Measurements of ultrasonic pulse arrival time differences produced by abdominal wall specimens.

The influence of propagation medium inhomogeneities on pulsatile ultrasonic fields has been investigated experimentally. The study employed a special curved transducer to produce a hemispherical wave pulse and a linear array to measure the resulting field along a line in a plane. Translation of the array in the elevation direction yielded data over a two-dimensional aperture. Time delay across the aperture was calculated by adding delay differences obtained by cross-correlating signals on adjacent elements and noting the position of the cross-correlation peaks. Received waveforms were shifted an amount given by the difference between the actual arrival time and a calculated geometric delay to isolate arrival time differences due to propagation path inhomogeneities. Waveform and time delay difference plots as well as histograms and statistics derived from them for propagation through a water path and for propagation through five specimens of human abdominal wall indicate that arrival time fluctuations in the presence of human abdominal wall specimens are significantly greater than for a water path and that degradation in focusing through human abdominal wall can be expected in ultrasonic imaging systems that operate in the low megahertz range and employ a relatively large aperture.     

Nonlinear propagation in ultrasonic fields: measurements, modelling and harmonic imaging

In high amplitude ultrasonic fields, such as those used in medical ultrasound, nonlinear propagation can result in waveform distortion and the generation of harmonics of the initial frequency. In the nearfield of a transducer this process is complicated by diffraction effects associated with the source. The results of a programme to study the nonlinear propagation in the fields of circular, focused and rectangular transducers are described, and comparisons made with numerical predictions obtained using a finite difference solution to the Khokhlov-Zabolotskaya-Kuznetsov (or KZK) equation. These results are extended to consider nonlinear propagation in tissue-like media and the implications for ultrasonic measurements and ultrasonic heating are discussed. The narrower beamwidths and reduced side-lobe levels of the harmonic beams are illustrated and the use of harmonics to form diagnostic images with improved resolution is described.     

Ultrasonic wave fluctuations through tissue: an experimental pilot study

The fundamental role of the assumption of plane wave ultrasonic propagation in attenuation and scattering measurements on soft tissues is discussed. The evidence for and against the validity of the assumption is shown to be inconclusive. An experimental procedure for direct assessment, by measurement of the ultrasonic amplitude and phase fluctuations occurring after an interrogating plane wave has passed through a tissue specimen, is described. The phase measurements impose significant demands on the mechanical precision, temperature stability and electronic timing capabilities of the measurement system. Measurements are reported on two specimens of fresh beef liver. One exhibited significant fluctuations of amplitude and phase which were reduced by 50% when the effects of thickness variations in the specimen were removed. The other specimen exhibited relatively small fluctuations. The feasibility of making such measurements has been demonstrated and the detailed features of the experimental procedure-which requires considerable care-have been outlined. Although more measurements are needed for definitive conclusions, these will require the development of a reliable technique for determining the presence of gaseous microbubbles in tissue in vitro. The preliminary conclusion that some samples of liver may exhibit low levels of wavefront disruption is consistent with recent evidence in the literature.     

Scattering and propagation of terahertz pulses in random soot aggregate systems

Scattering and propagation of terahertz pulses in random soot aggregate systems are studied by using the generalized multi-particle Mie-solution(GMM) and the pulse propagation theory. Soot aggregates are obtained by the diffusion-limited aggregation(DLA) model. For a soot aggregate in soot aggregate systems, scattering characteristics are analyzed by using the GMM. Scattering intensities versus scattering angles are given. The effects of different positions of the aggregate on the scattering intensities, scattering cross sections, extinction cross sections, and absorption cross sections are computed and compared. Based on pulse propagation in random media, the transmission of terahertz pulses in random soot aggregate systems is determined by the two-frequency mutual coherence function. Numerical simulations and analysis are given for terahertz pulses(0.7956 THz).     

