Effect of Different Stent Configurations using Lattice

Effect of Different Stent Configurations using Lattice Boltzmann Method and Particles Image Velocimetry on Artery Bifurcation Aneurysm Problem N. Hafizah Mokhtara,* Aizat Abas b, N.A. Razakc, Muhammad Najib Abdul Hamidd and Soon Lay Teonge a,b,d

School of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia

c

School of Aerospace Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia d

Universiti Kuala Lumpur Malaysian Spanish Institute, Kulim Hi-TechPark, 09000 Kulim, Kedah.

Correspondence should be addressed to Aizat Abas; [email protected]

Abstract

Proper design of stent for application at specific aneurysm effect arteries could help to reduce the issues with thrombosis and aneurysm. In this paper, four types of stent configuration namely half-Y (6mm), half-Y (4mm), cross-bar, and full-Y configuration will implanted on real 3D artery bifurcation aneurysm effected arteries. Comparisons were then conducted based on the flow patterns after stent placement using both LBM-based solver and PIV experimental findings. According to the data obtained from all 4 stent designs, the flow profiles and the computed velocity from both methods were in agreement with each other. Both methods found that half-Y (6mm) stent configuration is by far the best configuration in reducing the blood velocity at the vicinity of the aneurysm sac. The analysis also show that the half-Y (6mm) stent configuration recorded the highest percentage of velocity reduction and managed to substantially reduce the pressure at the bifurcation region. This high flow velocity reduction through the use of half-Y stent could consequently promote the formation of thrombus thereby reducing the risk of rupture in the aneurysm sac. "Keywords: Lattice Boltzmann Method, Particle Image Velocimetry, Aneurysm"

1. Introduction Aneurysm is an arterial condition characterized by weakened blood vessel walls, formed bulges, or distended arteries. This condition manifests two common types, namely, abdominal aortic aneurysm and thoracic aortic aneurysm. Aneurysm commonly occurs in the aorta, which is a large artery that originates from the left ventricle of the heart and passes through the chest and abdominal cavities, and in vital arteries carrying blood to the brain. An aneurysm grows substantially and forms blood-filled balloon-like structures. Although the precise cause of an aneurysm is illegible, specific factors might contribute to the condition. For example, damaged tissue in the arteries can play a role. The arteries can be harmed by blockages such as blood clot. These clot can trigger the heart to pump harder than necessary to force blood through the blockage arteries. Clotting in aneurysms may affect the arteries, consequently distends and ruptures [1]. As a result, subarachnoid hemorrhage occurs because stress caused by a high amount of force exerted by increased blood flow is induced on the arterial wall [2]. In severe cases, this condition may lead to death. Therefore, blood flow patterns in human vessels should be understood to prevent and treat aneurysm. In addition to medication or surgery applied to treat aneurysms, recent advancements in diagnostic imaging techniques have facilitated the rapid detection of aneurysms [3]. For instance, endovascular aneurysm repair (EVAR) is a novel minimally invasive technique performed as an alternative to conventional surgery and included in the management options for the elective 1

