| 1. |
- Hyhlik-Dürr, A., et al.
(författare)
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Finite Element Analysis of Abdominal Aortic Aneurysms : Preliminary Results of Intra and Inter observer Validation
- 2010
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Konferensbidrag (refereegranskat)abstract
- Objective: Treatment of abdominal aortic aneurysm (AAA) is indicated if risk for rupture exceeds the risk for aortic repair. Estimation of the individual risk for rupture in AAA is therefore essential. The diameter of AAA is known as an independent risk factor for rupture and therefore the base of indication for surgical or endovascular therapy. For more sensitive patient selection, other morphological or hemodynamic predictors such as volume or peak wall stress have to be evaluated. The purpose of this study was to analyze the reproducibility of diameter measurement, volume estimation and peak wall stress calculation in AAA by finite element analysis. Methods: Computed tomography angiography (CTA) scans of 10 patients with AAA and 4 volunteers with healthy infrarenal aortas were analyzed by three independent investigators. A semiautomatic reconstruction using two- and three-dimensional deformable (active) contour models was used to segment vascular bodies from CTA data. Centreline calculated maximal diameter and volume measurements, as extracted from the reconstructed abdominal aorta, as well as peak wall stress, as predicted by three-dimensional non-linear finite element models, were analyzed. Specifically, aortic wall and thrombus tissue were captured by isotropic, non-linear and finite strain constitutive models. Likewise, mean arterial pressure was applied at the luminal surface, the vessels were fixed at the renal arteries and the aortic bifurcation and no contact with surrounding organs was considered. Inter- and intra-observer variabilities for diameter, volume and peak wall stress measurements were assessed by calculating the coefficient of variation (CV=SD*100/mean in %) of the five fold determinations. The methodological variation was expressed as deviation of diameter (mm), volume (ml) and peak wall stress (kPa) amongst the three observers. Results: Reproducibility measurements in healthy vessels of aortic diameters between 16.1mm to 16.6mm varied from CV=2.5% to CV=4.9%. Abdominal aortic volumes of 14ml to 15ml were measured in the healthy cohort with a reproducibility of CV=5.8% to CV=11.5%. Peak wall stress varied between 53 kPa and 55 kPa, where CV ranged from 3-13%. Inter-observer variation was <10% for diameter, volume and peak stress in healthy volunteers. Aortic diameter in three AAAs was measured to 58.9 mm; 54.6 mm; and 71.2 mm respectively. The coefficient of variation showed high agreement with values less than 5%. AAA volume varied between 130 ml and 300 ml (CV < 10%) and Peak wall stress was predicted between 172 kPa and 296 kPa (CV <10%). Variability between the 3 observers in AAA measurements was 0.7 mm – 6.0 mm for diameter, 11 – 28 ml for volume and 4-27 kPa for peak wall stress, respectively. Conclusions: Volume and diameter measurements based on geometrical models reconstructed from CTA scans showed quit good reproducibility for serial measurements in normal and degenerative arteries. Peak wall stress predictions exhibited high accordance between different observers, and in serial measurements within one observer. Volume and peak wall stress analysis could be an additionally module for assessment of individual rupture risk in AAA in the future, which however needs to be validated by additional studies.
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| 2. |
- Hyhlik-Dürr, A., et al.
