NUI Galway – UL Alliance First Annual ENGINEERING AND - ARAN ...

3D Computational Modeling of Blood Flow in the Circle of Willis
Lynch, A.G., Walsh, M.T.
Centre of Applied Biomedical Engineering Research, Dept. of Mechanical, Aeronautical and
Biomedical Engineering and The Materials and Surface Science Institute, University of Limerick
email: adrian.lynch@ul.ie
Abstract
Carotid angioplasty and stenting (CAS) has been
developed in resent years as a minimally invasive
procedure for the treatment of carotid artery disease.
However, peri-operative complications can arise due to
one of the internal carotid arteries being blocked
during the procedure. In this study we examine how
variations in the arterial structure of the brain
contribute to hemodynamic compromise during CAS.
This analysis was achieved by developing a generic
model of the circle of Willis (CoW) from 50 MRA scans
and conducting a CFD study on the model. The results
suggest the variations in the arterial system have a
major effect on the CoW’s ability to maintain sufficient
cerebral perfusion and there exists a group of patients
that possess a specific CoW variation that may make
them unsuitable to undergo the CAS procedure.
1. Introduction
Modern surgical treatment of arterial disease is moving
towards minimally invasive procedures. However, one
area that is resisting this trend is the treatment of carotid
artery disease. This is due to the peri-operative
complications associated with the carotid angioplasty
and stenting procedure (CAS). During this procedure
blood flow in one of the internal carotid arteries is
interrupted. However, it has been shown that not all
patients can accommodate this interruption, with 3 to
13% of patients experiencing ischemic neurological
deficits [1] .
A key element in maintaining sufficient cerebral
perfusion is an arterial structure known as the circle of
Willis (CoW), figure 1. The role of the CoW is to
distribute oxygenated blood around the cerebral mass.
Studies have shown that among the general population
the CoW demonstrates significant morphological
variation, with less than 50% having a complete
CoW [2] . The remainder of the population have a
combination of absent or hypoplastic vessels, which
limit the ability of the CoW to operate effectively.
This study aims to determine the effect
morphological variations in the CoW have in relation to
hemodynamic compromise during the CAS procedure.
2. Methodology
Due to the high level of morphological variation
associated with the CoW, the approach adopted in this
study was to create a generic model of the CoW. To
develop this generic model, the arteries of the CoW in
50 MRI datasets were reconstructed using Mimics
12.11 software. Examining the 50 reconstructed
49
datasets resulted in 20 models being labelled as
complete as per published classifications [2] . These 20
complete models were then averaged to create a generic
CoW model. This generic model was then analysed
using Ansys 12 computational fluid dynamics software.
To develop physiologically correct boundary conditions
a porous zone was placed at each outlet from the CoW.
This porous zone represents the peripheral resistance
developed by the down stream arteries not included in
the model and also incorporated an autoregulation
mechanism.
Figure 1: Location and geometry of the circle of Willis.
To examine the effect variations in the CoW
have on the blood flow hemodynamics the diameters of
certain arteries were reduced to simulate the variation
found in literature. This resulted in the creation of 23
different CoW models.
3. Discussion
This analysis indicates that if the CAS procedure is
performed on a patient with a CoW incorporating a
certain variation of arteries, insufficient blood supply to
a particular region of the cerebral mass may occur. The
key elements of the CoW that have the largest influence
on ability of the arterial system to maintain adequate
cerebral perfusion are the three communication arteries.
Results suggest that there may be a group of people
with specific, identifiable CoW configurations, who
may be unsuitable for the CAS procedure. The ability to
identify high-risk patients for the CAS procedure may
prove important in a clinical environment and may
result in improved clinical outcomes.
4. References
[1] A.I. Qureshi, et al., “Carotid Angioplasty and Stent
Placement”, Neurosurgery, 2002, 50(3): p.466-475.
[2] B.J. Alpers, et al., “Anatomical studies of the circle of
Willis in normal brain”, AMA Arch Neurol Psychiatry, 1959,
81(4): p.409-418.