两种延迟线玻璃材料的超声衰减

本文应用超声脉冲回波技术,测量了两种延迟线玻璃材料SiO_2和FO_5的超声衰减,给出15K至300K温度条件下、10MHz至250MHz频率范围内两种材料的超声衰减系数随频率和温度的变化关系。实验结果表明。在50K温度附近出现一个大的吸收峰,在150K以上温区衰减系数与频率的关系近似成平方规律。最后,对实验结果进行了讨论。     

100kHz—10MHz智能超声衰减、声速综合测试仪

本文基于单片机技术,研制了频率范围为100kHz—10MHz的超声衰减、声速综合测试仪。文中声衰减系数的测量是采用两个不同反射脉冲回波序列包络的面积之比——即“面积比值法”来求得;声速的测量是在定距离下,用时差法测出声传播时间,再换算成声速值。     

用静电换能器实施超声脉冲回波重合法精密测量固体的声速

本文讨论采用静电换能器实施脉冲回波重合法以精密测量固体介质的声速.由于静电换能器的非接触特性,避免了耦合层修正且容易实现正确的周期重合,从而大大提高了测量系统的性能.本文推导并计算了使用静电换能器时的回波测量系统的衍射修正量,设计并制作了ESO型固体声速仪及其探头装置.对几块玻璃和铝试样的测量结果表明,本测量系统测量声时的精度达到射频周期的1/500,即当超声频率10MHz时,声时测量精度达到0.2ns.     

超声在人的正常与病变组织中的衰减

本文在2—9.3MHz和5—40℃的频率与温度范围内,利用脉冲传输法,测量了五人离体状态下的五种正常组织,和三种病变组织中的超声衰减系数。进而研究了衰减系数同频率与温度间的依赖关系;特别是研究了几种病变组织的声衰减特性;也对超声在同一人体的正常与病变组织中的衰减系数进行了对比,表明:病变组织中的衰减系数比正常组织中的偏大,最后给出了几点实验研究结论。     

材料非线性衰减系数的二次谐波测量方法研究

采用有限幅值法测量材料在基波和非线性引起的二次谐波作用下的衰减系数:利用准线性下的KZK方程推导基波和二次谐波的声压分布,并提取波束修正系数;采用短纯音信号进行非线性实验,对检测得到的基波和二次谐波声压进行衍射修正处理,有效抑制衍射对衰减系数测量的不利影响,继而通过线性拟合的方法计算得到更精确的基波和二次谐波的衰减系数。以水为例进行实验,研究了实验测量所得衰减系数的频率依赖关系,结果表明在非线性条件下水的衰减系数与频率间存在较强的线性关系,而线性条件下衰减系数随频率呈现二次方增长的特性则不适用于非线性条件。该研究提出了准确测量非线性声波衰减系数的方法,为更有效地应用非线性超声检测提供理论依据。      

用1-3及2-2连通复合换能器测量冻土

根据冻土弹性波测试要求，研制了一种测试冻土用的500kHz纵波，横波1-3及2-2连通复合超声换能器，其灵敏度高，频带宽，声阻抗低，波形简单及脉冲波形前沿徒削，亦适用于其它介质力学特性测试。文章介绍了换能结构原理及测量系统特性，实测了不同含水量（7%-25%）及负温（-2℃--20℃）条件下兰州黄土试块的纵，横波声速，获得很高测量精度（10ps级）。     

斜入射激光超声体波声场的实验研究

为研究微烧蚀状态下圆形光斑斜入射时激光超声声场的指向性,使用脉冲激光辐照半圆柱形铝质工件表面,并使用压电探头和电磁超声探头分别接收纵波和横波信号。根据速度参数确定了超声信号的类型,提取了信号的峰-峰值,并绘制出声场指向性曲线。通过分析获得以下结论:激光斜入射角度在0~60°范围内变化时,纵波声场的声束轴线方向保持不变,且与工件内法线平行;另外,横波声场的峰-峰值仍然在±35°的方向上取最大值,但是横波指向图形的主瓣易受激光入射方向的影响,当倾斜角度大于等于45°时,横波主瓣的范围变得较宽。     