(författare)
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Finite-Elemente-Analyse abdomineller Aortenaneurysmen : Erste Ergebnisse der Intra- und Interobserver Validierung
- 2010
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Konferensbidrag (refereegranskat)abstract
- Hintergrund: Die Therapie des abdominellen Aortenaneurysmas (AAA) ist indiziert, wenn das Rupturrisiko das Risiko der elektiven Operation übersteigt. Die Abschätzung des individuellen Rupturrisikos gilt als Basis der Indikationsstellung zur offenen oder endovaskulären Chirurgie. Bisher wird der Durchmesser des AAA als maßgeblicher Risikofaktor für die Ruptur herangezogen. Für eine sensitivere Indikationsstellung sollten jedoch andere morphologische oder biomechanische Faktoren wie die Volumenveränderung im Verlauf und/oder die Wandspannung im Aneurysma untersucht werden. Ziel dieser Studie ist die Analyse der Reproduzierbarkeit der Durchmesserbestimmung sowie der Volumen- und Wandspannungsberechnung anhand eines geometrischen Modells, basierend auf der Finite Elemente Methode. Methode: Computertomographische Daten von vier gesunden und zehn Patienten mit infrarenalen abdominellen Aneurysmen werden von drei unabhängigen Untersuchern analysiert. Die abdominelle Aorta wird semiautomatisch von Computertomographie-Angiographie (CTA) Bilddaten segmentiert, wobei zwei und drei-dimensionale aktive Konturmodelle, wie sie aus der Bildverarbeitung bekannt sind, zum Einsatz kommen. Der maximale Durchmesser (cernterline-basiert) sowie das aortale Volumen werden aus den rekonstruierten dreidimensionalen Modellen berechnet. Zusätzlich werden nicht-lineare Finite Elemente Modelle verwendet, um die mechanische Spannung in der Aortenwand zwischen der Aortenbifurkation und den Nierenarterien zu bestimmen. Zu diesen Zweck wird der mittlere arterielle Druck als Belastung angenommen und nicht-lineare isotrope Materialmodelle erfassen die mechanischen Eigenschaften der Aortenwand und des Thrombusgewebes. Die Intra- und Interobserver Variabilität der fünf Messungen des maximalen Durchmessers, des Volumens und der maximalen Wandspannung wurden durch die Berechnung des Variationskoeffizienten (CV=SD*100/Arithmethisches Mittel in %) ausgedrückt. Die methodische Variation berechnet sich aus der Abweichung des Duchmessers (mm), des Volumens (ml) und der maximalen Wandspannung (kPA) zwischen den drei Untersuchern. Ergebnisse: Die Reproduzierbarkeit gesunder Gefäßen lag bei einem Durchmesser zwischen 16.1mm und 16.6mm zwischen CV=2,5% und CV=4,9%. Das aortale Volumen lag zwischen 14ml und 15ml, die Reproduzierbarkeit bei den gesunden Gefäßen streute zwischen CV=5.8% und CV=11.5%. Die maximale Wandspannung variierte zwischen 53 kPA and 55 kPa, der CV% lag hierbei zwischen 3 und 13. Die Interobserver Variabilität lag < 10% für den Durchmesser, die Volumenbestimmung und die Bestimmung der maximale Wandspannung. Der maximale Durchmesser der Aorta bei 3 Patienten mit infrarenalem Aneurysma wurde mit durchschnittlich 58.9mm, 54.6mm und 71.2mm berechnet (Stand bei Abstracteinreichung). Der Variationskoeffizient zeigte dabei eine hohe Übereinstimmung mit Werten unter 5%. Das Volumen der Aneurysmen schwankte zwischen 130 ml und 300 ml (CV<10%), die berechnete Wandspannung lag zwischen 172 kPA und 296 kPA (CV<10%). Die Variabilität zwischen den drei Untersuchern betrug 0,7-6,0 mm für den Durchmesser, 11-28 ml für das Volumen und 4-27 kPA für die maximale Wandspannung. Zusammenfassung: Sowohl an gesunden als auch an degenerativ veränderten Gefäßen ergibt die Reproduzierbarkeit des Aortendurchmessers und des aortalen Volumens basierend auf dem dreidimensionalen rekonstruierten Modellen eine hohe Übereinstimmung. Die berechnete Wandspannung basierend auf den Finiten Elemente Modellen zeigt einen geringen Grad an Variabilität sowohl zwischen verschiedenen Untersuchern als auch bei wiederholter Messung. Daher könnten die Volumenbestimmung und die Analyse der Wandspannung zusätzliche Größen bei der Bestimmung des individuellen Rupturrisikos bei Patienten mit Aortenaneurysmen darstellen, um eine präzisere Indikationsstellung zu ermöglichen.
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| 3. |
- Auer, M., et al.
