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Research Papers

Exploring High Frequency Temporal Fluctuations in the Terminal Aneurysm of the Basilar Bifurcation

[+] Author and Article Information
Matthew D. Ford1

 Department of Mechanical and Materials Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canadamatthew.david.ford@gmail.com

Ugo Piomelli

 Department of Mechanical and Materials Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada

1

Corresponding author.

J Biomech Eng 134(9), 091003 (Aug 27, 2012) (10 pages) doi:10.1115/1.4007279 History: Received January 10, 2012; Revised June 26, 2012; Posted August 17, 2012; Published August 27, 2012; Online August 27, 2012

Cerebral aneurysms are a common cause of death and disability. Of all the cardiovascular diseases, aneurysms are perhaps the most strongly linked with the local fluid mechanic environment. Aside from early in vivo clinical work that hinted at the possibility of high-frequency intra-aneurysmal velocity oscillations, flow in cerebral aneurysms is most often assumed to be laminar. This work investigates, through the use of numerical simulations, the potential for disturbed flow to exist in the terminal aneurysm of the basilar bifurcation. The nature of the disturbed flow is explored using a series of four idealized basilar tip models, and the results supported by four patient specific terminal basilar tip aneurysms. All four idealized models demonstrated instability in the inflow jet through high frequency fluctuations in the velocity and the pressure at approximately 120 Hz. The instability arises through a breakdown of the inflow jet, which begins to oscillate upon entering the aneurysm. The wall shear stress undergoes similar high-frequency oscillations in both magnitude and direction. The neck and dome regions of the aneurysm present 180 deg changes in the direction of the wall shear stress, due to the formation of small recirculation zones near the shear layer of the jet (at the frequency of the inflow jet oscillation) and the oscillation of the impingement zone on the dome of the aneurysm, respectively. Similar results were observed in the patient-specific models, which showed high frequency fluctuations at approximately 112 Hz in two of the four models and oscillations in the magnitude and direction of the wall shear stress. These results demonstrate that there is potential for disturbed laminar unsteady flow in the terminal aneurysm of the basilar bifurcation. The instabilities appear similar to the first instability mode of a free round jet.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 2

Four patient-specific basilar tip aneurysms shown to relative scale

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Figure 3

Velocity magnitude contours, at mid-systolic acceleration, shown on the nominal central plane highlighting the location of the jet entering the aneurysm. Note the color bar where ⟨Ui⟩ is the cycle averaged mean inlet velocity, for the idealized models ⟨Ui⟩=0.515 m/s while for the patient specific models ⟨Ui⟩ is 0.43 m/s (model A), 0.39 m/s (model B) and 0.48 m/s (models C and D).

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Figure 10

Locations at which τw was probed (top row) along with |τw| (second row) and the angle over which τw oscillates (bottom row). |τw| has been normalized by |τw|=4.14 N/m2 (model A), |τw|=3.42 N/m2 (model B) and |τw|=5.05 N/m2 (models C and D) (calculated on the inlet vessel at the cycle averaged mean flow rate assuming a parabolic profile)

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Figure 1

Four idealized basilar tip aneurysm models (top row) shown in the lateral view (from left to right) with increasing aneurysm bulb offset from the parent vessel (here reported as the angle made between the line passing through the center of the parent vessel and the center of the aneurysm bulb). The anterior-posterior view is shown on the bottom row.

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Figure 4

Iso-surfaces of the jet into the aneurysm, at mid-diastole, along with the probe locations for each of the four idealized models (top row). Also shown are the fluctuations of the velocity magnitude (|U|, second row) with the inlet velocity magnitude shown in black, pressure (third row) and the power spectrum for |U| (bottom row) all color coded to probe location. The velocity magnitude has been normalized by the cycle averaged mean inlet velocity magnitude (values reported in Figs. 3) and time has been normalized by the period of the inlet waveform.

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Figure 5

Contour plots of the velocity magnitude (color maps chosen to highlight the fluctuation of the jet) on the central plane. Time range (8 ms) is approximately equal to time of one oscillation at the dominate frequency (120 Hz) shown at mid-diastole. Each frame, from left to right, is separated by 2 ms of normalized time.

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Figure 6

Contour plots of the Z vorticity for the 15 deg model on the central plane. The black arrow indicates the approximate location of the red probe in Fig. 7 (second column), at which we see the approximately ±180 deg oscillation in τw. Blue corresponds to negative vorticity, red to positive, and green is very near zero. Time range is the same as that described in Fig. 5.

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Figure 7

Locations at which τw was probed (top row) along with |τw| (second row) and the angle over which τw oscillates (bottom row). |τw| has been normalized by |τw|=5.87 N/m2 (calculated on the inlet vessel at the cycle averaged mean flow rate assuming a parabolic profile).

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Figure 8

Effect of Reynolds number on the dominant frequency

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Figure 9

Probe locations for each of the four patient-specific models (top row), along with the fluctuations of the velocity magnitude (|U|, second row) with the inlet velocity magnitude shown in black, pressure (third row) and the power spectrum for |U| (bottom row) all color coded to probe location. Normalization explained in Fig. 4.

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