平田 慎之介

Introduction: Assessing the stage of liver fibrosis during the diagnosis and follow-up of patients with diffuse liver disease is crucial. The tissue structure in the fibrotic liver is reflected in the texture and contrast of an ultrasound image, with the pixel brightness indicating the intensity of the echo envelope. Therefore, the progression of liver fibrosis can be evaluated non-invasively by analyzing ultrasound images. Methods: A convolutional-neural-network (CNN) classification of ultrasound images was applied to estimate liver fibrosis. In this study, the colorization of the ultrasound images using echo-envelope statistics that correspond to the features of the images is proposed to improve the accuracy of CNN classification. In the proposed method, the ultrasound image is modulated by the 3rd- and 4th-order moments of pixel brightness. The two modulated images and the original image were then synthesized into a color image of RGB representation. Results and Discussion: The colorized ultrasound images were classified via transfer learning of VGG-16 to evaluate the effect of colorization. Of the 80 ultrasound images with liver fibrosis stages F1–F4, 38 images were accurately classified by the CNN using the original ultrasound images, whereas 47 images were classified by the proposed method.

Abstract In contrast enhancement ultrasound (CEUS), the vasculature image can be formed from nonlinear echoes arising from microbubbles in a blood flow. The use of binary-coded pulse compression is promising for improving the contrast of CEUS images by suppressing background noise. However, the amplitudes of nonlinear echoes can be reduced, and sidelobes by nonlinear echoes can occur depending on the binary code. Optimal Golay codes with slight nonlinear-echo reduction and nonlinear sidelobe have been proposed. In this study, CEUS images obtained by optimal Golay pulse compression are evaluated through experiments using Sonazoid microbubbles flowing in a tissue-mimicking phantom.

Abstract In medical ultrasound imaging using microbubbles (MBs), the nonlinear echoes from the MBs are used for contrast-specific image construction. Techniques such as pulse inversion, amplitude modulation, and the combined method (PIAM) are employed to increase nonlinear components in the echoes. In addition, employment of pulse compression using binary-coded ultrasound can potentially increase the components. In the case of the nonlinear echo, however, a nonlinear sidelobe occurs around the compressed pulse (correlation peak), and nonlinear components for PIAM in the correlation peak are reduced. The shape of the nonlinear sidelobe and the nonlinear-component reduction in the correlation peak can be estimated from the binary codes used. In this study, the optimal binary codes for PIAM are determined from all patterns of 10, 16, and 20 bit Golay codes. Then, the performance of PIAM with pulse compression using each code is evaluated via computer simulations and experiments using the SonoVue MBs.

A method of ultrasonic position and velocity measurements for a moving object by M-sequence pulse compression has been proposed. In the proposed method, the Doppler velocity is estimated from the peak in the autocorrelation function of the received signal, which includes cyclic M-sequences reflected from the object. Then, the received signal is correlated with the reference M-sequence expanded or compressed in accordance with the estimated Doppler velocity. The position is determined from the acoustic image formed from cross-correlation functions obtained in different propagation paths of ultrasound. In this paper, the simultaneous transmission of preferred-pair M-sequences is studied to improve the spatial resolution of the image while keeping the original frame rate of the proposed method. Experiments using the moving pole, two loudspeakers and three microphones are conducted to evaluate the estimated Doppler velocities and determined positions. In most situations, Doppler velocities can be accurately estimated when two different Doppler components are included in the received signal. In some situations, however, accurate Doppler velocities inevitably cannot be estimated due to the peak overlapping in the autocorrelation function. The accuracy of determined positions may degrade when the signal-to-noise ratio of the received signal is insufficient or Doppler velocities are not accurately estimated.

In M-sequence pulse compression for acoustic distance measurement, a continuous signal modulated by the cyclic binary code in an M-sequence is transmitted. Then, the received signal is correlated with one cycle of the transmitted code. The time of flight for the estimation of the propagation distance is determined from the peak appeared in the cross-correlation function when both codes match. The temporal resolution of the measurement and the maximum measurable distance are determined by the cycle length of the transmitted signal and in a trade-off relationship. Alternate transmission of different M-sequence codes and cross-correlations with each code can extend the measurable distance without degradation of the temporal resolution. However, truncation noise in each code and truncated interference noise between codes are generated around correlation peaks. The patterns of these noises are varied by the combination of codes. Therefore, contamination by these noise can be suppressed by selecting suitable codes. (C) 2019 The Japan Society of Applied Physics

Ultrasonic distance measurement for obstacles has been recently applied in automobiles. The pulse-echo method based on the transmission of an ultrasonic pulse and time-of-flight (TOF) determination of the reflected echo is one of the typical methods of ultrasonic distance measurement. Improvement of the signal-to-noise ratio (SNR) of the echo and the avoidance of crosstalk between ultrasonic sensors in the pulse-echo method are required in automotive measurement. The SNR of the reflected echo and the resolution of the TOF are improved by the employment of pulse compression using a maximum-length sequence (M-sequence), which is one of the binary pseudorandom sequences generated from a linear feedback shift register (LFSR). Crosstalk is avoided by using transmitted signals coded by different M-sequences generated from different LFSRs. In the case of lower-order M-sequences, however, the number of measurement channels corresponding to the pattern of the LFSR is not enough. In this paper, pulse compression using linear-frequency-modulated (LFM) signals coded by M-sequences has been proposed. The coding of LFM signals by the same M-sequence can produce different transmitted signals and increase the number of measurement channels. In the proposed method, however, the truncation noise in autocorrelation functions and the interference noise in cross-correlation functions degrade the SNRs of received echoes. Therefore, autocorrelation properties and cross-correlation properties in all patterns of combinations of coded LFM signals are evaluated. (C) 2016 The Japan Society of Applied Physics