(författare)
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Reconstruction and Finite Element Mesh Generation of Abdominal Aortic Aneurysms From Computerized Tomography Angiography Data With Minimal User Interactions
- 2010
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Ingår i: IEEE Transactions on Medical Imaging. - : Institute of Electrical and Electronics Engineers (IEEE). - 0278-0062 .- 1558-254X. ; 29:4, s. 1022-1028
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Tidskriftsartikel (refereegranskat)abstract
- Evaluating rupture risk of abdominal aortic aneurysms is critically important in reducing related mortality without unnecessarily increasing the rate of elective repair. According to the current clinical practice aneurysm rupture risk is (mainly) estimated from its maximum diameter and/or expansion rate; an approach motivated from statistics but known to fail often in individuals. In contrast, recent research demonstrated that patient specific biomechanical simulations can provide more reliable diagnostic parameters, however current structural model development is cumbersome and time consuming. This paper used 2D and 3D deformable models to reconstruct aneurysms from computerized tomography angiography data with minimal user interactions. In particular, formulations of frames and shells, as known from structural mechanics, were used to define deformable modes, which in turn allowed a direct mechanical interpretation of the applied set of reconstruction parameters. Likewise, a parallel finite element implementation of the models allows the segmentation of clinical cases on standard personal computers within a few minutes. The particular topology of the applied 3D deformable models supports a fast and simple hexahedral-dominated meshing of the arising generally polyhedral domain. The variability of the derived segmentations (luminal: 0.50(SD 0.19) mm; exterior 0.89(SD 0.45) mm) with respect to large variations in elastic properties of the deformable models was in the range of the differences between manual segmentations as performed by experts (luminal: 0.57(SD 0.24) mm; exterior: 0.77(SD 0.58) mm), and was particularly independent from the algorithm's initialization. The proposed interaction of deformable models and mesh generation defines finite element meshes suitable to perform accurate and efficient structural analysis of the aneurysm using mixed finite element formulations.
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| 4. |
- Biasetti, Jacopo, et al.
(författare)
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An Integrated Fluid-Chemical Model Toward Modeling the Formation of Intra-Luminal Thrombus in Abdominal Aortic Aneurysms
- 2012
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Ingår i: Frontiers in Physiology. - : Frontiers Media SA. - 1664-042X. ; 3:266
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Tidskriftsartikel (refereegranskat)abstract
- Abdominal Aortic Aneurysms (AAAs) are frequently characterized by the presence of an Intra-Luminal Thrombus (ILT) known to influence their evolution biochemically and biomechanically. The ILT progression mechanism is still unclear and little is known regarding the impact of the chemical species transported by blood flow on this mechanism. Chemical agonists and antagonists of platelets activation, aggregation, and adhesion and the proteins involved in the coagulation cascade (CC) may play an important role in ILT development. Starting from this assumption, the evolution of chemical species involved in the CC, their relation to coherent vortical structures (VSs) and their possible effect on ILT evolution have been studied. To this end a fluid-chemical model that simulates the CC through a series of convection-diffusion-reaction (CDR) equations has been developed. The model involves plasma-phase and surface-bound enzymes and zymogens, and includes both plasma-phase and membrane-phase reactions. Blood is modeled as a non-Newtonian incompressible fluid. VSs convect thrombin in the domain and lead to the high concentration observed in the distal portion of the AAA. This finding is in line with the clinical observations showing that the thickest ILT is usually seen in the distal AAA region. The proposed model, due to its ability to couple the fluid and chemical domains, provides an integrated mechanochemical picture that potentially could help unveil mechanisms of ILT formation and development.
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| 5. |
- Biasetti, Jacopo, et al.
(författare)
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Blood flow and coherent vortices in the normal and aneurysmatic aortas : a fluid dynamical approach to intraluminal thrombus formation
- 2011
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Ingår i: Journal of the Royal Society Interface. - : The Royal Society. - 1742-5689 .- 1742-5662. ; 8:63, s. 1449-1461
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Tidskriftsartikel (refereegranskat)abstract
- Abdominal aortic aneurysms (AAAs) are frequently characterized by the development of an intra-luminal thrombus (ILT), which is known to have multiple biochemical and biomechanical implications. Development of the ILT is not well understood, and shear-stress-triggered activation of platelets could be the first step in its evolution. Vortical structures (VSs) in the flow affect platelet dynamics, which motivated the present study of a possible correlation between VS and ILT formation in AAAs. VSs educed by the lambda(2)-method using computational fluid dynamics simulations of the backward-facing step problem, normal aorta, fusiform AAA and saccular AAA were investigated. Patient-specific luminal geometries were reconstructed from computed tomography scans, and Newtonian and Carreau-Yasuda models were used to capture salient rheological features of blood flow. Particularly in complex flow domains, results depended on the constitutive model. VSs developed all along the normal aorta, showing that a clear correlation between VSs and high wall shear stress (WSS) existed, and that VSs started to break up during late systole. In contrast, in the fusiform AAA, large VSs developed at sites of tortuous geometry and high WSS, occupying the entire lumen, and lasting over the entire cardiac cycle. Downward motion of VSs in the AAA was in the range of a few centimetres per cardiac cycle, and with a VS burst at that location, the release (from VSs) of shear-stress-activated platelets and their deposition to the wall was within the lower part of the diseased artery, i.e. where the thickest ILT layer is typically observed. In the saccular AAA, only one VS was found near the healthy portion of the aorta, while in the aneurysmatic bulge, no VSs occurred. We present a fluid-dynamics-motivated mechanism for platelet activation, convection and deposition in AAAs that has the potential of improving our current understanding of the pathophysiology of fluid-driven ILT growth.
