Toward simultaneous flow and pressure assessment in large arteries using non-invasive ultrasound

Ultrageluid wordt in de kliniek vaak toegepast om op een niet-invasieve manier geometrische eigenschappen van grote vaten, zoals diameter en wanddikte en hemodynamische variabelen zoals bloedstroomsnelheid te bepalen. Om biomechanische parameters en hemodynamische variabelen die karakteristiek zijn voor de ontwikkeling van hart en vaatziekten, zoals compliantie en impedantie, te bepalen, is de bepaling van geometrie en bloedstroomsnelheid alleen onvoldoende. Daarvoor is een gelijktijdige en bij voorkeur niet invasieve meting van debiet en druk vereist. Met de huidige ultrageluidstechnieken is het onmogelijk om gelijktijdig debiet en druk nauwkeurig te bepalen. Debiet wordt vaak bepaald aan de hand van twee metingen: een diametermeting (geluidsbundel loodrecht op het vat) en een meting van de maximale axiale bloedstroomsnelheid met behulp van Doppler ultrageluid (geluidsbundel onder een hoek met het vat). Door een theoretische snelheidsverdeling aan te nemen, bijvoorbeeld een Poiseuille of Womersley profiel, wordt hieruit vervolgens het debiet berekend. In-vivo zijn vaten zelden recht: vaten zijn taps toelopend, gekromd en hebben vertakkingen. Dientengevolge zijn er secundaire snelheidscomponenten aanwezig die de axiale snelheidverdeling be¨invloeden. Dit resulteert in asymmetrische axiale snelheidsverdelingen. Omdat de aangenomen snelheidsverdelingen slechts geldig zijn voor rechte vaten, geeft een dusdanige bepaling een onnauwkeurige afschatting van het debiet. Verder is het onmogelijk om gelijktijdig met de snelheidsmeting nauwkeurig de wandbeweging te bepalen, waardoor de debietmeting nog verder verslechtert en het gelijktijdig bepalen van druk uit wandbeweging en debiet onmogelijk wordt. In dit onderzoek worden Particle Image Velocimetry (PIV) gebaseerde algoritmen toegepast op RF-data die verkregen zijn met behulp van een commercieel beschikbaar, voor klinische toepassing goedgekeurd ultrageluidssysteem. Dit maakt het mogelijk om snelheidscomponenten loodrecht op de ultrageluidbundel, en dus gelijktijdig wandpositie en axiale snelheid nauwkeurig te meten. Deze snelheidsmeettechniek is gevalideerd door metingen van het snelheidsprofiel in een experimentele opstelling te vergelijken met resultaten van computational fluid dynamics (CFD) berekeningen, voor stationaire en instationaire stromingen in een recht vat. Er werd een goede overeenstemming gevonden voor het axiale snelheidsprofiel. Integratie van het gemeten axiale snelheidsprofiel leverde een nauwkeurige afschatting van het debiet op. Omdat in de praktijk de meeste vaten gekromd zijn is de snelheids meetmethode vervolgens gevalideerd voor toepassing op stromingen in dit soort geometrieën. In de experimentele opstelling zijn axiale snelheidsprofielen gemeten voor stationaire en instationaire stroming in kromme buizen. Opnieuw zijn de gemeten profielen vergeleken met resultaten van CFD-berekeningen. Ook hier werd een goede overeenstemming gevonden tussen de gemeten profielen en de met behulp van CFD berekende snelheidsprofielen. Om nauwkeurig debiet te bepalen op basis van de gemeten asymmetrische axiale snelheidsprofielen, is een analytische en een op CFD gebaseerde studie gedaan naar stroming in kromme vaten. Deze studie heeft geresulteerd in de cos ¿-methode. Toepassing van de cos ¿-methode op de gemeten asymmetrische axiale profielen gaf een nauwkeurige afschatting van het debiet, voor stationaire en instationaire flow. Vergeleken met de huidig toegepaste afschattingsmethode voor het debiet werd een grote verbetering gevonden. Voor een fysiologisch relevant debiet gaf de cos ¿-methode een gemiddelde afwijking van 5% ten opzichte van het referentiedebiet terwijl deze voor de huidig toegepaste Poiseuille en Womersley benaderingen gelijk was aan 20%. Tenslotte is getracht om de lokale druk te bepalen uit enkel een niet-invasieve ultrageluidsmeting door een meting van de diameter te combineren met een gelijktijdige bepaling van de lokale compliantie. De lokale compliantie is bepaald door de lokale golfsnelheid (PWV) te meten. Verschillende methoden om lokaal de PWV te meten zijn getest in de experimentele opstelling. Hieruit bleek dat de QA-methode, een methode waarbij de lokale PWV bepaald wordt uit de verhouding tussen veranderingen in debiet en veranderingen in oppervlak van de dwarsdoorsnede van het vat, het mogelijk maakt om lokaal nauwkeurig PWV te meten. Door de PWV meting te combineren met een gelijktijdige meting van de diameter werd de lokale druk nauwkeurig afgeschat. Dit geeft aan dat het haalbaar is om op een niet-invasieve manier in-vivo druk te meten met behulp van ultrageluid. Hoewel de meettechnieken besproken in deze studie alleen getest zijn voor toepassing in een gecontroleerde experimentele omgeving, zijn de vooruitzichten voor klinische toepassing veelbelovend. De gepresenteerde methoden maken het mogelijk om de toestand van het vaatbed nauwkeuriger te bepalen, waardoor in de toekomst informatie verkregen kan worden over het effect van therapeutische ingrepen en factoren ge¨identificeerd kunnen worden die karakteristiek zijn voor de ontwikkeling van hart- en vaatziekten

