Flow visualisation and quantification using high frame rate ultrasound imaging and microbubble contrast agents

Abstract

Non-invasive techniques capable of visualising and quantifying blood flow in-vivo are highly desirable in studying a wide range of cardiovascular diseases. Although existing ultrasound imaging techniques have been widely used clinically to visualise and quantify blood flow, they have various limitations in terms of field of view, temporal and spatial resolution, imaging sensitivity, and beam-flow angle dependence. In this thesis, our aim is to develop flow quantification tools capable of non-invasively measuring the flow velocity, wall shear stress (WSS) as well as intraluminal mixing. Firstly, a high frame-rate ultrasound imaging velocimetry (UIV) system was developed based on tracking the speckle patterns of microbubbble contrast agents in contrast-enhanced ultrasound image sequences acquired from a plane wave imaging system. Initial evaluation of the system demonstrated the potential of the new system as a flow velocity mapping tool capable of tracking fast and dynamic flow and we improved our flow velocity measurement technique by introducing an incoherent ensemble correlation approach in the UIV tracking algorithm. Such a modified UIV technique avoids the motion artifact which could potentially affect the velocity measurement as compounded plane wave images are not coherently summed during the compounded plane wave image formation. Ultrasound flow simulations were conducted to fully evaluate our new modified-UIV technique. Together with some in-vitro experiments on physiologically relevant flow phantoms, we demonstrated the capability of our system to provide robust, angle independent, sensitive, and accurate two-dimensional velocity measurements. Secondly, as studies have revealed strong correlation between WSS and the initiation and development of atherosclerosis, we extended our UIV technique to the derive spatio-temporal wall shear rate from the velocity flow profile. The performance of the system to provide wall shear stress distributions was initially evaluated in simulation and demonstrated in-vitro using physiologically relevant flow phantoms. Thirdly, a novel approach which uses the high frame rate system and controlled microbubble destruction for flow visualisation and intraluminal mixing quantification was also proposed. Three different model vessel geometries: straight, planar curved and helical, with known effects on the flow field and mixing were evaluated against computational fluid dynamics (CFD) results. The findings indicated the technique is not only capable of visualising the secondary flows, but also able to quantify the degree of mixing in the different configurations. Finally, real time processing of the image formation and flow quantification technique were explored due to the large amount of data generated from the high frame rate ultrasound system. Initial development of a graphic processing unit (GPU) accelerated plane wave UIV system was demonstrated with the potential for real time measurements.