$290 Journal o f Biomechanics 2006, Vol. 39 (Suppl 1)

stretch promotes phenotypic changes of these cells to macrophages. MMP-9 production by isolated macrophages cultured on fibrin-coated substrates was increased by cyclic stretch. Consistent with our hypothesis, these observations suggest that AAA wall under a higher mechanical stress and/or lower oxygen tension has a more activated immune component and impaired remodeling mechanisms, which may contribute to persistent aneurysmal degeneration.

7187 We, 09:30-09:45 (P28) Fusion o f imaging and computat ional b iomechanics: A promising approach for improved analysis o f the b iomechanics o f atherosclerotic plaques

G.A. Holzapfel 1,2, M. Auer 2, D.E. Kiousis 1 , T.C. Gasser 1 . 1Royal Institute ef Technology, School of Engineering Sciences, Stockholm, Sweden, 2 Graz University of Technology, Computational Biomechanics, Graz, Austria

The combination of image analysis and biomechanics provides an efficient basis that permits the relation of morphological, structural and (in)elastic constitutive data to the mechanisms of interventional treatments such as balloon angioplasty. This promising combined approach may provide answers to clinical questions raised by cardiologists. We present a segmentation tool that permits a 3-D reconstruction of the most important tissue components of atherosclerotic human arteries by means of high-resolution magnetic resonance imaging [1]. We propose a model- based snake algorithm for identifying contours, which uses information about the plaque composition and geometric data of the tissue layers. Except for the identification of lipid-rich regions the proposed methodology is able to segment the tissue boundaries of atherosclerotic arteries in an automatic, robust and efficient way. NURBS-based computer-generated 3-D models of the individual tissue components of a patient-specific stenotic human iliac artery are then used to predict the changes of the stress-strain distributions in the heterogeneous arterial wall occurring during balloon angioplasty with stenting. Each tissue component undergoes anisotropic mechanical responses and is related to specific material parameters, which are obtained from experimental data. The finite element results provide novel insights for our understanding of the underlying mechanism of balloon angioplasty and highlight the critical wall regions which evolve due to balloon-stent-artery interaction.

References [1] M. Auer, R. Stollberger, P. Regitnig, F. Ebner, G.A. Holzapfel, 3-D reconstruc-

tion of tissue components for atherosclerotic human arteries based on high- resolution MRI, IEEE T. Med. Imaging, in press.

14.5.3. Atherescleresis and Aneurysms 2

7417 We, 11:00-11:15 (P31) Numerical s imulat ion o f b lood f low in the circle o f Will is with out f low boundary condi t ions using a one-d imensional model

M. Oshima 1 , S. Tokuda 1 , T. Unemura 2, S. Sugiyama 1 . 1Institute eflndustrial Science, University of Tokyo, Tokyo, Japan, 2AdanceSoft Corporation, Tokyo, Japan

Formation of cerebral aneurysm is reported to depend on hemodynamic factors, particularly on wall shear stress induced by blood flow. In order to obtain detailed information on the hemodynamic quantities in the cerebral vascular system, the authors have been developing a patient-specific modeling and numerical simulation system [1]. The purpose of this paper is to examine hemodynamics of the cerebral arterial circle of Willis, which includes all prefer- ential locations of the cerebral aneurysms using the present numerical system. Generally, the numerical simulation using a patient-specific model is conducted for a localized region such as an area near the cerebral aneurysm. Although the analysis region is only a part of the circulatory system, the simulation has to include the effects from the entire circulatory system. Thus, modeling of proper boundary conditions is essential for the simulation. In this paper, we focus on modeling out flow boundary conditions. First of all, the network of cerebral arteries are modeled including the arteries and arterioles, whose sizes can not be resolved by MIRA. The network is constructed combining anatomical structure and mathematical structure model. Second of all, one- dimensional model is designed considering peripheral resistance, compliance, and impedance. Finally, the present method for the outflow boundary condition is applied to two patient-specific models with or without occlusion in the cere- bral arterial circle of Willis. The results by the present method are compared to those using the free-stream boundary condition. As a result, particularly for the case with occlusion, the present method shows a significant difference in flow rate of each artery and also improvement in flow distribution in the network comparing to free-stream boundary condition.

References [1] Shojima M., Oshima M., et al. Stroke 2004; 35: 2500-2505.

