadenectomy: A digital in vivo anatomical study of 442 Chinese females.

OBJECTIVES: To investigate the distribution of iliac veins posterior to common iliac artery bifurcation (CIAB) for pelvic lymphadenectomy. METHODS: After IRB approval was obtained, computer tomography angiography data of 442 female pelvises were acquired. After vascular three-dimensional (3D) reconstructions, the structural types, frequencies and diameters of iliac veins immediately posterior to CIAB were investigated and measured. To quantify iliac vein courses, linear distances and their distances on sagittal, coronal and vertical axes from CIAB to external/internal iliac veins confluence (EIIVC) were geometrically measured. RESULTS: There were five structural types of iliac veins distribution immediately posterior to CIAB: common iliac vein (CIV, 13.8%), no occurrence of great vein (N, 71.27%, 0), EIIVC (1.58%) and external iliac vein (EIV, 13.35%) on the left side, while confluence of common iliac veins (CCIV, 8.82%), CIV (77.38%), N (1.58%, 0), EIIVC (6.11%), and EIV (6.11%) on right. The venous diameters immediately posterior to CIAB in "CCIV", "CIV" and "EIIVC" were significantly larger than that in "EIV" (P<0.05). Their linear distances and their distances on each axis from CIAB to external/internal iliac veins confluence (EIIVC) from CIAB to EIIVC were obtained. CONCLUSIONS: In this study, we presented new distribution of iliac veins posterior to CIAB, including structural types, frequencies, venous diameters immediately posterior to CIAB, and their quantified courses from CIAB to EIIVC. It could help surgeons reduce the risk of vascular injury, hemorrhage or transfusion in pelvic lymphadenectomy.

Greater hemodynamic stresses initiate aneurysms on major cerebral arterial bifurcations.

Objective: To retrospectively investigate the hemodynamic stresses in initiating aneurysm formation on major cerebral arterial bifurcations with computational fluid dynamics (CFD) analysis. Methods: The cerebral 3D angiographic data of major cerebral arterial bifurcations of the internal carotid, middle cerebral, anterior cerebral, and basilar arteries in 80 patients harboring bifurcation aneurysms and 80 control subjects with no aneurysms were retrospectively collected for the CFD analysis of hemodynamic stresses associated with aneurysm formation. Results: Bifurcation angles at major bifurcations in all patients were significantly positively (P < 0.001) correlated with the age. At the center of direct flow impingement (CDFI) on the bifurcation wall, total pressure was the highest but dropped rapidly toward the branches. Wall shear stress, dynamic pressure, strain rate, and vorticity were lowest at the CDFI but they increased quickly toward the branches. The bifurcation angle was significantly (P < 0.001) enlarged in patients with bifurcation aneurysms than those without them, for all major arterial bifurcations. Most aneurysms leaned toward the smaller arterial branch or the arterial branch that formed a smaller angle with the parent artery, where the hemodynamic stresses increased significantly (P < 0.05), compared with those on the contralateral arterial branch forming a larger angle with the parent artery. Following the aneurysm development, all the hemodynamic stresses on the aneurysm dome decreased significantly (P < 0.001) compared with those at the initiation site on the bifurcation wall after virtual aneurysm removal. With the decrease of bifurcation angles, all the hemodynamic stresses decreased. Conclusion: The formation of intracranial aneurysms on major intracranial arterial bifurcations is significantly associated with locally abnormally augmented hemodynamic stresses, which must be reduced.

Role of CD40 ligand-mediated endothelial cell-monocyte interaction at atherosclerosis predilection sites.

BACKGROUND: Monocyte recruitment into the vessel wall at atherosclerosis predilection sites is essential for lesion development in the early phase of atherosclerosis. Platelets interacting with ultra-large von Willebrand Factor (ULVWF) multimers deposited after CD40 receptor ligation on the endothelial surface form adhesive bridges and facilitate monocyte diapedesis. We hypothesise that enhanced endothelial CD40 expression at arterial bifurcations is responsible for monocyte recruitment and that its absence reduces susceptibility to atherosclerosis. METHODS: Y-shaped channel slides covered with endothelial cells (HUVEC) and isolated perfused carotid artery bifurcations from different mouse lines were used for adhesion studies with isolated fluorescent dye-labelled platelets and monocytes. Monocyte adherence was quantified via fluorescence imaging. Oil Red O staining visualised aortic atherosclerotic plaques, and mRNA expression was determined by qRT-PCR. RESULTS: In response to soluble CD40 ligand (sCD40L) stimulated ULVWF release, the number of monocytes bound distal to the bifurcation of the Y-slide was 1.8-fold greater than without stimulation. The number of adherent monocytes in sCD40L-treated carotid artery bifurcations was 6 to 12.3-fold greater in ApoE knockout mice as compared to bifurcations derived from CD40/ApoE-deficient or control mice. CD40 mRNA expression was 2-fold higher in carotid artery bifurcations of ApoE knockout mice as compared to the proximal unbranched segment. Introduction of the CD40 knockout into the ApoE-/- background reduced the atherosclerosis burden along the entire aorta of these mice by 60 %. CONCLUSIONS: Our data demonstrate the importance of endothelial CD40 expression at atherosclerosis predilection sites for endothelial cell-platelet-monocyte interaction in the early phase of atherosclerosis.

