Comparison of stress field forming methods for vibro-acoustography

Vibro-acoustography is a method that produces images of the acoustic response of a material to a localized harmonic motion generated by ultrasound radiation force. The low-frequency, oscillatory radiation force (e.g., 10 kHz) is produced by amplitude modulating a single ultrasound beam, or by interfering two beams of slightly different frequencies. Proper beam forming for the stress field of the probing ultrasound is very important because it determines the resolution of the imaging system. Three beam-forming geometries are studied: amplitude modulation, confocal, and x-focal. The amplitude of radiation force on a unit point target is calculated from the ultrasound energy density averaged over a short period of time. The profiles of radiation stress amplitude on the focal plane and on the beam axis are derived. The theory is validated by experiments using a small sphere as a point target. A laser vibrometer is used to measure the velocity of the sphere, which is proportional to the radiation stress exerted on the target as the transducer is scanned over the focal plane or along the beam axis. The measured velocity profiles match the theory. The theory and experimental technique may be useful in future transducer design for vibro-acoustography.     

超声激发振动声成像实验系统的设计

根据超声激发振动声成像的基本原理,设计并构建一个能获得振动声图像的实验系统.该系统由高频信号部分、超声发射及传播部分、低频信号处理和计算机处理与控制等四部分构成.实验表明,该系统能获得生物仿体或软组织内刚性物体的振动声图像,为进行生物组织的振动声成像研究提供了基础;通过采用更高灵敏度的低频水听器和改善共焦换能器的分辨力...     

一种新的超声成像方法振动声成像

文章介绍一种新的超声成像方法———振动声成像。首先从理论上分析了振动声成像的基本原理;然后分别介绍了该方法在钙化动脉血管成像、乳房中微钙化灶的检测以及受热疗和HIFU治疗的肿瘤组织焦斑检测等方面的应用;最后比较了振动声成像与传统超声成像不同点并指出了振动声成像独特的优势。     

Implementation of vibro-acoustography on a clinical ultrasound system

Vibro-acoustography is an ultrasound-based imaging modality that uses two ultrasound beams of slightly different frequencies to produce images based on the acoustic response caused by harmonic ultrasound radiation force excitation at the difference frequency between the two ultrasound frequencies. Vibro-acoustography has demonstrated feasibility and usefulness in imaging of breast and prostate tissue. However, previous studies have been performed either in controlled water tank settings or a prototype breast scanner equipped with a water tank. To make vibro-acoustography more accessible and relevant to clinical use, we report here on the implementation of vibro-acoustography on a General Electric Vivid 7 ultrasound scanner. In this paper, we will describe software and hardware modifications that were performed to make vibro- acoustography functional on this system. We will discuss aperture definition for the two ultrasound beams and beamforming using a linear-array transducer. Experimental results from beam measurements and phantom imaging studies will be shown. The implementation of vibro-acoustography provides a step toward clinical translation of this imaging modality for applications in various organs including breast, prostate, thyroid, kidney, and liver.     

Stress field forming of sector array transducers for vibro-acoustography

This paper presents a study of the stress field forming of sector array transducers for vibro-acoustography applications. The system point-spread function (PSF) is given in terms of the dynamic radiation stress exerted on a point target by a dual ultrasound beam with slightly different frequencies. The radiation stress is calculated by assuming that the resulting ultrasound beam is a plane wave. The stress is proportional to the product of the velocity potential of each incident ultrasound beam. The beamforming and stress field forming of sector array transducers are analyzed through linear acoustics. An expression for the velocity potential produced by sector array transducers is derived. The vibro-acoustography PSF is evaluated numerically. A comparison between the PSF of a sector array and a confocal transducers is presented. The compared characteristics of the PSF are sidelobe levels, transverse, and in-depth spatial resolution. Indeed, one motivation to study sector transducers is the fact the depth-of-field of these transducers should be smaller than that of same size confocal transducers. An experimental setup was used to validate the theoretical PSF of sector array transducers. Results show that the measured PSF is in good agreement with the theoretical predications. Vibro-acoustography images of a breast-phantom by both transducers are presented and discussed.     