repair of arterial aneurysms [4]. In EVAR, a stent graft is inserted into an aneurysm through small groin incisions by using an X-ray as a guide during insertion. The inserted graft expands inside the arterial aneurysm and fastens itself in place to stabilize blood flow. This process prevents arterial rupture caused by aneurysm. In early 2000s, stent-assisted coil embolization techniques were commonly used to enhance endovascular treatments for intracranial aneurysms and to stop blood flow into the aneurysm sac [5]. Stents alone have been utilized as flow diverters to treat uncoilable intracranial aneurysms and to divert blood flow from aneurysm-affected sites [6]. Y-configuration double stent-assisted coil embolization of this aneurysm type has been shown to have generally good clinical outcomes, although the technique is complex with various challenges described in the literature [7]. Many of aneurysm cases had been successful treated without complication [8]. However, stent-assisted coil embolization method should be improved to treat aneurysm problem [9]–[11]. Arduous effort have been concerted in treating wide-necked bifurcation aneurysms that often require the use of technical complex Y-stent techniques [12]. Wide-necked aneurysm often involved aneurysm sac that has wide opening that is dangerous to treat with neither surgical clipping nor coil embolization. Additionally, these procedures are prone to complications such as coil compaction followed by regrowth of the aneurysm or built of a secondary aneurysm. The safety and potential of flow diversion devices at bifurcation regions have not yet been demonstrated, unlike sidewall aneurysms which occur at non-branching sites of arteries [13]. The rising trend of aneurysm patients should gain more research attention in an attempt to improve aneurysm problem. Surgical clipping is not always a preferred choice due to the high risk involved. Consequently, coil embolization is difficult to be implemented for wide neck aneurysm. Despite these advancement and achievement to the current design, complex bifurcation aneurysm still remains technically challenging. Thus, alternative design configuration should be implemented to improve complications at the bifurcation aneurysm region that it known to be fatal due to risk of arterial rupture. Woowon Jeon et al. and Hashimoto et al. revealed that a complex interaction occurs during the growth of aneurysms in vessel walls and subsequently affects hemodynamic force and flow characteristics [14], [15]. Hence, hemodynamics and flow characteristics should be elucidated. CFD models can be used as clinical diagnostic tools to enhance our understanding on aneurysm development and treatment. CFD predictions should be validated on the basis of actual settings and should be compared with independent in vitro measurements [16]–[18]. The accuracy and reliability of CFD techniques should also be ensured by comparing numerical results with clinical and experimental data [19]. In this study, the CFD-predicted velocity field were determined and compared by considering particle imaging velocimetry (PIV) data. Nevertheless, using CFD alone is inadequate to examine blood flow in practical applications because of rapid advancements in high-resolution cameras and laser technologies. Undetermined errors, such as physical modeling, discretization, and usage errors, can be overlooked by CFD users because of numerous similarities among programming codes used in blood flow applications. As such, CFD assessments are impractical. Hence, whether CFD results accurately represent real-life conditions should be determined and compared with available experimental data to detect measurement errors in CFD codes [20]–[22]. In this regard, many methods, such as hot-wire anemometry, laser Doppler anemometry, pitot-tube, and particle image velocimetry (PIV), have been employed. PIV is an optimal method to verify numerical solutions in research on fluid mechanic applications [23], [24]. This technique also provides several advantages, such as relatively high-resolution and non-intrusive measurement of velocity. Thus, PIV is a reliable experimental approach that involves flow-measuring techniques that record the displacement of small particles infused in a fluid region via high-frequency lasers and high-frame rate cameras [18], [24]–[26]. This method can also be applied to evaluate the velocities of the main area of interest precisely and accurately. Most researchers focused on side wall aneurysms in straight vessels by using commercially available stents [27]–[30]. The flexibility of working fluid velocity is also enhanced for in vitro measurements to evaluate flow characteristics through CFD techniques. In this study, the unsteady blood flow inside an artery bifurcation aneurysm with an installed four designs stent configuration was investigated by using the LBM-based software Palabos. The computed LBM data were validated through a comparison with PIV results in terms of feasibility and efficacy in modeling aneurysm flow problems. 2. Lattice Boltzmann Models The results shown in this section are formulated using D3Q19 lattice model. The LBM equation can be summarized in Equation (1):

2

𝑓(𝑟 + 𝑐𝑑𝑡, 𝑐 + 𝐹𝑑𝑡, 𝑡 + 𝑑𝑡) − 𝑓(𝑟, 𝑐, 𝑡) = Ω𝑓(𝑟, 𝑐, 𝑡),

(1)

in which the right hand side represents the streaming step. The left hand term denotes the collision term which can be represented using the well-known Bhatnagar-Groos-Krook (BGK) model as given in Equation (2). 1

Ω = 𝜔(𝑓 𝑒𝑞 − 𝑓) = (𝑓 𝑒𝑞 − 𝑓)

(2)

𝜏

𝜔 and 𝜏 denote the relaxation frequency and time. 𝑓 𝑒𝑞 represents the equilibrium function that relates to the lattice arrangement. The equilibrium function, 𝑓 𝑒𝑞 , can be described as: 𝑓 𝑒𝑞 (𝜌, 𝑢) = 𝜌𝑤 [1 +

1 1 1 (𝑐. 𝑢) + 4 (𝑐. 𝑢)2 − 2 (𝑢. 𝑢)] 𝑐𝑠2 2𝑐𝑠 2𝑐𝑠

(3)

in which w represents weighting function across different lattice links. For the case of D3Q19 lattice model as depicted in Figure 1, the weighting functions can be described in Table 1 as:

Table 1. Weighting functions for D3Q19

Model

𝑐𝑠2

D3Q19

1/3

Node no. 𝑓0 𝑓1 − 𝑓6 𝑓7 − 𝑓18

Weight 1/3 1/18 1/36

Figure 1. 3D Lattice arrangements for D3Q19

3

Microscopic velocities for a D3Q19 lattice model is given as:

