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Sökning: WFRF:(Gasser Thomas) > Gasser Thomas Christian

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1.
  • Auer, Martin, et al. (författare)
  • Automatic Displacement and Strain measuring in the Aorta from dynamic electrocardiographically-gated Computed Tomographic Angiography
  • 2010
  • Konferensbidrag (refereegranskat)abstract
    • Introduction Image modalities like Duplex Ultrasound, Transesophageal Echocardiography, Intravascular Ultrasound, Computed Tomography and Magnetic Resonance provide vascular interventionists and surgeons with useful diagnostic information for treatment planning. Recent developments in cross-sectional imaging, including multi-modality image fusion and new contrast agents have resulted in improved spatial resolution. Specifically, dynamic Electrocardiographically-Gated Computed Tomographic Angiography (ECG-gated CTA) provides valuable information regarding motion and deformation of the normal and diseased aorta during the cardiac cycle. Extracting and presenting (visualization) of accurate quantitative information from the recorded image data, however remains a challenging task of image post processing. Method The algorithm proposed within this paper processes ECG-gated CTA data (here goes the scanner model and manufacturer) in DICOM (digital imaging and communication in medicine) format, within which the user manually defines an Eulerian Region of Interest (ROI). 2D deformable (active) contour models are used to pre-segment the luminal surfaces of the selected vessels at an arbitrary time point during the cardiac cycle. A tessellation algorithm is used to define the initial configuration of a 3D deformable (active) contour model, which in turn is used for the final segmentation of the luminal surfaces continuously during the cardiac cycle. Specifically, Finite Element (FE) formulations [1] for frames and shells, as known from structural mechanics, are used to define the deformable contour modes. This allows a direct mechanical interpretation of the applied set of reconstruction parameters and leads to an efficient FE implementation of the models [2]; parallel processor architecture is used to solve the global set of non-linear FE equations. Finally displacement and strain measures are derived from the dynamic segmentations and color coded plots are used to visualize them. Results and Conclusions The clinical relevance of dynamic imaging has not been fully exploited and accurate and fast image processing tools are critical to extract valuable information from ECG-gated CTA data. Such information is not only of direct clinical relevance but also critical to process our current understanding regarding normal and pathological aortic motions and deformations. The image processing concept proposed in this paper leads to efficient and clinically applicable software that facilitates an analysis of the entire aorta on a standard Personal Computer within a few minutes. Deformable (active) contour models are known to be more accurate compared to threshold based segmentation concepts [3] and the accuracy of the present approach is in the range of the in-plane image resolution. Apart from direct diagnostic information the extracted geometrical data could also be used (once enriched by accurate pressure measurements) for none invasive (minimal invasive) estimation of biomechanical aortic tissue properties. References [1] O. C. Zienkiewicz and R. L. Taylor, vol.1,2, 5th ed. Oxford: Butterworth Heinemann, 2000. [2] M. Auer and T. C. Gasser, IEEE T. Med. Imaging, 2010 (in press). [3] M. Sonka and J. M. Fitzpatrick, editors., Bellingham: Spie press, 2000
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2.
  • Biasetti, Jacopo, et al. (författare)
  • A Blood Flow based model for Platelet Activation in Abdominal Aortic Aneurisms
  • 2010
  • Konferensbidrag (refereegranskat)abstract
    • Introduction Thrombus formation is the physiological response to vascular injury, it prevents loss of blood and permits wound healing, however, it is also associated with pathological conditions like hypoxia, anoxia and infarction [1]. Consequently, thrombus development must be carefully modulated to avoid uncontrolled growth, which in turn could lead to organ malfunctions. Specifically, an Intra-Luminal Thrombus (ILT) is found in almost all larger (clinically relevant) Abdominal Aortic Aneurysms (AAAs) and multiple biochemical [2] and biomechanical [3] implications on the underlying wall tissue have been reported. Despite the dominant role played by the ILT in AAA disease little is known regarding its development, and hence, the present study investigates ILT formation with particular emphasis on platelet activation triggered by biomechanical and biochemical field variables. Method The proposed model assumes that platelet activation is defined by a single field variable representing the accumulation of mechanical [4] and chemical [5] factors as the platelet moves along its path line. Platelet activation is given as soon asovercomes a certain threshold thought to be