Numerical study on patient-specific haemodynamics subjected to embolisation and wall-distensibility

Computational fluid dynamics (CFD) simulations have been peformed to investigate the hemodynamics of patient-specific cerebral aneurysm treated with endovascular coils; and arteriovenous fistula (AVF) using Star-CCM+. Fluid-structure interactions (FSI) between the elastic vessel walls and the blood flow within were also taken into account to provide a more realistic environment and better understanding of haemodynamic effects on wall remodelling. The blood in both studies was modelled as non-Newtonian fluid and comprises of three phases to fully incorporate the effects of shear-thinning and distributions of blood cells, respectively. The use of a less invasive ultrasonic imaging texhnique for CFD simulations is shown to be a viable alternative to magnetic resonance imaging (MRI). This has proven to be beneficial especially for haemodialysis patients who require fistula check-up on a regular basis. Excessively enlarged sections of arteries, called aneurysms, are vulnerable to vessel wall degradation. When blood flows into a cerebral aneurysm, it causes abnormal haemodynamic changes, which increases the risk of aneurysm rupture and strokes. Patients diagnosed with a cerebral aneurysm are therefore treated by stenting the parent artery or aneurysmal coiling to achieve occlusion. Despite high coiling packing density, aneurysm may recanalise, which consequently leads to aneurysm recurrence. Our understanding of the relationship between coiling density and aneurysmal occlusion and aneurysm recurrence in a non-Newtonian environment are limited. The effects of coil packing density on aneurysmal haemodynamics and the mechanism behind aneurysmal recurrence are discussed in this thesis. In the present aneurysm study, the aneurysm dome was embolised with seven different coil configurations of different packing densities. A time-dependent passive scalar was added to the multiphase blood inflow to represent medical dyes which allows for the visualisation of blood flow penetrating into the coils. The observed relationship between passive scalar visualisations, white blood cells distribution, and hemodynamic quantities will be beneficial for clinical evaluation of aneurysm occlusion. It is shown that a packing density of 31% (7 coils) is the optimal coil density that can supress the aneurysmal volume-averaged velocity and wall shear stress. Furthermore, the temporal variation in streamwise velocity inside the aneurysm dome does not nescessarily decrease with coiling packing density during peak systole. Local packing density, distribution of red and white blood cells, and wall compliance have been correlated with aneurysm recurrence. Circumferential wall shear stress, radial wall displacement, adhesion of white blood cells on the wall, and whole blood velocity magnitude in the six and seven coils cases are compared. These two coiling cases are chosen to represent an event of aneurysm recurrence as unexpected increase in the mean inflow into the aneurysm is observed despite higher coil packing densities. To the best of the author’s knowledge, the present aneurysm study is the first to investigate the effects of coil packing densities and blood cells distribution on non-Newtonian aneurysmal flow reduction and aneurysm recurrence in both rigid and compliant cerebral aneurysms. Additionally, the effects of aneurysmal haemodynamics on coil movement and vice versa are investigated in a study featuring a compliant coil in a rigid aneurysm. Over-estimation in aneurysmal velocity and wall shear stress at multiple locations by the rigid coil model has been observed. An arteriovenous fistula (AVF) is a connection between a brachial artery and vein that is surgically created to provide haemodialysis patients with matured vascular access points. AVF maturation failure, however, often occurs and its underlying mechanisms still remain controversial. The present AVF study investigates the effects of the compliant wall and non-Newtonian blood viscosity in an end-to-end AVF. Four simulations were performed to compare Newtonian and non-Newtonian haemodynamics in both rigid and wall-compliant fistulas. Different ranges of wall shear stress parameters corresponding to certain endothelial changes are compared among the four cases. It is found that the effects of wall compliance is more significant than that of non-Newtonian rheology. Furthermore, non-Newtonian effects are more clear when the AVF walls are compliant. Volumetric quantities like flow recirculations and helicity, which are related to abnormal endothelial changes, are also found to be overestimated by the rigid wall assumption. The study also investigates the effects of multiphase haemodynamics on inward wall remodelling and thus AVF maturation failure. Low and oscillating wall shear stress index is introduced as a tool for predicting the risk of maturation failure. Wall shear stresses, both directional and magnitude, red blood cell viscosity, flow recirculations are correlated with wall remodelling and endothelial damages derived from von-Mises stresses

In-vitro analysis of haemodynamics in stented arteries.

