Ultrasonic wave speed measurement using the time-delay profile of rf-backscattered signals: simulation and experimental results

Conventional methods determine the ultrasonic wave speed measuring the medium path length propagated by a pulsed wave and the corresponding time-of-flight. In this work, the wave speed is determined without the need of the path length. A transmit transducer sends a pulsed wave into the medium (wave speed constant along the beam axis) and the backscattered signal is collected by a hydrophone placed at two distinct positions near the transmitted beam. The time-delay profile, between gated windows of the two rf-signals received by the hydrophone, is determined using a cross-correlation method. Also, a theoretical time-delay profile is determined considering the wave speed as a parameter. The estimated wave speed is obtained upon minimization of the rms error between theoretical and experimental time-delay profiles. A PZT conically focused transmitting transducer with center frequency of 3.3 MHz, focal depth of 30 mm, and beam full width (-3 dB) of 2 mm at the focus was used together with a PZT hydrophone (0.8 mm of aperture). The method was applied to three phantoms (wave speed of 1220, 1540, and 1720 m/s) and, in vitro, to fresh bovine liver sample, immersed in a temperature-controlled water bath. The results present a relative speed error less than 3% when compared with the sound speed obtained by a conventional method.     

Non-contact sound speed measurement by optical probing of beam deflection due to sound wave

We report a non-contact and non-invasive method of sound speed measurement by optical probing of deflected laser beam due to normally incident degenerated shock wave. In this study the shock wave from an exploding wire was degenerated to an ordinary sound wave at the distance exceeding 0.23 m. Temporal resolution of the deflected beam signal was improved by passing through an adequate electronic high-pass filter, as a result we obtained a better temporal resolution than that of the acoustic pressure detection by PZT transducer in terms of rising time. The spatial resolution was improved by passing the refracted beam signal into the edge of focusing lens to make a larger deflection angle. Sound speed was calculated by monitoring the time of flight of transient deflected signal at the predetermined position. Sound speed has been measured in air, distilled water and acryl, agreed well with the published values. The sound speed measured in the solution of glycerin, magnesium sulfate (MgSO4), and dimethylformamide with various mole fractions also agrees within 3% of relative error with those measured in the present work by ultrasonic pulse echo method. The results suggest that the method proposed is to be reliable and reproducible.     

Advantage analysis of PMN-PT material for free-flooded ring transducers

The acoustic radiation characteristics of free-flooded ring transducers made of PZT4 and PMN-PT materials are calculated and compared.First,the theoretical formulae for free-flooded ring transducers are studied.The resonant frequencies of a transducer made of PZT4 and PMN-PT materials are calculated.Then,the transmitting voltage responses of the free-flooded ring transducers are calculated using the finite element method.Finally,the acoustic radiation characteristics of the free-flooded ring transducers are calculated using the boundary element method.The calculated results show that the resonant frequencies of the free-flooded ring transducer made of PMN-PT are greatly reduced compared with those made of PZT4 with the same size.The transmitting voltage response of the transducer made of PMN-PT is much higher than that of the transducer made of PZT4.The calculated 3-dB beamwidth of the acoustic radiated far-field directivity of the free-flooded ring transducer made of PZT4 at the resonant frequency 1900 Hz is 63.6 and that of the transducer made of PMN-PT at the resonant frequency 1000 Hz is 64.6.The comparison results show that the free-flooded ring transducer made of PMN-PT material has many advantages over that made of PZT4.The PMN-PT is a promising material for improving the performance of free-flooded ring transducers.     

混合激励纵振换能器的研究

纵振式换能器是水声领域中不可或缺的一类中频段换能器,本文以超磁致伸缩材料Terfenol-D和压电陶瓷材料PZT作为驱动元件设计混合激励纵振式换能器。利用ANSYS有限元软件建立了有限元模型,对换能器的电声参数进行了优化,并制作一宽带混合激励纵振式换能器,实验测得的换能器与仿真的结果较为吻合,实测换能器的最大发送电压响应为145dB,其工作频带为4kHz-16kHz,频带内发送电压响应起伏为±4.5dB。     

Transfer functions of US transducers for harmonic imaging and bubble responses

Current medical diagnostic echo systems are mostly using harmonic imaging. This means that a fundamental frequency (e.g., 2 MHz) is transmitted and the reflected and scattered higher harmonics (e.g., 4 and 6 MHz), produced by nonlinear propagation, are recorded. The signal level of these harmonics is usually low and a well-defined transfer function of the receiving transducer is required. Studying the acoustic response of a single contrast bubble, which has an amplitude in the order of a few Pascal, is another area where an optimal receive transfer function is important.