单光束扩束扫描激光周视探测系统参数对探测能力的影响

针对现有单光束激光同步扫描周视探测对脉冲重复频率要求较高,难以实际应用的问题,提出单光束扩束扫描激光周视探测方法.基于单光束扩束扫描激光周视探测工作原理,推导了最低扫描频率和脉冲频率解析式;分析了圆柱目标回波特性及关键参数截面衰减系数,建立了脉冲扩束激光圆柱目标回波功率数学模型,讨论了系统参数对截面衰减系数的影响,得到最大相邻脉冲光束夹角表达式;重点分析了脉冲频率、光束角和光束入射角对不同直径目标的探测能力的影响;得到了探测系统对典型条件下最大光束角、最低脉冲频率的计算方法.结果表明,对扫描光束稍加扩束可有效降低脉冲重复频率要求.研究结果可为单光束脉冲激光周视探测系统设计、优化提供理论依据.     

固井水泥传输函数及其含水成份对声速和声衰减影响的实验研究

本文采用实验方法初步研究了固井水泥的超声传输函数、声衰减和频率的关系以及含水饱和度对声速和衰减的影响。实验中我们对若干种不同型号、水灰比和固化温度的水泥样品进行了声传输函数测量,测量频段为1.5MHz—4.5MHz。并使用中心频率为1.25MHz探头测量了固井水泥含水饱和度对声速和声衰减的影响。结果表明,在本测量频段内,各种固井水泥的声衰减系数主要与频率一次方成正比。随着固化温度的增加,固井水泥的声衰减系数有增大的趋势,水灰比由40％增至50％时,衰减系数有减小的趋势。固井水泥含水饱和度变化对声衰减影响剧烈,当含水饱和度由100％降至80％时,声衰减增加最快。含水饱和度变化对固井水泥声速度的影响不大。     

界面反射P-SV波转型超声散斑统计特性

对超声P波入射至强散射粗糙界面上时,反射声场中的转型SV波散斑的统计特性进行研究。根据Frensel-Huygens理论和自相关原理,得到了SV波散斑振幅概率密度函数的Rayleigh分布形式以及散斑平均横向尺寸的计算公式。进而又得到了,在使用具有一定尺寸孔径的横波探头时,所接收的SV波散斑的归一化声强的概率密度函数表达式,它与横波探头内接收到的平均散斑个数有关。在实验中,应用了两种不同孔径尺寸的横波探头对SV波的散斑声强进行测量。对测量所得的散斑合强度的数据进行统计分析的结果表明,实验值与理论值有较好的一致性。由此证实,上述理论分析的结果是正确的。     

Negative dispersion in bone: the role of interference in measurements of the apparent phase velocity of two temporally overlapping signals

In this study the attenuation coefficient and dispersion (frequency dependence of phase velocity) are measured using a phase sensitive (piezoelectric) receiver in a phantom in which two temporally overlapping signals are detected, analogous to the fast and slow waves typically found in measurements of cancellous bone. The phantom consisted of a flat and parallel Plexiglas plate into which a step discontinuity was milled. The phase velocity and attenuation coefficient of the plate were measured using both broadband and narrowband data and were calculated using standard magnitude and phase spectroscopy techniques. The observed frequency dependence of the phase velocity and attenuation coefficient exhibit significant changes in their frequency dependences as the interrogating ultrasonic field is translated across the step discontinuity of the plate. Negative dispersion is observed at specific spatial locations of the plate at which the attenuation coefficient rises linearly with frequency, a behavior analogous to that of bone measurements reported in the literature. For all sites investigated, broadband and narrowband data (3-7 MHz) demonstrate excellent consistency. Evidence suggests that the interference between the two signals simultaneously reaching the phase sensitive piezoelectric receiver is responsible for this negative dispersion.     