a constitutive property of blood. Specifically, the rate of the activation variable is determined by the maximum shear stress and the local concentrations of agonists and antagonists. To implement the model the fluid mechanical problem was solved in (COMSOL, COMSOL AB) and a particle tracking analysis (MATLAB, The MathWorks) was applied as a post processing step. The flow in a circular tube and the Backward Facing Step (BFS) problem under varying initial conditions were used for a basic investigation of the model and to relate its predictions to available data in the literature. Finally, platelet activation in patient specific AAAs was predicted and related to ILT development, which was estimated from Computer Tomography-Angiography (CT-A) data recorded from patient follow-up studies. Results and Conclusions The platelet activation variable  is complex distributed (highly heterogeneous) in the flow field, where, specifically, at the boundary of vortexes [6] and in the boundary layer of the non- endothelialized wall highest values were predicted. Continuous release of antagonists from the endothelialized wall lowers  in its vicinity, and hence, despite the high shear stress platelet activation is prevented. The proposed model links biomechanical and biochemical mechanisms of platelet activation and is able to predict the onset of thrombus formation of the BFS problem. The model is also able to predict some features of ILT development in the AAA, however, the change in luminal geometry is a cumulative effect of ILT growth, wall growth and their mechanical interactions, and hence, data recorded form patient follow-up studies needs to be analyzed carefully when validating the present model. References [1] J. D. Humphrey, Springer-Verlag, New York, 2002. [2] M. Kazi, et. al. J. Vasc. Surg., 38:1283-1292, 2003. [3] W. R. Mower et. al., J. Vasc. Surg., 33:602-608, 1997. [4] J. D. Hellums, Ann. Biomed. Eng., 22: 445-455, 1994. [5] B. Alberts et. al. Molecular Biology of the cell, 2002. [6] J. Biasetti et. al. Ann. Biomed. Eng., 38: 380–390 2010.
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3.
  • Biasetti, Jacopo, et al. (författare)
  • A Fluid-chemical model of thrombus formation
  • 2011
  • Ingår i: CMBE2011.
  • Konferensbidrag (refereegranskat)abstract
    • Our understanding of the genesis and evolution of Abdominal Aortic Aneurysms (AAAs), withparticular emphasis on Intra-Luminal Thrombus’ evolution, may be improved by studying thecomplex interplay between fluid-dynamics and biochemistry. To investigate the evolution of prothromboticchemicals inside the blood flow, in particular thrombin (factor IIa), a fluido-chemicalmodel has been developed. To this end a series of convection-diffusion-reaction (CDR) equationsdescribing the tissue factor pathway to thrombin have been solved on top of the biofluiddynamics problem. The proposed model integrates biochemistry and fluids dynamics, and hence,supports a comprehensive understanding of how ILT in AAAs may develop.
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4.
  • Erhart, P., et al. (författare)
  • Finite Element Analysis in Asymptomatic, Symptomatic, and Ruptured Abdominal Aortic Aneurysms : In Search of New Rupture Risk Predictors
  • 2015
  • Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 49:3, s. 239-245
  • Tidskriftsartikel (refereegranskat)abstract
    • Objectives: To compare biomechanical rupture risk parameters of asymptomatic, symptomatic and ruptured abdominal aortic aneurysms (AAA) using finite element analysis (FEA). Study design: Retrospective biomechanical single center analysis of asymptomatic, symptomatic, and ruptured AAAs. Comparison of biomechanical parameters from FEA. Materials and methods: From 2011 to 2013 computed tomography angiography (CTA) data from 30 asymptomatic, 15 symptomatic, and 15 ruptured AAAs were collected consecutively. FEA was performed according to the successive steps of AAA vessel reconstruction, segmentation and finite element computation. Biomechanical parameters Peak Wall Rupture Risk Index (PWRI), Peak Wall Stress (PWS), and Rupture Risk Equivalent Diameter (RRED) were compared among the three subgroups. Results: PWRI differentiated between asymptomatic and symptomatic AAAs (p < .0004) better than PWS (p < .1453). PWRI-dependent RRED was higher in the symptomatic subgroup compared with the asymptomatic subgroup (p < .0004). Maximum AAA external diameters were comparable between the two groups (p < .1355). Ruptured AAAs showed the highest values for external diameter, total intraluminal thrombus volume, PWS, RRED, and PWRI compared with asymptomatic and symptomatic AAAs. In contrast with symptomatic and ruptured AAAs, none of the asymptomatic patients had a PWRI value >1.0. This threshold value might identify patients at imminent risk of rupture: Conclusions: From different FEA derived parameter, PWRI distinguishes most precisely between asymptomatic and symptomatic AAAs. If elevated, this value may represent a negative prognostic factor for asymptomatic AAAs.
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5.
