多盲孔缺陷的超声兰姆波时域拓扑能量成像方法研究

传统超声成像方法受瑞利准则的约束，难以对缺陷间距小于成像分辨率阈值的多缺陷进行成像。提出了一种基于时域拓扑能量的超声兰姆波成像方法，将逆散射拓扑成像方法中的拓扑渐进过程转换成求解直接声场和伴随声场。然后通过将伴随声场进行时间反转，两个声场将具有在缺陷处聚焦，在非缺陷处不聚焦的特性。将直接声场和伴随声场进行融合，以时域拓扑能量值作为像素值进行成像，从而使表征缺陷的精度较高。建立了缺陷间距小于分辨率阈值的多盲孔缺陷有限元模型，通过“一发多收”的方式激发S0模式和采集缺陷散射信号，并进行时域拓扑能量成像。仿真结果表明：对于多盲孔缺陷，时域拓扑能量成像法能够获得比延时叠加法和时间反转成像法更高的分辨率，并且能在缺陷间距小于成像分辨率阈值时进行成像。     

超声透镜声场的数值计算

0引言平面换能器与声透镜结合可以形成有效的聚焦系统。声透镜聚焦换能器在超声显微镜、超声无损检测和成像声纳等方面有着广泛的应用。声透镜一般由曲面和平面共轴排列,由于声透镜材料声速往往大于传播介质声速,因此凹面镜会形成聚焦效果,如何准确计算透镜所形成的声场是声学透镜设计的重要基础。本文依据射线传播理论、混合传播理论和波动传播理论归纳出三种数值计算方法,分别进行声场计算,并与实测结果进行比较。     

各向异性单晶硅中声波时间反转自适应聚焦

1引言 在各向异性介质中,声波的传播速度与方向有关,这种传播特性会使聚焦声束产生散焦,研究如何克服这种因为传播速度各向异性而造成的相位畸变,具有重要的理论和实际意义.时间反转法(TR,time reversal)是一种新颖的聚焦方法,不需要了解介质的性质和换能器阵的结构就可以实现自适应聚焦.在时间反转处理过程中,用时间反转阵接收声波,并进行存储、时间反转后再发射,这时换能器阵各个阵元激发的声波将同时同相到达原声源处,实现自适应聚焦.近年来,这种聚焦方法引起越来越多的关注,Fink等在TR方法方面开展了大量而深入的研究,从理论上对时间反转法进行了论证,并开展了相应的实验研究.我们实验室也开展了水下波导介质和各向异性介质中时间反转法的研究,研制了一套数字式多通道超声成像系统.各向异性介质中声波时间反转法研究相当少见,在本研究中,选用本征单晶硅作为各向异性介质,研究时间反转法的自适应聚焦特性.     

基于磁共振图像的多层介质声学特性分析

基于磁共振图像,提出了一种多层介质声学特性分析方法,并推算了超声经过分层结构的焦点定位。首先对蔗糖溶液的磁共振T1加权图像进行区域提取、阈值分割、腐蚀膨胀等边缘处理后,得到局部区域的灰度均值;通过磁共振图像上的灰度信息与对应蔗糖溶液的浓度、声速计算关系拟合函数;最后建立模拟生物组织的蔗糖溶液分层结构,运用关系函数推算超声经过分层结构的声透射及聚焦超声的焦点前移量。实验结果表明,该方法从蔗糖溶液磁共振图像中分析得到的声速、声透射系数及焦点前移量与真实值相近,最大差异为0.6239mm,验证了可通过磁共振获取生物组织声参数的可能性。     

全局弱式无网格方法求解消声器横向模态

应用全局弱式无网格方法求解消声器的横向模态,使用径向基函数点插值法离散本征方程,使用伽辽金加权残数法进行数值积分。分别应用全局弱式无网格方法计算了圆形截面,不规则截面以及含有穿孔截面的本征值和本征向量,计算结果与解析方法和二维有限元方法计算结果吻合较好,并且与二维有限元方法相比,全局弱式无网格方法比较节省计算时间。进而分析了支持域的尺寸以及径向基函数中形状参数对计算精度的影响。     

基于振动声调制和时间反转法的微裂纹定位检测研究

针对常规超声检测对闭合微裂纹不敏感的现象,开展了基于振动声调制技术的非线性超声检测研究,并引入时间反转法对检测信号中的非线性信号进行聚焦处理,实现闭合微裂纹的检测及定位。利用ABAQUS有限元软件仿真振动声调制技术检测铝管微裂纹,用有限长冲激响应滤波器提取检测信号中的一阶旁瓣非线性信号,在时域反转后重新加载到无裂纹铝管模型进行时反信号聚焦仿真。结果表明,振动声调制检测信号中的一阶旁瓣非线性信号能够在裂纹位置附近聚焦,管道径向布置的时间反转信号加载方式信号聚焦效果优于轴向布置,并且根据聚焦像的大小和位置可以对裂纹进行定量和定位。     