简谐激励力作用下悬垂缆线的谐波共振

本文研究在简谐激励力作用下的悬垂缆线的谐波共振。用Hamilton原理导出悬垂缆线面内运动的非线性偏微分方程。通过假设悬垂缆线的挠度曲线，运用Galerkin方法将偏微分方程转化为常微分方程。用多尺度法研究悬垂缆线的超谐波共振和次谐波共振，得到了系统的定常周期解，平均方程和幅频曲线。研究了非线性对幅频曲线的影响和定常运动的稳定区域。     

包装的和谐化设计

文章归纳和总结了和谐美的精神所在,结合具体实例,从人与包装、包装与产品以及包装与环境3个层面论述了现代包装和谐化设计的必要性.提出作为设计师.只有更好地了解人的生理和心理需求,充分考虑到人类的生存环境这个大系统,才能设计出和谐包装符合人的需要适应社会的要求.     

Harmonic diffractive lenses

The harmonic diffractive lens is a diffractive imaging lens for which the optical path-length transition between adjacent facets is an integer multiple m of the design wavelength λ(o). The total lens thickness in air is mλ(o)/(n - 1), which is m times thicker than the so-called modulo 2π diffractive lens. Lenses constructed in this way have hybrid properties of both refractive and diffractive lenses. Such a lens will have a diffraction-limited, common focus for a number of discrete wavelengths across the visible spectrum. A 34.75-diopter, 6-mm-diameter lens is diamond turned in aluminum and replicated in optical materials. The sag of the lens is 23 μm. Modulation transfer function measurements in both monochromatic and white light verify the performance of the lens. The lens approaches the diffraction limit for 10 discrete wavelengths across the visible spectrum.     

Harmonic Analyzer/Generator

A method of measuring the phase angle and magnitude of "n" harmonics corresponding to a periodic signal is described. The method includes the generation of sinusoidal bursts to reproduce significant components of distorted and complex signals; burst amplitude, burst initial phase, and burst frequency correspond, respectively, to harmonic amplitude, harmonic phase angle, and harmonic frequency.     

Second Harmonic Reflected Light

A review is given of the laws of non-linear optical reflection which govern the direction, polarization and intensity of second harmonic light generated in reflection. The complex non-linear susceptibility of III-V and II-VI piezoelectric compounds has been measured over a range of frequencies and shows characteristic dispersive properties. Both the amplitude and the phase can be determined. When the medium has a centre of inversion, the reflected second harmonic production is several orders of magnitude smaller. The effect has magnetic dipole and electric quadrupole character and has been observed in silicon, germanium and several metals and alloys. In metals the contribution from core electrons and the conduction electron plasma have the same order of magnitude.     

Harmonic waves in thermoviscoelastic solids

The constitutive properties and time-harmonic wave propagation in thermoviscoelastic solids are investigated through the energy flux. First the dissipative character of the model is characterized by requiring that the divergence of the energy flux vector be non-positive. The attention is then restricted to planarly stratified solids and the governing equations are shown to satisfy a first-order system of the Stroh-form. The corresponding scalar energy flux is examined and shown to satisfy a decay property which, thanks to a previous result, provides existence and uniqueness for the reflection–transmission process generated by a layer. The required definiteness property of an appropriate matrix is shown to hold in particular cases.     

Angularly-resolved High-order Harmonic Generation in Helium

Abstract

Harmonics up to 119th have been generated in helium using a 1·053 μm, 1 ps chirped pulse amplification laser at intensities up to 3 × 10¹⁴W cm^?2, and their far-field spatial distributions have been measured. In the presence of considerable ionization, complex angular distributions are found for harmonics in the plateau region, while near the cut-off, their distribution narrows to approximately that predicted by lowest-order perturbation theory. The experimental results are found to be in good agreement with the predictions of a numerical propagation model which incorporates photoelectron dispersion and strong-field atomic data for the ionization rates and dipole moments.     

Harmonic wave generation in nonlinear thermoelasticity

The basic equations that describe nonlinear thermoelastic interactions in a continuous medium were derived under the simplifying monomode hypothesis to provide an effective basis for the investigation of nonlinear thermoelastic bulk wave propagation. The one-dimensional equations are solved for a semi-bounded region (half-space) subjected to a prescribed harmonic displacement at its boundary. Two methods of solution are used:

• 1.(i) The straightforward expansion in a small parameter for the near-field solution. The first two orders of approximation are obtained and briefly discussed. It appears, in particular, that the second order solution induces an alteration in the velocity of propagation of the fundamental mode due to thermal influence.
• 2.(ii) The multiple scale technique for the far-field solution. Here, the characteristic curves are investigated and the solution in the first approximation is obtained for points in the half-space not too far from the boundary. In particular, the formation of discontinuities is put in evidence. Comparison is established with the results obtained by the Poincaré expansion, thus establishing the limits of validity of the latter.

