CFD Analysis of Intracranial Aneurysms

December 8, 2009

Group Members: James Arter, Austin Ramme, Brian Walsh

Introduction

• Intracranial aneurysms have a prevalence of 2-6.5%1

• Saccular aneurysms spherical shape

• Anterior communicating artery (AcomA) are the most common2

• Rupture of intracranial aneurysms are devastating

• Available treatments– Conservative

– Surgical clipping

– Embolization coiling http://en.wikipedia.org/wiki/Anterior_communicating_artery

Introduction: Rupture Risk

http://emedicine.medscape.com/article/252142-overview

• Risk of rupture is 0-0.1% per year7

• Characteristics of ruptured aneurysms

• Symptomatic aneurysms3

• Posterior circulation3,7

• Greater than 5 mm in diameter3,7

• Surface irregularities and daughter sacks7

• Originating from parent arteries with larger diameters1

• Aspect ratio (height to neck ratio)7

• Low risk <1.4

• Intermediate risk 1.6-2.2

• High risk >3

Goals of the Study• Investigate the effect varying aspect ratio on wall

shear stress and flow patterns in parent vessel and aneurysm

• Investigate an aspect ratio of 2.6 that is not on the risk scale

• Hypotheses:

– As aspect ratio increases, we will also see an increase in aneurysm wall shear stress

– As aspect ratio increases, changes in fluid flow patterns will become more apparent

Materials and Methods

• Assumptions

– Laminar, Steady, Fully Developed flow

– Newtonian Fluid (ρ=1.06g/cc, µ=.035P)

• Use of 2D, Idealized Model Geometry

– Internal diameter of AcomA of 0.21cm1

– Average aneurysm neck diameter of 0.35cm1

– Average blood flow velocity of 30cm/s1

• Computational Analysis

– Mesh generation in Gambit

– CFD simulation in FLUENT

Materials and Methods (2)

Finite Element Model Number of Quad Elements

AcomA Normal Case 4000

AcomA with Aneurysm (1.0 Ratio) 6883

AcomA with Aneurysm (2.0 Ratio) 6863

AcomA with Aneurysm (2.6 Ratio) 7000

AcomA with Aneurysm (4.0 Ratio) 6790

*A convergence study was performed to ensure appropriate mesh density

• Dimensions correlated with in vivo measurements 1

• Height was only factor that was varied

Results & Discussion

• Normal AcomA Case• Fully developed flow reached

• Uniform WSS of 3Pa

• Maximum Axial Velocity of .4m/s

• Aneurysm Cases• Maximum WSS at distal aspect of aneurysm neck (Point A) observed in

all cases

Results & Discussion (2)• Typical aneurysm WSS plot

(4.0 ratio shown)

– Decreased wall shear stress on opposing vessel wall and dome of aneurysm

y = 0.187ln(x) + 5.168R² = 0.997

0

1

2

3

4

5

6

0 1 2 3 4 5

Max

imu

m W

SS (

Pa)

Aspect Ratio

• Max WSS comparison

– Increasing WSS at neck with increasing aspect ratio

Results & Discussion (3)

• Recirculation zone

• <15% of velocity magnitude observed in parent vessel

• Minimal velocity at:

• Center of recirculation zone

• Periphery of aneurysm

• Skewed flow profile

• Seen in all aneurysm cases

• Increased flow into aneurysm

• Reduced flow on opposing vessel wall

Conclusions• Results support low WSS theory of intra-cranial

aneurysm initiation & propagation

– Clinically, aneurysm propagation occurs in dome region opposed to neck region.

• Increasing aspect ratios demonstrate more exaggerated differences in flow patterns

– Aneurysm aspect ratio of 2.6 could be considered intermediate risk based on WSS values

• Regions of stagnant flow, are associated with regions of low WSS.

• Decreased regions of wall shear stress have been shown to be associated with lesion formation and subsequent atherogenesis.

Future Work

• Further validation of low WSS theory of aneurysm propagation required.

– 3D, patient specific models for CFD simulation & experimentation

– Biochemical analysis will become essential to explain effects of mechanical factors• Intra-aneurysm endothelium-dependent Nitric Oxide is released in

response to shear stress

• Directly influence cell morphology

References1. Chien A, Castro MA, Tateshima S, et al. Quantitative hemodynamic analysis of brain aneurysms at different locations. AJNR Am J

Neuroradiol. 2009;30:1507-1512.

2. Park JH, Park SK, Kim TH, et al. Anterior communicating artery aneurysm related to visual symptoms. J Korean Neurosurg Soc. 2009;46:232-238.

3. Lysack JT, Coakley A. Asymptomatic unruptured intracranial aneurysms: Approach to screening and treatment. Can FamPhysician. 2008;54:1535-1538.

4. Gentile S, Fontanella M, Giudice RL, et al. Resolution of cluster headache after closure of an anterior communicating artery aneurysm: The role of pericarotid sympathetic fibres. Clin Neurol Neurosurg. 2006;108:195-198.

5. Sforza DM, Putman CM, Cebral JR. Hemodynamics of cerebral aneurysms. Annu Rev Fluid Mech. 2009;41:91-107.

6. Cebral JR, Putman CM, Alley MT, et al. Hemodynamics in normal cerebral arteries: Qualitative comparison of 4D phase-contrast magnetic resonance and image-based computational fluid dynamics. J Eng Math. 2009;64:367-378.

7. Lall RR, Eddleman CS, Bendok BR, et al. Unruptured intracranial aneurysms and the assessment of rupture risk based on anatomical and morphological factors: Sifting through the sands of data. Neurosurg Focus. 2009;26:E2.

8. Qureshi AI, Janardhan V, Hanel RA, et al. Comparison of endovascular and surgical treatments for intracranial aneurysms: An evidence-based review. Lancet Neurol. 2007;6:816-825.

9. Chandran KB, Yoganathan AP, Rittgers SE. Biofluid Mechanics: The Human Circulation. 2007.