A large ultrasonic bounded acoustic pulse transducer for acoustic transmission goniometry: modeling and calibration

A large, flat ultrasonic transmitter and a small receiver are developed for studies of material properties in acoustic transmission goniometry. While the character of the wave field produced by the transmitter can be considered as a plane wave as observed by the receiver, diffraction effects are noticeable near critical angles and result in the appearance of weak but detectable arrivals. Transmitted ultrasonic waveforms are acquired in one elastic silicate glass and two visco-elastic acrylic glass sample plates as a function of the angle of incidence. Phase velocities are determined from modeling of the shape of curves of the observed arrival times versus angle of incidence. The waveform observations are modeled using a phase propagation technique that incorporates full wave behavior including attenuation. Subtle diffraction effects are captured in addition to the main bounded pulse propagation. The full propagation modeling allows for various arrivals to be unambiguously interpreted. The results of the plane wave solution are close to the full wave propagation modeling without any corrections to the observed wave field. This is an advantage as it places confidence that later analyses can use simpler plane wave solutions without the need for additional diffraction corrections. A further advantage is that the uniform bounded acoustic pulse allows for the detection of weak arrivals such as a low energy edge diffraction observed in our experiments.     

长脉冲高功率微波驱动源放电过程研究

从理论上分析了充放电时高功率微波(HPM)驱动源——水介质脉冲形成线(PFL)上的波过程,提出了形成线放电过程的等效分析方法,得到了输出脉冲波形平顶畸变与PFL充电电压波形的关系,并使用Pspice软件对等效分析的结果进行了模拟验证.开展了基于水介质脉冲形成线的高功率微波驱动源的实验研究,并依据等效分析法得出的结论,通过控制激光触发开关的导通时间,明显减小了输出脉冲波形平顶的畸变,得到了幅值数百kV、脉宽约150 ns、平顶约60 ns的脉冲电压. 关键词： 脉冲形成线(PFL) 高功率微波 等效分析 平顶畸变     

用压电聚合物构成的单极性声脉冲发生器

本文介绍了用阶跃电压的快速下降沿作用在PVDF压电膜上而产生的单极性声脉冲。实验结果表明，声脉冲的波形和脉冲宽度取决于PVDF压电膜的厚度，阶跃电压的下降沿宽度，以及背衬材料和声传播媒质的形状等多个因素。该声脉冲发生器所产生的压力波脉冲的宽度约为50ns，不仅适用于固体中空间电荷的测量，也可以用于其他方面的应用，例如超声医学诊断，对材料的非破坏性的检测等。文章以该声脉冲发生器在压电压力波中的应用为例进行论述，并且所有测量结果都是通过压电压力波测试系统获得的。     

Ultrasonics without a source: thermal fluctuation correlations at MHz frequencies

Noise generated in an ultrasonic receiver circuit consisting of transducer and amplifier is usually ignored, or treated as a nuisance. Here it is argued that acoustic thermal fluctuations, with displacement amplitudes of 3 fm, contain substantial ultrasonic information. It is shown that the noise autocorrelation function is the waveform that would be obtained in a direct pulse/echo measurement. That thesis is demonstrated in experiments in which direct measurements are compared to correlation functions. The thermal nature of the elastodynamic noise that generates these correlations is confirmed by an absolute measurement of their strength, essentially a measurement of the sample temperature.     

γ脉冲射线束在大气中的输运计算方法

在γ脉冲射线束测量中,束流波形是由许多不同能量的γ粒子叠加而成的,在大气介质传输中,由于γ粒子与空气原子的相互作用,测点的γ能通量强度随时间变化的波形、同源点的γ强度波形相比,在较大测点距离的情况下就会产生畸变。讨论了γ射线在大气中输运的传输函数以及用褶积计算测点γ能通量强度波形的方法,给出了解析表达式,对典型的γ源脉冲波形输运不同距离,计算了大气输运对测量的影响。最后讨论了减小或避免波形畸变的方法。     

Simulation of ultrasonic focus aberration and correction through human tissue

Ultrasonic focusing in two dimensions has been investigated by calculating the propagation of ultrasonic pulses through cross-sectional models of human abdominal wall and breast. Propagation calculations used a full-wave k-space method that accounts for spatial variations in density, sound speed, and frequency-dependent absorption and includes perfectly matched layer absorbing boundary conditions. To obtain a distorted receive wavefront, propagation from a point source through the tissue path was computed. Receive focusing used an angular spectrum method. Transmit focusing was accomplished by propagating a pressure wavefront from a virtual array through the tissue path. As well as uncompensated focusing, focusing that employed time-shift compensation and time-shift compensation after backpropagation was investigated in both transmit and receive and time reversal was investigated for transmit focusing in addition. The results indicate, consistent with measurements, that breast causes greater focus degradation than abdominal wall. The investigated compensation methods corrected the receive focus better than the transmit focus. Time-shift compensation after backpropagation improved the focus from that obtained using time-shift compensation alone but the improvement was less in transmit focusing than in receive focusing. Transmit focusing by time reversal resulted in lower sidelobes but larger mainlobes than the other investigated transmit focus compensation methods.     