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| 6. |
- Biasetti, Jacopo, et al.
(författare)
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Hemodynamics of the Normal Aorta Compared to Fusiform and Saccular Abdominal Aortic Aneurysms with Emphasis on a Potential Thrombus Formation Mechanism
- 2010
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Ingår i: Annals of Biomedical Engineering. - : Springer Science and Business Media LLC. - 0090-6964 .- 1573-9686. ; 38:2, s. 380-390
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Tidskriftsartikel (refereegranskat)abstract
- Abdominal Aortic Aneurysms (AAAs), i.e., focal enlargements of the aorta in the abdomen are frequently observed in the elderly population and their rupture is highly mortal. An intra-luminal thrombus is found in nearly all aneurysms of clinically relevant size and multiply affects the underlying wall. However, from a biomechanical perspective thrombus development and its relation to aneurysm rupture is still not clearly understood. In order to explore the impact of blood flow on thrombus development, normal aortas (n = 4), fusiform AAAs (n = 3), and saccular AAAs (n = 2) were compared on the basis of unsteady Computational Fluid Dynamics simulations. To this end patient-specific luminal geometries were segmented from Computerized Tomography Angiography data and five full heart cycles using physiologically realistic boundary conditions were analyzed. Simulations were carried out with computational grids of about half a million finite volume elements and the Carreau-Yasuda model captured the non-Newtonian behavior of blood. In contrast to the normal aorta the flow in aneurysm was highly disturbed and, particularly right after the neck, flow separation involving regions of high streaming velocities and high shear stresses were observed. Naturally, at the expanded sites of the aneurysm average flow velocity and wall shear stress were much lower compared to normal aortas. These findings suggest platelets activation right after the neck, i.e., within zones of pronounced recirculation, and platelet adhesion, i.e., thrombus formation, downstream. This mechanism is supported by recirculation zones promoting the advection of activated platelets to the wall.
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| 7. |
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| 8. |
- Biasetti, Jacopo
(författare)
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Physics of blood flow in arteries and its relation to intra-luminal thrombus and atherosclerosis
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Vascular pathologies such as Abdominal Aortic Aneurysm (AAA) and atherosclerosis are complex vascular diseases involving biological, mechanical, and fluid-dynamical factors. This thesis follows a multidisciplinary approach and presents an integrated fluid-chemical theory of ILT growth and analyzes the shear-induced migration of red blood cells (RBCs) in large arteries with respect to hypoxia and its possible role in atherosclerosis. The concept of Vortical Structures (VSs) is employed, with which a theory of uid-chemically-driven ILT growth is formulated. The theory proposes that VSs play an important role in convecting and activating platelets in the aneurysmatic bulge. In particular, platelets are convected toward the distal aneurysm region inside vortex cores and are activated via a combination of high residence times and relatively high shear stress at the vortex boundary. After vortex breakup, platelets are free to adhere to the thrombogenic wall surface. VSs also convect thrombin, a potent procoagulant enzyme, captured in their core, through the aneurysmatic lumen and force its accumulation in the distal portion of the AAA. This framework is in line with the clinical observation that the thickest ILT is usually seen in the distal AAA region. The investigation of the fluid-dynamics in arteries led to the study of the shear-induced migration of RBCs in large vessels such as the abdominal aorta and the carotid artery. Marked RBCs migration is observed in the region of the carotid sinus and in the iliac arteries, regions prone to atherogenesis. This leads to the hypothesis that oxyhemoglobin availability can decrease in the near-wall region thus contributing to wall hypoxia, a factor implicated in atherosclerosis. The thesis proposes a new potential mechanism of ILT growth, driven by fluid and chemical stimuli, which can be used to study ILT progression over physiologically relevant timeframes and be used as a framework to test new hypotheses; the thesis also provides new insights on the oxyhemoglobin availability in the near-wall region with direct inuence on atherosclerosis.
