Segmentation and reconstruction of 3D artery models for surgical planning

Master'sMASTER OF SCIENC

Blood

This book examines both the fluid and cellular components of blood. After the introductory section, the second section presents updates on various topics in hemodynamics. Chapters in this section discuss anemia, 4D flow MRI in cardiology, cardiovascular complications of robot-assisted laparoscopic pelvic surgery, altered perfusion in multiple sclerosis, and hemodynamic laminar shear stress in oxidative homeostasis. The third section focuses on thalassemia with chapters on diagnosis and screening for thalassemia, high blood pressure in beta-thalassemia, and hepatitis C infection in thalassemia patients

Studies of fluid flow in arterial bypass grafts by magnetic resonance imaging

This thesis is concerned primarily with the effects of graft geometry and flow pulsatility on the hemodynamics of arterial bypass grafts. Motivation for this work was the need of better understanding of the underlying flow related conditions that promote arterial bypass graft stenosis. The thesis describes techniques currently available for quantitative flow Magnetic Resonance Imaging (MRI). Methods to estimate and reduce measurement uncertainty in MRI phase contrast velocimetry are investigated. The MR phase-velocity mapping technique is calibrated in complex steady and transient flows by using highly resolved numerical computations as reference solutions. A novel splines based approximation method is introduced to model MRI velocity encoded phase. Applications of the proposed method to estimate wall shear stress and vorticity from MR phase velocity measurements are described and results from a series of validation studies are presented. The influence of out-of-plane geometry on the flow field in arterial bypass grafts is assessed by comparing two idealized model geometries: a planar and a non-planar configuration under steady and time-varying flow. Procedures for the acquisition of three-dimensional bypass graft geometries by MRI and their transformation to a com-puter-aided-design (CAD) representation are described. Stereolithographic replicas of bypass graft geometries (an in vivo extracted femoro-tibial distal anastomosis and an ex vivo imaged aorto-coronary distal anastomosis) and silicon negative models were fabricated for use in MRI flow studies. The effect of flow pulsatility on the flow features in the realistic aorto-coronary distal anastomosis model is investigated by comparing the flow field generated by a sinusoidal (single harmonic) and a physiological (multi-harmonic) waveform. The influence of the distal-proximal graft outflow division on the distal anastomotic sinus flow field in the femoro-tibial bypass graft model is examined. Finally, an account of the results from the comparative studies is given, and the physiological implications of the flow features of note are discussed

Analysis of Blood Flow in Patient-specific Models of Type B Aortic Dissection

Aortic dissection is the most common acute catastrophic event affecting the aorta. The majority of patients presenting with an uncomplicated type B dissection are treated medically, but 25% of these patients develop subsequent dilatation and aortic aneurysm formation. The reasons behind the long‐term outcomes of type B aortic dissection are poorly understood. As haemodynamic factors have been involved in the development and progression of a variety of cardiovascular diseases, the flow phenomena and environment in patient‐specific models of type B aortic dissection have been studied in this thesis by applying computational fluid dynamics (CFD) to in vivo data. The present study aims to gain more detailed knowledge of the links between morphology, flow characteristics and clinical outcomes in type B dissection patients. The thesis includes two parts of patient‐specific study: a multiple case cross‐sectional study and a single case longitudinal study. The multiple cases study involved a group of ten patients with classic type B aortic dissection with a focus on examining the flow characteristics as well as the role of morphological factors in determining the flow patterns and haemodynamic parameters. The single case study was based on a series of follow‐up scans of a patient who has a stable dissection, with an aim to identify the specified haemodynamic factors that are associated with the progression of aortic dissection. Both studies were carried out based on computed tomography images acquired from the patients. 4D Phase‐contrast magnetic resonance imaging was performed on a typical type B aortic dissection patient to provide detailed flow data for validation purpose. This was achieved by qualitative and quantitative comparisons of velocity‐encoded images with simulation results of the CFD model. The analysis of simulation results, including velocity, wall shear stress and turbulence intensity profiles, demonstrates certain correlations between the morphological features and haemodynamic factors, and also their effects on long‐term outcomes of type B aortic dissections. The simulation results were in good agreement with in vivo MR flow data in the patient‐specific validation case, giving credence to the application of the computational model to the study of flow conditions in aortic dissection. This study made an important contribution by identifying the role of certain morphological and haemodynamic factors in the development of type B aortic dissection, which may help provide a better guideline to assist surgeons in choosing optimal treatment protocol for individual patient