Ultrasonic shear wave properties of soft tissues and tissuelike materials

Determinations of shear wave speeds of sound and attenuation coefficients are reported for soft tissues, a silicone rubber reference material, and a gel used in manufacturing ultrasonically tissue-mimicking materials. Fresh bovine tissues were investigated, including calfskin, liver, cardiac muscle, and striated muscle. Because of the very large shear wave attenuation coefficients, reasonably accurate determinations of shear wave properties are difficult to make. The quantity measured directly was the complex reflection coefficient for shear waves at a planar interface between the sample and fused silica. Measurements were made at frequencies spanning the range 2-14 MHz. The shear wave attenuation coefficients increase with frequency and are of the order of 10(4) times the longitudinal wave attenuation coefficients. The shear wave speeds of sound also increase with frequency but are only a few percent of the longitudinal wave speeds of sound. The results are accurate enough to allow frequency dependencies to be proposed.     

Attenuation of a Stoneley wave and higher Lamb modes due to the scattering by two-dimensional irregularities of the walls of a fluid-filled borehole

Attenuation of Stoneley waves and higher Lamb modes propagating along an irregular surface of a fluid-filled borehole is investigated. This problem generalizes the problem on the attenuation of Rayleigh waves by an irregular surface of an empty borehole [10]. The technique used to evaluate the attenuation coefficient is based on the perturbation method (surface irregularity heights are considered to be small in comparison with the wavelength) and the mean field method. As a result, an expression is obtained for the partial coefficients of the eigenmode attenuation due to the scattering of eigenmodes by the irregularities of the borehole walls into the same or other eigenmodes, as well as into the bulk longitudinal and transverse waves. The frequency-dependent behavior of the partial attenuation coefficients of both Stoneley waves and higher modes is analyzed against the ratio between the irregularity correlation length and the borehole radius for different correlation functions of irregularities.     

On the measurement of frequency-dependent ultrasonic attenuation in strongly heterogeneous materials

This paper deals with the measurement of frequency-dependent ultrasonic attenuation in strongly heterogeneous materials, such as cementitious materials. To improve the measurement of this parameter on this kind of materials, a linear swept–frequency signal is used to drive an emitter transducer to conduct a through-transmission inspection in immersion. To filter out undesirable frequency content, time–frequency filtering and detection process are performed. The use of this method has been compared with two excitation techniques, the broadband and the narrowband pulses. The results obtained using the swept–frequency excitation together with the time–frequency filtering, allows the determination of the attenuation curves with high accuracy over a wide frequency range without the need for complicated equipment, and improves the effective bandwidth by using a unique pair of transducers.     

Frequency-dependent attenuation and backscatter coefficients in bovine trabecular bone from 0.2 to 1.2 MHz

The frequency-dependent attenuation and backscatter coefficients were measured in 25 bovine femoral trabecular bone samples from 0.2 to 1.2 MHz. When the average attenuation coefficient was fitted to a nonlinear power law α(f)=α(0)+α(1)f(n), the exponent n was found to be 1.65. In contrast, the average backscatter coefficient was fitted to a power law η(f)=η(1)f(n) and the exponent n was measured as 3.25. The apparent bone density was significantly correlated with the parameter α(1) (0.2-0.7 MHz: r = 0.852, 0.6-1.2 MHz: r = 0.832) as well as the backscatter coefficient (0.5 MHz: r = 0.751, 1.0 MHz: r = 0.808).     

