$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.

Track 14. Cardiovascular Mechanics 14.5. Cardiovascular Disease - Cardiac or Coronary Applications $291

References [1] I.E. Vignon-Clementel et al. Outflow boundary conditions for three-dimensional

finite element modeling of blood flow and pressure in arteries. CMAME in press. [2] C.A. Figueroa et al. A coupled momentum method for modeling blood flow in

three-dimensional deformable arteries. CMAME in press.

4242 We, 11:45-12:00 (P31) Experimental study of flow and wall pressure fluctuations behind a simulated vascular stenosis A. Borisyuk. Institute of Hydromechanics, Kiev, Ukraine

An in-vitro experiment is carried out in order to study flow and the wall pressure fluctuations behind a simulated vascular stenosis. The flow separation and flow re-attachment regions, as well as the region of flow stabilization and re- development into the undisturbed state are found behind a stenosis. Here sharp increase in the pressure fluctuations and the presence of a pronounced pressure maximum upstream of the point of jet re-attachment are found. The quantitative estimates for the lengths of the flow regions noted, as well as for the distance from a stenosis to the point of maximum pressure fluctuations and the pressure magnitude at this point are obtained. These estimates are more universal compared to those available in the scientific literature. Study of the wall pressure power spectrum reveals the low-frequency maxima in it. The maxima are found to be determined by the corresponding large- scale eddies in the flow regions noted above, and the frequencies of the maxima are close to the characteristic frequencies of the eddies' formation. These maxima are the main distinguishing features of the spectrum under investigation compared to that of the wall pressure fluctuations in a fully developed turbulent pipe flow. This is the new result that can be used for finding, in a non-invasive way, the stenosis diameter from an analysis of the appropriate recordings made from patients.

5172 We, 12:00-12:15 (P31) A method for patient-specific adjustment of the multi-branched model estimating hemodynamic parameters in the human arterial system P.V. Stroev 1, S.S. Zakirov 2, P.R. Hoskins 3, W.J. Easson 1 . 1School of Engineering and Electronics, Edinburgh University, Edinburgh, UK, 21ntel, Moscow, Russia, 3Medical Physics Section, Edinburgh University, Edinburgh, UK

Background: 1D models of the arterial system are based on averaged data. We propose a new method for patient-specific adjustment of model parameters based on measurement of pulse wave velocity (PWV) and blood flow. Method: We used a transmission line model proposed by Avolio (1980) in which PWV was used as the core parameter with an assumed relationship between PWV in different segments. Ideally, the model would use the aortic pressure and flow waveforms as input but these are very impractical to measure. It is proposed that pressure and flow waveforms measured non- invasively from 3 other sites (common carotid, brachial, popliteal) are used as input to the model, to provide adjusted PWV values. The patient specific model is then run in the forward manner to obtain flow and pressure and flow at every location. We investigated how typical measurement errors would affect predicted pressure and flow. Results: A 20% overestimation of the arterial diameter leads to overestimation of the peak value for reconstructed flow waveform by up to 40%. A 20% overestimation of PWV leads to overestimation of the peak value for recon- structed waveform by up to 25%. A 10% overestimation of pressure leads to overestimation of the peak value of flow waveforms by 10%; pressure waves in this case get shifted upwards along the axis of ordinate by up to 25%. Conclusion: An inverse method is proposed for patient specific adjustment of 1D model parameters. Realistic errors in measurement may lead to errors in estimated pressure and flow by typically 25-40%

References A. Avolio (1980). Multi-branched model of the human arterial system. Medical and

Biological Engineering and Computing 18: 709-718.

6294 We, 12:15-12:30 (P31) Fluid flow structure in a vein bypass graft R. Tran-Son-Tay 1,2, M. Hwang 1 , S. Berceli 3, K. Ozaki 3, M. Garbey 4, W. Shyy 5 . 1 Department of Mechanical & Aerospace Engineering, University of Florida, Gainesville, Florida, USA, 2Department of Biomedical Engineering, University of Florida, Gainesville, Florida, USA, 3Department of Surgery, University of Florida, Malcom Randafl VAMC, Gainesville, Florida, USA, 4Department of Computer Science, University of Houston, Houston, Texas, USA, 5Department of Aerospace Engineering, University of Michigan, Ann Arbor, Michigan, USA