Effects of size and elasticity on the relation between flow velocity and wall shear stress in side-wall aneurysms:A lattice Boltzmann-based computer simulation study

Blood flow in an artery is a fluid-structure interaction problem. It is widely accepted that aneurysm formation, enlargement and failure are associated with wall shear stress (WSS) which is exerted by flowing blood on the aneurysmal wall. To date, the combined effect of aneurysm size and wall elasticity on intra-aneurysm (IA) flow characteristics, particularly in the case of side-wall aneurysms, is poorly understood. Here we propose a model of three-dimensional viscous flow in a compliant artery containing an aneurysm by employing the immersed boundary-lattice Boltzmann-finite element method. This model allows to adequately account for the elastic deformation of both the blood vessel and aneurysm walls. Using this model, we perform a detailed investigation of the flow through aneurysm under different conditions with a focus on the parameters which may influence the wall shear stress. Most importantly, it is shown in this work that the use of flow velocity as a proxy for wall shear stress is well justified only in those sections of the vessel which are close to the ideal cylindrical geometry. Within the aneurysm domain, however, the correlation between wall shear stress and flow velocity is largely lost due to the complexity of the geometry and the resulting flow pattern. Moreover, the correlations weaken further with the phase shift between flow velocity and transmural pressure. These findings have important implications for medical applications since wall shear stress is believed to play a crucial role in aneurysm rupture

Transient Cardiovascular Hemodynamics In A Patient-Specific Arterial System

The ultimate goal of the present study is to aid in the development of tools to assist in the treatment of cardiovascular disease. Gaining an understanding of hemodynamic parameters for medical implants allow clinicians to have some patient-specific proposals for intervention planning. In the present study a full cardiovascular experimental phantom and digital phantom (CFD model) was fabricated to study: (1) the effects of local hemodynamics on global hemodynamics, (2) the effects of transition from bed-rest to upright position, and (3) transport of dye (drug delivery) in the arterial system. Computational three dimensional (3-D) models (designs A, B, and C) stents were also developed to study the effects of stent design on hemodynamic flow and the effects of drug deposition into the arterial wall. The experimental phantom used in the present study is the first system reported in literature to be used for hemodynamic assessment in static and orthostatic posture changes. Both the digital and experimental phantom proved to provide different magnitudes of wall shear and normal stresses in sections where previous studies have only analyzed single arteries. The dye mass concentration study for the digital and experimental cardiovascular phantom proved to be useful as a surrogate for medical drug dispersion. The dye mass concentration provided information such as transition time and drug trajectory paths. For the stent design CFD studies, hemodynamic results (wall shear stress (WSS), normal stress, and vorticity) were assessed to determine if simplified stented geometries can be used as a surrogate for patient-specific geometries and the role of stent design on flow. Substantial differences in hemodynamic parameters were found to exist which confirms the need for patient-specific modeling. For drug eluting stent studies, the total deposition time for the drug into the arterial wall was approximately 3.5 months

Flow Dynamics in Human Aorta with Coexisting Models of Bicuspid Aortic Stenosis and Coarctation of the Aorta