尾气净化消声装置结构设计与性能分析

基于结构形状和尺寸限制,考虑消声特性、阻力损失和载体端面速度均匀性等性能指标要求,设计一款柴油机尾气净化消声装置,使用声学有限元法和有限体积法计算该装置的传递损失和内部流场,分析腔体长度、进出口管穿孔率以及导流环对声学性能与流动特性的影响。基于计算分析,对结构进行优化,经优化设计的尾气净化消声装置能够满足不同频段下的消声需要和不同转速下的阻力损失要求,且载体内气体流动均匀性良好。     

经颅聚焦超声治疗脑血栓的数值仿真研究

脑卒中是致残和致死的首因,经颅脑卒中治疗具有无创和颅内出血风险低等优势,目前经颅聚焦超声治疗血栓性缺血脑卒中时使用参数尚不明确。基于志愿者头颅CT图像和82阵元相控换能器建立三维数值仿真模型,利用时域有限差分法数值解析Westervelt声波非线性传播方程,对0.5～1.0 MHz超声激励频率和输入声功率等参数进行数值仿真筛选。结果表明：频率相同时焦点处形成的负压越大所需输入声功率越大,经颅所需输入声功率约为开颅的1.5倍;频率越高焦域面积越小但焦域处的旁瓣增多;频率相同时经颅和开颅模型的焦域形状和大小相近但经颅时的旁瓣较强;焦点处负压达到具有溶栓效果的-6 MPa和具有显著溶栓效果的-8 MPa时所需声功率随频率的提高先减少后增加且频率为0.8 MHz时最小;辐照时间和占空比对焦点位置和焦域面积没有影响。     

聚焦超声的辐射力计算与高强度聚焦超声功率测量实验

本文研究聚焦超声场的辐射力计算，应用几何声学方法，推导了聚焦超声作用于测试靶上的辐射力的通用公式，讨论全反射靶和全吸收靶上的辐射力，最后给出了应用辐射力法测量高强度聚焦超声装置的声功率的实例，其结果与量热法测量的声功率接近，偏差不大于３％。     

功率超声聚焦波束发生器

本文介绍的功率超声聚焦波束发生器是采用六个面积相等的同心圆环形平面换能器作为声发射基阵。当改变对每个圆环的电激励信号从外向内递增的时延，就能实现改变等效球面换能器的曲率半径，从而达到可变聚焦焦距的目的。本系统经聚焦后在声焦点处的声束宽度（按－６分贝计）与平面换能器的声束宽度相比，声束缩小了大约５倍。本装置在水槽里工作时，可明显看到在声焦点附近由于空化所引起的气泡群。根据气泡群的分布可以直接观察到聚焦波束的大致形状。 本文还在理论上导出了由六个圆环构成的等效球面换能器在空间任意点Ｑ的声压表示式，由此可以计算出超声聚焦声场的形状，为确定时延系统的设计参数提供了依据。     

Ultrasonic beam focusing through tissue inhomogeneities with a timereversal mirror: application to transskull therapy

Time reversal of ultrasonic fields allows a very efficient approach to focus pulsed ultrasonic waves through lossless inhomogeneous media. Time reversal mirrors (TRMs) are made of large transducer arrays, allowing the incident field to be sampled, time reversed, and reemitted. Time reversal method corrects for phase, amplitude, and even shape aberration and thus, is more efficient than time shift compensation techniques. However, this technique needs the knowledge of the Green's function of a dominant scatterer available in the medium. Aberration correction for ultrasonic hyperthermia could be achieved by combining TRM with an artificial acoustic source or sensor implanted inside the treatment volume. In this paper, time reversal method has been experimentally applied to the focusing through the skull bone. It is shown that the skull induces severe attenuation of ultrasound and reduces the efficiency of the time reversal approach. Then, an amplitude correction method is proposed to focus through an attenuating layer located close to the array. This method consists in inversing the amplitude modulation and then time reversing these signals. Finally, this method is combined with numerical backpropagation to compensate for an attenuating layer located some distance away from the transducer array     

超声针灸相控阵声场及控制模式研究

超声“针灸”是利用二维相控阵实现声能在皮下不同深度的聚焦,刺激特定穴位,从而模拟传统针灸手法的治疗技术。对设计的超声针灸相控阵在不同聚焦深度下的垂直聚焦声场和偏转声场进行仿真,并对焦域进行统计分析,明确了所设计的相控阵探头延时聚焦声场可以满足超声“针灸”的“针形”声场要求。基于声场仿真,还对超声“针灸”声场控制模式进行了仿真研究,研究结果表明:控制超声相控阵聚焦声场聚焦位置的上下移动以及偏转聚焦,可以模拟传统针灸的提、插以及进针角度的变化,为超声“针灸”进一步模拟传统针灸手法提供了理论依据。     

