Doctor of Philosophy
Prof. Tamie L Poepping; Prof. James C Lacefield
Accurate estimation of low 3D blood velocities, such as near the wall in recirculation or disturbed flow regions, is important for accurate mapping of velocities to improve estimations of wall shear stress and turbulence, which are associated risk factors for vascular disease and stroke. Doppler ultrasound non-invasively measures blood-velocities but suffers from two major limitations addressed in this thesis. These are angle dependence of the measurements, which requires the knowledge of beam-to-flow angle, and the wall-filter. The high-pass wall filter that is applied to attenuate the high-intensity low-frequency signal from tissue and slowly moving vessel wall also attenuates any low velocity signals from blood thus causing inaccurate estimation of these velocities. This thesis presents two methods to alleviate the angle-dependence limitation and to minimize the effect of the wall filter on low blood-velocity estimates: a multi-receiver technique – vector Doppler ultrasound (VDUS), and a novel method called aperture-translation technique.
For the first method – VDUS, theoretical and experimental studies were performed to assess the comparative benefit of three to eight receivers (3R–8R) in Doppler ultrasound configurations in terms of the number of receiver beams, inter-beam angle, and beam- selection method (criterion for discriminating between tissue and blood Doppler signals) for a range of velocity orientations. Accuracy and precision for ≥5 receivers were consistently better over all flow velocity orientations and for all beam-selection methods. Asymmetry in the 5R configuration led to improved accuracy and precision compared to symmetrical 6R and 8R configurations.
Second, a novel 2D-VDUS aperture-translation technique using mechanical or electronic translation of the transmit-receive apertures was introduced and assessed experimentally. Both versions of the technique outperformed the conventional 2D-VDUS method for detection of low flow velocities in terms of accuracy and precision. The electronic version, which is more relevant and feasible clinically, showed comparable reliability and better accuracy compared with the idealized mechanical version, therefore suggesting its potential for future development. This work demonstrated that a minimum of five receivers, preferably with an inherent asymmetry with respect to the flow direction, should be considered when designing a 2D-array configuration for improved estimation of low velocities. For estimation of low velocities not measurable with conventional VDUS methods, the aperture-translation technique could be a potential candidate.
Hussain, Bushra, "Methods for Improved Estimation of Low Blood Velocities Using Vector Doppler Ultrasound" (2017). Electronic Thesis and Dissertation Repository. 4515.