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By Dr. Thomas Frauenfelder, Institute of Diagnostic Radiology,
University Hospital of Zürich, Zürich, Switzerland
View the pdf of this Supplement
An abdominal aortic aneurysm (AAA)
is a pathologic dilatation of the
abdominal aorta reaching a diameter
of more than 3cm. Two different types
of AAA are known: the suprarenal and the
infrarenal, situated above and below the renal
artery, respectively. Nine percent of all people
above the age of 65 suffer from an AAA.
Clinical studies reveal a very high risk for rupture
of the more frequent infrarenal AAAs
when they are larger than 5cm. In clinical
practice, a threshold of 5cm for the maximum
transverse diameter of an AAA is typically
used to recommend interventional
treatment. Recent studies have found, however,
that calculations of flow velocity and
pressure using CFD seem to be more conclusive
for assessing the risk of rupture than
the 5cm rule.
Pathlines (above)
and wall pressure
(below) suggest
problematic
conditions for an
abdominal aortic
aneurysm
Today, endoluminal stent-graft implantations,
which use fabric-covered stents, have
gained increased acceptance as a less invasive
alternative to the surgical treatment
of AAAs, with fewer complications. This
method requires a very accurate imaging
standard before the procedure, since the stent-graft
dimensions must be chosen prior to
implantation. Most imaging techniques result
in only static anatomical models, however.
Thus, while they allow for optimal and precise
stent planning, they do not take into
consideration the effects of the temporally-varying flow and pressure that are
always present in the vessel. To better address
these concerns, FIDAP has been used to investigate
the pressure and flow patterns in patient-specific
models of AAAs before and after
stent-graft implantation.
Three major steps are involved in the
process: (1) data acquisition by CT angiography;
(2) 3D solid model generation; and
(3) numerical simulation. Once the CT data
becomes available, Amira, visualization and
volume modeling software from TGS, is used
to construct the 3D geometry and delineate
structures such as the vessel lumen, vessel
wall, thrombus, calcifications, and post-operatively,
the stent. A triangular surface mesh
is generated and imported into GAMBIT, where
a volumetric mesh is built and the properties
of objects and boundaries are identified.
A turbulent fluid-structure interaction (FSI)
simulation is then run in FIDAP, where the
blood is treated as a non-Newtonian fluid.
The transient velocity profile at the inlet boundary
is taken from Doppler-ultrasound measurements.
Other values, like density, Young’s
modulus, and Poisson coefficient, are based
on material-specific data that have been adapted
during test simulations.
After deploying a
stent-graft, pathlines
through the artery
show improved flow
Many important observations have
been made from the FIDAP simulations to
date. Prior to implantation, the flow patterns
show major vortices in the concavities of the
aneurysm, which, in the context of blood
rheology, can explain the formation of intra-luminal
thrombi. After stent-graft implantation,
the flow velocity increases and the
vortices disappear inside the stent. New
eddies appear at the stent-end, however,
which can lead to the need for a second
intervention in some cases. The wall pressure
is very high before implantation at
locations where the blood hits the vessel.
This is a possible explanation for the growth
of the aneurysm and the formation of aortal
kinking. After implantation, the high pressure
areas are mostly found adjacent to the
stent bifurcation. This is probably due to the
funnel-like stent geometry. When the pressure
drop is found to be asymmetric inside
the stent limbs, asymmetric wall shear stresses
can develop, which can lead to stent
migration or rupture. After implantation, the
increase in blood volume can be assessed
in the leg that, prior to intervention, had a
weaker blood flow. The reason for this
increased flow is due to the more symmetric
geometry of the stent. In the future, CFD,
in combination with virtual stent placement,
can help choose the optimal stent.
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