Cardiovascular diseases (CVD) are the leading cause of death in the developed world. One of the most common management methods for CVD is through vascular implants such as stents to support arterial walls. However, determining the efficacy of stents can be difficult, particularly for high-risk stents, such as those used in the aorta. In-vitro modelling can provide safe insight into the haemodynamics changes within an artery due to specific stenting methods, without intrusive patient monitoring. The in-vitro studies presented in this thesis contribute to research on the haemodynamic changes within arteries using particle image velocimetry (PIV). In-vitro modelling can be used to investigate haemodynamics of arterial geometry and stent implants. However, in-vitro model fidelity is reliant on precise matching of in-vivo conditions. Flow distribution and wall shear stress depend on the Reynolds and Womersley numbers. This thesis reviewed currently published Reynolds and Womersley numbers for 14 major arteries in the human body. The results were presented both in a table and graphically for ease of understanding and future use. The results identified a paucity of information in smaller distal arteries compared to major arteries such as the aorta. Matching Reynolds and Womersley numbers for compliant in-vitro modelling may also be limited by model dimensional tolerances. A method for visualising the range of experimental conditions required for dynamic matching was developed and case studies for the ascending aorta and common carotid artery were presented. The assumed Sylgard 184 silicone would be used for phantom fabrication, and compared three working solutions: water/glycerine, water/glycerine/urea, and water/glycerine/sodium-iodide. To manufacture compliance matched silicone models of the ascending aorta and common carotid arteries, the models were scaled to 1.5x (ascending aorta) and 3x (common carotid) life scale, respectively. Modelling the ascending aorta with the comparatively high viscosity water/glycerine solution will lead to very high pump power demands. However, any of the working fluids considered could be dynamically matched with low pump demand for the common carotid model. The Frozen Elephant Trunk (FET) stent is a hybrid endovascular device that may be implemented in the event of an aneurysm or aortic dissection of the aortic arch or superior descending aorta. However, the FET stent is a high risk stent. In particular, the Type 1B endoleak can lead to intrasaccular flow due to an incomplete distal fit between the stent and artery during systole. Chapter 5 developed an in-vitro modelling technique to enable the investigation of the known failure. Recirculation zones and an asymmetric endoleak were identified distal to the surrogate stent graft. The endoleak developed at the peak of systole and was sustained until the onset of diastole. The endoleak geometry indicated a potential variation in the phantom artery wall thickness or stent alignment. Recirculation was identified on the posterior dorsal line during late systole which may induce an inflammatory response in an artery. The identification of the Type 1B endoleak proved that in-vitro modelling can be used to investigate complex compliance changes and wall motions. The kissing stent (KS) configuration is a low risk, stenting method often used to treat aorto-iliac occlusive disease (AIOD). However, long-term patency reduces by nearly 25% in the first five years potentially due to deleterious flow behaviour. The risk of harmful haemodynamics due to the KS configuration were investigated in-vitro. PIV experimentation identified peak proximal and distal velocity in-vitro was 0.71 m·s-1 and 1.90 m·s-1, respectively. A lumen wall collapse in the sagittal plane occurred during late systole to early diastole proximal the KS configuration. The collapse disturbed the flow proximal to the stented region producing potential recirculation zones and abnormal flow patterns. However, the systolic flow was as normal and undisturbed indicating the KS configuration is safe to use for repairing AIOD. The collapse had not been previously identified and would require further investigation. Thoracic extra-anatomic bypasses (EAB) are grafted stents that may be used to prophylactically revascularize supra-aortic arteries that may require blockage during thoracic endovascular aortic repair (TEVAR) methods. However, prophylactic use of EAB may introduce a risk of failure due to abnormally low or disrupted flow, known as competitive flow, within the bypasses. Competitive flow within the bypasses between supra-aortic arteries has not been captured previously. PIV was used to assess each model configuration for flow abnormalities and potential for flow competition. The investigation found potential for competitive flow in the bypasses when just the left subclavian artery (LSA), the left carotid artery (LCCA), or none of the arteries are blocked. In contrast, when the LSA and LCCA were both blocked, there was no evidence of competitive flow. Flow stagnated at the initiation of systole within the BC bypass in the 2 configurations with an unblocked LCCA, along with notable recirculation zones and reciprocating flow occurring throughout the rest of systolic flow. Flow stagnated in the CS bypass at early systole when only the LCCA was blocked. A large recirculation was identifiable in the CS bypass when just the LSA was blocked, particularly after peak systole. The potential of competitive flow indicated prophylactic used of EAB in the supra-aortic arteries may require location of proximal arteries to limit the number of pathways blood flow can take