Increased heating by diagnostic ultrasound due to nonlinear propagation

The heating of tissues by the absorption of ultrasound is an important safety consideration in the use of diagnostic ultrasound. This paper shows that models of ultrasonic heating for this situation need to take account of nonlinear propagation. Measurements were made of the temperature rise in a sample of tissue-mimicking gel, caused by the application of 3.6-MHz focused ultrasonic beams for 3 min. The propagation path to the focus was in water, to mimic the situation where the fetus is scanned through the full bladder. The effect of nonlinear propagation was seen by changing the pressure amplitude of the pulse, while altering the pulsing regime to preserve a constant spatial-peak temporal-average intensity of 1 W cm-2. When nonlinear distortion was present, an enhancement in the temperature rise was observed, which correlated with the value of the shock parameter. The enhancement ratio was typically up to a factor of 3, and the maximum temperature rise observed was 2 degrees C. This enhanced heating was seen both at the surface of the tissue-mimicking gel and after propagation through 23 mm of the material. Under conditions of nonlinear propagation, the maximum heating usually occurs in the prefocal region, rather than at the focus.     

Distortions of laser pulses in neodymium atomic vapor

An experiment on the simulation of the conditions for propagation of laser pulses in dense neodymium atomic vapor is proposed for the development of a technological setup for the laser separation of the ¹⁵⁰Nd isotope from a kilogram sample of natural mixture. The comparison of waveforms of laser pulses in the presence and in the absence of absorption shows that the absorption predominantly takes place at the leading edge of the pulse. The experimental distortion of the waveform is related to the saturation of absorption. The effect is well described using the calculations based on the formalism of the rate equations.     

The Distortion Characteristics of a Pulse Wave Propagating through Fog Medium at Millimeter Wave Band

When a pulse wave propagates through fog, the effects of distortion, being severer at millimeter wave band, is caused by fog because of its dispersive property. The transfer function of a pulse signal is obtained by the theory of radio wave propagation; the complex envelope is deduced by the solution of Fourier integral. The broadening and compressing effects of a pulse wave are discussed. The variations of distortion effects with pulse-width, propagation distance and visibility of fog are found. As an example, the simulating and calculating results of distortion effects for Gaussian pulse at millimeter wave band are given.     

Distortion of ultrashort laser pulses in a spatial filter

By means of Collins diffraction integral, the propagation expression of an ultrashort laser pulse passing through a spatial filter is derived. The effects of the magnification of spatial filter on pulse broadening and distortion of pulse-front for the ultrashort optical pulse are analytically deduced and numerically simulated. It is found that pulse broadening and propagation time difference of a laser pulse getting through spatial filter is proportional to the magnification. As a conclusion, in an ultrashort pulse laser system with a large aperture, the effect of pulse broadening and distortion of pulse-front should be considered.     

Heavy fragment production cross sections from 1.05 GeV/nucleon 56Fe in C, Al, Cu, Pb, and CH2 targets

We have obtained charge-changing cross sections and partial cross sections for fragmentation of 1.05 GeV/nucleon Fe projectiles incident on H, C, Al, Cu, and Pb nuclei. The energy region covered by this experiment is critical for an understanding of galactic cosmic ray propagation and space radiation biophysics. Surviving primary beam particles and fragments with charges from 12 to 25 produced within a forward cone of half-angle 61 mrad were detected using a silicon detector telescope to identify their charge and the cross sections were calculated after correction of the measured yields for finite target thickness effects. The cross sections are compared to model calculations and to previous measurements. Cross sections for the production of fragments with even-numbered nuclear charges are seen to be enhanced in almost all cases.     