𝑒0 = (0,0,0) 𝑒1,2 = (±1,0,0) 𝑒3,4 = (0, ±1,0) 𝑒5,6 = (0,0, ±1) 𝑒7−10 = (±1, ±1,0) 𝑒11−14 = (±1,0, ±1) 𝑒15−18 = (±1,0, ±1)

(4)

2.1. Conventional BGK collision model The underlying theory of LBM is based on the discrete Boltzmann equation. Due to the complicated collision term that exist in the RHS of the Boltzmann equation, it is difficult and burdensome to solve. To ease the computation effort, Bhatnagar, Gross and Krook [31] proposed a simplified version of the collision operator. The collision operator can be replaced as 1

𝛺 = 𝜔(𝑓 𝑒𝑞 − 𝑓) = (𝑓 𝑒𝑞 − 𝑓)

(5)

𝜏

1

where 𝜔 = . The coefficient 𝜔 denotes the collision frequency and 𝜏 is the relaxation factor. 𝑓 𝑒𝑞 is the Maxwell Boltzmann 𝜏 equilibrium distribution function. By substituting the approximate collision operator, the discrete Boltzmann equation can be shown as

(6)

𝑓𝑖 (𝑥⃗ + 𝑒⃗𝑖 , 𝑡 + 1) = 𝑓𝑖 (𝑥⃗, 𝑡) + 𝛺

where 𝑖 is the index selected between the possible discrete velocity directions and 𝑒𝑖 is the direction of the selected velocity. The fluid density and macroscopic velocity can be found from the moment of the distribution function as below

𝜌 = ∑𝑖 𝑓𝑖

(7)

1

(8)

𝑢 ⃗⃗ = ∑𝑖 𝑓𝑖 𝑒⃗𝑖 𝜌

Subsequently, the equilibrium distribution, 𝑓 𝑒𝑞 can be arranged according to the Maxwell-Boltzmann distribution form as

9

3

2

2

𝑒𝑞 2 𝑓𝑖 (𝑥⃗, 𝑡) = 𝑤𝑖 𝜌 [1 + 3(𝑒⃗⃗⃗𝑢 ⃗⃗⃗𝑢 ⃗⃗2 ] 𝑖 ⃗⃗) + (𝑒 𝑖 ⃗⃗) − 𝑢

(9)

Equation 5 will replace the commonly used Navier-Stokes equation in CFD simulations. It is also possible to derive N-S equation from Boltzmann equation using the Chapman-Enskog model.

4

2.2. Incompressible BGK model

In the fundamental form of BGK, the equilibrium term is multiplied by the fluid density as shown in equation 5. In incompressible model, the density tem, 𝜌 will be formulated as 𝜌 = 𝜌0 + 𝛿𝜌 where 𝜌0 is a constant value and 𝛿𝜌 is the infinitesimal changes in density. The equilibrium distribution function then becomes [16] 9

3

2

2

𝑒𝑞 2 𝑓𝑖 (𝑥⃗, 𝑡) = 𝑤𝑖 [𝜌 + 𝜌0 {3(𝑒⃗⃗⃗𝑢 ⃗⃗⃗𝑢 ⃗⃗2 }] 𝑖 ⃗⃗) + (𝑒 𝑖 ⃗⃗) − 𝑢

(10)

in which all terms containing 𝑢 ⃗⃗ are multiplied by the constant, 𝜌0 . Since the velocity is defined to be proportional to the momentum, the incompressible model is computationally cheaper than the standard BGK as it avoids the division by 𝜌 in equation (3). It also reduces the compressibility errors introduced by the quasi- incompressible nature of LBM.

3. Particle Image Velocimetry In early 1980’s onwards Particle image velocimetry (PIV) has been developed to map complete flow fields instantaneously [32]. Stamhuis et al and U.K Muller et al pointed out that PIV has been applied to biologically important flows characteristic [33], [34]. Particle image velocimetry (PIV) is an optical method to determine the instantaneous vector measurement in cross section view. In other words, fluid motion is made visible by adding small tracer particles and from the positions of these tracer particle at two instances of time such as particle displacement to infer the flow velocity field [35].Velocity vector of the fluid is calculated and determined from the movement of tracer particle between two light pulses:

V

x t

(11)

A primary source of measurement error is the influence of gravitational forces, 𝑔 when the density of the tracer particles is different to the density, 𝜌 of work fluid.

