Use of Pulse-Echo Ultrasound Imaging in Transcranial Diagnostics of Brain Structures

The results are presented from computer simulations of acoustic pulse propagation in heterogenous media mimicking the human head in two-dimensional and three-dimensional geometries. In the three-dimensional experiment, the cranial bone is presented as a liquid layer with a speed of sound corresponding to that of longitudinal waves in the bone. In the two-dimensional experiment, both longitudinal and transverse waves are considered. Based on data obtained in the numerical experiments, the possibility of obtaining ultrasound images of point scatterers by compensating for aberrations introduced by cranial bones is studied. It is shown that even a simple time delay correction along straight rays greatly improves the quality of an ultrasound image obtained through a nonuniform-thickness solid layer.     

Nonlinear elastodynamics in micro-inhomogeneous solids observed by head-wave based dynamic acoustoelastic testing

Dynamic acoustoelastic testing provides a more complete insight into the acoustic nonlinearity exhibited by micro-inhomogeneous media like granular and cracked materials. This method consists of measuring time of flight and energy modulations of pulsed ultrasonic waves induced by a low-frequency standing wave. Here pulsed ultrasonic head waves were employed to assess elastic and dissipative nonlinearities in a region near the surface of a solid. Synchronization of the ultrasound pulse sequence with the low-frequency excitation provided instantaneous variations in the elastic modulus and the attenuation as functions of the instantaneous low-frequency strain. Weak quadratic elastic nonlinearity and no dissipative nonlinearity were detected in duralumin. In limestone, distinction between tensile and compressive behaviors revealed an asymmetry in the acoustic nonlinearity and hysteresis in both the elastic modulus and the attenuation variations. Measured nonlinear acoustical parameters are in good agreement with values obtained by different techniques. Reversible acoustically induced conditioning modified the acoustic nonlinearity both quantitatively and qualitatively. It reduced tension-compression asymmetry, suggesting a nonequilibrium modification of the sources of acoustic nonlinearity. Additionally to the metrology of the acoustic nonlinearity, head wave based dynamic acoustoelastic testing may be a useful tool to monitor changes in the microstructure or the accumulation of damage in solids.     

时间反演聚焦经颅磁声电刺激仿真与实验研究*

无创脑神经调控技术是生物医学领域的研究热点,经颅磁声电刺激是利用静磁场和声场的耦合而产生的感应电场作用于神经组织,对大脑的目标位置进行刺激和调控的一项技术。颅骨的存在使超声在传播过程中发生相位畸变和幅值衰减,聚焦区域偏离,难以实现精准聚焦。该文基于时间反演法,模拟颅内点声源发射脉冲以及超声传播过程,计算各个阵元接收到的时间差,按照后到先发的原则发射脉冲进行聚焦刺激。与传统相控阵聚焦相比,焦点偏移现象基本得到解决,焦域横向、纵向分辨率均有所提高,提高了声束聚焦精度和感应电场峰值。通过搭建实验平台,将两种聚焦方法所测得的声场归一化处理,验证了时间反演法能补偿焦点偏移,并通过实验证实了超声换能器声场和产生感应电场分布存在较高的一致性。基于真实颅脑结构的虚拟点源时间反演聚焦可以实现无创、精准、灵活的经颅磁声电刺激,有助于推动精准神经调控技术的发展。     

New method of spatial superposition of attenuated waves for ultrasound field modelling

The objective of this work is the contrary issues of ultrasonic diagnostics in medicine when modern requirements for resolution are in conflict with strict safety issues. There is only one way to make progress by starting to take into account the attenuation in biological tissues and the wave diffraction phenomena. The aim of this work is to develop the flexible ultrasound field model implemented in routine algorithms of digital signal processing. The method consists of the calculation of plane wave propagation and the calculation of an ultrasound signal field. On the basis of the spatial impulse response of an aperture for calculation of space-spread ultrasound signals and the spectrum decomposition method for modelling plane wave propagation in lossy media, the modified method of spatial superposition of attenuated waves was developed. Using the method of equidistant line calculation the time and frequency features of the ultrasound signal field caused by the geometry and dynamics of the aperture, the attenuation and velocity dispersion in the medium are determined. The method was successfully applied to the investigation of the system for intracranial media monitoring, where a new measurement channel based on the changes of attenuation and dispersion in intracranial medium has been implemented.     