Coarctation of the aorta (COA) is an obstruction of the aorta and is usually associated with bicuspid and tricuspid aortic valve stenosis (AS). The main objective of this work is to understand the hemodynamic of COA from different perspectives. This was performed using a global approach including: numerical simulations, mathematical lumped parameter modeling and experimental measurements. Numerous investigations pointed to a relationship between the genesis and the progression of cardiovascular disease and the locally irregular flow occurring at the diseased zone. Therefore, to examine the relationship between arterial disease and hemodynamics conditions, a joint experimental and numerical investigation was performed to understand physics of fluid flow of COA. When COA coexists with AS, the left ventricle faces a double hemodynamic load: a valvular load plus a vascular load. First, a formulation describing the instantaneous net pressure gradient through COA was introduced and the predictions compared to in vitro results. The model was then used to determine left ventricular work induced by coexisting aortic stenosis and coarctation with different severities. The suggested model can be used to guide the choice of optimal operative procedure (aortic valve replacement and/or coarctation repaired surgery) and to predict the potential outcome for such patients. Early detection and accurate estimation of COA severity is the most important predictor of successful long-term outcome. However, current clinical parameters used for the evaluation of the severity of COA have several limitations. In this study, first, we evaluated the limitations of current existing parameters (Catheter trans-COA pressure gradients and Doppler echocardiographic trans-COA pressure gradients) for the evaluation of the severity of COA. Then, we suggested a new approach based on COA Doppler velocity index and COA effective orifice area capable of predicting more accurately the severity of COA. An original in-vitro study was performed using a mock flow circulation model with different COA severities and various aortic valve conditions under different flow rates. In conclusion, this study investigated the flow dynamics of COA and development of a lumped parameter model, based on non-invasive measurements, capable of accurately investigating the impact of coexisting AS and COA on left ventricular workload. In addition, this study proposed two innovative approaches to evaluate the severity of COA correctly

Axisymmetric and Three-Dimensional Lattice Boltzmann Models and Their Applications in Fluid Flows

Ph.DDOCTOR OF PHILOSOPH

Biomechanics of Retinal Venous Pulsations as Indicators of Intracranial Pressure

The origin of retinal venous pulsations remains an open problem in physiology and medicine; so too, their exact relationship to intracranial pressure. This study takes a mathematical modeling approach to explore details of blood flow through the eye to reveal the mechanism of pulsations. The intravaginal, intraneural, and intraocular segments of the retinal arteries and veins are modeled as connected resistive-capacitive segments. The analysis incorporates two critical mechanical properties of these small blood vessels, not heretofore studied, which become significant under conditions of negative transmural pressures: (1) dramatically reduced compliance during flattening and (2) cross-sectional shape change as internal volume decreases. Intraocular pressure acting on these veins close to the optic disc normally creates fluctuating negative transmural pressure. The observed long diameters of these venous segments become wider during diastole as they empty and flatten and narrower during systole as they refill with blood. Such visible pulsations occur only in models that include nonlinear compliance and constant-perimeter flattening. Further, the pulsations disappear when raised intracranial pressure, raised cavernous sinus pressure, or reduced arteriolar resistance elevates internal pressure in all retinal veins above the level of intraocular pressure. In this case transmural venous pressures are always positive, cross sectional shapes are circular, and compliance is greatly reduced. Then visible retinal venous pulsations disappear. Scenarios are suggested under which intracranial pressure can be estimated quantitatively from physical examination of retinal venous pulsations, if intraocular pressure is also measured

The effect of geometrical configurations on flows in idealised and realistic vascular geometries

This thesis reports the use of computational fluid dynamics (CFD) to investigate geometrical effects on flows in idealised non-branching double curved geometries (Study A) and in realistic distal anastomoses where geometries have been determined in vivo using magnetic resonance imaging (Study B). The purpose of this research is to improve understanding of the effects of geometrical configurations, especially curvature and non-planarity, on steady flow in idealised non-branching double curved geometries typical of arteries such as the aortic arch, the right coronary artery or the femoral arteries and on pulsatile flow in realistic distal anastomosis geometries. It is explained that the further knowledge gained from these idealised geometries can be useful to understand flows in anatomically correct geometries in order to optimise the design of end-to-side bypass graft vessels in clinical surgery. In the Study A, three-dimensional computations of steady flows in planar and non-planar double bends with 8 = 0.25 (curvature ratio) at Reynolds numbers of 125 and 500 were performed using the Navier-Stokes solver called Nektar that is based on spectral/hp element methods. The numerical haemodynamics analysis is presented in terms of the various mechanical factors which primarily involve axial velocity, transverse flows, vorticity, coherent vertical structure and wall shear stress. From these results, we can anticipate the wall shear stress distributions and secondary flow patterns in various double bend geometries with different non-planarity at low Reynolds numbers. Non-planarity plays a significant role on the wall shear stress distribution and the mixing properties of flow in the double bends, both of which are believed to be important factors for the patency of bypass grafts. In the Study B, Three sets of MRI data from patients undergoing tunnelled or superficial femoral bypass surgery were processed to give input velocity waveforms and geometries. From the latter and using the same Navier Stokes solver, detailed patient-specific pulsatile haemodynamics were calculated. Wall shear stress and velocity are influenced by the anatomical geometry, and wall shear stress distributions in pulsatile flow are compared with those in steady flow. The correlation is discussed between the haemodynamics and the remodelling of the vessel following bypass surgery determined in follow-up studies carried out a few months or a few years later. The results suggest that better designs should be possible for bypass grafts and indicate how inflow conditions can affect the flow field. The findings imply that proper anastomotical configurations might, induce haemodynamics environments that may prevent cardiovascular disorders or delay the progression of vascular disease.Open acces