The Haemodynamics of a Stented Arteriovenous Fistula through Experiments and Flow Modelling

The arteriovenous fistula (AVF) is a vasculature created for end-stage renal disease patients who undergo haemodialysis. This vasculature is often affected by stenosis in the juxta-anastomotic (JXA) region and the presence of disturbed haemodynamics within the vessel is known to initiate such diseased conditions. A novel treatment involving the implantation of a flexible stent in the JXA region has shown potential for retaining healthy AVFs. Only a limited number of experimental studies have been conducted to understand the disturbed flow conditions, while the impact of stent implantation on the haemodynamics within the AVF is yet to be explored. The study was initiated by developing a benchtop patient-specific AVF model to conduct a Tomographic Particle Image Velocimetry (Tomo-PIV) measurement. The subsequent temporally resolved volumetric velocity field was phase-averaged to quantify fluctuations occurring over the inlet pulsatile conditions. It was noted that high turbulent kinetic energy (TKE) was generated at the JXA region. To study the effects of the stent implantation, Large Eddy Simulations (LES) comparing the AVF geometry with and without the presence of the stent implantation were conducted. The trajectory of the flow in the stented case was funnelled within the stent encapsulated region which in turn, contained the disturbed flow within the stent lumen while mitigating the generation of turbulence. Consequently, the distribution of adverse wall shear stress (WSS) in the stented region was much lower compared to that of the `stent-absent' case. Simulations were also conducted on the diseased patient AVF, before the stent implantation, to make an overall assessment of the effect of treatment. Larger and persistent regions of high TKE were noted in the vessel downstream of the stenosis despite the lower velocity of flow in the diseased model. In summary, the stent implantation in the patient AVF showed the ability to funnel flow disturbances away from the vessel wall, thereby leading to lower adverse WSS distributions. The presence of the stent also mitigated turbulence generation. These findings provide valuable insight into the favourable haemodynamic effects of this novel endovascular procedure, thus, substantiating this treatment strategy to treat vascular disease in AVFs

Hemodynamics of Native and Bioprosthetic Aortic Valves: Insights from a Reduced Degree-of-Freedom Model

