Papers by Evangelos Boutsianis
Radiologe, 2007
Das Ziel der numerischen Simulation des Blutflusses in der Aorta ist, die Mechanik der Entstehung... more Das Ziel der numerischen Simulation des Blutflusses in der Aorta ist, die Mechanik der Entstehung von Aortenaneurysmen im Hinblick auf das Rupturrisiko zu untersuchen und die Wirkungen interventioneller Maßnahmen zu beschreiben. Die Grundlage der numerischen Simulation sind virtuelle Modelle von Gefäßen und die physikalischen Eigenschaften der Gefäßbestandteile, des Blutes und der Strömung. Basierend auf diesen Angaben werden mit Hilfe numerischer Methoden die strömungsmechanischen Probleme des Blutflusses näherungsweise gelöst. Die Ergebnisse können dann quantitativ und qualitativ dargestellt werden. Die Ergebnisse der numerischen Flusssimulation zeigen, dass in abdominellen Aortenaneurysmen die Höhe des Wanddrucks, der von entscheidender Bedeutung für das Rupturrisiko ist, von verschiedenen Faktoren, wie z. B. der Lage des Wandthrombus, abhängt. In Modellen mit Stentgrafts wurden mit Hilfe der numerischen Simulation Faktoren, welche die Stentgraftmigration beeinflussen, untersucht. Obwohl die numerische Simulation des Blutflusses noch einige Limitationen aufweist, zeigen aktuelle Studien, dass die Methode das Potenzial hat, um in Zukunft eine dedizierte Beurteilung des Rupturrisikos von Aortenaneurysmen vorzunehmen. The goal of numeric analysis of aortic blood flow is to evaluate the mechanisms leading to an aortic aneurysm with regard to the risk of a rupture and to describe the effect of interventional therapy. Numeric analysis is based on virtual models of vascular structures and the physical characteristics of the vessel wall, of blood as fluidum, and the blood flow. Using this information, numeric analysis solves the appropriate equations. The results can be displayed quantitatively and qualitatively. The results of numeric flow simulation show that in abdominal aortic aneurysms the wall pressure, which is of vital importance for the risk of rupture, depends on several factors, one being the location of the intraluminal thrombus. In models of aneurysms after stent grafting, numeric analysis can be used to evaluate factors leading to stent migration. Although numeric analysis of aortic blood flow still has several limitations, recent studies have shown that this method has the potential for improved estimation of the rupture risk of aortic aneurysms in the near future.
Radiologe, 2007
Das Ziel der numerischen Simulation des Blutflusses in der Aorta ist, die Mechanik der Entstehung... more Das Ziel der numerischen Simulation des Blutflusses in der Aorta ist, die Mechanik der Entstehung von Aortenaneurysmen im Hinblick auf das Rupturrisiko zu untersuchen und die Wirkungen interventioneller Maßnahmen zu beschreiben. Die Grundlage der numerischen Simulation sind virtuelle Modelle von Gefäßen und die physikalischen Eigenschaften der Gefäßbestandteile, des Blutes und der Strömung. Basierend auf diesen Angaben werden mit Hilfe numerischer Methoden die strömungsmechanischen Probleme des Blutflusses näherungsweise gelöst. Die Ergebnisse können dann quantitativ und qualitativ dargestellt werden. Die Ergebnisse der numerischen Flusssimulation zeigen, dass in abdominellen Aortenaneurysmen die Höhe des Wanddrucks, der von entscheidender Bedeutung für das Rupturrisiko ist, von verschiedenen Faktoren, wie z. B. der Lage des Wandthrombus, abhängt. In Modellen mit Stentgrafts wurden mit Hilfe der numerischen Simulation Faktoren, welche die Stentgraftmigration beeinflussen, untersucht. Obwohl die numerische Simulation des Blutflusses noch einige Limitationen aufweist, zeigen aktuelle Studien, dass die Methode das Potenzial hat, um in Zukunft eine dedizierte Beurteilung des Rupturrisikos von Aortenaneurysmen vorzunehmen. The goal of numeric analysis of aortic blood flow is to evaluate the mechanisms leading to an aortic aneurysm with regard to the risk of a rupture and to describe the effect of interventional therapy. Numeric analysis is based on virtual models of vascular structures and the physical characteristics of the vessel wall, of blood as fluidum, and the blood flow. Using this information, numeric analysis solves the appropriate equations. The results can be displayed quantitatively and qualitatively. The results of numeric flow simulation show that in abdominal aortic aneurysms the wall pressure, which is of vital importance for the risk of rupture, depends on several factors, one being the location of the intraluminal thrombus. In models of aneurysms after stent grafting, numeric analysis can be used to evaluate factors leading to stent migration. Although numeric analysis of aortic blood flow still has several limitations, recent studies have shown that this method has the potential for improved estimation of the rupture risk of aortic aneurysms in the near future.