Review of Journal of Cardiovascular Magnetic Resonance 2015

There were 116 articles published in the Journal of Cardiovascular Magnetic Resonance (JCMR) in 2015, which is a 14 % increase on the 102 articles published in 2014. The quality of the submissions continues to increase. The 2015 JCMR Impact Factor (which is published in June 2016) rose to 5.75 from 4.72 for 2014 (as published in June 2015), which is the highest impact factor ever recorded for JCMR. The 2015 impact factor means that the JCMR papers that were published in 2013 and 2014 were cited on average 5.75 times in 2015. The impact factor undergoes natural variation according to citation rates of papers in the 2 years following publication, and is significantly influenced by highly cited papers such as official reports. However, the progress of the journal's impact over the last 5 years has been impressive. Our acceptance rate is <25 % and has been falling because the number of articles being submitted has been increasing. In accordance with Open-Access publishing, the JCMR articles go on-line as they are accepted with no collating of the articles into sections or special thematic issues. For this reason, the Editors have felt that it is useful once per calendar year to summarize the papers for the readership into broad areas of interest or theme, so that areas of interest can be reviewed in a single article in relation to each other and other recent JCMR articles. The papers are presented in broad themes and set in context with related literature and previously published JCMR papers to guide continuity of thought in the journal. We hope that you find the open-access system increases wider reading and citation of your papers, and that you will continue to send your quality papers to JCMR for publication

Investigation of blood flow in the superior mesenteric artery and its potential influence on atheroma and gut ischaemia.

Atherosclerosis is the underlying process in coronary heart disease leading to myocardial infarction, and in arterial damage leading to cerebrovascular accidents. It accounts for almost 50% of deaths in the western world. Atherosclerosis is characterised by the presence of fibro-lipid plaques (atheroma) within the vessel wall. Whilst the initiation and progression of atheroma are not fully understood, it is generally accepted that the time-varying haemodynamic wall shear stress (WSS) that the vessel wall is exposed to is important in determining the likelihood of development of an atherosclerotic plaque The superior mesenteric artery (SMA) is the major blood vessel feeding the small intestine; compared to other vessels of similar size, it is largely spared the effects of atherosclerosis

Blood flow dynamics in surviving patients with repaired Tetralogy of Fallot

Tetralogy of Fallot (TOF) is a congenital heart disease that causes structural abnormalities in the pulmonary arteries, which in turn disrupt the blood flow. Surgical repair is necessary early in childhood, but chronic complications are common in the adult surviving patients. Pulmonary valve replacement is an operation performed in the repaired TOF (rTOF) patients to overcome the right ventricular overload, but the optimal timing remains a challenge. The main research question is whether the haemodynamic environment of the pulmonary junction can clarify the interplay between the upstream and downstream pulmonary vasculature. Therefore, an extensive analysis of the effect of morphological and flow characteristics in healthy and rTOF models was performed, under various boundary conditions (BCs). The effects of branch angle and origin, branch stenosis, flow splits and pulmonary resistance were investigated in idealised two-dimensional geometries, representative of healthy and rTOF cases, explaining the elevated pressure in the LPA, and clearly showing that downstream pressure and peripheral resistance alter the flow development and the flow split between the two daughter branches. Various modelling parameters were also tested, demonstrating the importance of the valve, and how it disturbs the flow patterns along the MPA. The elasticity of arterial wall had a minimal effect on the flow development while the WSS deviated based on the rheological model assumed. Finally, anatomically realistic three-dimensional models of rTOF patients and healthy volunteers were reconstructed and morphological and flow features were analysed. Higher curvature and tortuosity were correlated with more complex secondary flow patterns, and higher Reynolds and Dean numbers, with increased regions of time-averaged wall shear stress. More importantly, the importance of patient-specificity in the rTOF models, and the variability of the geometric and flow characteristics within the population was highlighted, contrary to the observations in the healthy models. The results of this work could help clinicians evaluate the haemodynamic environment in the rTOF population and potentially predict patients at higher risk, prior to the appearance of severe complications.Tetralogy of Fallot (TOF) is a congenital heart disease that causes structural abnormalities in the pulmonary arteries, which in turn disrupt the blood flow. Surgical repair is necessary early in childhood, but chronic complications are common in the adult surviving patients. Pulmonary valve replacement is an operation performed in the repaired TOF (rTOF) patients to overcome the right ventricular overload, but the optimal timing remains a challenge. The main research question is whether the haemodynamic environment of the pulmonary junction can clarify the interplay between the upstream and downstream pulmonary vasculature. Therefore, an extensive analysis of the effect of morphological and flow characteristics in healthy and rTOF models was performed, under various boundary conditions (BCs). The effects of branch angle and origin, branch stenosis, flow splits and pulmonary resistance were investigated in idealised two-dimensional geometries, representative of healthy and rTOF cases, explaining the elevated pressure in the LPA, and clearly showing that downstream pressure and peripheral resistance alter the flow development and the flow split between the two daughter branches. Various modelling parameters were also tested, demonstrating the importance of the valve, and how it disturbs the flow patterns along the MPA. The elasticity of arterial wall had a minimal effect on the flow development while the WSS deviated based on the rheological model assumed. Finally, anatomically realistic three-dimensional models of rTOF patients and healthy volunteers were reconstructed and morphological and flow features were analysed. Higher curvature and tortuosity were correlated with more complex secondary flow patterns, and higher Reynolds and Dean numbers, with increased regions of time-averaged wall shear stress. More importantly, the importance of patient-specificity in the rTOF models, and the variability of the geometric and flow characteristics within the population was highlighted, contrary to the observations in the healthy models. The results of this work could help clinicians evaluate the haemodynamic environment in the rTOF population and potentially predict patients at higher risk, prior to the appearance of severe complications