We have developed three methods to determine the absolute transfer function of a transducer. The first is based on a well-defined wave generated by a calibrated source in the far field. The receiving transducer receives the calibrated wave and from this the transfer functions can be calculated. The second and third methods are based on the reciprocity of the transducer. The second utilizes a calibrated hydrophone to measure the transmitted field. In the third method, a pulse is transmitted by the transducer, which impinges on a reflector and is received again by the same transducer. In both methods, the response combined with the transducer impedance and beam profiles enables the calculation of the transfer function.

The proposed methods are useful to select the optimal piezoelectric material (PZT, single crystal) for transducers used in reception only, such as in certain 3D scanning designs and superharmonic imaging, and for selected experiments like single bubble behavior.

We tested and compared these methods on two unfocused single element transducers, one commercially available (radius 6.35 mm, centre frequency 2.25 MHz) the other custom built (radius 0.75 mm, centre frequency 4.3 MHz). The methods were accurate to within 15%.     

A KLM-circuit model of a multi-layer transducer for acoustic bladder volume measurements

In a preceding study a new technique to non-invasively measure the bladder volume on the basis of non-linear wave propagation was validated. It was shown that the harmonic level generated at the posterior bladder wall increases for larger bladder volumes. A dedicated transducer is needed to further verify and implement this approach. This transducer must be capable of both transmission of high-pressure waves at fundamental frequency and reception of up to the third harmonic. For this purpose, a multi-layer transducer was constructed using a single element PZT transducer for transmission and a PVDF top-layer for reception. To determine feasibility of the multi-layer concept for bladder volume measurements, and to ensure optimal performance, an equivalent mathematical model on the basis of KLM-circuit modeling was generated. This model was obtained in two subsequent steps. Firstly, the PZT transducer was modeled without PVDF-layer attached by means of matching the model with the measured electrical input impedance. It was validated using pulse-echo measurements. Secondly, the model was extended with the PVDF-layer. The total model was validated by considering the PVDF-layer as a hydrophone on the PZT transducer surface and comparing the measured and simulated PVDF responses on a wave transmitted by the PZT transducer. The obtained results indicated that a valid model for the multi-layer transducer was constructed. The model showed feasibility of the multi-layer concept for bladder volume measurements. It also allowed for further optimization with respect to electrical matching and transmit waveform. Additionally, the model demonstrated the effect of mechanical loading of the PVDF-layer on the PZT transducer.     

The 10 MHz ultrasonic near-field: calculations, hydrophone and optical diffraction tomography measurements

The near-field from a 10 MHz ultrasound transducer has been studied using three different techniques; theoretical calculations, miniature hydrophone measurements and optical diffraction tomography (ODT). The motive was twofold; to evaluate the measurement possibilities of ODT for high-frequency ultrasound and to verify the calculated complexity of near-fields from real ultrasound transducers designed for use in blood perfusion measurements. Calculations of the field from an ideal piston transducer were done with surface integrals of the Kirchhoff function and the measured transducer was designed to approximate this ideal. Contour maps, from the three methods, of pressure amplitude from 1 to 30 mm along the beam are presented with beam profiles at 5 and 45 mm. The results imply that ODT has an increased spatial resolution compared with the 0.5 and 1.0 mm hydrophones. However, even better spatial resolution is needed to draw definite conclusions regarding the complexity of the near-field of the 10 MHz transducer.     

Increase in the efficiency of the shear wave generation in gelatin due to the nonlinear absorption of a focused ultrasonic beam

Experimental results and theoretical estimates are presented to demonstrate the prospects of using the acoustic nonlinearity of a gel-like medium for increasing the efficiency of the shear wave generation in it by a pulsed ultrasonic beam. The experiment is based on the propagation of a focused beam of longitudinal acoustic waves at a frequency of 1.1 MHz in a gelatin sample and on the detection of shear waves by the optical method [1]. It is demonstrated that the amplitude of the shear wave excited by a nonlinear acoustic pulse can be increased by an order of magnitude owing to the formation of shock fronts in the profile of this pulse.     

Generation of a vortex ultrasonic beam with a phase plate with an angular dependence of the thickness

Vortex-wave beams are beams that carry angular momentum. Their specific feature is a ring-like transverse distribution of wave intensity with zero intensity at the axis. A method for generating an ultrasonic vortex beam by combining a single-element transducer and a phase plate with a nonuniform thickness is proposed. The method is examined theoretically and tested experimentally. In the theoretical analysis, the acoustic field was calculated using the Rayleigh integral. Experiments were performed in water with a focusing piezoceramic source with a frequency of the order of 1 MHz; the radiation from it was transmitted through a 12-sector organic-glass phase plate. The beam vorticity was established by setting the correct thickness of sectors. The results of scanning the field with a miniature hydrophone confirmed that the amplitude and phase distributions of the generated wave field were in fact consistent with a vortex beam. The capacity of the obtained beam to induce the rotation of scatterers positioned in the focal region was demonstrated.     