Heart disease is the leading cause of deaths in the US with aortic valve (AV) diseases being major contributors. Valve replacement is the primary therapeutic indication for AV diseases and transcatheter aortic valve replacement (TAVR) provides a safe and minimally invasive option. However, post-TAVR patient outcomes show considerable variability with deployment parameters. TAVR valves are also susceptible to failure mechanisms like leaflet thrombosis which increase the risk for serious thromboembolic events. Early detection and intervention can avert such outcomes, but symptoms often manifest at advanced stages of valve failure. Continuous monitoring can facilitate early detection, but regulatory and technological challenges may hinder developing such technology through experimental or clinical means. Computer simulations enable unprecedented predictive capabilities which can help gain insights into the pathophysiology of valvular diseases, conduct in silico trials to design novel monitoring technologies and even guide surgeries for optimal valve deployment. However, accurate, yet efficient numerical models are required. This study describes the implementation of a versatile, efficient AV dynamics model in a previously developed fluid-structure interaction solver, and its application to each of these tasks. The model accelerates simulations by simplifying the constitutive parameter space and equations governing leaflet motion without compromising accuracy. It can simulate native and prosthetic valve dynamics exhibiting physiological and pathological function in idealized and personalized aorta anatomies. This computational framework is used to generate canonical and patient-specific simulation datasets describing hemodynamic differences secondary to healthy and pathological AVs. These differences help identify biomarkers which reliably predict the risk of valvular and vascular diseases. Changes in these biomarkers are used to assess whether TAVR can deter aortic disease progression. Next, statistical differences in such biomarkers recorded by virtual wearable or embedded sensor systems, between normal and abnormal AV function, are analyzed using data-driven methods to infer valve health. This lays the groundwork for inexpensive, at-home diagnostic technologies, based on digital auscultation and in situ embedded-sensor platforms. Finally, a simulation describing the deployment of a commercially available TAVR valve in a patient-specific aorta anatomy and the associated hemodynamics is presented. Such simulations empower clinicians to optimize TAVR deployment and, consequently, patient outcomes

In-silico clinical trials for assessment of intracranial flow diverters

In-silico trials refer to pre-clinical trials performed, entirely or in part, using individualised computer models that simulate some aspect of drug effect, medical device, or clinical intervention. Such virtual trials reduce and optimise animal and clinical trials, and enable exploring a wider range of anatomies and physiologies. In the context of endovascular treatment of intracranial aneurysms, in-silico trials can be used to evaluate the effectiveness of endovascular devices over virtual populations of patients with different aneurysm morphologies and physiologies. However, this requires (i) a virtual endovascular treatment model to evaluate device performance based on a reliable performance indicator, (ii) models that represent intra- and inter-subject variations of a virtual population, and (iii) creation of cost-effective and fully-automatic workflows to enable a large number of simulations at a reasonable computational cost and time. Flow-diverting stents have been proven safe and effective in the treatment of large wide-necked intracranial aneurysms. The presented thesis aims to provide the ingredient models of a workflow for in-silico trials of flow-diverting stents and to enhance the general knowledge of how the ingredient models can be streamlined and accelerated to allow large-scale trials. This work contributed to the following aspects: 1) To understand the key ingredient models of a virtual treatment workflow for evaluation of the flow-diverter performance. 2) To understand the effect of input uncertainty and variability on the workflow outputs, 3) To develop generative statistical models that describe variability in internal carotid artery flow waveforms, and investigate the effect of uncertainties on quantification of aneurysmal wall shear stress, 4) As part of a metric to evaluate success of flow diversion, to develop and validate a thrombosis model to assess FD-induced clot stability, and 5) To understand how a fully-automatic aneurysm flow modelling workflow can be built and how computationally inexpensive models can reduce the computational costs

Soluções analíticas e semi-analíticas de escoamentos instacionários em meio confinado para fluidos newtonianos generalizados

Tese de mestrado integrado. Engenharia Mecânica. Faculdade de Engenharia. Universidade do Porto. 201

The Modeling of pulsatile blood flow as Cross-Williamson and Carreau fluids in an artery with a partial occlusion

In the present article behavior of pulsatile blood flow through stenoses is studied using the incompressible non-Newtonian models. The non-Newtonian models chosen are characterized by the Carreau and Cross-Williamson models incorporating the effect of tapering due to the pulsatile nature of blood flow. The flow mechanism in the stenosed artery subject to a pulsatile pressure gradient arising from the normal functioning of the heart has been considered. An improved shape of the time-variant stenoses present in the tapered arterial lumen is given mathematically in order to update resemblance to the in vivo situation. Results were compared with powerlaw model and the differential approximation for the heat flux is invoked in the energy equation. The effect of heat transfer on the velocity is computed and discussed.6 page(s