  • Gasser, Thomas Christian (författare)
  • Aorta
  • 2017
  • Ingår i: Biomechanics of Living Organs. - : Elsevier. - 9780128040607 - 9780128040096 ; , s. 169-191
  • Bokkapitel (refereegranskat)abstract
    • The aorta is a dynamic structure that is able to maintain conditions for optimal mechanical operation through the continuous turnover of its internal structure. The aorta's properties are critical to the entire cardiovascular system, and the study of its biomechanics may help us to better understand the role of tissue stress and strain in aortic aging and pathology, help to optimize medical devices, and improve therapeutic and diagnostic methods that are currently used in clinics. The present chapter reviews aortic wall histology and morphology in relation to its key mechanical properties. Specifically, the biomechanical role of cells (endothelial cells, smooth muscle cells, fibroblasts, etc.), as well as the extracellular matrix components (elastin, collagen, proteoglycans, water, etc.), will be discussed. Then this information is related to reported constitutive descriptions for aortic tissues. The focus is on histo-mechanical approaches and modeling frames, related to hyperelasticity as well as a superposition of fiber contributions according to a general theory of fibrous connective tissue. Concluding remarks relate to open problems in aorta biomechanics, such as uncertainty and variability of input information. Remarks are also made on the admissible degree of complexity in aortic simulations, in the context of such uncertainties.
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6.
  • Giampaolo, Martufi, et al. (författare)
  • Abdominal Aortic Aneurysm development over time : Experimental evidence and constitutive modeling
  • 2010
  • Ingår i: Proceedings of the 6th World Congress of Biomechanics. - : Springer. - 9783642145148
  • Konferensbidrag (refereegranskat)abstract
    • Abdominal Aortic Aneurysms (AAAs) are defined as a localized permanent dilatation of the infrarenal aorta at least 50 % of its normal diameter. AAAs are frequently diagnosed in the elderly male population and evaluating rupture risk is critically important as aneurysm rupture carries high mortality rates. Growth predictors might be helpful to assess AAA rupture risk and could therefore give a better graded indication for elective repair in order to reduce related mortality without unnecessarily increasing the rate of interventions. Factors associated with AAA growth are still limited but there are some evidence that higher initial AAA diameter is related to faster AAA expansion [1]. The initial dilatation is dependent on elastin degradation, but strength of the AAA is maintained by increased production of collagen. It has been suggested that rupture occurs when collagen production is insufficient to counteract load-bearing at high pressure [2]. AAA growth quantification 30 patients with infrarenal AAAs were included in this study. Criteria for inclusion were 1-year follow-up and availability of at least two high-resolution Computer Tomography-Angiography (CTA) scans. Consequently, 60 CT-A scans were systematically segmented, reconstructed and analyzed (A4research, VASCOPS GmbH), in order to investigate geometrical and mechanical factors likely to be correlated with AAA growth. Derived results were analyzed with an especially developed (automatic) analyzing schema (MatLab, The MathWorks), and the derived information aims at guiding the development of an analytical growth model for AAAs. Constitutive Modeling Collagen is a structural protein responsible for the mechanical strength, stiffness and toughness of biological tissues like skin, tendon, bone, cornea, lung and vasculature. In the present study we considered the enlargement of the aneurysm as a consequence of a pathological degradation and synthesis of collagen, i.e. malfunction of collagen turn-over. Consequently, the vascular wall is modeled by an (inert) matrix material representing the elastin, which is reinforced by a dynamic structure of bundles of collagen. Specifically, collagen is formed by a continuous stress-mediated process and deposited in the current configuration [3] and removed by a constant degradation rate. Finally the micro-plane concept [4] is used for the Finite Element implementation [5] of the constitutive model. Results and conclusions The quantitative description of AAA growth by examining patient follow-up data revealed novel insights into the natural history of this disease. Most interestingly not all portions of the AAA seem to enlarge, some might be stable or even shrink over time; a feature that has not yet been considered by models reported in the literature. The model proposed within this study has a strong biological motivation and captures saline feature of AAA growth. Besides that, the micro-plane approach allows a straight forward FE implementation and preliminary results indicate its numerical robustness. References [1] F.J.V. Schlösser, et al., J Vasc Surg, 47:1127–1133 2008. [2] E. Choke, et al., Eur.j.Vasc.endovasc.surg, 30(3):227-44 2005. [3] J.D.Humphrey, J Biomech Eng, 121:591–597 1999. [4] Z.P. Bazant and P.C. Prat, J Eng Mech, 113(7) 1050-1064 1987. [5] S. Federico and T.C Gasser, J R Soc Interface (in press)
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7.
