Simulation of sound field in a tissue medium generated by a concave spherically annular transducer

The concave spherically annular transducer is regarded as a negative and a positive concave spherical transducer, and the spheroidal beam equation is used to simulate the linear and nonlinear sound field in a tissue medium generated by this transducer. It is found that the acoustic focus of the ring does not coincide with the acoustic focus of its central part. If the width of the ring increases, its acoustic focus will move toward the geometric focus and the amplitudes of nonlinear harmonics will increase obviously. If there are several coaxial rings placed on the concave spherical surface, more than one peak will appear along the axial direction for the fundamental, and high harmonics focus better. The distribution of sound field will change with the number and the excited signals of rings, so it maybe is a potential approach to treat locally big tumors.     

A prototype for a tomography system using third-order acoustic nonlinear effects

A prototype of a tomography system for reconstructing the distributions of acoustic nonlinear parameters is developed and manufactured on the basis of the effect of nonlinear noncollinear interaction of three primary waves. Application of coded primary signals with further correlation processing of a detected combination signal makes it possible to reconstruct the complete image of an object as a result of a single experiment using a small number of transducers, i.e., three radiators and one receiver. A mirror system is proposed, consisting of two coaxial conical acoustic mirrors that make it possible to transform the front of a wave from a cylindrical transducer into a homogeneous quasi-plane beam with a large width close to the real medical diagnostics requirements. Results of physical experiments are given.     

Testing for inhomogeneity of the nonlinear parameter in an acoustic medium by ultrasonic phase conjugation

The principle of applying a selective phase conjugation of the second harmonic of a focused ultrasonic wave to diagnosing inhomogeneity of the nonlinear parameter in an acoustic medium is considered. A solution to the three-dimensional problem of harmonic generation by phase-conjugated waves in a nonlinear medium with a localized isoechogenous inclusion is obtained. The signal amplitudes detected by a transmitting-receiving transducer at the second and forth harmonics of a probe wave are calculated for varying position of the inclusion relative to the focus.     

剪切波对HIFU经颅聚焦形成温度场影响的数值仿真研究

在高强度聚焦超声经颅治疗时,既有纵波又有剪切波,为了保障该治疗方法的安全有效性,有必要分析剪切波对HIFU治疗温度场的影响。该文基于人体头颅CT数据和曲率半径为150 mm的256阵元的半球相控换能器建立三维高强度聚焦超声经颅声波传播模型,利用时域有限差分法结合Westervelt声波非线性传播方程、动量方程、质量守恒方程和Pennes生物热传导方程数值仿真其形成温度场,研究在相同输入功率、不同聚焦角度条件下对应阵元数进行激励时,剪切波对换能器形成温度场的影响。结果表明,随换能器聚焦角度减小,在几何焦点处形成的焦域面积逐渐增大,考虑剪切波形成的温度场达到65?C所需时间逐渐延长,焦点前移程度越大;在相同聚焦角度条件下,考虑剪切波的温度场达到65?C所需时间更短,旁瓣更少,在颅骨处的温度更高,对焦点前移几乎没有影响;随换能器聚焦角度减小,考虑剪切波的模型形成的焦域面积变化范围更大;幂指数函数形式对不同聚焦角度下焦域面积大小的拟合优度高,可预测不同聚焦角度换能器形成的焦域面积。     

Wideband focused film transducer for optoacoustic tomography

The response of a focused film transducer to wideband acoustic signals is studied both theoretically and experimentally. The transducer has the form of a narrow PVDF strip placed on a concave cylindrical surface. A software package is developed for calculating the impulse transient response functions depending on the position of the point source of spherical waves. The experiments are performed using laser thermooptical sources of acoustic spherical wave pulses excited by a pulsed diode-pumped Nd: YAG laser. The theoretical and measured temporal profiles of signals recorded by the transducer are shown to be in good agreement for the source positioned near the transducer’s focus. For this region, a transducer sensitivity map is investigated. For the case of the source positioned at the focus of the transducer, the absolute value of the transducer sensitivity is 8 µV/Pa.     

Narrow band laser-generated surface acoustic waves using a formed source in the ablative regime

A narrow band laser-generated acoustic signal was created using a 4-element lenticular array. This arrangement of the array produces an acoustical signal with frequency content that is compatible with the response of a noncontact and remote broadband receiver, such as a capacitive air-coupled transducer. To support the experimental observations, a simplified concept is presented to explain the effect of a line array source on the frequency content of a surface acoustic wave. The analytical model solution for the wave front shape is derived from the point load solution of Lamb's problem that represents the displacement of a surface acoustic wave generated by an ablative line array. The distribution function, which was used for the model to represent the laser light energy, was tailored to depict the actual energy distribution that illuminates the lenticular array. Filtering functions are applied to the resultant surface displacement function to retain frequencies similar to those detected by the broadband 50 kHz-2.25 MHz receiver. The theoretical model showed good agreement with experimental results.     