Analysis of non-Newtonian effects within an aorta-iliac bifurcation region.

The geometry of the arteries at or near arterial bifurcation influences the blood flow field, which is an important factor affecting arteriogenesis. The blood can act sometimes as a non-Newtonian fluid. However, many studies have argued that for large and medium arteries, the blood flow can be considered to be Newtonian. In this work a comprehensive investigation of non-Newtonian effects on the blood fluid dynamic behavior in an aorta-iliac bifurcation is presented. The aorta-iliac geometry is reconstructed with references to the values reported in Shah et al. (1978); the 3D geometrical model consists of three filleted cylinders of different diameters. Governing equations with the appropriate boundary conditions are solved with a finite-element code. Different rheological models are used for the blood flow through the lumen and detailed comparisons are presented for the aorta-iliac bifurcation. Results are presented in terms of the velocity profiles in the bifurcation zone and Wall Shear Stress (WSS) for different sides of the bifurcation both for male and female geometries, showing that the Newtonian fluid assumption can be made without any particular loss in terms of accuracy with respect to the other more complex rheological models.

Atherosclerosis at arterial bifurcations: evidence for the role of haemodynamics and geometry.

Atherosclerotic plaques are found at distinct locations in the arterial system, despite the exposure to systemic risk factors of the entire vascular tree. From the study of arterial bifurcation regions, emerges ample evidence that haemodynamics are involved in the local onset and progression of the atherosclerotic disease. This observed co-localisation of disturbed flow regions and lesion prevalence at geometrically predisposed districts such as arterial bifurcations has led to the formulation of a 'haemodynamic hypothesis', that in this review is grounded to the most current research concerning localising factors of vascular disease. In particular, this review focuses on carotid and coronary bifurcations because of their primary relevance to stroke and heart attack. We highlight reported relationships between atherosclerotic plaque location, progression and composition, and fluid forces at vessel's wall, in particular shear stress and its 'easier-to-measure' surrogates, i.e. vascular geometric attributes (because geometry shapes the flow) and intravascular flow features (because they mediate disturbed shear stress), in order to give more insight in plaque initiation and destabilisation. Analogous to Virchow's triad for thrombosis, atherosclerosis must be thought of as subject to a triad of, and especially interactions among, haemodynamic forces, systemic risk factors, and the biological response of the wall.

The effect of basilar artery bifurcation angle on rates of initial occlusion, recanalization, and retreatment of basilar artery apex aneurysms following coil embolization.

BACKGROUND: Arterial bifurcations are common locations for aneurysm development given the altered hemodynamic forces and shear stress variations present at these locations. Recent reports indicate that a wide basilar artery bifurcation angle is an independent predictor of aneurysm development, growth, and subsequent rupture. METHODS: To determine the effect of basilar artery bifurcation angle on rates of initial occlusion, recanalization, and retreatment of basilar artery apex aneurysms following coil embolization, the records of 46 patients with basilar artery apex aneurysms treated with endovascular coil embolization from 2007 to 2013 were analyzed. RESULTS: A wide basilar artery bifurcation angle was associated with a Raymond-Roy Occlusion Classification (RROC) III occlusion in univariate analysis, but was not a statistically significant factor in multivariate modeling. An increasing basilar artery bifurcation angle was not associated with aneurysm recanalization or retreatment following coil embolization. Increasing packing density (p < .01) was the only statistically significant predictor of a RROC I or II closure. The initial RROC designation was the most powerful predictor of both eventual aneurysm recanalization (p = .01) and retreatment (p = .02). While increasing aneurysm size (p < .01), increasing aneurysm volume (p < .01), and increasing neck size (p < .01) were associated with wide basilar artery bifurcation angles, neck size (p = .03) was the only statistically significant predictor of basilar artery bifurcation angle on multivariate analyses. CONCLUSION: Basilar artery bifurcation angle fails to predict rates of initial occlusion, recanalization, and retreatment on multivariate modeling in our series. Basilar artery apex aneurysm neck size independently correlates with basilar artery bifurcation angle.

Crossing Y-stent technique with dual open-cell stents for coiling of wide-necked bifurcation aneurysms.

