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Title: 3D phase contrast MRI : velocity-field visualisation and wall shear rate calculation in major arteries
Author: Köhler, Uwe
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2000
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Approximately half of all deaths in the developed world arise from cardiovascular disease, primarily caused by the deposition of atheroma within major arteries. It has been observed that atheroma is deposited preferentially in regions along the outer wall of bifurcations, and along the distal part of the inner wall of bends. These are regions associated with disturbances of the blood flow that display abnormal shear rate (spatial velocity gradient at the vessel wall). Thus, in order to facilitate clinical diagnoses, it is important to visualise the structure and haemodynamic properties of arteries and veins. Magnetic resonance imaging (MRI) is well suited for volume imaging and can be made sensitive to flow. Quantitative velocity measurements are possible using phase contrast (PC) MRI. The aim of this project was the provision of a method that provides information on wall shear rate vectors using MRI. To handle the large number of images acquired in PC MRI automated flow detection algorithms were developed. Three different algorithms were identified: one operating on magnitude MRI images only and two methods which additionally use the velocity information generated from in-vivo and in-vitro acquisitions. These algorithms are based on an edge detection method and were tested on phantoms. The post processing steps necessary to calculate wall shear stress involved the fit of smooth functions to the velocity data, the detection of walls and the calculation of the wall shear rate vector based on that information. Fitting a smooth function removed residual noise and allowed the calculation of spatial derivatives. The velocity data was satisfactorily described by a segmented fifth order polynomial fit. One method of vessel wall reconstruction was based on the fitted velocity field, while another one utilised the detected flow regions. Using the surface position and normals, the wall shear rate was calculated from the shear stress tensor. All post-processing steps were integrated in a purpose built program that enabled graphical user interactions. The calculated wall shear rate values were quantitatively verified with experiments on various phantoms and simulations, and qualitatively compared with computational fluid dynamics calculations. It is shown that a method to calculate reliably wall shear rate directly from time averaged PC MRI acquisitions has been established.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available