The effects of stent porosity on the endovascular treatment of intracranial aneurysms located near a bifurcation

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Development of a novel uncovered stent system for the management of complex aortic aneurysms

Endovascular aortic repair (EVAR) is a minimally invasive alternative to open surgery for the treatment of aortic aneurysms (AA). However, standard EVAR is not applicable to complex AA with involvement of vital branches, which could be occluded by the endograft. As an emerging technique, the concept of multiple overlapping uncovered stents (MOUS) have been proposed to manage complex lesions. MOUS was used to modulate the flow pattern inside the aneurysm sac, and promote the thrombus formation followed by the aneurysm shrinkage. In this dissertation, we sought to investigate the mechanism of MOUS-induced flow modulation and key factors associated with the success of this novel technique: - The mechanical behaviour of AA was characterised by uniaxial material tests (Chapter 4). A Bayesian framework was proposed for material constants identification. They were found correlated to the microstructure of tissue fibre network and were capable in differentiating tissue types. - Solid-to-solid interaction and one-way fluid-solid interaction (FSI) analysis was performed based on patient-specific computer tomography angiography (Chapters 5&6). Structural stress concentrations were observed within the landing zones, which increased with the number of stents deployed. In the parameter studies (Chapter 6), the overall porosity was identified as the dominant factor of the flow-diverting outcome, while cross-stent structures of MOUS had limited influence. - The pathological effect of structural stress concentration induced by an implanted device was further studied in rabbit models (Chapter 7). The wall structural stress and fluid shear stress were obtained from FSI analysis based on magnetic resonance imaging (MRI), and correlated to plaque characteristics. Both high structural stress and low fluid shear stress were found correlated to plaque initialisation and increased inflammation. Overall, MOUS modulates the blood flow with robust performance under different overlapping patterns. Image-based biomechanical analysis can optimise MOUS design and can contribute to personalised pre-surgery planning.Shuo Wang was sponsored by China Scholarship Council (2014-2018)

Stenting as porous media in anatomically accurate geometries: A comparison of models and spatial heterogeneity

Modelling intracranial aneurysm blood flow after flow diverter treatment has proven to be of great scientific and clinical interest. One of the reasons for not having CFD as an everyday clinical tool yet is the time required to set-up such simulations plus the required computational time. The speed-up of these simulations can have a considerable impact during treatment planning and device selection. Modelling flow diverters as a porous medium (PM) can considerably improve the computational time. Many models have been presented in literature, but quantitative comparisons between models are scarce.In this study, the untreated case, the explicit definition of the flow diverter wires as no-slip boundary condition and five different porous medium models were chosen for comparison, and evaluated on intracranial aneurysm of 14 patients with different shapes, sizes, and locations. CFD simulations were made using finite volume method on steady flow conditions. Velocities, kinetic energy, wall shear stress, and computational time were assessed for each model. Then, all models are compared against the no-slip boundary condition using non parametric Kolmogorov–Smirnov test.The model with least performance showed a mean K-S statistic of 0.31 and deviance of 0.2, while the model with best values always gave K-S statistics below 0.2. Kinetic energy between PM models varied between an over estimation of 218.3% and an under estimation of 73.06%. Also, speedups were between 4.75x and 5.3x (stdev: 0.38x and 0.15x) when using PM models.Flow diverters can be simulated with PM with a good agreement to standard CFD simulations were FD wires are represented with no-slip boundary condition in less than a quarter of the time. Best results were obtained on PM models based on geometrical properties, in particular, when using a heterogeneous medium based on equations for flat rhomboidal wire frames.Fil: Dazeo, Nicolás Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil; Argentina. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Grupo de Plasmas Densos Magnetizados. Provincia de Buenos Aires. Gobernación. Comision de Investigaciones Científicas. Grupo de Plasmas Densos Magnetizados; ArgentinaFil: Dottori, Javier Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil; Argentina. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Grupo de Plasmas Densos Magnetizados. Provincia de Buenos Aires. Gobernación. Comision de Investigaciones Científicas. Grupo de Plasmas Densos Magnetizados; ArgentinaFil: Boroni, Gustavo Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil; Argentina. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Grupo de Plasmas Densos Magnetizados. Provincia de Buenos Aires. Gobernación. Comision de Investigaciones Científicas. Grupo de Plasmas Densos Magnetizados; ArgentinaFil: Narata, Ana Paula. Universite de Tours; FranciaFil: Larrabide, Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil; Argentina. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Grupo de Plasmas Densos Magnetizados. Provincia de Buenos Aires. Gobernación. Comision de Investigaciones Científicas. Grupo de Plasmas Densos Magnetizados; Argentin

