Ryuhei Yamaguchi

Hemodynamic Study of the Anterior Communicating Artery

BACKGROUND AND PURPOSE The anterior communicating artery (ACoA) is a site of predilection for intracranial saccular aneurysms causing subarachnoid hemorrhage. ACoA aneurysms are frequently associated with an asymmetrical circle of Willis. In such cases, the ACoA is probably exposed to high hemodynamic stress caused by a considerable shunt flow across the ACoA to the distal segment of the contralateral anterior cerebral artery (ACA). In the present study, the flow pattern and flow-induced shear stress in the ACoA complex that may initiate aneurysmal lesions were studied under steady and pulsatile flow conditions. METHODS Flow visualization was studied with dye injection and birefringent flow visualization in symmetrical and asymmetrical models of various sizes of ACoA. The distribution of wall shear stress was measured using an electrochemical method based on a diffusion-controlled reaction of ferricyanide ion to ferrocyanide ion at a platinum electrode embedded in the wall of the ACoA model. RESULTS With equal flow rate (Reynolds number 150 to 600), vortical flow was formed in the mouth of the ACoA, and no cross flow through the ACoA was observed. The wall shear stress on the mid-wall of the ACoA was almost zero. However, as soon as the flow rate became unequal, a cross flow through the ACoA was observed. The stagnation point also appeared at the medial junction of the ACoA and ACA. The wall shear stress increased to a very high level at the wall of the ACoA and around the stagnation point. CONCLUSIONS Geometric changes from the symmetrical to the asymmetrical ACoA develop higher shear stress on the ACoA than critical values and the stagnation point at the ACoA junction. A combination of these hemodynamic factors is considered to play an important role in initiation of aneurysm.

Effect of elasticity on wall shear stress inside cerebral aneurysm at anterior cerebral artery

BACKGROUND Many numerical studies have been published with respect to about flow structures around cerebral aneurysm assuming to be rigid. Furthermore, there is little experimental research concerning aneurysm with elastic wall. Wall shear stress in elastic wall comparing with rigid wall should be clarified in experimental approach and verified in CFD. OBJECTIVE We have experimentally realized elastic aneurysm model accompanying with wall deformation. Wall shear stress was examined for both rigid and elastic aneurysm models in pulsatile flow. METHODS Effect of elasticity on wall shear stress inside aneurysm induced at the apex of anterior cerebral artery was experimentally examined by particle image velocimetry in vitro. In order to adjust the wall deformation, the pressure adjustment chamber was specially equipped outside the aneurysm wall. RESULTS Effect of elasticity on wall shear stress was noticed on the comparison with that of rigidity. Wall elasticity reduced the peak magnitude, the spatial and temporal averaged wall shear stress comparing with those of wall rigidity experimentally. These reductions were endorsed by fluid-structure interaction simulation. CONCLUSION Elastic wall comparing with rigid wall would reduce the peak magnitude, the spatial and temporal averaged wall shear stress acting on vascular wall.

Variation of Wall Shear Stress and Periodic Oscillations Induced in the Right-Angle Branch During Laminar Steady Flow

We report on flow phenomena such as wall shear stress and periodic oscillations that occur in the right-angle branch during laminar steady flow in the upstream trunk. The side-branch bifurcates from the trunk at a 90-deg angle, and both the upstream and the downstream corners of the entrance of the side-branch are square-edged. As the flow approaches the entrance of the side-branch through the trunk, the wall shear stress steeply increases along the near wall, and this stress at the upstream corner is comparable to that generated around the flow divider. Periodic velocity oscillations occur downstream in the separation region in the side-branch. The Strouhal number based on the flow conditions in the side-branch is independent of the flow division ratio for the side-branch radii ranging from 4 to 9 mm

Repression of wall shear stress inside cerebral aneurysm at bifurcation of anterior cerebral artery by stents

The effect of a simple bare metal stent on repression of wall shear stress inside a model cerebral aneurysm was experimentally investigated by two-dimensional particle image velocimetry in vitro. The flow model simulated a cerebral aneurysm induced at the apex of bifurcation between the anterior cerebral artery and the anterior communicating artery. Wall shear stress was investigated using both stented and non-stented models to assess the simple stent characteristics. The flow behavior inside the stented aneurysm sac was unusual and wall shear stress was much smaller inside the aneurysm sac. Stent placement effectively repressed the temporal and spatial variations and the magnitude of wall shear stress. Hence, there is an effective possibility that would retard the progress of cerebral aneurysms by even simple stent.

