Cardiovascular diseases are the most common cause of morbidity and mortality in the developed countries. Understanding the main mechanisms responsible for the onset and the subsequent development of these pathologies is essential in providing accurate and reliable representations of the physiological and pathological flow conditions in the cardiovascular system. Recent progress in medical imaging and geometry reconstruction techniques allowed scientists to obtain accurate descriptions of a vascular compartment of a specific individual. Nevertheless, despite the significant amount of biological data available, the translation of these data into more effective therapies, surgical planning and optimization of medical devices represents a very important issue. In this regard, computational models of the cardiovascular system have the potential to deliver a concrete aid in diagnosis and therapy in health-care. Indeed, numerical simulations of blood flow and the mechanical response of blood vessels may help both biomedical engineers and medical doctors to better characterize the patient conditions and increase the confidence of the estimate. This thesis focuses in this direction, by producing several novel outcomes. The first contribution consists in a deep investigation into the role of haemodynamics in the development of abdominal aortic aneurysms (AAAs) and the deposition of an intraluminal thrombus in the aneurysm sac. Indeed, several in vivo and in vitro observations suggested that non-physiological Wall Shear Stress (WSS, i.e. the tangential force per unit area exerted by blood on the arterial wall) may regulate the expression of an inflammatory, hypertrophic and thrombotic state. WSS signal may exhibit complex spatiotemporal patterns and the best way to extract quantitative synthetic information on the clinical risk is still an open question. Nevertheless, there is a recurring need to provide medical doctors with haemodynamic risk indicators/predictors supporting the decision-making protocol. In this dissertation, two novel synthetic risk predictors based on the Three-Band Decomposition (TBD) analysis are introduced and tested by performing geometrical multiscale fluid-structure interaction simulations in both idealized and patient-specific scenarios. In particular, the preliminary WSS analysis carried out using axially symmetric geometries suggests that the averaging operations involved in the definition of the risk indices commonly adopted in the Literature may induce a low level of information with respect to the complexity of the WSS spatiotemporal distribution. In this regard, the obtained results demonstrate that the TBD analysis can highlight the significant features of the shear stress and its role in the development of AAAs. Moreover, the two novel risk predictors related to the TBD analysis improve the risk assessment for both thrombus deposition and bulge expansion in the aneurysms models. The results obtained in such simplified geometries are confirmed by accurate numerical simulations performed for patient-specific models of both a healthy abdominal aorta and an abdominal aortic aneurysm. In particular, as expected, the healthy scenario does not show a significant risk pattern. On the contrary, in the patient-specific AAA model, a good agreement is found between the sites of platelets deposition, computed by performing a particle tracking, and the risk information provided by the two TBD-based risk predictors. The results also show good agreement with clinical data, thus confirming the ability of the TBD analysis and the associated risk indices to give access to the risk information related to the thrombus development. The second novel outcome of this dissertation is to compare the biomechanical performances of the stented and the stentless biological prostheses used for the replacement of the aortic valve. In particular, a stented prosthesis consists of a porcine aortic valve or bovine pericardial tissue leaflets mounted on a rigid frame surrounded by a synthetic sewing ring. The stentless biological prosthesis is obtained from the stented one by eliminating any rigid support. In particular, the Sorin Freedom SOLO stentless prostheses require a minimal surgical implantation technique with a single suture running around the three sinuses of Valsalva. In this dissertation, a finite element analysis based on the fluid-structure interaction between the blood and the surrounding aortic wall is carried out. To this aim, three patient-specific aortic root geometries have been reconstructed and for each of these, the stented, the stentless, and the native (healthy) configurations have been drawn. The obtained results reveal that the stentless prosthesis leads the aortic root to recover a more physiological dynamics with respect to the stented one, both in term of displacement magnitude and von Mises stresses. Finally, the third novel contribution of this thesis concerns more analytical aspects related to the role of the spatial variations of WSS in the development of vascular pathologies. Indeed, experimental studies demonstrated that the spatial gradient of Wall Shear Stress (WSSG) may promote inflammatory processes in the arterial wall of the blood vessels. In the Literature, the WSSG has been defined as the directional derivative of the WSS. However, by using the rigid frame formalism, one can show that the geometrically correct definition must involve the evaluation of the intrinsic derivative. The relevant differences found between the two definitions, by adopting analytical and numerical estimates, support the necessity to re-evaluate the past considerations on the WSSG in the Literature, specifically on the haemodynamic risk.