𝑉𝑔 = 𝑑𝑝 2

(𝜌𝑝 −𝜌) 18𝜇

𝑔

(12)

The velocity, 𝑉 lag of a particle in a continuously acceleration fluid will be:

⃗𝑠 = 𝑉 ⃗𝑃 − 𝑉 ⃗ = 𝑑𝑝 2 (𝜌𝑝 −𝜌) 𝑔 𝑉 18𝜇

(13)

⃗𝑝 (𝑡) = 𝑉 ⃗ (1 − exp(− 𝑡 ) 𝑉 𝜏

(14)

τs =

(15)

ρp dp 2 18μ

𝑠

The first derivative of position with respect to time, the technique consists in measuring the displacement of fluid (∆x) over a 5

given time interval (∆t). The position of the fluid is imaged through the light scattered by liquid or solid particles illuminated by a laser light sheet [36]. The camera lens images the target area onto the sensor array of a digital camera.

Figure 2: Illustration of the basic principle of PIV [37].

In general, interrogation areas contain multiple particle that are divided into small consecutive image and every size of the areas is defines the spatial resolution. After division of the images, the consecutive images are cross- correlated with each other. The correlation produces a signal peak, the highest peak in cross- correlation plane of consecutive image in interrogation areas produce velocity vector for the particular flow as explained in Figure 2. A two- dimensional velocity is obtained by dividing the displacement vector of all interrogation areas with time between consecutive images [37]. In other words, a velocity vector map over the whole target area is obtained by repeating the cross-correlation for each interrogation area over the two image frames captured by the camera.

4. Methodology 4.1. Simulation Setup

Y-shaped artery bifurcation aneurysm geometry is created using computer-aided design (CAD) software is shown Figure 3. The circular area represents the region of interest, where aneurysm grows. This solid model consists of one inlet and two outlets, which yield different diameters. All of the arterial branches are cylindrical. The diameter of the inlet is 8 mm, and the diameters of the outlets are 6 and 4 mm. The length of the arterial model is 90 mm. The length of the main artery is 65 mm and is measured from the inlet to the opening of the aneurysm. The lengths of the bifurcating branches are approximately 40 mm. The angle between the two branches is 90°. The diameter of the hemispherical and wide-necked aneurysm sac is 8 mm. For simplicity, a model with a 0° angle is considered optimal in this study.

6

Figure 3. Artery bifurcation aneurysms geometry.

4.1.1: Stent configuration The stents are drawn using CAD software and saved in the same part file as the artery model. The four type stent configuration is drawn and placed in the artery bifurcation model. The four stent configuration are namely: (a) half-Y (6mm), (b) half-Y (4mm), (c) cross-bar, and (d) full-Y are created as shown in Figure 4. This configuration covers the main artery and both branches. The stents are completely in contact with the arterial wall such that no gap is present between the stents and the arteries. The dimensions of the wire are set at 0.2 mm wide and 0.2 mm long. The distance between the adjacent wires is 1 mm each in the horizontal and vertical directions.

(a)

(b)

7

(c)

(d)

Figure 4. Stent configuration of (a) half-Y (6mm), (b) half-Y (4mm), (c) cross-bar and (d) full-Y.

4.1.2: Mesh Generation Mesh generation is a critical part of computational fluid simulation. The quality of meshes determines the resolution of geometric models and thus affect the accuracy of simulations. The mesh creation process of Palabos is written in the source code, and the mesh is automatically generated when the Palabos code is executed. Figures 5 and 6 show the discretized geometry of the flow and wall artery models on a 2D plane. The time step of the simulation dt is set at 0.01 s. The grid is fairly small, and the contour can be observed in detail. The size of one grid dx is calculated by dividing the length of the boundary in the x direction by the reference resolution. In our work, dx is 0.00710059. The resolution is set to 400 with reference to the x direction (72.142 mm). A high resolution corresponds to an enhanced detail of the results. Thus, mesh creation is prolonged and large output files are required.

Figure 5. Cross sectional view of discretized model on a plane

8

Figure 6. Mesh figure of the model

4.1.3: Parameter and Boundary conditions In the current simulation setup, the blood fluid is set as an incompressible Newtonian fluid. Table 2 summarizes the blood flow properties considered in this simulation. Table 2. Flow parameters Parameters

Value

Density of blood

1059 kg/m3

Inlet diameter

0.008m

Average inlet velocity

0.06m/s

Kinematic viscosity

1.587 x 10-6 m2/s

Flow rate

3.117e-06 m3/s

The density and dynamic viscosity of the blood are obtained on the basis of the properties of a glycerol solution to mimic actual blood: 1059 kg/m3 and 1.5873 × 106 m2/s, respectively [38]. A constant parabolic velocity profile is imposed on the inlet, and both outlets are subjected to zero pressure. The wall boundary is established under non-slip boundary conditions in Figure 7. An incompressible Bhatnagar–Gross–Krook (BGK) model is used and a Poiseuille profile is selected as an inflow boundary condition. The Poiseuille flow provides the pressure drop in the incompressible fluid in the laminar flow through a long cylindrical pipe of the constant cross section.