OBJECTIVE: Double stenting in a Y-configuration is a promising therapeutic option for wide-necked cerebral aneurysms not amenable to reconstruction with a single stent. We retrospectively evaluated the efficacy and safety of the crossing Y-stent technique for coiling of wide-necked bifurcation aneurysms. METHODS: By collecting clinical and radiological data we evaluated from January 2007 through December 2013, 20 wide-necked bifurcation aneurysms. RESULTS: Twelve unruptured and eight ruptured aneurysms in 20 patients were treated with crossing Y-stent-assisted coiling. Aneurysm size and neck size ranged from 3.2 to 28.2mm (mean 7.5mm) and from 1.9 to 9.1mm (mean 4.5mm). A Y-configuration was established successfully in all 20 patients. All aneurysms were treated with a pair of Neuroform stents. The immediate angiographic results were total occlusion in 17 aneurysms, residual neck in two, and residual sac in one. Peri-operative morbidity was only 5%. Fifteen of 18 surviving patients underwent follow-up conventional angiography (mean, 10.9 months). The result showed stable occlusion in all 15 aneurysms and asymptomatic in-stent occlusion in one branch artery. At the end of the observation period (mean, 33.5 months), all 12 patients without subarachnoid hemorrhage had excellent clinical outcomes (mRS 0), except one (mRS 2). Of eight patients with subarachnoid hemorrhage, four remained symptom free (mRS 0), while the other four had were dependent or dead (mRS score, 3-6). CONCLUSION: In this report on 20 patients, crossing Y-stent technique for coiling of wide-necked bifurcation aneurysms showed a good technical safety and favorable clinical and angiographic outcome.

Development of a general method for designing microvascular networks using distribution of wall shear stress.

In the present study, theoretical formulations for calculation of optimal bifurcation angle and relationship between the diameters of mother and daughter vessels using the power law model for non-Newtonian fluids are developed. The method is based on the distribution of wall shear stress in the mother and daughter vessels. Also, the effect of distribution of wall shear stress on the minimization of energy loss and flow resistance is considered. It is shown that constant wall shear stress in the mother and daughter vessels provides the minimum flow resistance and energy loss of biological flows. Moreover, the effects of different wall shear stresses in the mother and daughter branches, different lengths of daughter branches in the asymmetric bifurcations and non-Newtonian effect of biological fluid flows on the bifurcation angle and the relationship between the diameters of mother and daughter branches are considered. Using numerical simulations for non-Newtonian models such as power law and Carreau models, the effects of optimal bifurcation angle on the pressure drop and flow resistance of blood flow in the symmetric bifurcation are investigated. Numerical simulations show that optimal bifurcation angle decreases the pressure drop and flow resistance especially for bifurcations at large Reynolds number.

Preliminary Results of Y-Stent-Assisted Coil Embolization of Wide-Necked Intracranial Aneurysms: 8 Consecutive Patients / 대한뇌혈관외과학회지

OBJECTIVE: The endovascular treatment of wide-necked intracranial aneurysms is challenging. The stent-assisted coil embolization has facilitated treatment of such complex aneurysms. However, the single stent-assisted technique has a limitation for the wide-necked intracranial aneurysm at the arterial bifurcation. The Y-stent-assisted technique could be an alternative solution for these aneurysms. We present a case series where stent-assisted coil embolization where the Y-configuration stent was used. METHODS: Between January 2007 to December 2010, 8 wide-necked bifurcation aneurysms in 8 patients were treated with the Y-stent technique. Among the eight patients, there were six unruptured aneurysms and the remaining two patients presented with subarachnoid hemorrhage (SAH). Three out of eight aneurysms were located at the anterior communicating artery (ACOM), three at the top of the basilar artery (BA), one at the middle cerebral artery (MCA) bifurcation and one at the pericallosal artery. The size of aneurysm ranged from 3.6 mm to 28.2 mm (mean 8.7 mm, neck size from 3 to 7 mm). Four patients were female and aged ranged from 52 to 73 years. RESULTS: The Y-stent-assisted coil embolization was successfully performed in all 8 cases. The immediate angiographic results were complete occlusion in 7 cases with a remnant neck the remaining case. Angiographic follow-up was done in six patients and stable occlusion was confirmed in all aneurysms. Acute thromboembolism (TE) during the procedure occurred in 4 patients. There were one acute cerebral infarction due to distal coil migration and one delayed cerebral infarction due to in-stent thrombosis after 2 months. CONCLUSION: Traditionally microsurgery has been the treatment of choice for wide-necked intracranial aneurysms at the arterial bifurcation. However, with the advancement of new techniques and instruments for endovascular treatment, the Y-stent-assisted coil embolization seems to be a feasible treatment option for reconstruction of these complex aneurysms.