A CFD Approach to Quantifying the Hemodynamic Forces in Giant

Cerebral Aneurysms

Anna Hoppe1, Brian Walsh1

1Department of Biomedical Engineering, University of Iowa

Dr. Ching-Long Lin2

2Department of Mechanical Engineering,

University of Iowa

 

The Hemodynamics of Aneurysms

What is an aneurysm?

A localized, blood-filled dilation of a blood vessel caused by

disease or weakening of the vessel wall3

The diagnostic implications:4

1) High risk of hemorrhage

- causing stroke or death

- mortality rate: 68% (after 2 yrs.), 85% (after 5 yrs.)

2) Cerebral compression or distal thromboembolism

- leading to progressive neurologic deficits

The Hemodynamics of Aneurysms and Its Therapeutic Implications

An understanding of the hemodynamics within a particular aneurysm could aid:4

- a prediction of when rupture is likely to occur- assessment of possible treatment options

Therapeutic treatments are dependent upon aneurysm geometry4

Possible Aneurysm Geometries

Figure 1: Illustration of a saccular aneurysm6

Figure 2: Illustration of a fusiform aneurysm5

Therapeutic Treatment Options

• Stent used for artery reconstruction4

– Used for saccular aneurysms– Dangerous with giant fusiform aneurysms (> 25 mm

in diameter)• Coil Embolization Technique4

– Used for saccular aneurysms– Dangerous with giant fusiform aneurysms

Figure 3: Illustration of the coiling technique2

Therapeutic Treatment Options

• Surgically remove the aneurysm4

– Only done with saccular aneurysms

• Occlude a proximal supplying artery to the aneurysm4

– Most commonly done with giant fusiform aneurysms– NO quantitative validation for using this method

• A CFD approach for quantitative validation is outlined in an article in AJNR by Jou, L., et al.

Quantitative Validation for Proximal Artery Occlusion

• A CFD approach for quantitative validation of proximal artery occlusion:– Article in Am. J. of Neuroradiology by Jou, L., et al.4

– Determined velocity field, wall shear stress and pressure distribution within a giant saccular cerebral aneurysm of a patient (MR image)4

• Before and after simulated proximal artery occlusion4 • Conclusion:4

– Important to know hemodynamic conditions when planning treatment– Flow patterns: dependent upon flow in each feeding artery

» May not improve the long-term stability of the aneurysm

Our Project: Quantitative Validation for Proximal Artery Occlusion

Use a patient-specific aneurysm geometry to determine the affect of proximal artery occlusion on:

Wall shear stressPressure distributionVelocity (in the aneurysm sac)

To answer the question: Is proximal artery occlusion a valid treatment option for giant aneurysms (does it reduce hemodynamic forces in the aneurysm sac)?

In general: We would also like to compare wall shear stress, pressure distribution, and velocity in diseased artery with and without the proximal feeding artery occluded.

We obtained:- patient-specific MR image of a saccular aneurysm in the Circle of Willis in the brain - courtesy of Professor Raghavan, Dept. of Biomedical Engineering at the U of I

Our Project: Quantitative Validation for Proximal Artery Occlusion

• Like the article by Jou, L., et al.4 – Our MR image is a saccular aneurysm and is in the same

general location in the brain• In the article: aneurysm occurred off the basilar artery (feeds

into the Circle of Willis)4 • Our saccular aneurysm: occurred in the Circle of Willis

Figure 4: Illustration of the Circle of Willis in the human brain1

Methods

• Meshing started with an STL file.• Using Magics software hard edges were removed• Once the STL was loaded into Gambit the surface mesh

was deleted, leaving just a surface.• Next the edges, faces and volume were created then

meshed; yielding a volumetric mesh. Boundary conditions:– Inlets- Velocity Inlet– Outlets- Outflow– Aneurysm body- Wall– No slip condition assumed (See Figure 5)

Material: Blood(assumed to be Newtonian fluid)Viscosity: .0035 PoiseDensity: 1056 kg/ m3

Time: TransientModel: k-omega

Methods (cont.)

Outlet25 Nodes

Velocity Inlet 75 Nodes

Outlet50 Nodes

Velocity Inlet25 Nodes

Velocity Inlet50 Nodes

Wall Figure 5: Geometry of aneurysm used in simulation

Each of the three inlets was occluded by changing the boundary condition from velocity inlet to wall in order to examine the effects on the velocity vectors, pressure, and wall shear stress in the aneurysm.