9

Figure 7. Boundary condition of inlet, outlet and wall.

4.2: Experimental Setup using Particle Image Velocimetry An artery bifurcation model identical to that in the simulation is fabricated with a high-precision computer numeric control machine by using two pieces of Perspex with the following dimensions: width of 90 mm, length of 100 mm, and thickness of 10 mm. The two pieces are then joined together with a bolt and a nut to form the Y-shaped artery bifurcation in Figure 8. The stent configuration is constructed using a stainless steel wire mesh with a diameter of 0.2 mm and a size of 1 mm × 1 mm. The finished mesh wire stent is inserted properly into the Perspex artery bifurcation in Figure 9 to simulate an actual aneurysm bifurcation.

Figure 8. Perspex artery bifurcation model

10

(a)

(b)

(c)

(d)

Figure 9. Stent placement in the Perspex model (a) half-Y (6mm), (b) half-Y (4mm), (c) cross-bar and (d) full-Y.

A custom PIV setup is developed to obtain the flow field in the artery bifurcation model. The setup consisting of an imaging sensor and an illumination system is established in accordance with the methods of Willert et al. (2010). A DMK230618 monochrome CCD camera (Imaging Source Company) is used as an imaging device. The pixel sizes of the images are 640 × 480. The camera is paired with a macro lens to focus the CCD to the area of interest and thus can record approximately 120 frames per second and can be triggered externally. Seeding particles are illuminated by using a high-power light emitting diode (LED) and a driver circuit. The LED is modified to generate sufficient light at a backscattering mode. A synchronizer is used to control the imaging device and the LED driver circuit. The vector field of the flow in the Y bifurcation model is calculated by correlating the two image frames in PIVLab software. The interrogation window used for the analysis is 32 × 32 pixels and the overlap is 50%. For further validation, an additional area that includes the subpixel interrogation is used to improve accuracy. The subsequent processes are performed using Matlab.

4.3: Experiment Setup and Measurement The schematic of the experimental setup of the aneurysm model is shown in Figure 10. The black lines represent the wire connection, while the red lines represent the flow path of the fluid connected by pipes. The working fluid used in the experiment is composed of 22% by mass of glycerol in water solution. The properties of the glycerol solution mimic those of the blood (Miner and Dalton) [38]. The aqueous glycerol solution is characterized by a density of 1059 kg/m3 and a kinematic viscosity of 1.587 × 106 m2/s. PIV seeding particles are then added to the solution. The seeding particles used in this study are PSP-50, which are polyamide seeding particles (Dantec Dynamics) with a diameter of 50 µm and a mass of 250 g. The glycerol solution in the tank is pumped through the system via a 12-V DC water pump. The fluid is initially allowed to pass through a flow meter with a flow rate of 0.2–30 L/min. The instantaneous flow rate and fluid velocity are measured by using an Arduino Uno R3 board, and the obtained measurements are displayed on a computer. The voltage supply to the water pump determines the velocity of the fluid, that is, a high voltage corresponds to a high blood flow velocity. The inlet velocity is retained at 0.06 m/s. The fluid passing through the flow meter is then allowed to flow into the inlet of the Perspex model to 11

fill the artery. Afterward, the fluid is directed to flow back into the tank via the two outlets. A TSI C49 camera is fixed on a retort stand, connected to the computer, and used to record the flow at a maximum frame rate of 120 fps. This camera is controlled by using IC Capture 2.4. An LED light is also installed at the bottom of the Perspex model to illuminate the PIV seeding particles and to increase the image clarity. The LED light is connected to a LED driving circuit, which regulates the power to the LED and prevents overheating and instability.

Figure 10. Schematic diagram of flow system

Figure 11. Perspex model in PIV experiment setup

12

TSI C49 Camera

Arduino Set-up

Figure 12: Experiment set- up

5. Results and Discussion 5.1. Validation between PIV and LBM methods 1 2 3 4 5 6 7 8 9 10

In this study, the effectiveness of the LBM-based code is validated through PIV experimental observation. The effects of stent placement on the velocity and pressure of blood flow are investigated. PIV measurement and LBM simulation results are extracted using PIVlab and ParaView, respectively. PIVlab can calculate velocity contours and vector fields. ParaView in turn can post-process the computed LBM results in terms of velocity contours, pressure contours, and stream lines. In our work, nine points of interest are selected on the middle plane of the artery, and all of these points are near the artery bifurcation region. The points are indicated in red dots and labeled as A, B, C, D, E, F, and G in Figure 13. For validation, similar points at the same position are collected for the PIV measurement and LBM simulation. The selected points include the region outside the aneurysm (G), on the aneurysm neck (D, E, F), and inside the aneurysm (A, B, C). The coordinates of the points in ParaView are shown in Table 3.