High frequency ultrasonic characterization of human vocal fold tissue

Recently, endolaryngeal sonography at frequencies ranging from 10 to 30 MHz has been found to be useful in diagnosing diseases of the vocal folds (VFs). However, image resolution can be further improved by ultrasound at higher frequencies, necessitating the measurement of high-frequency acoustic properties of VF tissue. The ultrasonic parameters of integrated backscatter, sound velocity, and frequency-dependent attenuation coefficient were measured in both the lamina propria (LP) and vocalis muscle (VM) of human VFs using a 47 MHz high-frequency ultrasonic transducer. The integrated backscatter was -173.44+/-6.14 (mean+/-s.d.) and -195.13+/-3.58 dB in the LP and VM, respectively, the sound velocity was 1667.68+/-44.9 and 1595.07+/-39.33 ms, and the attenuation coefficient at 47 MHz was 8.28+/-1.72 and 7.17+/-1.30 dBmm. The difference between these ultrasonic parameters may be attributed to variations in the structure and fiber concentrations in VF tissue. These results could serve as a useful clinical reference for the further development of high-frequency ultrasound devices for endolarynx sonography applications.     

A generalized method of converting CT image to PET linear attenuation coefficient distribution in PET/CT imaging

The accuracy of attenuation correction in positron emission tomography scanners depends mainly on deriving the reliable 511-keV linear attenuation coefficient distribution in the scanned objects. In the PET/CT system, the linear attenu- ation distribution is usually obtained from the intensities of the CT image. However, the intensities of the CT image relate to the attenuation of photons in an energy range of 40 keV-140 keV. Before implementing PET attenuation correction, the intensities of CT images must be transformed into the PET 511-keV linear attenuation coefficients. However, the CT scan parameters can affect the effective energy of CT X-ray photons and thus affect the intensities of the CT image. Therefore, for PET/CT attenuation correction, it is crucial to determine the conversion curve with a given set of CT scan parameters and convert the CT image into a PET linear attenuation coefficient distribution. A generalized method is proposed for con- verting a CT image into a PET linear attenuation coefficient distribution. Instead of some parameter-dependent phantom calibration experiments, the conversion curve is calculated directly by employing the consistency conditions to yield the most consistent attenuation map with the measured PET data. The method is evaluated with phantom experiments and small animal experiments. In phantom studies, the estimated conversion curve fits the true attenuation coefficients accurately, and accurate PET attenuation maps are obtained by the estimated conversion curves and provide nearly the same correction results as the true attenuation map. In small animal studies, a more complicated attenuation distribution of the mouse is obtained successfully to remove the attenuation artifact and improve the PET image contrast efficiently.     

Rayleigh wave attenuation due to the scattering by two-dimensional irregularities of the walls of an empty borehole

Attenuation of the Rayleigh waves propagating along an irregular surface of an empty borehole is investigated. This problem generalizes the problem on the attenuation of Rayleigh waves by an irregular boundary of a half-space. The technique used to evaluate the attenuation coefficient is based on the perturbation method and the mean field method. As a result, an expression is obtained that relates the partial attenuation coefficients of the surface Rayleigh wave to the scattering by the irregular surface of an empty borehole into the bulk longitudinal and transverse waves (the R → P and R → S processes) and into the surface Rayleigh waves (the R → R processes). The frequency-dependent behavior of the partial attenuation coefficients is analyzed for different correlation functions of irregularities.     

Ultrasonic waves in diluted and densified suspensions

A theory of propagation of stress waves in diluted and densified suspensions is developed to make the theoretical basis for analysis of ultrasonic waves through these media. The formulae for the phase velocity and the attenuation coefficient are determined as the function of wave frequency and the suspension structure parameter, which is the volume or mass fraction of the solid phase. These formulae can be use, after suitable calibration, for determination of the solid volume fraction in diluted suspensions, and the solid mass fraction or the water content in densified suspensions, that is, parameters that characterize the structure of a suspension. These structure parameters can be determined by measuring the transition time of ultrasonic wave through a given distance of suspension. The phase velocity dispersion curves and the attenuation coefficients determined theoretically and experimentally are plotted as a function of the volume fraction of the solid phase for dilute suspension, or the solid mass fraction for densified suspension.