Effect of laser focus in two-color synthesized waveform on generation of soft x-ray high harmonics

Synthesis of multi-color laser pulses has been developed as a promising way to improve low conversion efficiency of high-order harmonic generation (HHG). Here we systematically study the effect of laser focus in a two-color waveform on generation of macroscopic HHG in soft x-rays. We find that the dependence of HHG yields on laser focus at low or high gas pressure is sensitive to the characteristics of single-atom harmonic response, in which "short"-or "long"-trajectory emissions can be selectively controlled by changing the waveform of two-color synthesized laser pulse. We uncover the phase-matching mechanism of HHG in the gas medium by examining the propagation of the two-color waveform and the evolution of time-frequency emissions of high-harmonic field. We further reveal that the nonlinear effects, such as geometric phase, atomic dispersion, and plasma defocusing, are responsible for modification of two-color waveform upon propagation. This work can be used to find better macroscopic conditions for generating soft x-ray HHG by employing two-color optimized waveforms.     

脉冲波沿多导线传输系统传播的矩阵分析

本文从电压和电流的基本方程出发,分析脉冲波在多导线传输系统的传播现象和多导线传输系统导线间的相互耦合作用。分析结果说明,在一轮换的双迴三相输电系统里,系统扰动将会产生四种不同速度的脉冲波,对这四种不同速度的脉冲波将有四个不同的特性阻抗。应用这些关系,将能解释线路上脉冲波传播的某些特殊现象,和按照给定的边际条件计算出波的畸变、反射和折射等等,从而对线路实测结果提供理论依据。文中举例说明这分析方法。这脉冲波传播的理论分析被进一步推广应用到稳态交流运行情况,并推导出双迴输电线的等效电路。     

反向追迹法获取水下对空全景图像

在水下透过波浪对海面及空中目标成像，所获取的目标图像会发生畸变。为定量分析波浪对图像畸变的影响，建立一种理想的水下对空成像模型，得到透过波浪的全景图像。首先，在给定的成像模型下，运用主光线反向追迹法，由斯涅耳定律计算出与水下入射光线对应的水上折射光线的方向矢量，从而得到天球上目标点与成像靶面上单像素点的物像对应关系。然后，对整个成像靶面上的像素点进行遍历，形成水下对空全景图像。最后，在4种特殊的海面波浪波形下，利用MATLAB环境计算出天空圆形目标及背景的水下对空全景图像。计算结果表明:运用反向追迹法能够有效地计算出水下对空全景图像，通过调整波浪参数，可以定量分析图像的畸变程度。主光线反向追迹法适用于分析透过具有一阶可导波浪的水下对空全景图像的畸变特征，为研究更复杂波浪下的水下对空图像畸变特征奠定了基础。     

Monitoring of temperature variation in the focal region of an ultrasonic transducer

A method for remote monitoring of temperature in the focal region of a high-intensity ultrasonic transducer is described. Results of measurements and theoretical simulation are presented. The measurements were conducted on a polymer sample with thermophysical and acoustic parameters close to the properties of a soft biological tissue. The sample was heated by a focused piezoelectric transducer with different values of radiation power. The delay of a probe pulse transmitted through the heated region perpendicularly to the axis of the intense ultrasonic beam was detected. The local character of temperature measurements was provided by focusing the probe pulse at the heated region. The application of an additional transducer installed confocally with the probing one provided an opportunity to enhance the precision of measurements. An analysis was conducted on the basis of a numerical solution of the heat conduction equation. The function of thermal sources in the heat conduction equation was calculated according to the results of measuring the pressure distribution in the focal region of the heating transducer. The experimental data obtained agree well with the results of simulation and demonstrate a fundamental possibility of using the proposed ultrasonic technique for remote temperature measurements.     

周期极化铌酸锂波长转换器脉冲抽运时的波形畸变

理论分析了周期性结构的LiNbO3晶体(PPLN)光波导在脉冲光抽运时的级联二阶非线性效应,并通过数值计算得到了抽运、倍频、信号及转换四个光脉冲的波形在波导中的演化。研究表明,使用脉冲光抽运时,由于抽运光脉冲与倍频光脉冲之间的群速度失配导致的走离,将使倍频光脉冲产生展宽;再通过差频转换使得转换光脉冲产生了波形畸变和峰值偏移。最后通过实验验证了转换光脉冲的波形畸变。