11 12

Figure 13. Coordinates of 9 points of interest

13

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Table 3: Coordinates point in LBM

Point

Coordinates x

y

A

1.49822

0.177515

2.67692

B

1.54083

0.177515

2.62722

C

1.45562

0.177515

2.62722

D

1.57633

0.177515

2.57751

E

1.49822

0.177515

2.57751

F

1.42012

0.177515

2.57751

G

1.49822

0.177515

2.4213

5.2. Effect of stent on flow pattern Recirculation occurs inside the aneurysm sac, as depicted by the stream line plots in Figure 14, and comprises two vortices. The vortex near the 6 mm branch is large, whereas the other vortex near the 4 mm branch is slightly small. Vortex size is proportional to arterial size, that is, large arteries correspond to large vortex formations. Thus, the vortex formed in the 6 mm artery is larger than that formed in the 4 mm artery. This finding demonstrates that the size of the branches affects the recirculation pattern inside the aneurysm sac. The branch with a large diameter causes a large internal vortex in the aneurysm sac. Consequently, the branch of the artery influences the blood flow in the axis artery [32]–[38]. As vortex size increases, the fluid shear stress near the aneurysm sac increases and thus exacerbates aneurysm because a high fluid shear stress may pose high risks of aneurysm deformation and arterial wall weakening. These conditions may consequently cause aneurysm rupture and probably death [2], [39]. Figures 15 and 16 illustrate the velocity contours from the PIV and LBM of the configurations without stent. Flow separation occurs at each branch near the upper wall of the artery, and this condition may weaken arterial walls. This condition also indicates an increased pressure at the aneurysm region. Persistent flow separation near the aneurysm region can substantially increase the wall shear stress. As a result, aneurysm grows and ruptures [3], [40]. Substantially, with half-Y (6mm) stent deployed, the flow separation region at the 6mm branch side is significantly reduced in size. Similar trend occurs with half-Y (4mm) stent, where the separation region at the 4mm branch is diminished. For cross-bar and full-Y stent configurations, the separation regions at both sides are minimized, but not as much compared to those in half-Y configurations. In general, stent placement can reduce the size of the separation region and prevent wall weakening at the bifurcation region. Consequently, the accumulation of the complex flow, such as flow separation in arteries, weakens the arterial wall and thus may lead to pressureinduced arterial rupture in the aneurysm [37]–[40].

37 38

z

Figure 14 : Recirculation in aneurysm sac

14

(a)

(b)

(d)

No stent

Half-Y (6mm)

(c)

Cross-bar

Half-Y (4mm)

(e)

Full-Y

Figure 15: Velocity Contour obtained from PIV measurement: (a) no stent, (b) half-Y (6mm), (c) half-Y (4mm), (d) cross-bar and (e) full-Y.

Flow Separation region

(a)

No stent

15

(b)

(d)

Half-Y (6mm)

(c)

Cross-bar

Half-Y (4mm)

(e) Full-Y

Figure 16: Velocity Contour obtained from LBM measurement: (a) no stent, (b) half-Y (6mm), (c) half-Y (4mm), (d) cross-bar and (e) full-Y.