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| 9. |
- Biasetti, Jacopo, et al.
(författare)
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Shear-induced migration of red blood cells in the abdominal aorta and thecarotid bifurcation : considerations on oxygen transport
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Shear-induced migration of red blood cells (RBCs) is a well known phenomenon characterizing blood flow in the small vessels (micron to mm size) of the cardiovascular system. In large vessels, like the abdominal aorta and the carotid artery (mm to cm size), the extent of this migration has not been fully elucidated. RBCs migration exerts its influence primarily on platelet concentration, oxygen transport and oxygen availability at the luminal surface; this being of primary importance in, for example, intra-luminal thrombus (ILT) growth, atherosclerosis and intima hyperplasia. Phillips’ shear-induced particle migration model coupled to the Quemada viscosity model was employed to simulate the macroscopic behavior of RBCs in four patient-specific geometries: a normal abdominal aorta, an abdominal aortic aneurysm (AAA), a normal carotid bifurcation and a stenotic carotid bifurcation. Simulations show a migration of RBCs from the near wall region with a lowering of wall hematocrit (volume fraction of RBCs) on the posterior side of the normal aorta and in the iliac arteries. A marked migration is observed on the outer wall of the carotid sinus, the inner curvature wall of the common carotid artery and in the carotid stenosis. No significant migration is observed in the AAA. The spatial and temporal patterns of wall hematocrit are correlated with the near-wall shear layer and with the secondary flow induced by the vessel curvature. The results reinforce data in literature showing a decrease in oxygen partial pressure on the inner curvature wall of the carotid sinus and, more in general, on the inner curvature wall. The lowering of wall hematocrit is postulated to induce a decrease in oxygen availability at the luminal surface through a diminished concentration of oxyhemoglobin, hence contributing, with the lowered oxygen partial pressure, to local hypoxia.
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| 10. |
- Biasetti, Jacopo, et al.
(författare)
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Synergy between shear-induced migration and secondary flows on red blood cells transport in arteries : considerations on oxygen transport
- 2014
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Ingår i: Journal of the Royal Society Interface. - : The Royal Society. - 1742-5689 .- 1742-5662. ; 11:97, s. 20140403-
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Tidskriftsartikel (refereegranskat)abstract
- Shear-induced migration of red blood cells (RBCs) is a well-known phenomenon characterizing blood flow in the small vessels (micrometre to millimetre size) of the cardiovascular system. In large vessels, like the abdominal aorta and the carotid artery (millimetre to centimetre size), the extent of this migration and its interaction with secondary flows has not been fully elucidated. RBC migration exerts its influence primarily on platelet concentration, oxygen transport and oxygen availability at the luminal surface, which could influence vessel wall disease processes in and adjacent to the intima. Phillips' shear-induced particle migration model, coupled to the Quemada viscosity model, was employed to simulate the macroscopic behaviour of RBCs in four patient-specific geometries: a normal abdominal aorta, an abdominal aortic aneurysm (AAA), a normal carotid bifurcation and a stenotic carotid bifurcation. Simulations show a migration of RBCs from the near-wall region with a lowering of wall haematocrit (volume fraction of RBCs) on the posterior side of the normal aorta and on the lateral-external side of the iliac arteries. A marked migration is observed on the outer wall of the carotid sinus, along the common carotid artery and in the carotid stenosis. No significant migration is observed in the AAA. The spatial and temporal patterns of wall haematocrit are correlated with the near-wall shear layer and with the secondary flows induced by the vessel curvature. In particular, secondary flows accentuate the initial lowering in RBC near-wall concentration by convecting RBCs from the inner curvature side to the outer curvature side. The results reinforce data in literature showing a decrease in oxygen partial pressure on the inner curvature wall of the carotid sinus induced by the presence of secondary flows. The lowering of wall haematocrit is postulated to induce a decrease in oxygen availability at the luminal surface through a diminished concentration of oxyhaemoglobin, hence contributing, with the reported lowered oxygen partial pressure, to local hypoxia.
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