Atti del XXXV Convegno Nazionale di Idraulica e Costruzioni Idrauliche

La XXXV edizione del Convegno Nazionale di Idraulica e Costruzioni Idrauliche (IDRA16), co-organizzata dal Gruppo Italiano di Idraulica (GII) e dal Dipartimento di Ingegneria Civile, Chimica, Ambientale, e dei Materiali (DICAM) dell’Alma Mater Studiorum - Università di Bologna, si è svolta a Bologna dal 14 al 16 settembre 2016. Il Convegno Nazionale è tornato pertanto ad affacciarsi all’ombra del “Nettuno”, dopo l’edizione del 1982 (XVIII edizione). Il titolo della XXXV edizione, “Ambiente, Risorse, Energia: le sfide dell’Ingegneria delle acque in un mondo che cambia”, sottolinea l’importanza e la complessità delle tematiche che rivestono la sfera dello studio e del governo delle risorse idriche. Le sempre più profonde interconnessioni tra risorse idriche, sviluppo economico e benessere sociale, infatti, spronano sia l’Accademia che l’intera comunità tecnico-scientifica nazionale ed internazionale all’identificazione ed alla messa in atto di strategie di gestione innovative ed ottimali: sfide percepite quanto mai necessarie in un contesto ambientale in continua evoluzione, come quello in cui viviamo. La XXXV edizione del Convegno di Idraulica e Costruzioni Idrauliche, pertanto, si è posta come punto d’incontro della comunità tecnico-scientifica italiana per la discussione a tutto tondo di tali problematiche, offrendo un programma scientifico particolarmente ricco e articolato, che ha coperto tutti gli ambiti riconducibili all’Ingegneria delle Acque. L’apertura dei lavori del Convegno si è svolta nella storica cornice della Chiesa di Santa Cristina, uno dei luoghi più caratteristici e belli della città ed oggi luogo privilegiato per l’ascolto della musica classica, mentre le attività di presentazione e discussione scientifica si sono svolte principalmente presso la sede della Scuola di Ingegneria e Architettura dell’Università di Bologna sita in Via Terracini. Il presente volume digitale ad accesso libero (licenza Creative Commons 4.0) raccoglie le memorie brevi pervenute al Comitato Scientifico di IDRA16 ed accettate per la presentazione al convegno a valle di un processo di revisione tra pari. Il volume articola dette memorie in sette macro-tematiche, che costituiscono i capitoli del volume stesso: I. meccanica dei fluidi; II. ambiente marittimo e costiero; III. criteri, metodi e modelli per l’analisi dei processi idrologici e la gestione delle acque; IV. gestione e tutela dei corpi idrici e degli ecosistemi; V. valutazione e mitigazione del rischio idrologico e idraulico; VI. dinamiche acqua-società: sviluppo sostenibile e gestione del territorio; VII. monitoraggio, open-data e software libero. Ciascuna macro-tematica raggruppa più sessioni specialistiche autonome sviluppatesi in parallelo durante le giornate del Convegno, i cui titoli vengono richiamati all’interno del presente volume. La vastità e la diversità delle tematiche affrontate, che ben rappresentano la complessità delle numerose sfide dell’Ingegneria delle Acque, appaiono evidenti dalla consultazione dell’insieme di memorie brevi presentate. La convinta partecipazione della Comunità Scientifica Italiana è dimostrata dalle oltre 350 memorie brevi, distribuite in maniera pressoché uniforme tra le sette macro-tematiche di riferimento. Dette memorie sono sommari estesi di lunghezza variabile redatti in lingua italiana, o inglese. In particolare, la possibilità di stesura in inglese è stata concessa con l’auspicio di portare la visibilità del lavoro presentato ad un livello sovranazionale, grazie alla pubblicazione open access del volume degli Atti del Convegno. Il volume si divide in tre parti: la parte iniziale è dedicata alla presentazione del volume ed all’indice generale dei contributi divisi per macro-tematiche; la parte centrale raccoglie le memorie brevi; la terza parte riporta l’indice analitico degli Autori, che chiude il volume

