Hitomi Anzai

Optimization of Strut Placement in Flow Diverter Stents for Four Different Aneurysm Configurations

A modern technique for the treatment of cerebral aneurysms involves insertion of a flow diverter stent. Flow stagnation, produced by the fine mesh structure of the diverter, is thought to promote blood clotting in an aneurysm. However, apart from its effect on flow reduction, the insertion of the metal device poses the risk of occlusion of a parent artery. One strategy for avoiding the risk of arterial occlusion is the use of a device with a higher porosity. To aid the development of optimal stents in the view point of flow reduction maintaining a high porosity, we used lattice Boltzmann flow simulations and simulated annealing optimization to investigate the optimal placement of stent struts. We constructed four idealized aneurysm geometries that resulted in four different inflow characteristics and employed a stent model with 36 unconnected struts corresponding to the porosity of 80%. Assuming intracranial flow, steady flow simulation with Reynolds number of 200 was applied for each aneurysm. Optimization of strut position was performed to minimize the average velocity in an aneurysm while maintaining the porosity. As the results of optimization, we obtained nonuniformed structure as optimized stent for each aneurysm geometry. And all optimized stents were characterized by denser struts in the inflow area. The variety of inflow patterns that resulted from differing aneurysm geometries led to unique strut placements for each aneurysm type.

Optimization of flow diverters for cerebral aneurysms

Abstract A modern technique to treat cerebral aneurysms is to insert a flow diverter in the parent artery. In order to produce an optimal design of such devices, we consider a methodology combining simulated annealing optimization and lattice Boltzmann simulations. Our results surpass, in terms of stent efficiency, those obtained in the recent literature with an other optimization method. Although our approach is still in 2D, it demonstrates the potential of the method. We give some hint on how the 3D cases can be investigated.

Computational fluid dynamic analysis following recurrence of cerebral aneurysm after coil embolization

Hemodynamic factors are thought to play important role in the initiation, growth, and rupture of cerebral aneurysms. However, hemodynamic features in the residual neck of incompletely occluded aneurysms and their influences on recanalization are rarely reported. This study characterized the hemodynamics of incompletely occluded aneurysms that had been confirmed to undergo recanalization during long-term follow-up using computational fluid dynamic analysis. A ruptured left basilar-SCA aneurysm was incompletely occluded and showed recanalization during 11 years follow-up period. We retrospectively characterized on three-dimensional MR angiography. After subtotal occlusion, the flow pattern, wall shear stress (WSS), and velocity at the remnant neck changed during long-term follow-up period. Specifically, high WSS region and high blood flow velocity were found near the neck. Interestingly, these area of the remnant neck coincided with the location of aneurysm recanalization. High WSS and blood flow velocity were consistently observed near the remnant neck of incompletely occluded aneurysm, prone to future recanalization. It will suggest that hemodynamic factors may play important roles in aneurismal recurrence after endovascular treatment.

In vitro strain measurements in cerebral aneurysm models for cyber-physical diagnosis

The development of new diagnostic technologies for cerebrovascular diseases requires an understanding of the mechanism behind the growth and rupture of cerebral aneurysms. To provide a comprehensive diagnosis and prognosis of this disease, it is desirable to evaluate wall shear stress, pressure, deformation and strain in the aneurysm region, based on information provided by medical imaging technologies.

Towards the patient-specific design of flow diverters made from helix-like wires: an optimization study