The fluid flow through a bypass graft can substantially influence the outcome of the bypass surgery. Vein bypass graft is the primary treatment for patients

with peripheral vascular occlusive disease. Though the vein grafts are highly effective at improving the limb perfusion at the early stage of the treatment, occlusive adaptations of the vein graft occur in the timeframe of months for many patients resulting in the narrowing of the lumen and distal limb ischemia. To help improve our understanding of this and related issues, pulsatile flows are computed in a (1) idealized arterial bypass system with partially occluded host artery, and (2) physical bypass graft. In the first case, both the residual flow issued from the stenosis - which is important at earlier stage after grafting - and the complex flow structure induced by the bypass graft are investigated. In the second case, computed tomography (CT) scans are collected from a patient undergoing bypass grafting, and 3D geometry of the lumen within the vein graft is reconstructed from the cross sectional CT images. Blood flow simulation is performed for the entire graft length. Physiological flow rate waveform obtained from duplex ultrasonography is applied at the inlet of the graft. CT images of the vein graft from the same patient are obtained at different time points and reconstructed in 3D in order to investigate the change of hemodynamic environment at the sites of lesions as the vascular remodeling proceeds. Flow patterns in the vein graft and hemodynamic factors such as wall shear stress distribution are examined to correlate these factors with the remodeling of the vein graft.

14.5.4. Cardiac or Coronary Applications

6515 We, 14:00-14:30 (P34) The role of cardiac mechanics in diagnosis and treatment of myocardial ischemia and infarction J.W. Holmes. Cardiac Biomechanics Group, Columbia University, New York NY, USA

In its infancy, the field of cardiac mechanics consisted by necessity largely of descriptive studies. The first step required to understand the biomechanics of disease was to characterize normal heart structure, deformation, and function. Next came descriptions of changes during disease. Now, cardiac mechanics is entering an exciting new phase. The body of information currently available, particularly its encoding into sophisticated finite element models of the heart, has opened enormous possibilities for cardiac mechanics to impact the diagnosis and treatment of heart disease. In this new phase, one essential requirement for unlocking the full potential of cardiac mechanics as a field is improved communication between engineers and clinicians, including the framing of biomechanics models and concepts in a language more accessible to clinicians. Ischemic heart disease is an important health problem in most modernized countries. Much progress has been made in noninvasive diagnosis of is- chemic disease, largely as a result of innovations in methods for imaging heart structure and function. However, these imaging techniques have arisen primarily from consideration of the physics of the imaging modalities, with little regard to the underlying biomechanics of myocardial ischemia. Both recent modeling and older experimental studies of regional ischemia suggest that existing approaches such as endocardial wall motion analysis would be much more effective in diagnosing ischemia if combined with an understanding of the underlying mechanics of ischemia. The potential for exciting new contributions extends well beyond diagnosis to treatment of ischemic heart disease. For example, once patients suffer a myocardial infarction, the structure and mechanics of the damaged region change dramatically. These changes in mechanical properties are important determinants of heart function and remodeling and are now fairly well un- derstood, suggesting the possibility of intelligent therapeutic manipulation. Devices such as left ventricular assist devices (LVADs) now make previously undreamed mechanical manipulations possible, enabling and challenging us to apply biomechanics to cardiac therapy more directly and successfully than ever before.

5242 We, 14:30-14:45 (P34) Myocardial mechanics during epicardial versus endocardial left ventricular pacing: simulations and experiments

R. Kerckhoffs 1 , P. Bordachar 2, S. Healy 1 , J. Omens 1,3, A. McCulloch 1 . 1Department of Bioengineering, UC, San Diego, USA, 2University Victor Segallen, Bordeaux, France, 3Department of Medicine, UC, San Diego, USA

Cardiac resynchronization therapy has resulted in increased use of epicardial left ventricular (LV) pacing in patients with heart failure [1]. We hypothesized that endocardial LV pacing (vs epicardial pacing) reduces dyssynchrony and heterogeneity of regional deformation and improves global LV function. A 3D model of normal canine cardiac electromechanics was used, and computer simulations were compared to experimental results (7 dogs). A model of canine LV and right ventricular (RV) anatomy with Purkinje network and myofiber architecture served as a domain to solve impulse propagation (modified FitzHugh-Nagumo-monodomain formulation) and mechanics [2]. Each ventricle was coupled to a three-element windkessel model of aortic