Spherical focusing of acoustic pulses in a liquid

Nonlinear processes accompanying the focusing of a microsecond acoustic pulse produced by an electromagnetic source shaped as a spherical segment are investigated. The processes are considered to be far from the boundaries of a liquid, in the absence of cavitation. Detailed measurements of the pressure field by a fiber-optic sensor and high-speed photography of the shock front are performed. The pressure field is found to be determined by the nonlinear effects that occur in the course of the propagation of the initial converging compression wave and an edge rarefaction wave. The peak pressure amplitudes at the focus are 75 and ?42 MPa for the compression and rarefaction waves, respectively, at the maximum voltage of the pulse generator in use. The measured length of the compression wave front is equal to the response time of the sensor (8 ns). The pressure amplitude is shown to be limited by the irregularity of the propagation of a shock wave in the form of Mach’s disk. At the focus, the pressure gradient across the radiator axis reaches 0.5 atm/μm, while the diameter of the focal spot is 2.5±0.2 mm. The focus of the edge rarefaction wave formed due to diffraction is located closer to the radiator than the focus of the compression wave, which may facilitate the study of the biological effect of cavitation independently of the shear motion of the medium.     

Sonar-induced pressure fields in a post-mortem common dolphin

Potential physical effects of sonar transmissions on marine mammals were investigated by measuring pressure fields induced in a 119-kg, 211-cm-long, young adult male common dolphin (Delphinus delphis) cadaver. The specimen was instrumented with tourmaline acoustic pressure gauges used as receiving sensors. Gauge implantation near critical tissues was guided by intraoperative, high-resolution, computerized tomography (CT) scanning. Instrumented structures included the melon, nares, ear, thoracic wall, lungs, epaxial muscle, and lower abdomen. The specimen was suspended from a frame equipped with a standard 50.8-mm-diameter spherical transducer used as the acoustic source and additional receiving sensors to monitor the transmitted and external, scattered field. Following immersion, the transducer transmitted pulsed sinusoidal signals at 5, 7, and 10 kHz. Quantitative internal pressure fields are reported for all cases except those in which the gauge failed or no received signal was detected. A full necropsy was performed immediately after the experiment to examine instrumented areas and all major organs. No lesions attributable to acoustic transmissions were found, consistent with the low source level and source-receiver distances.     

PVDF梳状换能器接收非线性兰姆波的实验研究*

该文基于PVDF压电薄膜对梳状换能器进行设计制作,并将该换能器应用于接收非线性兰姆波信号实验研究。首先通过输出矢量和方法分析PVDF梳状换能器的工作特性,然后将设计制作的PVDF梳状换能器应用于铝板中非线性兰姆波信号的接收,并与传统压电陶瓷换能器经斜劈接收的非线性兰姆波信号进行比较分析。实验结果表明,两种换能器所接收到的信号幅值随传播距离的变化趋势相近,线性增长的积累效应表征结果相似,且PVDF梳状换能器可以对基波和二次谐波信号同时响应。除此之外,PVDF梳状换能器用于接收非线性兰姆波信号更加稳定。因此,PVDF梳状换能器有望应用于复杂构件的在线检测与监测研究。     

球形腔聚焦换能器的非线性声场建模

球形腔聚焦换能器是一种特殊形式的聚焦换能器。为理论证实球形腔聚焦换能器能突破传统超声聚焦在聚焦精度和聚焦增益上的限制,采用Westervelt非线性方程并结合时域有限差分法,建立了球形腔聚焦换能器的非线性声场的数值模型。数值计算了直径为120 mm的0.6 MHz球形腔聚焦换能器的非线性声场,并与传统球壳形聚焦换能器进行了对比。当激励声压为100 kPa时,球形腔聚焦换能器与同尺寸壳形聚焦换能器相比,焦点正声压增益提高约8.5倍,且焦域精度更高,-6 dB聚焦区域在z方向减小约20倍,达到次波长尺度。研究表明球形腔聚焦换能器在高强度聚焦超声精细治疗上具有潜在的应用前景。      

一维非线性声波传播特性

针对一维非线性声波的传播问题进行了有限元仿真和实验研究. 首先推导了一维非线性声波方程的有限元形式, 含有高阶矩阵的非线性项导致声波具有波形畸变、谐波滋生、基频信号能量向高次谐波传递等非线性特性. 编制有限元程序对一维非线性声波进行了计算并对仿真得到的畸变非线性声波信号进行处理, 分析其传播性质和物理意义. 为验证有限元计算结果, 开展了水中的非线性声波传播的实验研究, 得到了不同输入信号幅度激励下和不同传播距离的畸变非线性声波信号. 然后对基波和二次谐波的传播性质进行详细讨论, 分析了二次谐波幅度与传播距离和输入信号幅度的变化关系及其意义, 拟合出二次谐波幅度随传播距离变化的方程并阐述了拟合方程的物理意义. 结果表明, 数值仿真信号及其频谱均与实验结果有较好的一致性, 证实计算方法和结果的正确性, 并提出了具有一定物理意义的二次谐波随传播距离变化的简单数学关系. 最后还对固体中的非线性声波传播性质进行了初步探讨. 本研究工作可为流体介质中的非线性声传播问题提供理论和实验依据.     

