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会場青葉山キャンパス 工学研究科機械・知能系内 共同棟 6階611室
Lecture 1:

"Constitutive modeling of the initiation and early stage development of cerebral aneurysms"

Prof. Anne M. Robertson
(Department of Mechanical Engineering and Materials Science, U. of Pittsburgh, USA
McGowan Center for Regenerative Medicine, U. of Pittsburgh, USA)

This talk will focus on research in our laboratory directed at modeling the development of ICA from a segment of arterial tissue. Early stage cerebral aneurysms are characterized by disruption of the internal elastic lamina (IEL). The cause of this breakdown is still not understood. We conjecture the arterial wall is first weakened by hemodynamic derived damage to the IEL. The weakened wall is then vulnerable to mechanical failure even under normal loading conditions. In vivo animal experiments suggest this initial damage can arise from exposure to hemodynamical forces which are in some sense abnormal. Rather than causing direct damage to the arterial wall, they appear to lead to destructive remodeling. In this talk, we discuss a multi-mechanism damage constitutive equation we have developed to model the destruction of the IEL [1]. In particular, we consider the damage as arising from both mechanical and hemodynamic factors.
The damage constitutive model builds on a previously introduced nonlinear, inelastic multi-mechanism model for cerebral arteries [2], as well as a recent generalization to include the wall anisotropy [3]. In the multi-mechanism model, elastin and collagen fibers are treated as separate components (mechanisms) of the artery. The aniostropic material response of the wall is introduced through the collagen mechanism which is composed of helically distributed families of collagen fibers. The orientation of these families is described using either a finite number of fiber orientations or using a fiber distribution function. The current model includes subfailure damage of elastin, represented by changes in tissue mechanical properties and unloaded reference length. A structural model is used to characterize gradual elastin degradation, elastin failure and recruitment of anisotropic collagen fibers. Available inelastic experimental data for cerebral arteries are used in the evaluation of the constitutive model.

[1] D. Li and A. M. Robertson, A Structural Multi-Mechanism Damage Model for Cerebral Arterial Tissue, J Biomech Eng-T ASME, (in press, 2009).
[2] R. Wulandana and A. M. Robertson, An Inelastic Multi-Mechanism Constitutive Equation for Cerebral Arterial Tissue, Biomech Model Mechanobiol, 4(4), p 235-248, 2005.
[3] D. Li and A.M. Robertson, A Structural Multi-Mechanism Constitutive Equation for Cerebral Arterial Tissue, Int J Solids Struct, 46(14-15), p. 2920-2928, 2009.

Lecture 2:

"A parametric model for side wall and bifurcation cerebral aneurysms"

Prof. Zijing Zeng
(Department of Mechanical Engineering and Materials Science, U. of Pittsburgh, USA)

Background and purpose:

Regional differences in hemodynamic loads on artery walls have been associated with localized vascular disease such as atherosclerosis and cerebral aneurysms. Due to their intrinsic geometric relevance, 3D reconstructions of patient specific geometries are frequently used in hemodynamic studies of these diseases. However, it is not possible to use them to systematically vary geometric features for parametric studies. Idealized artery and aneurysm models are inherently suited for parametric studies, but are limited by their tendency to oversimplify the vessel geometry. The goal of this work is develop and validate a more realistic parametric model for intracranial aneurysms (ICA) and the surrounding vasculature. We consider two classes of ICA: side wall and bifurcation aneurysms with aspect ratios (aneurysm height/effective diameter of the neck) ranging from 0.9 to 2.8.

Materials and Methods:

Four human cerebral aneurysm cases were selected for parametric modeling. Surface geometries were reconstructed for the aneurysm and surrounding vessels (Mimics; Materialise. Inc) using 3D DSA angiogram data. Building on the work in [1], a parametric model was then created for the surrounding arteries, bifurcation region and aneurysm sac. Measurements from the aneurysm sac included centerline and areas of the neck cross section, largest cross section, and an intermediate cross section between largest cross section and aneurysm apex. After model creation, CFD analyses of flow in the parametric and 3D reconstructed geometries were performed and compared.

Results and Discussion:

The geometric comparisons between parametric model and 3D reconstruction show a good match with a maximum error of 10% of the cross sectional diameters. Hemodynamic features of the parametric models were also modeled well.

Conclusion: This parametric model captures important geometric and flow features of both the aneurysm and neighboring vasculature. These models will be used for studies of remodeling and rupture in ICA and can also guide 3D reconstruction of poor quality clinical data.

[1] H. Zakaria, A. M. Robertson, C. W. Kerber, Parametric Models for Studies of Flow in Arterial Bifurcations, Ann Biomed Eng, 36(9), pp1515-1530, 2008.
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