Papaharilaou, Yannis; (2002) Studies of fluid flow in arterial bypass grafts by magnetic resonance imaging. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

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.

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