Blood flow computational characterization inside an idealized saccular aneurysm in presence of magnetic field

The blood flow of a wide neck saccular cerebral aneurysm is modeled using numerical methods, before and after positioning a diverter flow or stent as endovascular treatment. Blood, as a magnetic fluid, is modeled computationally under the influence of different external magnetic fields. For this, a list of factors that can be varied in the external magnetic field configuration and, in turn, can affect the velocity field of the blood flow of the stented aneurysm is selected. By varying the amplitude, direction and frequency of the magnetic field, it is concluded that the amplitude has an incidence on blood velocity and shear stress in the regions where the aneurysm starts and in the stent spires. After studying the changes in blood flow, a suspension of idealized endothelial cells is computationally injected, the trajectory is modeled and the cells that are trapped on the stent spires are quantified. With this, it is possible to understand the changes in the flow conditions of the cells and to examine whether, when the blood flow is subjected to an external magnetic field, it is possible to trap endothelial cells in the region of the stent. Trapped cells finally would promote complete occlusion of the neck of the aneurysm through the stimulation of tissue growth called endothelialization.Resumen: El flujo de sangre de un aneurisma cerebral, sacular de cuello amplio, es modelado usando métodos numéricos, antes y después de posicionar un diversor de flujo o stent como tratamiento endovascular. La sangre, como fluido magnético, es modelada computacionalmente bajo la influencia de diferentes campos magnéticos externos. Para esto, se selecciona una lista de factores que pueden ser variados en la configuración del campo magnético externo y a su vez pueden afectar el campo de velocidades del flujo de sangre del aneurisma con stent. Al variar la magnitud, dirección y frecuencia del campo magnético, se concluye que la magnitud tiene incidencia en la velocidad de la sangre y en el esfuerzo cortante en las regiones donde se inicia el aneurisma y en las espiras del stent. Después de estudiar los cambios en el flujo de sangre, se inyectan computacionalmente una suspensión de células endoteliales idealizadas, se modela su trayectoria y se cuantifican las células que son atrapadas por las espiras del stent en la región del cuello del aneurisma. Con esto, es posible comprender los cambios en las condiciones de flujo de las células y examinar si cuando el flujo de sangre es sometido a un campo magnético externo, es posible atrapar células endoteliales en la región del stent para finalmente promover la oclusión completa del cuello del aneurisma a través del estímulo del crecimiento de tejido llamado endotelizaciónMaestrí

Computational investigations of a shape‐memory polymer foam embolization device for intracranial aneurysms

The objective of this research is to determine the efficacy of an intracranial aneurysm treatment option. An open‐source computational fluid dynamics software is used to simulate blood flow through 6 patient‐specific intracranial aneurysm geometries for 42 different cases. Virtual shape memory polymer foam embolization devices are created and implanted into the geometries. Different porous media parameters are considered for the embolic devices, and it is found that devices with a permeability of ∼5e-9 m2 can reduce aneurysmal inflow by 90% for various geometries of the treated aneurysm and its surrounding parent arterial vessel. For a wide‐necked aneurysm, devices with a permeability of 60%, indicating that they may be able to provide that level of performance for most aneurysm morphologies. As such, a permeability range of 5e-9–5e-8 m2 is recommended for the device. Furthermore, material removal from the center of the device is found to be feasible for larger aneurysm devices if compressibility is deemed a concern. For a high‐inflow case, the average aneurysmal velocity reduction is within 2% of the uncored device for all cored devices with a material thickness of at least 1.5 mm occluding the inlet area. Convective heat transfer is also modeled to determine the safety of the thermally stimulated shape memory polymer device. Steady‐state simulations identify the worst‐case geometry, a deep aneurysm with little opportunity for convection. Transient heat transfer during the device deployment process for 2 stimulus temperatures is modeled with this aneurysm, demonstrating that the vessel walls can reach the stimulus temperature of 40 °C and 45 °C within seconds and take over a minute to cool back to near body temperature. The threshold for brain tissue damage is not reached, but nonetheless, it is suggested that the temperature and heating time be kept as low as possible. Full model validation is not available, but general verification of the flow fields in untreated aneurysms is achieved by comparing simulation results to those obtained by other research groups in a modeling competition