5.3: Effect of stent on velocity distributions Figures 15 and 16 show the velocity contours from the PIV and LBM. In both methods, the stent obstructs the blood flow in the aneurysm sac and significantly alters flow pattern. The blood flow in the aneurysm sac is obstructed and its speed at the vicinity of the aneurysm sac is reduced when the stent is placed in the main artery [41]. The velocities in the 6 and 4 mm branches are higher than those in the main artery (8 mm) [42]. This result indicates that reducing the size of the branches increases the flow velocity. Therefore, velocity is inversely proportional to arterial diameter [43]. The overall trend shown in Figures 17 is confirmed in Figures 15 and 16 showing that the placement of the stent effectively reduces the velocity at all selected points. PIV results are consistent with LBM findings, as indicated by their similar plot trend. This effect may be attributed to the difference between the densities of the seeding particle and glycerol in the PIV measurement and the discretization of the lattice in the LBM simulation. Therefore, the LBM simulation results are slightly lower than the PIV measurement results. The blood flow velocities of the points far from the aneurysm sac region are higher than those of the points near that region. The peak velocity occurs at point G, which is outside the aneurysm sac in the artery bifurcation geometry, and no obstacles are observed at the entrance (point G) of the artery [44]. Before the particles reach the branch of the artery, the blood flow velocity eventually increases within that area. In Figure 17, a maximum velocity of 0.043 m/s is observed at point G without a stent, whereas a minimum velocity of 0.0046 m/s is obtained at point A with half-Y (6mm) stent placement in the PIV measurement. A maximum velocity of 0.016 m/s is observed at point F. By comparison, a minimum velocity of 0.0029 m/s is obtained at point A with half-Y stent placement in the LBM measurement shows in Figure 17. The high flow stagnancy occurring at point A likely promotes thrombosis formation inside the aneurysm sac and can completely clot the aneurysm area. Arterial circulation can be effectively diverted from the aneurysm area and aneurysm rupture can be prevented because the half-Y stent substantially reduces the flow velocity by eightfold [9], [34]. Points D, E, and F are located at the opening of the aneurysm sac. Point D, which is the point nearest the 4 mm branch, yields the lowest velocity, whereas point F, which is the point nearest the 6 mm branch, manifests the highest velocity among the three points. This result is attributed to the distribution of velocity in the recirculation vortex in the aneurysm sac [45]. Low distribution velocity in the vortex circulation in the aneurysm sac likely increases the frequency of shear stress because of a 16

pressure increase and thrombus formation within the area [44]. Therefore, thrombus can form at point D. Point F is characterized by a relatively high flow velocity. As such, aneurysm likely ruptures at the left side. In Figures 18 and 19, the reduction in velocity after stent placement is high at points relatively far from the aneurysm sac. Generally, both the half-Y stent configurations are more effective in reducing velocity compared to other configurations. In the PIV measurements, the highest percentage of velocity reduction occurs at points G (70 %) with half-Y (6mm) stent configuration. In the LBM simulation, half-Y (6mm) stent configuration provides the highest percentage of velocity reduction at point G (55 %). For point E and F, half-Y (4mm) appears to be most effective in reducing velocity with maximum of 68% reduction. Cross-bar configuration has the highest percentage velocity reduction at points inside the aneurysm sac which is point A (27%), B (20%), and C (24%). The full-Y configuration seems to be less effective in reducing velocity as its percentage reduction is comparatively low at all points. Therefore, stent placement successfully reduces the flow velocity in the aneurysm sac and thus possibly promotes thrombus formation effectively. Consequently, the risk of bleeding is reduced.

Figure 17: Graph of blood velocity in the aneurysm sac at selected coordinates using various stent configurations obtained by LBM and PIV.

17

Graph of Percentage Reduction in Velocity After Stent Placement for PIV Percentage reduction (%)

80 70 60 50 40 30 20 10 0 A

B

C

D

E

F

G

Coordinate half-Y (6mm)

half-Y (4mm)

cross-bar

full-Y

Figure 18: Graph of percentage reduction in velocity after stent placement for PIV measurement.

Graph of Percentage Reduction in Velocity After Stent Placement for LBM

Percentage reduction (%)

70 60 50 40 30 20 10 0 A

B

C

D

E

F

Coordinate half-Y (6mm)

half-Y (4mm)

cross-bar

full-Y

Figure 19: Graph of percentage reduction in velocity after stent placement for LBM measurement.

18

G

5.4: Effect of stent placement on pressure Figure 20 shows the pressure contours obtained from LBM simulation. From Figure 21, the maximum pressure occurs at point G (7.1Pa) with cross-bar and full-Y stent configuration. The minimum pressure happens at point A (2.0Pa) with half-Y (6mm) configuration. The effect of stent placement on the pressure can best be explained by Figure 21. It is obvious to see that both the half-Y stent configurations generally reduce the pressure while the deployment of cross-bar and full-Y stent configuration raises the pressure. From Figure 22, the maximum reduction in pressure can be found at point F with half-Y (4mm) configuration which is up to 57%. On the other hand, the maximum increment of pressure at point D with full-Y configuration is up to 57% as well. The half-Y (6mm) stent configuration can be said as the best configuration as it consistently reduces the pressure at the bifurcation region and decrease the risk of aneurysm rupturing.

no stent

half-Y (6mm)

half-Y (4mm)

cross-bar

Full-Y

Figure 20: Pressure contours obtained from LBM simulation.

19

Graph of Pressure at Selected Coordinates using Various Stent Configuration 8 7

Pressure (Pa)

6 no stent

5

half-Y (6mm)

4

half-Y (4mm)

3

cross-bar

2

full-Y

1 0 A

B

C

D

E

F

G

Coordinate

Figure 21: Graph of blood pressure at selected coordinates using various stent configurations from LBM simulation.