Atti del XXXV Convegno Nazionale di Idraulica e Costruzioni Idrauliche

La XXXV edizione del Convegno Nazionale di Idraulica e Costruzioni Idrauliche (IDRA16), co-organizzata dal Gruppo Italiano di Idraulica (GII) e dal Dipartimento di Ingegneria Civile, Chimica, Ambientale, e dei Materiali (DICAM) dell’Alma Mater Studiorum - Università di Bologna, si è svolta a Bologna dal 14 al 16 settembre 2016. Il Convegno Nazionale è tornato pertanto ad affacciarsi all’ombra del “Nettuno”, dopo l’edizione del 1982 (XVIII edizione). Il titolo della XXXV edizione, “Ambiente, Risorse, Energia: le sfide dell’Ingegneria delle acque in un mondo che cambia”, sottolinea l’importanza e la complessità delle tematiche che rivestono la sfera dello studio e del governo delle risorse idriche. Le sempre più profonde interconnessioni tra risorse idriche, sviluppo economico e benessere sociale, infatti, spronano sia l’Accademia che l’intera comunità tecnico-scientifica nazionale ed internazionale all’identificazione ed alla messa in atto di strategie di gestione innovative ed ottimali: sfide percepite quanto mai necessarie in un contesto ambientale in continua evoluzione, come quello in cui viviamo. La XXXV edizione del Convegno di Idraulica e Costruzioni Idrauliche, pertanto, si è posta come punto d’incontro della comunità tecnico-scientifica italiana per la discussione a tutto tondo di tali problematiche, offrendo un programma scientifico particolarmente ricco e articolato, che ha coperto tutti gli ambiti riconducibili all’Ingegneria delle Acque. L’apertura dei lavori del Convegno si è svolta nella storica cornice della Chiesa di Santa Cristina, uno dei luoghi più caratteristici e belli della città ed oggi luogo privilegiato per l’ascolto della musica classica, mentre le attività di presentazione e discussione scientifica si sono svolte principalmente presso la sede della Scuola di Ingegneria e Architettura dell’Università di Bologna sita in Via Terracini. Il presente volume digitale ad accesso libero (licenza Creative Commons 4.0) raccoglie le memorie brevi pervenute al Comitato Scientifico di IDRA16 ed accettate per la presentazione al convegno a valle di un processo di revisione tra pari. Il volume articola dette memorie in sette macro-tematiche, che costituiscono i capitoli del volume stesso: I. meccanica dei fluidi; II. ambiente marittimo e costiero; III. criteri, metodi e modelli per l’analisi dei processi idrologici e la gestione delle acque; IV. gestione e tutela dei corpi idrici e degli ecosistemi; V. valutazione e mitigazione del rischio idrologico e idraulico; VI. dinamiche acqua-società: sviluppo sostenibile e gestione del territorio; VII. monitoraggio, open-data e software libero. Ciascuna macro-tematica raggruppa più sessioni specialistiche autonome sviluppatesi in parallelo durante le giornate del Convegno, i cui titoli vengono richiamati all’interno del presente volume. La vastità e la diversità delle tematiche affrontate, che ben rappresentano la complessità delle numerose sfide dell’Ingegneria delle Acque, appaiono evidenti dalla consultazione dell’insieme di memorie brevi presentate. La convinta partecipazione della Comunità Scientifica Italiana è dimostrata dalle oltre 350 memorie brevi, distribuite in maniera pressoché uniforme tra le sette macro-tematiche di riferimento. Dette memorie sono sommari estesi di lunghezza variabile redatti in lingua italiana, o inglese. In particolare, la possibilità di stesura in inglese è stata concessa con l’auspicio di portare la visibilità del lavoro presentato ad un livello sovranazionale, grazie alla pubblicazione open access del volume degli Atti del Convegno. Il volume si divide in tre parti: la parte iniziale è dedicata alla presentazione del volume ed all’indice generale dei contributi divisi per macro-tematiche; la parte centrale raccoglie le memorie brevi; la terza parte riporta l’indice analitico degli Autori, che chiude il volume