环形阵列超声断层扫描高分辨率成像方法

介绍了针对环形阵列超声断层扫描的高分辨率全波形反演成像方法,不同于传统的延时叠加和渡越时间断层扫描,该方法充分考虑声传播的透射、反射、散射等特点,通过数据匹配的方法重构目标体声学参数图像。开发了环形超声换能器阵列构建超声断层扫描成像系统,并设计了适合检验成像算法的数值和物理模型,分别使用基于射线理论的回波延时叠加和初至波渡越时间断层扫描、以及基于波动理论的全波形反演方法对目标体进行图像重建,并对比其原理、分辨率、计算量等方面的特点,系统性分析了波动方法相对传统射线方法的优点和不足。结果表明,全波形反演这类充分考虑接收信号多种物理特征的方法可用于较高分辨率的声参数成像,适合与传统方法结合设计有效的超声断层扫描成像系统。     

弯折波导杆的超声斜入射方法*

波导杆技术是一种有效的超声辅助检测方法,在提升高温关键承压设备安全运行方面具有重要作用。现有的波导换能器因直入式发射声波,常被用来在线监测壁厚的腐蚀进程,难以对指定方向的缺陷进行有效检测。为了解决这个难题,基于斯涅耳定理以及导波的频散特性提出了一种弯折结构的波导杆来实现超声斜入射。首先分析了弯折波导杆中水平剪切波的传播特性,探究了波导杆的厚度、弯折角度对杆中声波传播的影响规律;然后,进一步研究了弯折波导杆在半无限空间内的声束指向性;最后通过一对弯折波导杆在线检测结构内部缺陷的试验证明了波导超声斜入射的应用价值。研究结果为基于斜入射技术的波导超声在线监测提供了坚实的理论基础。     

On the specifics of measuring attenuation and velocity of ultrasound in media with microstructure

We examine specific problems of measuring attenuation and velocity of ultrasound in engineering materials using the pulse method of exciting and receiving elastic oscillations. These problems are related to the influence of the internal structure of metals and alloys on the propagation in them of elastic longitudinal and shear waves in the megahertz frequency range. Using the example of a probing signal in the form of a radio pulse with a smooth envelope of Gaussian shape, additional sources of measurement error of the echo method of evaluation of the main acoustic characteristics used in problems of nondestructive testing and ultrasonic diagnostics of materials with microstructure were revealed.     

Combined surface-focused acoustic microscopy in transmission and scanning ultrasonic holography

Employment of ultrasound techniques in nondestructive testing may require identification of the acoustic modes contributing to imaging. Such identification can be achieved, with some restrictions, by time-of-flight analysis. Another approach is acoustic holography that reveals the propagation properties of any selected mode. In anisotropic media, the propagation features are distinct and allow for a reliable classification of the selected mode. Both techniques were applied for classification of bonded, disbonded, and weakly bonded areas in directly bonded semiconductor wafers.     

应用弹性振子点阵模型数值模拟表面带裂痕材料中的激光激发声表面波

本文将激光等效为垂直力源,建立弹性振子点阵模型,并采用数值模拟的方法对线源激光在金属材料中所产生超声及其传播进行了研究,着重针对有无裂痕材料表面上接收到的波形进行了对比和分析。该方法特别适合处理边界问题,因此也易于处理有缺陷材料中的激光超声问题。     

Three-dimensional reconstruction of biological objects' internal structure heterogeneity from the set of ultrasonic tomograms

The paper presents the method of the three-dimensional reconstruction of biological objects' internal structure heterogeneity based on the ultrasonic examination of a woman's breast biopsy phantom. The phantom is made of quasi-homogeneous dense gel in which drops of lesions, characterized by fixed sizes and two different acoustic impedances, were dipped at random. For the purpose of this research a special measurement setup was elaborated, enabling a non-invasive in vitro imaging of biological objects' internal structure in cross-sections for fixed levels, by means of ultrasound transmission tomography (UTT) using the parallel-ray projection geometry of scanning. The two-dimensional images of the local values of ultrasonic wave's propagation velocity in the phantom's internal structure (ultrasonic tomograms) were reconstructed for fixed levels (by using the convolution and back-projection algorithm) from the measurements of average values of ultrasonic signals' runtime propagated from many directions around the object dipped in water. Analyzing the values of particular pixels and using an appropriate image processing technique, in effect the three-dimensional image of heterogeneity boundaries in the examined phantom's internal structure was computer-reconstructed. The obtained results are compatible with the specification provided by the phantom's producer in terms of sizes and acoustic parameters of lesions, which can simulate pathological changes and of the gel imitating the healthy tissue. It means that the method presented, after an appropriate modification and development of the measurement setup with an aim to accelerate the object scanning process and thus provide an opportunity for non-invasive in vivo examinations, could be applied for detecting and diagnosing tumors in women's breasts.     