BackgroundFlow diverter (FD) intervention is an emerging endovascular technique for treating intracranial aneurysms. High flow-diversion efficiency is desired to accelerate thrombotic occlusion inside the aneurysm; however, the risk of post-stenting stenosis in the parent artery is posed when flow-diversion efficiency is pursued by simply decreasing device porosity. For improving the prognosis of FD intervention, we develop an optimization method for the design of patient-specific FD devices that maintain high levels of porosity.MethodsAn automated structure optimization method for FDs with helix-like wires was developed by applying a combination of lattice Boltzmann fluid simulation and simulated annealing procedure. Employing intra-aneurysmal average velocity as the objective function, the proposed method tailored the wire structure of an FD to a given vascular geometry by rearranging the starting phase of the helix wires.ResultsFD optimization was applied to two idealized (S and C) vascular models and one realistic (R) model. Without altering the original device porosity of 80%, the flow-reduction rates of optimized FDs were improved by 5, 2, and 28% for the S, C, and R models, respectively. Furthermore, the aneurysmal flow patterns after optimization exhibited marked alterations. We confirmed that the disruption of bundle of inflow is of great help in blocking aneurysmal inflow. Axial displacement tests suggested that the optimal FD implanted in the R model possesses good robustness to tolerate uncertain axial positioning errors.ConclusionsThe optimization method developed in this study can be used to identify the FD wire structure with the optimal flow-diversion efficiency. For a given vascular geometry, custom-designed FD structure can maximally reduce the aneurysmal inflow with its porosity maintained at a high level, thereby lowering the risk of post-stenting stenosis. This method facilitates the study of patient-specific designs for FD devices.

The Effect of 3D Visualization on Optimal Design for Strut Position of Intracranial Stent

Cerebral aneurysms generally occur at arterial bifurcations and arterial curves in or near the circle of Willis. For the treatment of this disorder, stent placement has been valued as a minimal invasive therapy. The effect of stents on flow reduction in cerebral aneurysms has been examined in several computed fluid dynamics (CFD) studies, suggesting that the stent position or the strut shape may affect flow reduction. However, the position of the stent with the best effect on flow reduction is still unknown because of the flow complexity. Three-dimensional visualization may help to easily specify the inflow zone from the parent artery to the aneurysm and to find the relationship between the effective strut position and the flow pattern. However, confirmation of the ability of 3D visualization to determine the effective position of a stent has not been achieved. In this study, we simulated blood flow with several aneurysm geometries to confirm the effect of 3D visualization on determination of optimal stent position. First, flow simulation using real aneurysm geometries without a stent was performed as a “pre-stenting situation.” Meshes were generated using a commercial code (Gambit 2.3, Fluent Inc., NH). CFD was carried out using a commercial code (Fluent 6.3, Fluent Inc., NH) based on steady flow. The streamlines around an aneurysm were visualized using a 3D visualization system (EnSight Gold 8.2, Comuputational Engineering Inc., NC) in Realization Workspace (RWS) to visualize the inflow zone. Secondly, a rectangular solid as a strut model was set in the inflow zone using computer-aided design (CAD) techniques. CFD was then performed as a “post-stenting situation” under the same conditions as the pre-stenting situation using the same mesh generator and CFD code. Three-dimensional visualization showed an inflow zone in the aneurysm. A bundle of flow streamlines hit the wall of the neck of the aneurysm and entered it. The inflow zone was a narrow local part in contrast to the outflow. After setting a strut, a change of flow pattern could be observed. The flow speed and the wall shear stress (WSS) were both reduced. When the strut position was moved away from the original position, the flow speed and the WSS were not reduced. These results may suggest that 3D visualization can provide information useful for strut positioning to realize effective reduction of flow into an aneurysm, especially a side wall aneurysm.Copyright

Comparative Study Between Different Strut’s Cross Section Shape on Minimizing Low Wall Shear Stress Along Stent Vicinity via Surrogate-Based Optimization