Journal of Biomechanical Engineering-transactions of The Asme, 2008
A computational model incorporating physiological motion and uniform transient wall deformation o... more A computational model incorporating physiological motion and uniform transient wall deformation of a branchless right coronary artery (RCA) was developed to assess the influence of artery compliance on wall shear stress (WSS). Arterial geometry and deformation were derived from modern medical imaging techniques, whereas the blood flow was solved numerically employing a moving-grid approach using a well-validated inhouse finite element code. The simulation results indicate that artery compliance affects the WSS in the RCA heterogeneously, with the distal region mostly experiencing these effects. Under physiological inflow conditions, coronary compliance contributed to phase changes in the WSS time history, without affecting the temporal gradient of the local WSS nor the bounds of the WSS magnitude. Compliance does not cause considerable changes to the topology of WSS vector patterns nor to the localization of WSS minima along the RCA. We conclude that compliance is not an important factor affecting local hemodynamics in the proximal region of the RCA while the influence of compliance in the distal region needs to be evaluated in conjunction with the outflow to the myocardium through the major branches of the RCA.
European Journal of Cardio-thoracic Surgery, 2004
Objective: To assess the feasibility of computationally simulating intracoronary blood flow based... more Objective: To assess the feasibility of computationally simulating intracoronary blood flow based on real coronary artery geometry and to graphically depict various mechanical characteristics of this flow. Methods: Explanted fresh pig hearts were fixed using a continuous perfusion of 4% formaldehyde at physiological pressures. Omnipaque dye added to lead rubber solution was titrated to an optimum proportion of 1:25, to cast the coronary arterial tree. The heart was stabilized in a phantom model so as to suspend the base and the apex without causing external deformation. High resolution computerized tomography scans of this model were utilized to reconstruct the threedimensional coronary artery geometry, which in turn was used to generate several volumetric tetrahedral meshes of sufficient density needed for numerical accuracy. The transient equations of momentum and mass conservation were numerically solved by employing methods of computational fluid dynamics under realistic pulsatile inflow boundary conditions. Results: The simulations have yielded graphic distributions of intracoronary flow stream lines, static pressure drop, wall shear stress, bifurcation mass flow ratios and velocity profiles. The variability of these quantities within the cardiac cycle has been investigated at a temporal resolution of 1/100th of a second and a spatial resolution of about 10 mm. The areas of amplified variations in wall shear stress, mostly evident in the neighborhoods of arterial branching, seem to correlate well with clinically observed increased atherogenesis. The intracoronary flow lines showed stasis and extreme vorticity during the phase of minimum coronary flow in contrast to streamlined undisturbed flow during the phase of maximum flow. Conclusions: Computational tools of this kind along with a state-of-the-art multislice computerized tomography or magnetic resonance-based noninvasive coronary imaging, could enable realistic, repetitive, non-invasive and multidimensional quantifications of the effects of stenosis on distal hemodynamics, and thus help in precise surgical/interventional planning. It could also add insights into coronary and bypass graft atherogenesis. q
Annals of Biomedical Engineering, 2008
Appropriate velocity boundary conditions are a prerequisite in computational hemodynamics. A meth... more Appropriate velocity boundary conditions are a prerequisite in computational hemodynamics. A method for mapping analytical or experimental velocity profiles on anatomically realistic boundary cross-sections is presented. Interpolation is required because the computational and experimental domains are seldom aligned. In the absence of velocity information one alternative is the adaptation of analytical profiles based on volumetric flux constraints. The presented algorithms are based on the Schwarz-Christoffel (S-C) mapping of singly or doubly connected polygons to the unit circle or an annulus with unary external radius. S-C transformations are combined to construct a one-to-one invertible map between the target surface and the measurement domain or the support of the source analytical profile. The proposed technique permits us to segment each space separately and map one onto the other in its entirety. Tests are performed with normal velocity boundary conditions for computational simulations of blood flow in the ascending aorta and cerebrospinal fluid flow in the spinal cavity. Mappings of axisymmetric velocity profiles of the Womersley type through a simply connected circular pipe as well as through a doubly connected circular annulus, and interpolations from in-vivo phase-contrast magnetic resonance imaging velocity measurements under instantaneous volumetric flux constraints are considered.