Multiple focused-waves production by parametric mixing in a layer with hysteretic quadratic nonlinearity

Nonlinear scattering of a weak probe wave by a strong pump wave in a layer with nonlinearity hysteresis is analysed. It is demonstrated that a spherical probe wave from a point source after nonlinear process of difference frequency excitation is scattered in several foci. Different foci correspond to different frequencies of the transformed wave spectrum. In the case of a plane pump wave and under the condition that nonlinear scattering proceeds without acoustic mode conversion wave front reversal is possible. To achieve wave front reversal in a layer with hysteretic quadratic nonlinearity the pump wave should have a frequency equal to the frequency of the probe wave or to any of its subharmonics.     

Temporal and spatial apodization in defocused acoustic transmission microscopy

Two non-confocally adjusted spherical transducers are employed to implement an acoustic microscope operating in transmission with an approximately line-shaped point spread function (PSF). Such a PSF is of advantage in acoustic transmission line tomography and spatially resolved velocity measurements in solids. The foci of the transducers are viewed as diffraction-limited point transducers and appropriate time-selective signal acquisition is designed to restrict the ultrasound wave paths to the line connecting them. It is found that for typical commercially available transducers the largest contribution to the detected signal is not due to the direct ultrasound wave but due to the edge waves emanating from the rim of the focusing transducer. This poses constraints on achieving a line-shaped PSF in defocused acoustic transmission microscopy. It is shown that, due to the strong contribution from edge waves, it is impossible to achieve a line-shaped PSF in the case of application of a long exciting toneburst. The influence of the exciting pulse length, as well as the position of the time gate on the obtainable PSF is investigated.     

Signal enhancement in Rayleigh wave interactions using a laser-ultrasound/EMAT imaging system

Enhancement of signal amplitudes from Rayleigh wave interaction at solid surface features has been investigated when signals were detected by an in-plane electromagnetic acoustic transducer (EMAT). A laser-ultrasound system was used to inspect surface-breaking slots, serving as artificial defects. Nd:YAG laser pulses were delivered onto a metal surface via an optical fiber and focused to a line source by a cylindrical lens. An in-plane EMAT receiver detected transient surface acoustic waves. A-scan signals and B-scan images from surface defects revealed increased signal amplitude up to 2.8+/-0.3 depending on the distance of the transducer from a slot. An explanation is based on the interaction of the EMAT sensor with the Rayleigh wave. A supporting computer model was derived to show that experimental signal enhancements were consistent with numerical predictions.     

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。      

球形集声器在生物组织中形成的组织损伤

球形集声器可在亚波长焦域内形成高强度声压,在高强度聚焦超声治疗中具有潜在应用前景.本文结合非线性声传播理论及生物传热学理论,研究球形集声器在生物组织中形成的组织损伤.实验中采用430 kHz,内径为240 mm的球形集声器对肝组织作用,结果表明:集声器表面声压为53 kPa时作用2 s,可以形成小于波长尺度的组织损伤.理论计算结果与实验结果符合得较好,并且理论模型可优化球形集声器的开口孔径.研究结果表明,球形集声器可应用于肿瘤的精细超声治疗.     

基于球壳内壁反射的球形换能器自易校准方法

基于球形换能器发射的脉冲球面声波及其在同心球壳内壁的反射,应用机电互易定理,导出了在球面波发射回波声场中,用于校准球形换能器的互易常数.以此为基础,提出一种使用封闭球壳反射器的自易校准方法,通过一次测量即可得到球形换能器的5个基本电声参数.进一步,将自易法校准所得的电压灵敏度换算成平面波声场中定义的标准灵敏度,给出了换...     

Focused antenna array in a strongly inhomogeneous medium

A focused acoustic antenna array is considered in a strongly inhomogeneous stationary medium. An opportunity is indicated to determine the coordinates of a number of objects by active location. It is assumed that, in insonifying the objects by a wave with an arbitrary wave front, they scatter spherical waves and are sufficiently separated in distance and angle to be resolved by the same array in a homogeneous medium. The procedure of determining the coordinates of the objects involves a wave front inversion for distinguishing between the signals from different objects. The coordinates are determined by estimating the parameters for each individual object. The parameter estimation procedure is shown to provide a high efficiency of extracting the argument of a complex signal. The results of the numerical modeling and solution of the problem are presented.     

脉冲激光在液体中激发的声波特性研究

理论上分析了脉冲激光在液体中产生的声波波阵面随光声源形状的变化,以及不同激发机制下光声脉冲波形的差别,并从实验中得出了脉冲CO2激光光声脉冲频谱特性.结果表明光声波波阵面为球面或柱面,热弹机制激发双极性的光声脉冲,汽化机制激发单极性的光声脉冲,CO2脉冲激光在水中激发的声波频谱峰值主要在100 kHz以下.通过选择光声源的形状和激发机制可以获得所需的光声信号.     

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.