  • Holzapfel, Gerhard A., et al. (författare)
  • Computational stress-deformation analysis of arterial walls including high-pressure response
  • 2007
  • Ingår i: International Journal of Cardiology. - : Elsevier BV. - 0167-5273 .- 1874-1754. ; 116:1, s. 78-85
  • Tidskriftsartikel (refereegranskat)abstract
    • Background: Changes in the mechanical behavior of arteries after balloon angioplasty cause cell reactions that may be responsible for restenosis. Hence, the study of the stress-deforination changes in arterial walls following supraphysiological tissue loading is an essential task. Methods: A normal LAD coronary artery was modeled and computationally analyzed as a two-layer, thick-walled, anisotropic and inelastic circular tube including residual strains. Each layer was treated as a fibre-matrix composite. The tube was subjected to an axial stretch of 1. 1 and a transmural pressure of 750 min Hg. Since overstretch of rerrmant non-diseased tissue in lesions is a primary mechanism of lumen enlargement this model approach represents a reasonable first step. Results: At physiological loading, the residual stresses led to a significant reduction of the high circumferential stress values at the inner wall, and the stress gradients. At low pressure level the media was the mechanically relevant layer, while at supraphysiological loading, the adventitia was the predominant load-carrying constituent providing a stiff support for 'redistribution' of soft plaque components by means of radial compression. After unloading to physiological loading conditions the stress state in the arterial wall differed significantly from that before inflation; the stress gradient in the media even changed its sign. Complete unloading indicated lumen enlargement, material softening and energy dissipation, which is in agreement with experimental studies. Conclusions: This method may be useful to improve interventional protocols for reducing the dilatational trauma, and thereby the adverse biological reaction in arterial walls following balloon angioplasty.
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8.
  • Hyhlik-Dürr, A., et al. (författare)
  • Finite-Elemente-Analyse abdomineller Aortenaneurysmen : Erste Ergebnisse der Intra- und Interobserver Validierung
  • 2010
  • 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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9.
  • Man, V., et al. (författare)
  • Impact of isotropic constitutive descriptions on the predicted peak wall stress in abdominal aortic aneurysms
  • 2018
  • Ingår i: Medical Engineering and Physics. - : Elsevier. - 1350-4533 .- 1873-4030. ; 53, s. 49-57
  • Tidskriftsartikel (refereegranskat)abstract
    • Biomechanics-based assessment of Abdominal Aortic Aneurysm (AAA) rupture risk has gained considerable scientific and clinical momentum. However, computation of peak wall stress (PWS) using state-ofthe-art finite element models is time demanding. This study investigates which features of the constitutive description of AAA wall are decisive for achieving acceptable stress predictions in it. Influence of five different isotropic constitutive descriptions of AAA wall is tested; models reflect realistic non-linear, artificially stiff non-linear, or artificially stiff pseudo-linear constitutive descriptions of AAA wall. Influence of the AAA wall model is tested on idealized (n = 4) and patient-specific (n = 16) AAA geometries. Wall stress computations consider a (hypothetical) load-free configuration and include residual stresses homogenizing the stresses across the wall. Wall stress differences amongst the different descriptions were statistically analyzed. When the qualitatively similar non-linear response of the AAA wall with low initial stiffness and subsequent strain stiffening was taken into consideration, wall stress (and PWS) predictions did not change significantly. Keeping this non-linear feature when using an artificially stiff wall can save up to 30% of the computational time, without significant change in PWS. In contrast, a stiff pseudo-linear elastic model may underestimate the PWS and is not reliable for AAA wall stress computations.
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10.
  • Martufi, Giampaolo, et al. (författare)
  • Review : The role of biomechanical modeling in the rupture risk assessment for abdominal aortic aneurysms
  • 2013
  • Ingår i: Journal of Biomechanical Engineering. - : ASME International. - 0148-0731 .- 1528-8951. ; 135:2, s. 021010-
  • Forskningsöversikt (refereegranskat)abstract
    • AAA disease is a serious condition and a multidisciplinary approach including biomechanics is needed to better understand and more effectively treat this disease. A rupture risk assessment is central to the management of AAA patients, and biomechanical simulation is a powerful tool to assist clinical decisions. Central to such a simulation approach is a need for robust and physiologically relevant models. Vascular tissue senses and responds actively to changes in its mechanical environment, a crucial tissue property that might also improve the biomechanical AAA rupture risk assessment. Specifically, constitutive modeling should not only focus on the (passive) interaction of structural components within the vascular wall, but also how cells dynamically maintain such a structure. In this article, after specifying the objectives of an AAA rupture risk assessment, the histology and mechanical properties of AAA tissue, with emphasis on the wall, are reviewed. Then a histomechanical constitutive description of the AAA wall is introduced that specifically accounts for collagen turnover. A test case simulation clearly emphasizes the need for constitutive descriptions that remodels with respect to the mechanical loading state. Finally, remarks regarding modeling of realistic clinical problems and possible future trends conclude the article.
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