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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
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, 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
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
Improving surface mesh quality
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
Elastic, distance-based shape deformation
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
The stent geometry has been deployed on a trial and error basis after bending the stent geometry.
Deployment of the stent after deformation
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
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Computational grid
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
• 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
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
• 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
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
Wall shear stress distribution
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
Streamlines in the stented geometry
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
Determination of the inflow rate
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
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
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
• 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
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
• 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
ANSYS Conference & 27th CADFEM Users' Meeting 2009November 18-20, 2009 – Congress Center Leipzig, Germany
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)
Thank you for your attention!