Journal of Biomechanical Engineering-transactions of The Asme, 2009
There is considerable interest in computational and experimental flow investigations within abdom... more There is considerable interest in computational and experimental flow investigations within abdominal aortic aneurysms (AAAs). This task stipulates advanced grid generation techniques and cross-validation because of the anatomical complexity. The purpose of this study is to examine the feasibility of velocity measurements by particle tracking velocimetry (PTV) in realistic AAA models. Computed tomography and rapid prototyping were combined to digitize and construct a silicone replica of a patient-specific AAA. Three-dimensional velocity measurements were acquired using PTV under steady averaged resting boundary conditions. Computational fluid dynamics (CFD) simulations were subsequently carried out with identical boundary conditions. The computational grid was created by splitting the luminal volume into manifold and nonmanifold subsections. They were filled with tetrahedral and hexahedral elements, respectively. Grid independency was tested on three successively refined meshes. Velocity differences of about 1% in all three directions existed mainly within the AAA sack. Pressure revealed similar variations, with the sparser mesh predicting larger values. PTV velocity measurements were taken along the abdominal aorta and showed good agreement with the numerical data. The results within the aneurysm neck and sack showed average velocity variations of about 5% of the mean inlet velocity. The corresponding average differences increased for all velocity components downstream the iliac bifurcation to as much as 15%. The two domains differed slightly due to flow-induced forces acting on the silicone model. Velocity quantification through narrow branches was problematic due to decreased signal to noise ratio at the larger local velocities. Computational wall pressure and shear fields are also presented. The agreement between CFD simulations and the PTV experimental data was confirmed by three-dimensional velocity comparisons at several locations within the investigated AAA anatomy indicating the feasibility of this approach. Fig. 2 Overview of the PTV setup. Panel "a…: Photograph of the image acquisition system. Its main components are labeled as "1… camera, "2… four way splitter prism, "3… mirrors and "4… optical rail. Panel "b…: Photographic output showing the four different perspectives.
Biomedical Engineering Online, 2007
Purpose: Coronary artery bypass graft (CABG) surgery represents the standard treatment of advance... more Purpose: Coronary artery bypass graft (CABG) surgery represents the standard treatment of advanced coronary artery disease. Two major types of anastomosis exist to connect the graft to the coronary artery, i.e., by using an end-to-side or a side-to-side anastomosis. There is still controversy because of the differences in the patency rates of the two types of anastomosis. The purpose of this paper is to noninvasively quantify hemodynamic parameters, such as mass flow and wall shear stress (WSS), in end-to-side and side-to-side anastomoses of patients with CABG using computational fluid dynamics (CFD).
European Radiology, 2007
The purpose of this paper was to non-invasively assess hemodynamic parameters such as mass flow, ... more The purpose of this paper was to non-invasively assess hemodynamic parameters such as mass flow, wall shear stress (WSS), and wall pressure with computational fluid dynamics (CFD) in coronary arteries using patient-specific data from computed tomography (CT) angiography. Five patients (two without atherosclerosis, three with atherosclerosis) underwent retrospectively electrocardiogram (ECG) gated 16-detector row CT using ECG-pulsing and geometric models of coronary arteries were reconstructed for CFD analysis. Blood flow was considered laminar, incompressible, Newtonian, and pulsatile. The mass flow, WSS, and wall pressure were quantified and flow patterns were visualized. The wall pressure continuously decreased towards distal segments and showed pressure drops in stenotic segments. In coronary segments without atherosclerotic wall changes, WSS remained low, even during phases of high flow velocity, whereas in atherosclerotic vessels, the WSS was elevated already at low flow velocities. Stenoses and post-stenotic dilatations led to flow acceleration and rapid deceleration, respectively, including a distortion of flow. Areas of high WSS and high flow velocities were found adjacent to plaques, with values correlating with the degree of stenosis. CFD provided detailed mass flow measurements. CFD analysis is feasible in normal and atherosclerotic coronary arteries and provides the rationale for further investigation of the links between hemodynamic parameters and the significance of coronary stenoses.
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Papers by Evangelos Boutsianis