Universality of Periodic Oscillation Induced in Side Branch of a T-Junction in Numerical Simulation

The flow instability through the side branch of a T-junction is analyzed in a numerical simulation. In a previous experimental study, the authors clarified the mechanism of fluid-induced vibration in the side branch of the T-junction in laminar steady flow through the trunk. However, in that approach there were restrictions with respect to extracting details of flow behavior such as the flow instability and the distribution of wall shear stress along the wall. Here the spatial growth of the velocity perturbation at the upstream boundary of the side branch is investigated. The simulation result indicates that a periodic velocity fluctuation introduced at the upstream boundary is amplified downstream, in good agreement with experimental result. The fluctuation in wall shear stress because of the flow instability shows local extrema in both the near and distal walls. From the numerical simulation, the downstream fluid oscillation under a typical condition has a Strouhal number of 1.05, which approximately agrees with the value obtained in experiments. Therefore, this periodic oscillation motion is a universal phenomenon in the side branch of a T-junction.

Suppression of Wall Shear Stress inside Intracranial Aneurysms by Simple Stents

The effect of a simple bare-metal stent on suppression of wall shear stress inside a spherical intracranial aneurysm model was experimentally investigated by two dimensional particle image velocimetry in vitro . The flow model simulated an intracranial aneurysm induced both at the side wall and at the apex of bifurcation between the anterior cerebral artery and the anterior communicating artery. Wall shear stress was investigated using both stented and non-stented models to assess the simple bare metal stent characteristics. The flow behavior inside the stented aneurysm sac was unusual and wall shear stress was much smaller inside the aneurysm sac. For both models, the maximum and the temporal and spatial averaged wall shear stress in the stented model is reduced by at least 50 % from those in the non-stented model. Stent placement effectively suppresses the temporal and spatial variations, and the magnitude of wall shear stress. Consequently, there is an effective possibility that would retard the progress of cerebral aneurysms by even simple stent.

Study for Flow Structure within Real Aneurysm Induced in Anterior Communicating Artery

The mechanism involved in the formation and subsequent rupture of cerebral aneurysm has not been clarified adequately. The aneurysm frequently appears at the flow divider where the flow bifurcates to the anterior communicating artery and the anterior cerebral artery. In the present study, the flow pattern and the wall shear stress (WSS) within the real aneurysm model having bleb are measured using PIV. On the basis of the MRI data of human cerebral artery with aneurysm, the aneurysm model is reconstructed by the optical 3D design with CAD. WSS abruptly changes along the aneurysmal wall and minimum WSS appears at bottom wall of the bleb. Particularly, WSS is small along the bleb wall. It would be associated with the degeneration of bleb wall such as the formation of thrombosis.

Mechanism of Periodical Fluid Oscillation Induced by Separating High Shear Rate Flow in Side Branch of Right-Angle Branch

In the previous study, it has been clarified in the right-angle branch that the periodical oscillation flow is induced in the side branch and the Strouhal number based on the variables in the side branch is independent of the Reynolds number, the flow division ratio, and the radius of the side branch. This periodical oscillation appears along the boundary between two pairs of vortices with the opposite rotation at the cross section of the side branch across the separation flow. In the present study, another source of the periodical oscillation is experimentally clarified that the high shear rate flow is induced along the separation shearing layer between the separation flow of low velocity and the main flow with high velocity along the distal wall in the side branch since the high shear rate along this separation shearing layer is several times as large as that in the upstream trunk or in the downstream region of the side branch.

Effect of Secondary Flow on Wall Shear Stress and Velocity Profile in Right-Angle Branch.

The axial and transverse velocity components in a right-angle branch have been measured using a two-dimensional laser Doppler velocimeter. The radius Rs of curvature along the upstream corner of the side branch is one half of trunk diameter D0. The experiment has been carried out for two flow division ratios at the Reynolds number of 800. Consequently, it is clarified that the sinusoidal variation of wall shear stress along the proximal wall of the side branch is induced by the secondary helical flow transferred from the trunk. The wall shear stress estimated from the axial velocity profile near the tube wall at the common median plane agrees fairly well with the result measured by the electrochemical method, employed in the previous study. Therefore, the electrochemical method is one of the most suitable methods for the measurement of wall shear stress in the range of liquid laminar flow.

Variation of Wall Shear Stress in Daughter Tube of Asymmetrical Arterial Branch Model.

In the present study, the effect of the flow distribution ratio and the pulsation on the wall shear stress through an asymmetrical branch model has been studied experimentally. The experiment has been carried out for a branch model, such as the inferior mesenteric artery from the abdominal aorta, so that a daughter tube asymmetrically branches out from the parent tube at 45 degrees. The wall shear stress is quite affected by the difference in flow distribution ratio for each downstream tube. In steady flow, the wall shear stress periodically changes along the proximal wall of the daughter tube. In pulsating flow, a< 7, this periodic change in wall shear stress markedly occurs and its magnitude is several times larger than that in the upstream parent tube. The distribution of the amplitude of wall shear stress in pulsating flow at a quasi-steady state is similar to that of wall shear stress in steady flow.