Graph of Percentage change in Pressure After Stent Placement for LBM

Percentage Pressure Change %

60 50 40 30 20 10 0 A

B

C

D

E

F

Coordinates Half-Y(6mm)

Half- Y(4mm)

Cross-bar

Full- Y

Figure 22: Graph of percentage change in pressure after stent placement from LBM simulation.

20

1 2 3 4 5 6 7 8 9 10 11 12 13 14

5.5 Highlight of best configuration

Based on the results above, the overall best configuration that significantly reduces the velocity and pressure at the bifurcation region is the half-Y (6mm) stent configuration. It provides better area coverage at the opening of the aneurysm and diverts the flow into the aneurysm effectively. Based on Figure 23, after placing the halfY (6mm) stent, the velocity is reduced from 0.0064 m/s to 0.0046 m/s at point A which is inside the aneurysm sac close to the wall. The half-Y (6mm) stent configuration also significantly reduces the velocity at point G outside the aneurysm sac from 0.0434 m/s to 0.0129. From LBM simulation as depicted in Figure 23, the halfY (6mm) stent configuration reduces the velocity at point A from 0.0037 m/s to 0.0029 m/s, and 0.0375 m/s to 0.017 m/s at point G. Both PIV and LBM simulation methods agree that the half-Y (6mm) configuration reduces the velocity of points at region of interest by considerable amount. From Figure 24, the pressure is reduced at point A from 3.025 Pa to 2.086 Pa, at point E from 4.039 Pa to 2.688 Pa, and at point F from 5.121 Pa to 3.752 Pa.

0.05

Graph of Velocity at Selected Coordinates With No Stent and HalfY Stent Configuration From PIV and LBM

0.045

Velocity (m/s)

0.04 0.035 0.03 0.025 0.02 0.015 0.01 0.005 0 A

E

G

Coordinate 15 16

Figure 23: Graph of velocity at selected coordinates with no stent and half-Y stent configuration obtained from PIV and LBM.

17 18 19 20

21

Graph of Pressure at Selected Coordinates With No Stent and Half-Y Stent Configuration From LBM 6

Pressure (Pa)

5 4 3

no stent

2

half-Y

1 0 A

E

G

Coordinate 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Figure 24: Graph of pressure at selected coordinates with no stent and half-Y stent configuration from LBM.

6. Conclusions Experiments and simulations are carried out to study the flow characteristics of blood through an artery bifurcation aneurysm model. The effect of stent configurations on the flow is analyzed by using PIV measurement and LBM simulation. The flow pattern, velocity and pressure of the blood are studied and compared. LBM simulation results are consistent with PIV results in all of the computed velocities at points A– G. In terms of the flow patterns, the placement of a stent in arteries effectively reduces the blood flow in the aneurysm sac domain. The size of flow separation regions that causes an increase of pressure in the aneurysm sac is reduced after a stent is placed. The decrease in size of these regions is attributed to flow diversion. Consequently, this effect reduces the possibility of aneurysm rupture in the aneurysm sac. Both the half-Y (6mm) and half-Y (4mm) stent configurations are able reduce the pressure at most of the points. On the other hand, cross-bar and full-Y stent configurations were found to increase the pressure of the region. This will increase the risk of aneurysm rupturing or bleeding which is undesirable. The half-Y (6mm) stent configuration is chosen to be the best configuration in reducing the blood velocity and pressure in the region of interest. The high velocity reduction occur at the half-Y (6mm) stent will enhance the flow stagnancy in the aneurysm sac and thus promote thrombus formation. These thrombus formation will enclose the affect ed area but still permitting blood flow into covered arteries thereby excluding aneurysm from the circulation. This will avoid weakening of wall arteries that may lead to aneurysm rupture. The huge reduction in pressure also lowers the risk of bleeding and aneurysm rupturing. In this research study, both half-Y (6mm) and half-Y (4mm) configuration have higher velocity reduction than full- Y and crossing configuration. It is shown that half-Y configuration is able to reduce the flow separation region at the branch artery and promote thrombus formation within the aneurysm sac significantly which prevent the weakening of the wall of arteries. This study successfully demonstrates the feasibility of stent placement for aneurysm treatments. 22

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Acknowledgements The work was partly supported by the Short Term Grant from the Division of Research and Innovation, Universiti Sains Malaysia, FRGS Grant 6071322 from the Ministry of Higher Education, Malaysia and USM Fellowship Scheme P-CM0005/16(R).

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