Advanced Theoretical Modelling and Fluid-Structure Interaction Analysis of Patient-Specific Cardiovascular Haemodynamics / Maria Giuseppina Chiara Nestola , 2015 Jun 11. 27. ciclo

Advanced Theoretical Modelling and Fluid-Structure Interaction Analysis of Patient-Specific Cardiovascular Haemodynamics

2015-06-11

Abstract

Cardiovascular diseases are the most common cause of morbidity and mortality in the developed countries. Understanding the main mechanisms responsible for the onset and the subsequent development of these pathologies is essential in providing accurate and reliable representations of the physiological and pathological flow conditions in the cardiovascular system. Recent progress in medical imaging and geometry reconstruction techniques allowed scientists to obtain accurate descriptions of a vascular compartment of a specific individual. Nevertheless, despite the significant amount of biological data available, the translation of these data into more effective therapies, surgical planning and optimization of medical devices represents a very important issue. In this regard, computational models of the cardiovascular system have the potential to deliver a concrete aid in diagnosis and therapy in health-care. Indeed, numerical simulations of blood flow and the mechanical response of blood vessels may help both biomedical engineers and medical doctors to better characterize the patient conditions and increase the confidence of the estimate. This thesis focuses in this direction, by producing several novel outcomes. The first contribution consists in a deep investigation into the role of haemodynamics in the development of abdominal aortic aneurysms (AAAs) and the deposition of an intraluminal thrombus in the aneurysm sac. Indeed, several in vivo and in vitro observations suggested that non-physiological Wall Shear Stress (WSS, i.e. the tangential force per unit area exerted by blood on the arterial wall) may regulate the expression of an inflammatory, hypertrophic and thrombotic state. WSS signal may exhibit complex spatiotemporal patterns and the best way to extract quantitative synthetic information on the clinical risk is still an open question. Nevertheless, there is a recurring need to provide medical doctors with haemodynamic risk indicators/predictors supporting the decision-making protocol. In this dissertation, two novel synthetic risk predictors based on the Three-Band Decomposition (TBD) analysis are introduced and tested by performing geometrical multiscale fluid-structure interaction simulations in both idealized and patient-specific scenarios. In particular, the preliminary WSS analysis carried out using axially symmetric geometries suggests that the averaging operations involved in the definition of the risk indices commonly adopted in the Literature may induce a low level of information with respect to the complexity of the WSS spatiotemporal distribution. In this regard, the obtained results demonstrate that the TBD analysis can highlight the significant features of the shear stress and its role in the development of AAAs. Moreover, the two novel risk predictors related to the TBD analysis improve the risk assessment for both thrombus deposition and bulge expansion in the aneurysms models. The results obtained in such simplified geometries are confirmed by accurate numerical simulations performed for patient-specific models of both a healthy abdominal aorta and an abdominal aortic aneurysm. In particular, as expected, the healthy scenario does not show a significant risk pattern. On the contrary, in the patient-specific AAA model, a good agreement is found between the sites of platelets deposition, computed by performing a particle tracking, and the risk information provided by the two TBD-based risk predictors. The results also show good agreement with clinical data, thus confirming the ability of the TBD analysis and the associated risk indices to give access to the risk information related to the thrombus development. The second novel outcome of this dissertation is to compare the biomechanical performances of the stented and the stentless biological prostheses used for the replacement of the aortic valve. In particular, a stented prosthesis consists of a porcine aortic valve or bovine pericardial tissue leaflets mounted on a rigid frame surrounded by a synthetic sewing ring. The stentless biological prosthesis is obtained from the stented one by eliminating any rigid support. In particular, the Sorin Freedom SOLO stentless prostheses require a minimal surgical implantation technique with a single suture running around the three sinuses of Valsalva. In this dissertation, a finite element analysis based on the fluid-structure interaction between the blood and the surrounding aortic wall is carried out. To this aim, three patient-specific aortic root geometries have been reconstructed and for each of these, the stented, the stentless, and the native (healthy) configurations have been drawn. The obtained results reveal that the stentless prosthesis leads the aortic root to recover a more physiological dynamics with respect to the stented one, both in term of displacement magnitude and von Mises stresses. Finally, the third novel contribution of this thesis concerns more analytical aspects related to the role of the spatial variations of WSS in the development of vascular pathologies. Indeed, experimental studies demonstrated that the spatial gradient of Wall Shear Stress (WSSG) may promote inflammatory processes in the arterial wall of the blood vessels. In the Literature, the WSSG has been defined as the directional derivative of the WSS. However, by using the rigid frame formalism, one can show that the geometrically correct definition must involve the evaluation of the intrinsic derivative. The relevant differences found between the two definitions, by adopting analytical and numerical estimates, support the necessity to re-evaluate the past considerations on the WSSG in the Literature, specifically on the haemodynamic risk.
11-giu-2015
Haemodynamic risk assessment; Wall Shear Stress; Fluid-Structure Interaction; Bioprosthetic Heart Valves; Wall Shear Stress Gradient
Advanced Theoretical Modelling and Fluid-Structure Interaction Analysis of Patient-Specific Cardiovascular Haemodynamics / Maria Giuseppina Chiara Nestola , 2015 Jun 11. 27. ciclo
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.12610/68786
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