Computational modelling of monocyte deposition in abdominal aortic aneurysms

Abdominal aortic aneurysm (AAA) disease involves a dilation of the aorta below the renal arteries. If the aneurysm becomes sufficiently dilated and tissue strength is less than vascular pressure, rupture of the aorta occurs entailing a high mortality rate. Despite improvements in surgical technique, the mortality rate for emergency repair remains high and so an accurate predictor of rupture risk is required. Inflammation and the associated recruitment of monocytes into the aortic wall are critical in the pathology of AAA disease, stimulating the degradation and remodeling of the vessel wall. Areas with high concentrations of macrophages may experience an increase in tissue degradation and therefore an increased risk of rupture. Determining the magnitude and distribution of monocyte recruitment can help us understand the pathology of AAA disease and add spatial accuracy to the existing rupture risk prediction models. In this study finite element computational fluid dynamics simulations of AAA haemodynamics are seeded with monocytes to elucidate patterns of cell deposition and probability of recruitment. Haemodynamics are first simulated in simplified AAA geometries of varying diameters with a patient averaged flow waveform inlet boundary condition. This allows a comparison with previous experimental investigations as well as determining trends in monocyte adhesion with aneurysm progression. Previous experimental investigations show a transition to turbulent flow occurring during the deceleration phase of the cardiac cycle. There has thus far been no investigation into the accuracy of turbulence models in simulating AAA haemodynamics and so simulations are compared using RNG κ − ε, κ − ω and LES turbulence models. The RNG κ − ε model is insufficient to model secondary flows in AAA and LES models are sensitive to inlet turbulence intensity. The probability of monocyte adhesion and recruitment depends on cell residence time and local wall shear stress. A near wall particle residence time (NWPRT)model is created incorporating a wall shear stress-limiter based on in vitro experimental data. Simulated haemodynamics show qualitative agreement with experimental results. Peaks of maximum NWPRT move downstream in successively larger geometries, correlating with vortex behaviour. Average NWPRT rises sharply in models above a critical maximum diameter. These techniques are then applied to patient-specific AAAs. Geometries are created from CT slices and velocity boundary conditions taken from Phase Contrast-MRI (PC-MRI) data for 3 patients. There is no gold standard for inlet boundary conditions and so simulations using 3 velocity components, 1 velocity component and parabolic flow profiles at the inlet are compared with each other and with PC-MRI data at the AAA midsection. The general trends in flow and wall shear stress are similar between simulations with 3 and 1 components of inlet velocity despite differences in the nature and complexity of secondary flow. Applying parabolic velocity profiles, however, can cause significant deviations in haemodynamics. Axial velocities show average to good correlation with PC-MRI data though the lower magnitude radial velocities produce high levels of noise in the raw data making comparisons difficult. Patient specific NWPRT models show monocyte infiltration is most likely at or around the iliac bifurcation