同振球型矢量水听器声波接收理论研究

对同振球型矢量水听器声压和质点振速的声波接收理论进行了研究。以同振球型振速水听器测量原理为基础,推导了自由运动刚性球体和弹性球体的声波接收响应数学表达式,分析了振速水听器几何尺寸、平均密度与其频响特性曲线之间的关系;另外,根据球面接收器的声波接收理论,推导了矢量水听器声压接收响应数学表达式,通过理论分析和数值计算,研究了振速水听器表面上的声压分布规律以及声压水听器的声波接收压力系数与其接收面的大小、质点振速水听器的半径、布放的位置和半径等参数之间的关系;从理论上建立了矢量水听器声波接收理论模型和分析方法,为矢量水听器的设计和研制提供了理论依据。      

Estimation of mechanical properties of a viscoelastic medium using a laser-induced microbubble interrogated by an acoustic radiation force

An approach to assess the mechanical properties of a viscoelastic medium using laser-induced microbubbles is presented. To measure mechanical properties of the medium, dynamics of a laser-induced cavitation microbubble in viscoelastic medium under acoustic radiation force was investigated. An objective lens with a 1.13 numerical aperture and an 8.0 mm working distance was designed to focus a 532 nm wavelength nanosecond pulsed laser beam and to create a microbubble at the desired location. A 3.5 MHz ultrasound transducer was used to generate acoustic radiation force to excite a laser-induced microbubble. Motion of the microbubble was tracked using a 25 MHz imaging transducer. Agreement between a theoretical model of bubble motion in a viscoelastic medium and experimental measurements was demonstrated. Young's modulii reconstructed using the laser-induced microbubble approach were compared with those measured using a direct uniaxial method over the range from 0.8 to 13 kPa. The results indicate good agreement between methods. Thus, the proposed approach can be used to assess the mechanical properties of a viscoelastic medium.     

激光激发水中产生超声波的实验研究

采用压电陶瓷水听器研究了Nd：YAG脉冲激光激发水中产生瞬态超声波的特性,给出了声压信号随距离的变化关系。研究结果表明：用激光产生水下声波是完全可行的。当观测点与光击穿区的距离r远大于柱体长度时,垂直于光传播方向的激光瞬态超声波幅值与r成反比;当观测点与光击穿区的距离r很小时,垂直于光传播方向的声压幅值与产成反比。此外,当激光入射角度发生变化,超声脉冲的幅值也随之发生变化,其幅值在激光束垂直入射的时候最大。     

瞬态波位移场计算方法在相控阵声场模拟中的实验验证

通过实验研究了声束偏转不同角度和声场半径变化时线性相控阵的声场特性,并与基于瞬态波位移解析解的相控阵声场计算方法得到的理论结果进行了对比。首先研究了基于半圆圆弧离面瞬态位移的声场计算方法,并进行了数值计算可靠性的优化。然后建立了一套相控阵瞬态波位移场的测量系统,以半圆形钢板为试件,纵波传感器为接收传感器,测量了偏转角度为-10°,-30°,-50°,-70°,及声场半径为50 mm,75 mm,100 mm和125 mm时线性超声相控阵的声束指向性;同时以基于瞬态波位移解析解的相控阵声场计算方法,计算了相同条件下该相控阵的声束指向性。最后将实验结果与理论结果进行了对比,取得了较好的一致。说明基于瞬态波位移的相控阵声场计算方法可以较好地计算线阵的声场,对于声束偏转较大角度和声场半径变化时也同样有效。      

超声吸收体物理参数对红外热成像测量高强度聚焦超声声场的影响

超声吸收体的物理参数对利用水听器和红外热成像技术的高强度聚焦超声(High-Intensity Focused Ultrasound,HIFU)声场测量结果具有重要影响。为了探索超声吸收体的物理参数(密度、声速、衰减系数、热扩散系数、定压比热容)对测量结果的影响规律,本文根据层状模型计算出相同声源功率输出时不同物理参数对应的超声吸收体内部声场和热场,利用有限差分法计算出超声吸收体表面在辐照过程中的温度变化;利用基于水听器和红外热成像技术的聚焦声场测量方法测量出焦域内不同位置上声场特征值(轴线声强和-6 dB声束宽度),与通过理论计算得到的声源在纯水中声场特征值进行比较,分析了不同物理参数对测量结果的影响。超声吸收体的声、热学参数中除了声速外,其它物理参数的变化引起声场特征值的测量造成最大相对差异率小于15%。因为声波传播速度的改变会导致超声吸收体内部热场分布变化,使测量结果与理论计算值有较大偏差,其中-6 dB声束宽度和轴线声强最大相对差异率为95.37%和69.97%。因此在选择超声吸收体的声、热学参数时应重点关注声波在吸收体内声速的影响。超声吸收体的声学参数与水的声学参数相近时,可以在焦域内获得较好的测量结果。      