楔块中脉冲声场及相控阵成像自检研究

相控阵横波检测必须在换能器前方加上楔块,对于楔块中的声场进行研究具有重要意义。本文通过数值计算和物理实验研究了超声相控阵楔块中的脉冲声场,分析并解释了超声相控阵系统自检成像过程中各种波形的产生机理和转换过程。首先从固体中超声传播理论出发,采用时域有限差分方法对相控阵楔块中各阵元独立发射条件下的脉冲声场进行仿真计算,给出了超声脉冲在楔块中的传播过程;然后开展了对应边界条件下相控阵楔块的B模式成像实验。数值模拟和物理实验的结果基本一致。本文的研究初步展示了楔块中的脉冲声场,其结果可为超声相控阵系统自检和楔块设计提供参考。     

经颅超声刺激人脑海马的数值仿真研究

颞叶癫痫、阿尔兹海默症等神经系统疾病与大脑海马的异常放电相关。近年兴起的经颅超声刺激治疗脑疾病具有无创、能深入脑组织和可调控海马神经元放电的特点。该文基于128阵元相控阵换能器、人头颅CT数据、水体建立超声经颅刺激海马数值仿真模型,数值仿真优选适应于海马刺激的换能器结构参数,并结合电容模型,探究超声声学参数对海马神经元放电的影响。结果表明曲率半径90 mm、开口半径56 mm、阵元半径2.0 mm、频率0.9 MHz的128阵元相控超声换能器可形成适应于刺激人脑海马大小的焦域;且在超声频率为0.9 MHz时对海马神经元放电有较好的抑制作用;在较小的占空比和较低的空间峰值时间平均声强条件下对神经元放电有较好的抑制作用。     

非侵入式超声波浆料浓度测量技术研究*

设计了一种基于非侵入式超声波透射衰减法的浆料浓度测量系统,根据超声传播衰减原理,建立超声衰减值与浆料浓度之间的关系。实验中采用生物显微镜和激光粒度仪对颗粒标称粒径进行验证,采用中心频率为200 kHz的超声波换能器,利用一发一收模式对超声波在有机玻璃管内的浆料进行非侵入式测量并分析透射波信号,对体积百分浓度小于25%、不同粒径的石英砂浆料进行测量,通过拟合方法获得浆料温度、体积百分浓度与声衰减对应的关系,并据此构造浓度求解方程,通过现场实时在线测量并与取样结果进行对比验证方程的准确性,结果显示,本方法可有效测量浆料浓度。     

Review on second-harmonic generation of ultrasonic guided waves in solid media(Ⅰ): Theoretical analyses

Considering the high sensitivity of the nonlinear ultrasonic measurement technique and great advantages of the guided wave testing method, the use of nonlinear ultrasonic guided waves provides a promising means for evaluating and characterizing the hidden and/or inaccessible damage/degradation in solid media. Increasing attention on the development of the testing method based on nonlinear ultrasonic guided waves is largely attributed to the theoretical advances of nonlinear guided waves propagation in solid media. One of the typical acoustic nonlinear responses is the generation of second harmonics that can be used to effectively evaluate damage/degradation in materials/structures. In this paper, the theoretical progress of second-harmonic generation(SHG) of ultrasonic guided wave propagation in solid media is reviewed. The advances and developments of theoretical investigations on the effect of SHG of ultrasonic guided wave propagation in different structures are addressed. Some obscure understandings and the ideas in dispute are also discussed.     

Fast volumetric integral-equation solver for acoustic wave propagation through inhomogeneous media

Elements are described of a volumetric integral-equation-based algorithm applicable to accurate large-scale simulations of scattering and propagation of sound waves through inhomogeneous media. The considered algorithm makes possible simulations involving realistic geometries characterized by highly subwavelength details, large density contrasts, and described in terms of several million unknowns. The algorithm achieves its competitive performance, characterized by O(N log N) solution complexity and O(N) memory requirements, where N is the number of unknowns, through a fast and nonlossy fast Fourier transform based matrix compression technique, the adaptive integral method, previously developed for solving large-scale electromagnetic problems. Because of its ability of handling large problems with complex geometries, the developed solver may constitute an efficient and high fidelity numerical simulation tool for calculating acoustic field distributions in anatomically realistic models, e.g., in investigating acoustic energy transfer to the inner ear via nonairborne pathways in the human head. Examples of calculations of acoustic field distribution in a human head, which require solving linear systems of equations involving several million unknowns, are presented.     