层状无砟轨道结构脱层缺陷超声成像

无砟轨道是典型的层状混凝土结构,脱层缺陷是其最常见的损伤形式,影响着高速列车的安全运行。传统的合成孔径聚焦成像方法是基于恒定声速的超声检测方法,忽略层间的声阻抗差异以及声波在层间界面处的折射,导致声束难以在缺陷处聚焦,声波在层状结构中传播的时间误差较大。鉴于此,提出了一种多层结构合成孔径聚焦成像方法,充分考虑多层结构中的层间声速差异,采用射线追踪方法准确获取声波在多层结构中的传播时间。在此基础上,分析了不同入射波模式以及不同激发频率对多层结构合成孔径聚焦成像结果的影响。结果表明：采用多层结构合成孔径聚焦成像方法,使用频率为50 kHz的横波入射成像分辨率更高,对无砟轨道脱层缺陷检测效果更好。该研究为该类缺陷检测提供了理论支撑。     

Transcranial ultrasound focus reconstruction with phase and amplitude correction

Therapeutic and diagnostic ultrasound procedures performed noninvasively through the skull require a reliable method for maintaining acoustic focus integrity after transmission through layered bone structures. This study used a multiple-element, phased-array transducer to reconstruct ultrasound foci through the human skull by amplitude and phase correction. It was previously demonstrated that adaptive phase correction using a multiple-element, focused transducer array yields a significant correction to an acoustic field that has been distorted by the heterogeneities of the skull bone. The introduction of amplitude correction, in a regime in which acoustic pressures from individual transducer array elements are adjusted to be normalized at the focus, has demonstrated a 6% (-0.27 dB) average decrease in acoustic sidelobe acoustic intensity relative to the focal intensity and a 2% (-0.09 dB) average decrease in the full-width-at-half-maximum (FWHM) of the acoustic intensity profile at the focus. These improvements come at the expense of significant ultrasound intensity loss--as much as 30% lower (-1.55 dB)--at the focus because the amplitude correction method requires that, at constant power, a larger proportion of energy is absorbed or reflected by regions of the skull that transmit less energy. In contrast, a second correction method that distributes pressure amplitudes such that the sections of the skull which transmit more ultrasound energy are exposed with higher ultrasound intensities has demonstrated an average sidelobe intensity decrease of 3% (-0.13 dB) with no change in the FWHM at the focus. On average, there was a 2% (0.09 dB) increase in the acoustic intensity at the focus for this inverse amplitude correction method. These results indicate that amplitude correction according to the transmission properties of various segments of the skull have a clear effect on ultrasound energy throughput into a target site within the brain parenchyma.     

Shape calibration of a conformal ultrasound therapy array

A conformal ultrasound phased array prototype with 96 elements was previously calibrated for electronic steering and focusing in a water tank. The procedure for calibrating the shape of this 2D therapy array consists of two steps. First, a least squares triangulation algorithm determines the element coordinates from a 21×21 grid of time delays. The triangulation algorithm also requires temperature measurements to compensate for variations in the speed of sound. Second, a Rayleigh-Sommerfeld formulation of the acoustic radiation integral is aligned to a second grid of measured pressure amplitudes in a least squares sense. This shape calibration procedure, which is applicable to a wide variety of ultrasound phased arrays, was tested on a square array panel consisting of 7-×7-mm elements operating at 617 kHz. The simulated fields generated by an array of 96 equivalent elements are consistent with the measured data, even in the fine structure away from the primary focus and sidelobes. These two calibration steps are sufficient for the simulation model to predict successfully the pressure field generated by this conformal ultrasound phased array prototype     

基于时反选择性聚焦的信混比增强方法

将时反选择性聚焦方法应用于浅海波导中的信混比增强,提出了一种基于时反选择性聚焦的信混比增强方法。在没有探测声源的条件下,通过对返回的回波信号进行时反算子分解获得时反阵与目标及海底散射体之间的传输函数信息,取一个与目标对应的特征向量作为时反阵的激励权向量,可以实现声场在目标处的选择性聚焦,并抑制相应距离上的海底混响,从而增强回波信混比。该处理方法解决了直接时反处理需要探测声源和混响零点设置方法在海底处设置混响零点的同时也会在目标位置设置零点的问题。利用典型浅海波导环境,进行了计算机仿真实验．验证了该方法的有效性。     

Micro-receiver guided transcranial beam steering

A new method for focusing ultrasound energy in brain tissue through the skull is investigated. The procedure is designed for use with a therapeutic transducer array and a small catheter-inserted hydrophone receiver placed in the brain to guide the array's focus. When performed at high-intensity, a focal intensity on the order of several hundred watts per centimeter-squared is achieved, and cells within a target volume are destroyed. The present study tests the feasibility and range of the method using an ex vivo human skull. Acoustic phase information is obtained from the stationary receiver and used to electrically shift the beam to new locations as well as correct for aberrations due to the skull. The method is applied to a 104-element 1.1 MHz array and a 120-element 0.81 MHz array. Using these array configurations, it is determined that the method can reconstruct and steer a focus over a distance of 50 mm. Application of this minimally invasive technique for ultrasound brain therapy and surgery also is investigated in vitro with a 64-element 0.664 MHz hemisphere array designed for transskull surgery. Tissue is placed inside of a skull and a catheter-inserted receiver is inserted into the tissue. A focus intense enough to coagulate the tissue is achieved at a predetermined location 10 mm from the receiver, the maximum distance that this large element array can electronically steer the focus.