No abstract

High-frame-rate ultrasound is an emerging technique for functional ultrasound imaging. However, the lateral spatial resolution and contrast in high-frame-rate ultrasound with an unfocused transmit beam are inherently lower than those in conventional ultrasonic imaging based on the line-by-line acquisition using a focused ultrasonic beam because of the low directivity of the transmit beam. Coherence-based beamforming methods were introduced in ultrasound imaging for improvement of image quality. Such methods improve the lateral spatial resolution using the coherence among ultrasonic echo signals received by individual transducer elements. In this study, a new method based on the signal-to-noise ratio (SNR) among the element echo signals was developed for enhancement of the effect of the coherence factor (CF), which was previously developed for improvement in spatial resolution and contrast. In the proposed method, a new factor, namely, SNR factor, was introduced, and the relationship between the previously developed CF and SNR factor was discussed. The proposed method was implemented in plane wave imaging, and the performance was evaluated by simulated and phantom experiments. In simulation, the lateral spatial resolution and contrast obtained with the conventional CF were 0.23 mm and 47.0 dB, respectively, which were significantly better than 0.39 mm and 15.3 dB obtained by conventional delay-and-sum (DAS) beamforming. Using the proposed method, the lateral spatial resolution and contrast were further improved to 0.12 mm and 69.8 dB, respectively. Similar trends were found also in phantom experiments.

We prepared crystals of (C6H5C n H2n NH3)2PbBr4 (n = 2, 3, 4), and evaluated their photoluminescence and scintillation characteristics. According to the photoluminescence spectra under 340 nm excitation light, a photoluminescence peak at around 410 nm due to free exciton emissions appeared in (C6H5C2H4NH3)2PbBr4 (2PEA), (C6H5C3H6NH3)2PbBr4 (3PPA) and (C6H5C4H8NH3)2PbBr4 (4PBA). The quantum efficiencies of 2PEA, 3PPA and 4PBA were 0.24, 0.01 and 0.04, respectively. In scintillation, a sharp scintillation peak originating from the exciton emissions was detected at approximately 436 nm for 2PEA, 459 nm for 3PPA and 442 nm for 4PBA. In addition, the scintillation light yield of 4PBA under 59.5 keV gamma-ray irradiation from 241Am was determined to be approximately 2200 photons MeV−1 in comparison with the yield of 2PEA (14 000 photons MeV−1), whereas the yield of 3PPA could not be determined due to its low scintillation intensity.

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.