| The atherosclerosis, which indicates that plaque have developed to the point where it significantly narrows the vessels and includes a sequence of inflammatory cells, fibroblasts, calcification and intracellular or extracellular lipids, is one of the most common forms of arterial chronic-disease. The clinical studies show that the vulnerable plaque which is especially susceptible to disruption is the higher risk for the ischemic cerebrovascular events and may be the fundamental reasons for thrombus generation and plaque cap fracture directly leading to heart attack and apoplexy. Early and accurate detection of the vulnerable plaque is one of the most important diagnoses of cardiovascular diseases to reduce their leading morbidity and mortality. The pathological studies expose that smaller and oscillatory wall shear rates (WSR) as the gene expression and regulation mechanism of endothelial cells are essential to the foundation and development of atherosclerosis. The ultrasound technique, which may be used for the study of various types of motion within the body, is one of the most important noninvasive diagnostic methods to detect and quantify the pulsatile blood flow in vessels. The detection of WSR by the ultrasound technique to obtain the sensitivity parameters for atherosclerosis diagnosis has attracted the interest of many investigators in recent years.In this process, there are two main challenges:the first one is how to effectively suppress the strong clutter components scattered from the vessel walls and the surrounding stationary or slow-moving tissues caused corruption in blood flow signals, especially those echoed from blood flow near the edge of vessel wall, which could embody the information about the existence of diseases for improving the diagnosis efficiency at an early stage; the second one is how to improve the accuracy of the estimation of blood flow velocity profile, especially the slower blood flow velocities approaching the vessel walls for meliorating the detection correctness of the WSR.In view of the two main challenges for detection of WSR by ultrasound technique, the corresponding solutions are proposed in this thesis. A novel quadrature clutter rejection approach based on multivariate empirical mode decomposition (MEMD), which is an extension of empirical mode decomposition (EMD) to multivariate for processing multichannel signals, is proposed in this paper to suppress the quadrature clutter signals induced by the vascular wall and the surrounding stationary or slowly moving tissues in composite Doppler ultrasound signals, and extract more blood flow components with low velocities. In this approach, the MEMD algorithms, which include the bivariate empirical mode decomposition (BEMD), the BEMD with a nonuniform sampling scheme for adaptive selection of projection directions (NS_BEMD) and the tri-variate empirical mode decomposition with noise assistance (NA_TEMD), are directly employed to adaptively decompose the complex-valued quadrature composite signals echoed from both bidirectional blood flow and moving wall into a small number of zero-mean rotation components, which are defined as complex intrinsic mode functions (CIMFs). Then the relevant CIMFs contributed to blood flow components are automatically distinguished in terms of the break of the CIMFs power, and then directly added up to give the quadrature blood flow signal. Specific simulation and human subject experiments are taken up to demonstrate the advantages and limitations of this novel method for quadrature clutter rejection in bidirectional Doppler ultrasound signals. Due to eliminating the extra errors induced by the Hilbert transform or complex FIR filter algorithms used in the traditional clutter rejection approaches based on the directional separation process, the proposed method provides improved accuracy for clutter rejection, and preserve more slow blood blow components, which could be helpful to early diagnose arterial diseases.An improved autocorrelation approach (IAA) based on the pulse-echo RF lines generated with adaptively varying pulse-shooting intervals (VPI) at different radial positions of arteries is presented in this paper to enhance the estimation accuracy of blood flow velocity profile. In order to guarantee complete periodicity of the Doppler signals in time domain, the IAA employs the short pulse-shooting intervals for generating the compulsory radio frequency (RF) lines at lumen center and relatively longer one for those approaching the vessel walls. The pulse-shooting intervals at the determined radial positions are inversely varying with the blood flow velocities. To evaluate the performance of the proposed method, a computer ultrasound simulation model for normal common carotid artery (CCA) is setup to generate the pulse wave (PW) Doppler blood flow signals from the pulse-echo RF lines with the fixed pulse-shooting interval (FPI) and VPI manners. In the verification experiment, the RF lines generated respectively with1/10000s of FPI, and VPIs according to the predefined parabolic velocity profile of blood flow (VPI_PV) are simulated to produce the corresponding PW Doppler signals, from which the blood flow velocity profiles are estimated by using the autocorrelation algorithm. It can be found that the oscillation frequency of the PW Doppler signals based on the FPI are truncate with the detection positions in the radial direction away from axial center, and lead to their incomplete periodicity to the region near the vessel walls, which is responsible for the significant estimation deviations for slowly-moving blood flow velocities. However, the fluctuation frequency of the PW Doppler signals by the VPI_PV present consistence ensuring the integrity of their cycles at all radial positions in the artery, and essentially improving the velocity profile estimation accuracy. However, the predefine velocity profile of blood flow used to determine the VPIs in the VPI_PV approach is unknown in practice. Hence, a scheme based on a certain number of zero-crossing points in Doppler signals is proposed to determine the varying durations in the VPI (VPI_ZC), with which the pulse echo-RF lines are adaptively generated. Results from a comparison experiment clarify that the accuracy of the velocity profile of blood flows, especially for that near the vessel walls, obtained by the VPI_ZC is improved significantly and has a similar form with the theoretical one.Finally, the WSR estimation of the CCA is carried out by calculating the FPI and VPI velocity shear rate. It can be clearly found that the WSR obtained by the VPI velocity shear rate presents better consistency to the theoretical one, from which the sensitive information can pick up for improving the diagnosis efficiency at an early stage.In conclusion, the quadrature clutter rejection approach based on MEMD and improved VPI autocorrelation approach for the estimation of blood flow velocity profile can help to overcome the main challenges for detection of WSR by ultrasound technique. The more slow blood flow components preserved during the clutter rejection and their velocity with higher accuracy received by VPI could be potentially utilized as fundamental contents for further estimations of relevant diagnosis parameters such as wall shear stress, blood flow pressure, volume flow rate etc. |
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