Pulse compression using a maximum-length sequence (M-sequence) can improve the signal-to-noise ratio (SNR) of the reflected echo in the pulse-echo method. In the case of a moving object, however, the echo is modulated owing to the Doppler effect. The Doppler-shifted M-sequence-modulated signal cannot be correlated with the reference signal that corresponds to the transmitted M-sequence-modulated signal. Therefore, Doppler velocity estimation by spectrum-pattern analysis of a cyclic M-sequence-modulated signal and cross correlations with Doppler-shifted reference signals that correspond to the estimated Doppler velocities has been proposed. In this paper, measurements of the position and velocity of a moving object by the proposed method are described. First, Doppler velocities of the object are estimated using a microphone array. Secondly, the received signal from each microphone is correlated with each Doppler-shifted reference signal. Then, the position of the object is determined from the B-mode image formed from all cross-correlation functions. After that, the velocity of the object is calculated from velocity components estimated from the Doppler velocities and the position. Finally, the estimated Doppler velocities, determined positions, and calculated velocities are evaluated. (C) 2015 The Japan Society of Applied Physics

The coding of a transmitted signal by a maximum-length sequence (M-sequence) is employed in the pulse-echo method with pulse compression for ultrasonic distance measurement. To estimate the distance, the time of flight (TOF) of an echo is determined from the cross-correlation function between the received signal and the reference signal that corresponds to the transmitted signal. When codes of the received signal and the reference signal match, a high correlation value is obtained in the cross-correlation function. On the other hand, when they do not match, a low correlation value is obtained. In the case of the finite-length M-sequence code, however, truncation noise is generated at the beginning and end of the cross-correlation function. In this study, truncation noise in autocorrelation functions of all patterns of M-sequence codes is quantitatively evaluated from the 4th order to the 15th order. The peak amplitude and intensity of truncation noise vary depending on the patterns of M-sequence codes. Therefore, they can be reduced by selecting suitable M-sequence codes.

Microdroplet dispensation is required in current systems and industrial equipment. However, dispensing microdroplets from high-viscosity liquids using dropon-demand inkjet technologies is difficult. Therefore, a needle-type dispenser comprising a thin needle and glass capillary containing the liquid to be dispensed has been developed for microdroplet dispensation. When the needle passes through the capillary, a droplet of the liquid adheres to the needle tip. A microdroplet can be transcribed by bringing the needle-tip droplet into contact with the target surface. The needle-type dispenser can dispense sub-pico-liter droplets with viscosities of several hundred pascalseconds at a throughput of a droplet per 1 s. When a microdroplet is dispensed, a gap between the needle tip and target surface may be formed. Droplet volumes depend on the dispensing gap, and unstable if the gap fluctuates. Thus, a contact-detection method for the needle-tip droplet and target surface is proposed where the needle is vibrated by a piezoelectric actuator using a leaf spring. The needle-vibration characteristics depend on conditions between the needletip droplet and target surface. Contact of the needletip droplet with the target surface can be detected by the needle-vibration characteristics. The dispensing gap can thus be kept constant to dispense precise droplet volumes. In this study, the needle-vibration characteristics of the fabricated mechanism were evaluated experimentally for needle diameters of 500 and 100 µm. The needle-vibration displacements decreased depending on the dispensing gap for liquids with viscosities of 0.1 to 100 Pa•s.

Pulse compression using maximum-length sequence (M-sequence) can improve the signal-to-noise ratio (SNR) of the reflected echo and distance resolution in the pulse–echo method. In the case of a moving object, however, the echo is modulated due to the Doppler effect. The Doppler-shifted M-sequence-modulated signal cannot be correlated with the reference signal, which corresponds to the transmitted M-sequence-modulated signal. Therefore, Doppler velocity estimation before the correlation and cross correlation of the received signal with Doppler-shifted reference signals has been proposed. In this paper, the proposed Doppler velocity estimation based on spectral characteristics of cyclic M-sequence-modulated signals is described. Then, the Doppler velocity estimation is evaluated based on computer simulations. The Doppler velocity can be estimated from the Fourier-transformed spectral density of cycles of the M-sequence-modulated signal with high resolution even in noisy environments. According to the evaluation, furthermore, the cycle number and the number of carrier waves in 1 digit of the M-sequence-modulated signal should be decreased to improve the resolution and accuracy when the length of the transmitted signal is determined.

Micro-droplet dispensation is required in current systems or industrial equipments. However, drop-ondemand inkjet technologies are difficult to use in dispensing micro droplets from high-viscosity liquids. Therefore, the needle-type dispenser using a thin needle and a glass capillary has been developed for microdroplet dispensation. When the needle passes through the capillary with a sample liquid, a droplet of the liquid adheres to the needle tip. The micro droplet can be transcribed to the target surface by bringing the needle-tip droplet into contact with the target surface. The needle-type dispenser can dispense sub-pico-liter amounts of droplets from liquids of several hundred Pa?s. When a micro droplet is dispensed, a gap is formed between the needle tip and the target surface. Amounts of droplets are unstable, as the dispensing gaps fluctuate. Therefore, a contact-detection method of the needle-tip droplet and the target surface is proposed. The needle is vibrated by a piezoelectric actuator through a leaf spring in the proposed method. The needle-vibration characteristic is made variable by the differences in conditions between the needle-tip droplet and the target surface. Therefore, contact of the needle-tip droplet with the target surface can be detected by needle-vibration characteristics. Then, the dispensing gap can be kept constant by contact detection for dispensing accurate amounts of droplets.