Biomedical Engineering Reference
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Fig. 12.10. Oscillatory Shear Index (OSI, [74] ) on the arterial wall of the proximal abdominal
aorta. Left: Results of the rigid wall simulation. Right: Results of the moving wall simulation
conditions for the fluid equations. More precisely, resistance conditions were pre-
scribed at the outflow sections, yielding a normal stress proportional to the flow rate.
The proportionality constant represents the downstream resistance and was tuned so
that the desired distribution of abdominal flow among the branches was obtained.
Should more data on the downstream network be available, more complex bound-
ary conditions (including Windkessel-like conditions) could be considered as well.
Again, we compared the results obtained from a simulation of blood flow in a
mouse aorta under the assumption that the vessel geometry is fixed, with the results
of a simulation in moving domain with the “reduced” 4DIB approach.
The results of the rigid wall simulation (Fig. 12.10, left) showed that areas of
disturbed flow characterize the branching points of the proximal abdominal aorta.
High values of the oscillatory shear index (OSI - see e.g. [74]) were computed in
very localized regions at the ostia of the main aortic branches. The hemodynamic
environment was characterized overall by relatively low shear load. The results of
the moving domain simulation (Fig. 12.10, right) provided an insight into the effects
of the vessel dilatation in the region of interest. As a measure of the dilatation, the
difference between the maximum and minimum radius (over the cardiac cycle) of
each section was computed, and normalized by the minimum radius. The average
value of this indicator on the eight slices was 35 %, being maximum in the proximal
abdominal aorta (even more than 40 %). When taking into account the movement of
the vessel, the computed WSS showed a similar spatial pattern but overall a smaller
magnitude compared to the rigid wall case. The ostia of the main aortic branches
were not included in the moving domain simulation due to the lack of information
on their movement. However, the computed WSS was significantly more oscillating
with respect to the rigid wall simulation in the entire proximal abdominal aorta,
and in particular in the region surrounding the branching points. This was indeed
experimentally found to be a typical site for atherosclerosis development.
Despite being only in a preliminary stage, these results suggest that neglecting
the movement of the arterial wall may have a significant impact on the estimation
of clinically relevant features, such as the presence of oscillatory flow. Validation of
these results is ongoing (see [75]).
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