激光通道传输热特性对远场光束质量的影响

 通过仿真计算分析了激光在光束控制系统通道内传输所产生的热效应及其对远场光束质量的影响。激光传播由近轴波方程描述，用快速傅里叶变换技术求解；激光热效应引起的流场密度变化采用完全Navier-Stokes方程计算。计算给出了不同波长、不同吸收系数条件下的远场光斑情况。计算结果表明，在典型的工作条件和状态下，较高能量激光在光束控制系统通道内产生的热效应影响不容忽视，它会明显降低远场目标处的能量集中度，增大光斑的发散。     

A hybrid magnetostrictive-piezoelectric barrel-stave projector

A hybrid magnetostrictive-piezoelectric barrel-stave projector is designed and developed. The new type transducer is driven by the union of rare-earth giant magnetostrictive material Terfenol-D and PZT piezoelectric ceramic. Combining the advantages of low frequency performance within a compact size, it proves that the hybrid projector has a much broader band and higher sound power than that with single magnetostrictive or piezoelectric driver by the simulated and measured results. The prototype of the hybrid projector has a size of 88 mm in outside diameter and 316 mm in length, with an underwater resonant frequency of 1.30 kHz, Q factor of 1.43 for -3 dB bandwidth, and transmitting voltage response level of 135.1 dB at the resonant frequency.     

Optimization Design for the Sandwich Piezoelectric Transmitters in Acoustic Logging

An optimized transducer prototype with a sandwich structure vibrated longitudinally is proposed for a transmitter in acoustic logging,especially in acoustic logging while drilling,by taking account of drilling environments with high temperature and pressure,as well as strong collar drilling vibration during the drilling process.Aimed to improve the transmitting performance,numerical and experimental studies for the transducer optimization are conducted.The impact of location and length of the piezoelectric stack on resonance characteristics and effective electromechanical coupling coefficient is calculated and analyzed.Admittance and transmitting performance of the proposed transducer are measured in laboratory experiments,and the results are compared with simulated ones.It is shown that the newly proposed transducer has higher transmitting performance with lower resonance frequencies.This work provides theoretical and experimental bases for transducer designing and acoustic wave measurements in acoustic logging,especially in acoustic logging while drilling.     

压电陶瓷水听器加速度响应的有限元分析

本文用有限元法分析了压电陶瓷圆柱形水听器的材料参数与几何尺寸对其加速度响应的影响。针对理论分析，进行了水听器加速度响应实验，实验结果与理论分析基本相符。     

Slant path average intensity of finite optical beam propagating in turbulent atmosphere

The average intensity of finite laser beam propagating through turbulent atmosphere is calculated from the extended Huygens Fresnel principle. Formulas are presented for the slant path average intensity from an arbitrarily truncated Gaussian beam. The new expressions are derived from the modified von Karman spectrum for refractive-index fluctuations, quadratic approximation of the structure function,and Gaussian approximation for the product of Gaussian function and Bessel function. It is shown that the form of average intensity is not a Gaussian function but a polynomial of the power of the binomial function, Gaussian function, and the incomplete gamma function. The results also show that the mean irradiance of a finite optical beam propagating in slant path turbulent atmosphere not only depends on the effective beam radius at the transmitting aperture plane, propagation distance, and long-term lateral coherence length of spherical wave, but also on the radius of emit aperture.     

Analysis of the space-time dynamics of acoustic fields on the surface of a solid

The space-time dynamics of an acoustic field produced by a piezoelectric transducer in a pulsed mode is studied. The detection of acoustic fields is achieved using a Doppler laser interferometer. It is shown that, for a pictorial representation of the dynamics of a pulsed process, it is convenient to use the patterns of instantaneous spatial field distributions within the scanning area, the observation of which at successive instants makes it possible to trace the acoustic field variations on a time scale considerably smaller than the period of the ultrasonic wave. Experimental data demonstrating the process of phase propagation along the sample boundary as a function of time are presented. They are in good agreement with theoretical results obtained by using various methods of acoustic field calculation and different scalar potential distributions over the transducer surface. It is shown that the velocity of phase propagation along the sample boundary, which is mainly determined by the wave front curvature of the elastic wave incident on the sample surface, can considerably exceed the wave velocity in the unbounded medium.