3-D Doppler imaging in cardiac applications using high frame-rate sequences
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Several decades after the establishment of Doppler ultrasound, quantification and imaging of blood flow using Doppler-based techniques continue to face multiple challenges. Out-of-plane flow, transit-time broadening and angledependency compromise the reliability of quantitative blood flow measurements. Moreover, state-of-the-art focused 2-D imaging may fail at capturing the 3-D features of the blood vessels. These limitations may become particularly important in cardiac applications because the heart is a complex 3-D structure characterised by fast occurring events. Plane wave acquisition sequences would allow for 3-D imaging at frame rates suitable for Doppler processing, potentially addressing several of the aforementioned limitations. The goal of this work was to investigate the feasibility of 3-D plane wave sequences for imaging and quantification of blood flow in cardiac application and to develop methods that could be combined with plane wave sequences for improved quantitative blood flow measurements. We investigated the methods and the acquisition sequences using simulations, in vitro recordings and extensive in vivo feasibility studies on both healthy volunteers and patients with heart diseases. The first part of the thesis is focused on the application of 3-D plane wave sequences for imaging and quantification of blood flow in the coronary arteries. We introduced and investigated 3-D tracking Doppler, a wideband spectral estimator, which follows the blood scatterers along a straight line to increase the observation window and generate spectra with reduced transittime broadening. The second part of the thesis is focused on the application of 3-D plane wave sequences for aortic stenosis assessment. The study investigated the feasibility of automatic beam-to-flow angle correction using blood speckle tracking. Results indicated a strong feasibility of 3-D plane wave sequences for blood flow imaging and quantification in cardiac applications, but also revealed potential pitfalls, motivating further investigation.