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By Tada Shigeru, Department of Mechanical Engineering and Science, Tokyo Institute of Technology, Tokyo, Japan;
and John M. Tarbell, Department of Biomedical Engineering, The City College of New York / CUNY, New York, NY
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The common carotid artery (CCA) divides
in the neck into the external and internal
carotid arteries. The external carotid
(ECA) supplies the tissues of the face and
neck with oxygenated blood from the heart.
The internal carotid (ICA) supplies the anterior
portion of the brain. The carotid artery bifurcation
is one of the common sites where
atherosclerotic plaque and stenosis are prone
to develop. Immediately above the bifurcation,
there is a sinus, or bulb, in the internal carotid,
which plays a role in the progression of atherosclerosis,
depending on a complex spatiotemporal
pattern of mechanical forces.
Although recent advances in computational
modeling techniques have allowed the
incorporation of realistic vascular geometries,
providing information that is otherwise difficult
to get, most of these studies have focused
on hemodynamics alone. At the Tokyo Institute
of Technology, a fluid-wall coupled modeling
technique incorporated in the latest FIDAP
solver has been used to elucidate the relationship
between wall shear stress and mechanical
stress in the arterial wall. These
fluid-structure interaction (FSI) simulations have
demonstrated how local mechanical factors
involved in the fluid-wall coupling participate
in the pathogenesis of atherosclerosis.
Contours (grayscale) of the displacement magnitudes of the
thick-walled carotid bifurcation, along with line contours
of the velocity (color) at the time of peak pressure in the
cardiac cycle; the ECA branch is below and the ICA branch,
containing the carotid sinus, is above
Contour plot of the wall shear stress at the time of peak
pressure in the cardiac cycle showing a significant difference
in the level of wall shear stress between the ICA (upper) and
ECA (lower) branches
Transient simulations that track the blood
flow through a complete cardiac cycle have
been performed. Although a cast model from
a specific individual may provide detailed information
on the flow and shear stress fields near
the apex (flow divider), a representative bifurcation
geometry was adapted for this study
to extract the common features of hemodynamics
and wall motion, and to illuminate
the time- and spatially-varying characteristics
of wall shear stress and circumferential
strain. For the outlet flow boundary conditions
of the ICA and ECA, waveforms
obtained from phase-contrast magnetic resonance
velocity measurements were used to
construct the time variation of fully developed
parabolic velocity profiles. An isobaric time-varying
pressure was applied at the inlet of
the CCA. A modified waveform of the CCA
wall distention was applied for the inlet CCA
pressure transient.
Results illustrate that the maximum displacement
occurs at the side wall of the bifurcation
region. The wall stretching that
occurs suggests that the outer side of the ICA
wall may experience more cyclic strain than
other locations. The wall shear stress distribution
at the time of peak pressure shows
that the carotid sinus experiences very low
wall shear stress, whereas much higher wall
shear stress occurs in the ECA.
The authors are grateful to Dr. Peter J. Yim
from the Radiology Department of the
University of Medicine and Dentistry of New
Jersey for providing the carotid velocity measurements,
and to Ms. Midori Ohki, Fluent Asia
Pacific Co., Ltd., for her support.
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