Hemodynamic study in a real intracranial aneurysm: an in vitro and in silico approach

Mestrado de dupla diplomação com o Centro Federal de Educação Tecnológica Celso Suckow da Fonseca - Cefet/RJIntracranial aneurysm (IA) is a cerebrovascular disease with high rates of mortality and morbidity when it ruptures. It is known that changes in the intra-aneurysmal hemodynamic load play a significant factor in the development and rupture of IA. However, these factors are not fully understood. In this sense, the main objective of this work is to study the hemodynamic behavior during the blood analogues flow inside an AI and to determine its influence on the evolution of this pathology. To this end, experimental and numerical studies were carried out, using a real AI model obtained using computerized angiography. In the experimental approach, it was necessary, in the initial phase, to develop and manufacture biomodels from medical images of real aneurysms. Two techniques were used to manufacture the biomodels: rapid prototyping and gravity casting. The materials used to obtain the biomodels were of low cost. After manufacture, the biomodels were compared to each other for their transparency and final structure and proved to be suitable for testing flow visualizations. Numerical studies were performed with the aid of the Ansys Fluent software, using computational fluid dynamics (CFD), using the finite volume method. Subsequently, flow tests were performed experimentally and numerically using flow rates calculated from the velocity curve of a patient's doppler test. The experimental and numerical tests, in steady-state, made it possible to visualize the three-dimensional behavior of the flow inside the aneurysm, identifying the vortex zones created throughout the cardiac cycle. Correlating the results obtained in the two analyzes, it was possible to identify that the areas of vortexes are characterized by low speed and with increasing the fluid flow, the vortexes are positioned closer to the wall. These characteristics are associated with the rupture of an intracranial aneurysm. There was also a good qualitative correlation between numerical and experimental results.O aneurisma intracraniano (AI) é uma patologia cerebrovascular com altas taxas de mortalidade e morbidade quando se rompe. Sabe-se que alterações na carga hemodinâmica intra-aneurismática exerce um fator significativo no desenvolvimento e ruptura de AI, porém, esses fatores não estão totalmente compreendidos. Nesse sentido, o objetivo principal deste trabalho é o de estudar o comportamento hemodinâmico durante o escoamento de fluidos análogos do sangue no interior de um AI e determinar a sua influência na evolução da patologia. Para tal, foram realizados estudos experimentais e numéricos, utilizando um modelo de AI real obtido por meio de uma angiografia computadorizada. Na abordagem experimental foi necessário, na fase inicial, desenvolver e fabricar biomodelos a partir de imagens médicas de um aneurisma real. No fabrico dos biomodelos foram utilizadas duas técnicas: a prototipagem rápida e o vazamento por gravidade. Os materiais utilizados para a obtenção dos biomodelos foram de baixo custo. Após a fabricação, os biomodelos foram comparados entre si quanto à sua transparência e estrutura final e verificou-se serem adequados para testes de visualizações do fluxo. Os estudos numéricos foram realizados com recurso ao software Ansys Fluent, utilizando a dinâmica dos fluidos computacional (CFD), através do método dos volumes finitos. Posteriormente, foram realizados testes de escoamento experimentais e numéricos, utilizando caudais determinados a partir da curva de velocidades do ensaio doppler de um paciente. Os testes experimentais e numéricos, em regime permanente, possibilitaram a visualização do comportamento tridimensional do fluxo no interior do aneurisma, identificando as zonas de vórtices criadas ao longo do ciclo cardíaco. Correlacionando os resultados obtidos nas duas análises, foi possível identificar que as áreas de vórtices são caracterizadas por uma baixa velocidade e com o aumento do caudal os vórtices posicionam-se mais próximos da parede. Essas características apresentadas estão associadas com a ruptura de aneurisma intracraniano. Verificou-se, também, uma boa correlação qualitativa entre os resultados numéricos e experimentais

An innovative approach to simulate thrombosis with smoothed particle hydrodynamics

