CFD Simulation in a Real Cerebral Aneurysm after Stenting · CFD Simulation in a Real Cerebral...

CFD Simulation in a Real Cerebral Aneurysm after Stenting

ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany

Gábor JANIGA, Oliver BEUING, Santhosh SESHADHRI, Mathias NEUGEBAUER, Rocco GASTEIGER, Bernhard PREIM, Georg ROSE,

Martin SKALEJ, Dominique THÉVENIN

University of Magdeburg “Otto von Guericke”, Germany


the two provided cases have been submitted to the

organizers. A comparison of the treatment plans

provided by the different research groups will be

performed. Verification of the predicted outcome

and comparison with the real outcome will be

addressed as well.

The two patient data and the stent data have

been provided and the MOBESTAN group has

performed the virtual stent deployment and

haemodynamic simulation with the provided flow

conditions. In this paper, the main solution steps

and difficulties for the first test case provided by the

VISC09 challenge are discussed.

1.2. Medical Background

Over the past years, intracranial stenting has

been proven to be effective in several different

situations as a supporting procedure in the treatment

of ruptured and unruptured cerebral aneurysms.

Now, a substantial number of aneurysms can be

treated by an endovascular approach (i.e., coiling),

where surgical clipping was necessary in the past.

This includes fusiform or broad-necked aneurysms

and even cases in which arterial vessels arise from

the aneurysm itself.

In the recent years, considerable progress has

been made. This comprises different stent designs,

easier deployment and an improved navigation even

through tortuous vessels. Complication rates thus

decreased over time and technical success increased

considerably.

By establishing intracranial stenting in the

therapy strategy for cerebral aneurysms it was soon

noticed that substantial flow alteration occurred in

some aneurysms, while it remained unchanged in

others. Even gradually, but frequently complete

thrombosis has been observed.

As a consequence, many scientists now focus

on developing new stents, which allow aneurysm

occlusion without coiling. The major advantage of

such a therapy would be that it is not necessary to

probe the vulnerable aneurysm sack with the micro

catheter, guide wire or coils. It is highly probable

that complication rates (i.e., perforation, coil

displacement) can be reduced by the use of

intracranial stenting alone. Intervention time can be

decreased, which would also be a benefit, especially

for the patient.

One of the actual problems is that the treating

physician does not exactly know where to place the

stent for an optimal result, i.e., flow alteration and

consecutive thrombosis of the aneurysmal lumen.

Simulation of blood flow will not only help to

improve the technical and clinical results, it is also

necessary to develop better stent designs for this

purpose.

The modification of different haemodynamic

parameters is analyzed in this work. Here, the

commonly investigated wall shear stress has not

been studied in detail, because the maximum value

of this parameter may not correlate with the

position of the rupture. Instead, the inflow rate at

the aneurysm neck and the stasis in the aneurysm

has been investigated.

A successful stenting treatment should change

the haemodynamics in the aneurysm producing

thrombogenic conditions, i.e., reducing the flow

velocity and elongating the stasis. The numerical

flow simulation provides various haemodynamic

quantities. The qualitative examination can be

performed showing contour or vector plots of the

velocity. However, a quantitative analysis is

essential in order to accurately quantify the effect of

the stent deployment. The flow stasis has been

computed applying the turnover time, as it has been

done in [2] for idealized and patient-specific

geometries.

It is supposed that the stent deployment may

potentially decrease the blood flow from the

aneurysm excluding it from the arterial circulatory

system. To stimulate the aneurysmal thrombosis

[3], the increase of the stasis in the aneurysm is

targeted.

Previous studies [4-6] have shown that

increasing aneurysmal flow turnover time can

produce thrombus formation in cerebral aneurysms.

The turnover time is applied as an indicator of stasis

[7] in this work.

Figure 1. The original flow configuration of the

first case provided by the VISC09 challenge

2. METHODS

To prevent rupture, the aneurysm should be

treated. The motivation of this work is to study the

effect of the stent deployment in a real patient-

specific geometry in order to prevent the rupture of

the intracranial aneurysm.

In this section, the methods to solve the first

test case provided by the VISC09 challenge are

summarized.

Computational geometry


Improving surface mesh quality


Elastic, distance-based shape deformation


The stent geometry has been deployed on a trial and error basis after bending the stent geometry.

Deployment of the stent after deformation


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Computational grid


• A hybrid tetra/prism body conformed numerical mesh has been used.

• A three-layer-prism mesh has been built on the smoothed triangle surface on the wall producing an average cell wall distance of the first element of the wall as 1.5 µm.

• This allows an extremely good resolution of the boundary layer.

• The computational mesh involves 4,590,290 finite volume cells.

Computational grid


• The computations have been performed in parallel using four cores applying the commercial CFD solver ANSYS Fluent 6.3 based on a finite-volume discretization.

• A double precision solver using a second-order upwind solution has been applied.

• Normalized residuals of 10-6 are obtained wihtin less than 6 hours of computing time.

Computational details


Wall shear stress distribution


Streamlines in the stented geometry


Determination of the inflow rate


After deploying the stent, the inflow rate at the neck of the aneurysm is reduced by almost a factor of 4 resulting in an increase of the turnover time by the same factor.

Inflow rate [cm3/s]

Tu r n o v e r time [s]

Ratio of t u r n o v e r time [-]

Without stent 0.955 0.295 -

With stent 0.26 1.085 3.67

Quantitative comparison of the inflow rate and the turnover time


• Developed suitable CFD methodology, for model as well as for real geometry

• Generation of excellent numerical grid

• Able to investigate and quantify flow modifications induced by stents

• Developed quantitative criteria suitable for flow analysis

Conclusions


• Optimization of stents for aneurysm treatment

• Development of alternative stent geometries

• Checking fluid–structure interactions

• Development of further models, in particular for blood clogging

Further challenges


Financial support of:

• Land Saxony-Anhalt (Germany) for MOBESTAN project

• IMPRS (International Max-Planck Research School for Analysis, Design and Optimization in Chemical and Biochemical Process Engineering)

[email protected]

Thank you for your attention!

CFD Simulation in a Real Cerebral Aneurysm after Stenting · CFD Simulation in a Real Cerebral...

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