Endovascular stent has been employed to treat patients with intravascular diseases. Research on stent optimization is currently performed in order to find the best design in increasing the treatment efficacy. In this research, stent optimization is performed based on a finite element analysis method via Kriging surrogate model to observe the wall shear stress (WSS) conditions on the strut vicinity. Two configurations, rectangle and triangle are adopted as the cross section of a stent strut and compared to see the effects of the cross section on WSS condition. Strut gap in the range from 1 mm to 3 mm and the strut size length from 0.05 mm to 0.45 mm are considered as the design variables for each cross section. Structure contact simulation between stent and vessel wall is carried out to obtain the 5% vessel expansion. Afterward, computational fluid dynamics simulation is performed to analyze the hemodynamic effect of stent design along with wall deformation. Minimizing the percentage of low WSS area (WSS < 1 Pa) relative to the length of stent deployment area is set as the objective function of this optimization since low WSS is believed to promote some problems such as atherosclerosis. In total, 45 and 42 simulation iterations are conducted respectively for both cross sections to develop the Kriging surrogate models for efficient global optimization. Besides the prediction of the optimized configuration, broader observation on its behavior within the design range is also well predicted. The optimized configuration has 2.99 mm gap and 0.1 mm width for the rectangular strut, and 2.00 mm gap and 0.99 mm width for the triangular strut. The triangular strut has better performance in reducing the low WSS area with 14.6% of low WSS area on its optimized design, compared to 18.3% of the rectangular strut. Moreover, the triangular shape strut produces more stable performance; most design configuration with the strut width of less than 0.35 mm can keep low WSS area at the minimum value.

Stent design optimization based on kriging surrogate model under deformed vessel wall: Pulsatile inlet flow

The cardiovascular stent is one of the medical devices which has been commonly used for curing many vascular diseases. Nowadays, research on many aspects of stent development has been conducted to improve the devices efficacy. Mechanical and flow dynamics analysis on stent performance are useful to understand the impact of the device deployment. Many assumptions have been applied for constructing the stent simulation model including the blood vessel wall condition and its inlet flow conditions. Recently, common assumptions of the stent simulation model are mainly worked under the assumption of rigid wall condition and steady or pulsatile inlet flow. These different assumptions may lead to different simulation results. These differences may also affect the further analysis such as optimization process. This research tries to investigate the pulsatile effect on the stent optimization results based on computational simulation with wall deformation. Comparison with the previous optimization with a steady flow was conducted to find out about the differences between the two conditions. We found that the difference in optimization results from both inflow conditions is insignificant.

A cyber-physical system for strain measurements in the cerebral aneurysm models

For the development of artificial intelligent diagnosis for cerebrovascular intervention, it is desirable to forecast the growth of cerebral aneurysms. In order to achieve such purpose, it is needed to evaluate wall shear stress, strain, pressure, deformation and flow velocity in the aneurysm region. In this research, we focus on in-vitro strain and deformation measurements of cerebral aneurysm models, and propose a cyber-physical system, in which a scaled-up membranous silicone model of cerebral aneurysm was built and integrated with a specialized pump for the pulsatile blood flow simulation, and a vision system was constructed to measure the strain on different regions on the model with pulsatile blood flow circulated inside. Experimental results show that both distance and area strain maxima were larger for the aneurysm neck (0.042 and 0.052), followed by the aneurysm dome (0.023 and 0.04) and then by the main blood vessel section (0.01 and 0.014), which were complemented with computer fluid dynamics simulation for the inclusion of wall shear stress, oscillatory shear index and aneurysm formation index. Medical imaging data of the cerebral aneurysm in 2008 and 2011 was obtained. Diagnosis results have concordance with the aneurysm growth in 2011. The presented measurement method offers an option for measuring strain and deformation to be complementary with computer fluid dynamics and photoelastic stress analysis for advanced diagnostic in the endovascular surgery.

In search for a better stent: Surrogate based multi-objective optimization of stent design under influence of vessel wall deformation

Stenting is known as one of the main treatment procedure for some intravascular abnormalities such as stenosis and aneurysm. In recent years, stent optimization has been conducted by several research groups in order to increase its treatment efficacy. If we can observe post-deployment behavior on the blood vessel with respect to different stent designs, this observation will be useful in the design process. Kriging surrogate model based on fluid flow simulation on a deformed vessel wall was developed in order to observe this behavior. Multi-objectives optimization was performed with configurations of gap and size as design variables. In this research, percentage of low wall shear stress (WSS) area and average mechanical stress along the deployment area were set as the objective functions. We can recommend that strut with medium size around 100 – 250 micron with a relatively big inter-strut gap is suitable for achieving the optimize criteria. This is because on this range, acceptable optimized value of both objectives functions are successfully obtained.