La trombosi è una patologia che porta alla formazione di coaguli, che possono provocare ostruzioni arteriose e, infine, migrano attraverso il sistema cardiocircolatorio causando infarto, ictus o embolia polmonare. Il processo è abbastanza complesso ed il suo meccanismo non è ancora chiaro, essendo il risultato dell’interazione tra diversi fattori, compresa l'attivazione e l'aggregazione piastrinica, le reazioni chimiche e l'emodinamica.È fondamentale considerare e ridurre al minimo la formazione di trombi nella progettazione e realizzazione di organi artificiali, come valvole cardiache artificiali o protesi. Lo studio dell'emodinamica può fornire un supporto efficace per identificare e prevenire il rischio di trombosi.A causa della mancanza di soluzioni analitiche adeguate e della complessità degli studi sperimentali, la ricerca evolve sempre più verso l’utilizzo delle simulazioni numeriche Questa tesi mira a modellare la formazione, la crescita e l'evoluzione del trombo mediante il metodo numerico accoppiato Smoothed Particle Hydrodynamics (SPH) usando un modello di interazione fluido-struttura (FSI). Il modello proposto descrive le principali fasi della cascata coagulativa attraverso l'equilibrio di quattro specie biochimiche e tre tipologie di piastrine. Le particelle SPH possono passare dalla fase fluida a quella solida se sono soddisfatte delle specifiche condizioni biochimiche e fisiche. L'accoppiamento fluido-solido è modellato introducendo legami elastici tra le particelle solide senza nessuna interfaccia. Per raggiungere questo obiettivo, in primo luogo il modello viene validato confrontando i risultati numerici con i dati sperimentali disponibili in letteratura, in secondo luogo, il nuovo codice numerico è applicato per descrivere la trombosi in appendice atriale in caso di fibrillazione o come trombosi indotta in aneurismi cerebrali.Thrombosis is a pathology leading to the formation of clots, that can result in arterial obstructions and, eventually, migrate through the cardiocirculatory system causing heart attack, stroke or pulmonary embolism. The process is complex and its mechanism is still unclear, being the result of the interaction between various factors, including platelet activation and aggregation, chemical reactions, and hemodynamics.It is crucial to consider and minimise thrombosis in the design and implementation of artificial organs, such as artificial heart valves, and vascular prostheses. The study of hemodynamics can provide effective support to identify and prevent the risk of thrombosis.Due to the lack of adequate analytical solutions and the complexity of experimental studies, research increasingly evolves towards the use of computational methods.This thesis aims at modelling the formation, growth and evolution of thrombus by means of a Smoothed Particle Hydrodynamics (SPH) numerical method coupled with a fluid-structure interaction (FSI) model. The proposed model describes the main phases of the coagulative cascade through the balance of four biochemical species and three types of platelets. SPH particles can switch from fluid to solid phase whenspecific biochemical and physical conditions are satisfied. Fluid-solid coupling is modelled by introducing elastic binds between solid particles, without requiring detention and management of the interface between the two media.In order to reach this goal, firstly the model is validated by comparing the numerical prediction with experimental data available in the literature, secondly, it is applied to describe thrombosis formation due to relevant pathologies such as atrial fibrillation and cerebral aneurysms where the insertion of flow diverter creates thrombogenic stasis zone

Heterogeneous System-on-Chip based Lattice-Boltzmann Visual Simulation System

Cerebral aneurysm is a cerebrovascular disorder caused by a weakness in the wall of an artery or vein, that causes a localised dilation or ballooning of the blood vessel. It is life-threatening, hence an early and accurate diagnosis would be a great aid to medical professionals in making the correct choice of treatment. HemeLB is a massively parallel lattice-Boltzmann simulation software which is designed to provide the radiologist with estimates of flow rates, pressures and shear stresses throughout the relevant vascular structures, intended to eventually permit greater precision in the choice of therapeutic intervention. However, in order to allow surgeries and doctors to view and visualise the results in real-time at medical environments, a cost-efficient, practical platform is needed. In this paper, we have developed and evaluated a version of HemeLB on various heterogeneous system-on-chip platforms, allowing users to run HemeLB on a low cost embedded platform and to visualise the simulation results in real-time. A comprehensive evaluation of implementation on the Zynq SoC and Jetson TX1 embedded graphic processing unit platforms are reported. The achieved results show that the proposed Jetson TX1 implementation outperforms the Zynq implementation by a factor of 19 in terms of site updates per second