Oral Presentations

6407 We, 11:15-11:30 (P31) Statistical and numerical invest igat ions o f cerebral aneurysms' morpho logy and haemodynamics

A. Veneziani 1 , E. Boccardi 3, L. Antiga 4, M. Piccinelli 1,4, P. Secchi 1 , S. Vantini 1 , T. Passerini 1 , M. De Luca 1 , S. Bacigaluppi 2. 1MOX, Dept. Of Mathematics, Politecnico di Milano, Italy, 2 University of Milan, Neurosurgery, Ospedale Maggiore Policlinico - Fondazione IRCCS, Milano, Italy, 3Azienda Ospedaliera Ospedale Niguarda Ca' Granda, Milano, Italy, 4Bioengineering Department, Marie Negri Institute, Bergamo, Italy

The indications for the proper medical treatment of cerebral aneurysms are still quite controversial. The main features considered in decision making are the site and the size of the aneurysm (see [1]), besides the age of the patient. Both the risks of spontaneous rupture and of procedure-induced morbidity can be related to many other features, still to be investigated. In the present work, we in particular study the role of anatomical and fluid dynamical factors. We start from a data-base of about 70 patients collected at the Ca' Granda Hospital in Milan, in order to outline an appropriate morphological classification of the aneurysms. Statistical classification and pattern recognition are based on CART and logistic regression, in order to establish possible relations between the development and rupture of the aneurysm and the following features: 1. site 2. size (angiographic volume) 3. inlet aneurysm area 4. shape Statistical analysis are extended also to the vascular districts in the neighbor- hood of the aneurysm site by analyzing length, radius, curvature of the vessels involved, as well as the angles and the radii of the bifurcations. To this aim, a semi-automatic tools for recognition and quantification of these geometrical features has been developed [4]. In order to corroborate and improve the morphological classification, an exten- sive CFD analysis is carried out. In particular, numerical simulation obtain data on wall shear stress and residence time in the aneurysm. The simulations are performed by assuming to have rigid geometries. The complex haemodynam- ics of the Willis circle upstream the aneurysm site will be accounted by means of the geometrical multiscale approach advocated in [2,3].

References [1] T. Hassan, et al. J. Neurosurg. 103: 662~80. [2] A. Quarteroni, A. Veneziani, SIAM Multiscale Models and Simulation 2003; 1(2):

173-195. [3] R Migliavacca, R. Balossino, G. Pennati, G. Dubini, T.Y. Hsiab, M.R. De Leval,

E.L. Bove, J. Biomech. 2005; 38: 1129-1141. [4] M. Piccinelli et al. Computational geometric analysis of cerebral aneurysms

and their parent vasculature from CRA, 5th World Congress of Biomechanics (Abstract).

6625 We, 11:30-11:45 (P31) Three-dimensional s imulat ions o f hemodynamic factors in pu lmonary hyper tens ion

T.A. Fonte 1 , I.E. Vignon-Clementel 1 , C.A. Figueroa 1 , J.A. Feinstein 2, C.A. Taylor 1,2,3,4. 1Department of Mechanical Engineering, 2Department of Pediatrics, 3Department of Bioengineering and 4Department of Surgery, Stanford University, Stanford, USA

Introduct ion: Idiopathic pulmonary arterial hypertension (PAH) is a uniformly fatal disease manifested by severe pulmonary artery (PA) pressure elevation. Inflammation is believed to promote PAH, and wall shear stress modulates in- flammation, but little is known about hemodynamic conditions in PAH patients. We used image-based CFD to quantify shear stress in the PAs of PAH patients and normal controls. Methods: Three-dimensional models with 50-70 vessels per subject were created from contrast-enhanced MRI data of two healthy and three PAH subjects. Anisotropic, adaptive meshes were generated with approximately 1.3 million elements. A transient inflow waveform from PCMRI was prescribed at the main PA inlet. Resistance boundary conditions were set at all outlet vessels as a function of their diameters, the subject's pulmonary arterial pressure, and the left/right PA flow split [1]. The distensibility of the PAs was measured from PCMRI to set a Young's modulus for the vessel wall. The 3-D equations of blood flow in elastic vessels were solved on parallel computers using custom finite element software [2]. Results: Complex flow and vortices were observed in the proximal and distal PAs, which correlated with regions of low shear stress. The average MWSS (dynes/cm 2) in the central PAs was 17.8±1.4 for healthy subjects and 3.0±0.4 for PAH subjects (p<0.001). In the distal arteries, the average MWSS was 10.0±2.1 for healthy subjects and 9.8±1.0 for PAH subjects (p = NS). Conclusions: With the first three-dimensional simulations of pulsatile flow in human PAs, we have demonstrated that PAH patients experience significantly low shear stress in their proximal PAs and normal levels of shear stress in their distal PAs.