In this paper we investigate the estimation accuracy of two-step block matching methods, which perform a rough detection corresponding to pixellevel displacement in the first step and then a finer detection using a spatial interpolation in the second step. In the present study, four strategies were examined: (1) RF signals in both steps; (2) envelopes of RF signals in both steps; (3) envelopes in the first step and RF signals in the second step; (4) RF signals in both steps with an extended interpolation region. Their performances were evaluated by phantom and simulation experiments. In terms of the estimation accuracy and calculation time, strategy (3) was preferable for two-dimensional blood flow vector imaging. In in vivo measurements of the common carotid artery using this method, a blood flow velocity of approximately 650 mm s −1 was observed at the center and vortex flow at the bifurcation was visualized.
Visualization of blood flow is important to evaluate the cardiac function. In this study, we propose two signal processing techniques based on singular value decomposition (SVD) for the visualization of blood flow: one is a filtering for clutter reduction and the other is a regularization method for color Doppler images. In the clutter filtering, contrast-to-noise ratio obtained by the SVD filtering was better (15.7 dB) than that by conventional finite impulse response filtering (−0.5 dB). In color flow imaging, the standard deviation of cardiac blood flow was decreased from 0.022 to 0.014 mm s −1 by the proposed regularization method.
Methods for the estimation of two-dimensional (2D) velocity and displacement of physiological tissues are necessary for quantitative diagnosis. In echocardiography with a phased array probe, the accuracy in the estimation of the lateral motion is lower than that of the axial motion. To improve the accuracy in the estimation of the lateral motion, in the present study, the coordinate system for ultrasonic beamforming was changed from the conventional polar coordinate to the Cartesian coordinate. In a basic experiment, the motion velocity of a phantom, which was moved at a constant speed, was estimated by the conventional and proposed methods. The proposed method reduced the bias error and standard deviation in the estimated motion velocities. In an in vivo measurement, intracardiac blood flow was analyzed by the proposed method.
High-frame-rate ultrasound imaging with plane wave transmissions is a predominant method for blood flow imaging, and methods for estimation of blood flow velocity vectors have been developed based on high-frame-rate imaging. On the other hand, in imaging of soft tissues, such as arterial walls and atherosclerotic plaques, high-frame-rate imaging sometimes suffers from high-level clutters. Even in observation of the arterial wall with a focused transmit beam, it would be highly beneficial if blood flow velocity vectors could be estimated simultaneously. We conducted a preliminary study on estimation of blood flow velocity vectors based on a multi-angle Doppler method with focused transmit beam and parallel receive beamforming. It was shown that the lowest estimation error was achieved at a steering angle of 25 degrees by simulation. Also, velocity vectors with typical velocity magnitudes and directions could be obtained by the proposed method in in vivo measurement of a carotid artery.
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