A hybrid FDTD-Rayleigh integral computational method for the simulation of the ultrasound measurement of proximal femur

The development of novel quantitative ultrasound (QUS) techniques to measure the hip is critically dependent on the possibility to simulate the ultrasound propagation. One specificity of hip QUS is that ultrasounds propagate through a large thickness of soft tissue, which can be modeled by a homogeneous fluid in a first approach. Finite difference time domain (FDTD) algorithms have been widely used to simulate QUS measurements but they are not adapted to simulate ultrasonic propagation over long distances in homogeneous media. In this paper, an hybrid numerical method is presented to simulate hip QUS measurements. A two-dimensional FDTD simulation in the vicinity of the bone is coupled to the semi-analytic calculation of the Rayleigh integral to compute the wave propagation between the probe and the bone. The method is used to simulate a setup dedicated to the measurement of circumferential guided waves in the cortical compartment of the femoral neck. The proposed approach is validated by comparison with a full FDTD simulation and with an experiment on a bone phantom. For a realistic QUS configuration, the computation time is estimated to be sixty times less with the hybrid method than with a full FDTD approach.     

Review on second-harmonic generation of ultrasonic guided waves in solid media (I):Theoretical analyses

Considering the high sensitivity of the nonlinear ultrasonic measurement technique and great advantages of the guided wave testing method, the use of nonlinear ultrasonic guided waves provides a promising means for evaluating and characterizing the hidden and/or inaccessible damage/degradation in solid media. Increasing attention on the development of the testing method based on nonlinear ultrasonic guided waves is largely attributed to the theoretical advances of nonlinear guided waves propagation in solid media. One of the typical acoustic nonlinear responses is the generation of second harmonics that can be used to effectively evaluate damage/degradation in materials/structures. In this paper, the theoretical progress of second-harmonic generation (SHG) of ultrasonic guided wave propagation in solid media is reviewed. The advances and developments of theoretical investigations on the effect of SHG of ultrasonic guided wave propagation in different structures are addressed. Some obscure understandings and the ideas in dispute are also discussed.     

An ultrasound technique for monitoring the alcoholic wine fermentation

In this paper an ultrasonic technique, usually used in non-destructive-testing (NDT), is applied to the monitoring of alcoholic wine fermentation. This technique consists in placing a test tube, containing the analyzing fluid, between two matched ultrasonic piezoelectric transducers, one used as transmitter and the other as receiver. The transmitter generates an ultrasonic wave in the liquid sample which is received by the receiver; the attenuation and the delay of the received signal in respect to the transmitted one are used to characterize the testing fluid. As first experiments this technique was applied to some test solutions of saccharose in water, ethanol in water and of both solvents in water, in order to evaluate the method applicability and resolution. Best results are obtained by the propagation velocity measurement, with a resolution less than 2%. The method was then applied to monitor the wine fermentation: the propagation velocity in the must was measured every day and the results were related to chemical analyses. The obtained results show that the propagation velocity has the same behaviour of the saccharose concentration, putting in evidence the possibility of monitoring the process state by measuring the ultrasound propagation velocity.     

Modeling of acoustic wave propagation in time-domain using the discontinuous Galerkin method – A comparison with measurements

Simulations of acoustic wave propagation in time-domain are presented. In the simulations, the discontinuous Galerkin method for spatial derivatives and the low-storage Runge–Kutta approach for time derivatives are used. Three different simulation cases are studied. First, the directivity of loudspeaker is simulated. In the second case, acoustic wave propagation in free space is studied using a short pulse. In the last case, acoustic wave scattering from a metallic cylinder is simulated. All simulation results are compared with measurement results. The measurements for the acoustic wave scattering from the metallic cylinder are made in 2D planes using an automated measurement system. Comparison between the simulation and measurement results are made both temporally and spatially and a good agreement between the simulation and measurement results is found. The results suggest that the discontinuous Galerkin method coupled with the low-storage Runge–Kutta approach is a viable tool for modeling acoustic wave propagation in the time-domain.     

Thermographic measurement of the effect of humidity in mortar porosity

The objective of this analysis is to examine the influence of the moisture in the porosity measurement by means of thermal non-destructive test and ultrasound techniques. It is possible to determine the concrete durability by the calculation of its porosity. Porosity is determined in an indirect way, measuring mortar diffusivity by means of active thermography. Using ultrasound techniques, the porosity is related with the ultrasonic propagation of velocity. The diffusivity has been calculated using the W.J. Parker equation. In the ultrasound technique, using the pulse transmission method, ultrasonic propagation velocity was measured as a function of the water content. The conclusions express the correlation between both methods.