Secondary Boundary Conditions:

Assumptions

• Rigid Walls– Valid because the aneurysm is approximately twice as

stiff as other vessels due lack of elastin.– Yields higher WSS values than an actual patient

• Steady flow at peak systole(60 cm/s)– Yields maximum hemodynamic forces which have the

most clinical relevance• Velocity vectors normal to inlet surface

• Assumed to be turbulent flow because Re72413(1.056 )(60 )(0.4 )

Re0.0035

g cmcmvd cm s

gcm s

• Scheme– SIMPLE

• Spatial Discretization– Gradient- Least Squares Cell Based– Pressure- Standard– Momentum- second order upwind– Turbulent Kinetic Energy- Second Order Upwind– Specific Dissipation Rate- Second Order Upwind

• Transient Formulation– Second Order Implicit

Solution Methods

• Calculations– Time Step- .1 seconds– Number of Time Steps- 10– Iterations/Time Step- 20

Pressure Distribution Unoccluded Aneurysm Blocked Large Inlet

Blocked Medium Inlet Blocked Small Inlet

Wall Shear Stress Distribution Unoccluded Aneurysm Blocked Large Inlet

Blocked Medium Inlet Blocked Small Inlet

Velocity Distribution No Blocked Inlets

Velocity Distribution Blocked Large Inlet

Velocity Distribution Blocked Medium Inlet

Velocity Distribution Blocked Small Inlet

Is Proximal Artery Occlusion A Valid Treatment Option?

Pressure Distribution (in aneurysm sac)

- Unoccluded aneurysm: ≈ 959 Pa

- Blocked small inlet: 202 Pa – 927 Pa

- Blocked medium inlet: -7.71 Pa – 233 Pa

- Blocked large inlet: -5.23 Pa – 116 Pa

The unoccluded aneurysm had the highest pressure distribution. Blocking the small inlet increased the pressure distribution to a level similar to that before occlusion.

Increased pressure within aneurysms has been known to lead to subsequent aneurysm growth4

Thus, blocking the medium and large inlets constitute a better treatment modality from a pressure standpoint. Choosing which feeding artery to block is highly geometry dependent.4

Jou, L., et al. found: prior to proximal artery occlusion, increased pressure distribution due to primary flow diversion

Findings by Jou, L., et al.

(A) Calculated velocity field (m/s). Note highly asymmetric flow secondary to near occlusion of the right vertebral artery. There is a large region of slow re-circulating flow (blue) in the pouch of the aneurysm.

(B) Calculated distribution of pressure (range from 0 to 150 Pascal). The pressure distribution has no regions of pronounced increased pressure, with a smooth drop from inlet to outlet vessels.

Is Proximal Artery Occlusion A Valid Treatment Option?

Wall Shear Stress (WSS) Distribution (in aneurysm sac)

- Unoccluded aneurysm: ≈ 0 – 4.39 Pa

- Blocked small inlet: 0 Pa – 3.66 Pa

- Blocked medium inlet: 0 Pa – 2.06 Pa

- Blocked large inlet: 0 Pa – 1.14 Pa

The unoccluded aneurysm had the highest WSS distribution. Blocking the small inlet increased the WSS distribution to a level similar to that before occlusion.

Increased WSS within aneurysms has been known to lead to endothelial cell lining damage4

Thus, blocking the medium and large inlets constitute a better treatment modality from a WSS standpoint

Jou, L., et al. found: prior to proximal artery occlusion, increased WSS due to primary flow diversion

Is Proximal Artery Occlusion A Valid Treatment Option?

Velocity Distribution (in aneurysm sac)

- Unoccluded aneurysm: large re-circulating flow region, high velocity out of small outlet

- Blocked small inlet: large re-circulating flow region, high velocity out of small outlet

- Blocked medium inlet: large re-circulating flow region, high velocity out of small outlet

- Blocked large inlet: large re-circulating flow region, high velocity out of small outlet

The velocity and flow recirculation patterns are not largely effected by sacrificing any of the proximal feeding arteries however the velocity magnitudes are decreased by proximal atery

occlusion

Is Proximal Artery Occlusion A Valid Treatment Option?

Due to the high dependence of geometry on blood flow patterns within an aneurysm, sacrificing a proximal feeding artery is a viable treatment modality in limited cases, to be determined on a patient-specific basis.

References

[1] The A.D.A.M Editorial Team . Stroke Image. Retrieved November 29, 2009, from The How Stuff Works website: healthguide.howstuffworks.com/stroke-picture-....

[2] The AVM Foundation. What is Embolization? Retrieved November 29, 2009, from The AVM Foundation: www.avmfoundation.org/index.php?go=learn:avm:....

[3] Houghton Mifflin Company. (2007). The American Heritage® Medical Dictionary. Houghton Mifflin Harcourt Publishing Company.

References (continued)

[4] Jou, L., et al. (2003). Computational Approach to Quantifying Hemodynamic Forces in Giant Cerebral Aneurysms [Electronic Version.] American Journal of Neuroradiology , 24, 1804-1810.

[5] The University Hospital. Types of Stroke. Retrieved November 28, 2009, from The University Hospital: www.theuniversityhospital.com/stroke/types.htm.

[6] University of Illinois College of Medicine. (2007). What is a Brain Aneurysm? Retrieved November 28, 2009, from The University of Illinois College of Medicine website: chicago.medicine.uic.edu/.../brain_aneurysm/.