     

Harmonic oscillators of carbon nanotube arrays

The harmonic properties of single-walled carbon nanotube (10, 10) arrays in cantilever geometry of lengths, L = 1,000 to 10,000 nm and diameters, D = 15 to 70 nm have been measured recently, and a linear relationship between the first natural frequency and the ratio of array diameter and the square of the span length, D/L2 was postulated. In the present work the authors show that this relationship is highly nonlinear, especially for large values of the ratio, D/L2. In addition, for a given array length, L = 1000 nm, the first natural frequency of the cantilever is shown to vary little with diameters more than 30 nm and to become asymptotic to a value of 22 MHz as it is further increased. The present study is based on earlier work of the authors wherein the flexural stiffness of the single-walled carbon nanotube (CNT) array of hexagonal symmetry and of non-covalent bonding, due to van der Waals interactions, was predicted in terms of the chirality of the nanotubes and the shearing transfer efficiency between nanotubes when subjected to flexural deformation. In addition, predictions are shown to be in agreement with the experimental evidence wherein the flexural modulus of the CNT array decreases with an increase in array diameter.     

Acoustic Harmonic Generation at Diffusion Bonds

The distortion of a sinusoidal acoustic wave at unbonded interfaces has been determined in terms of the first and second harmonic amplitudes. The results demonstrate for the first time that the second harmonic can reach the theoretically predicted maximum value. As also predicted, the harmonic generation efficiency at unbonded interfaces first increases and then decreases with an externally applied compressive load. The technique has been applied to diffusion bonded specimens in an attempt to quantify their achieved strength. As already demonstrated earlier, the energy reflected from such diffusion bonds is also useful to characterize their strength. Indications are that a combination of reflected energy and harmonic generation data could be a powerful tool to quantify the strength of diffusion bonds, particularly those of nearly perfect strength. A strength determination of diffusion bonds by nondestructive evaluation is a necessity for the qualification of such bonds in critical applications.     

Harmonic admittance and dispersion equations--the theorem

The harmonic admittance is known as a powerful tool for analyzing the excitation and propagation of surface acoustic waves (SAWs) in periodic electrode arrays. In particular, the dispersion relationships for open- and short-circuited systems are indicated, respectively, by the zeros and poles of the harmonic admittance. Here, we show that a strict reverse relationship also exists: the harmonic admittance of a periodic system of electrodes may always be expressed as the ratio of two determinants, which have been specifically constructed to describe the eigenmodes of the open- and short-circuited systems. There is no need to solve these equations to find the admittance. The existence of a connection between the excitation and propagation problems was recognized within the coupling-of-modes theory by Chen and Haus (1985) and was recently used to model surface transverse waves by Koskela et al. (1998), but a rigorous mathematical proof was only found later by Biryukov (2000). Here, we reproduce this theorem in detail, give some examples of calculations based on this theorem, and compare the results with measured admittance curves.     

脉冲谐波混频器及应用

脉冲谐波混频器是一种具有广泛应用前景的频率变换电路。本文对脉冲谐波混频器的工作原理进行了详细分析，并给出了谐波混频器的工作条件及应用实例     

谐波治理技术应用研究

对谐波治理技术进行了分析和总结.首先对电能质量、谐波治理措施以及谐波治理技术在电力系统中的应用情况进行介绍;然后,重点分析有源滤波器的工作原理、系统组成及其主要部件;最后,通过具有代表性的实验,借助电能质量分析仪,对实验数据进行了分析总结.研究表明,采用有源滤波器的谐波治理效果可达92％ 以上.     

Harmonic patterns in fine granular vibrated beds

We examine the spontaneous formation of numerous previously unreported patterns in deep beds of fine grains vibrated at frequencies ranging from 10 to 100 Hz and accelerations ranging from 1 to 14 times gravity. Similarly to shallow beds, we find stripes, closed cells and labyrinthine patterns. In addition, we observe traveling waves, cannibalizing cells and other behaviors that are unexpected and remain unexplained. All of these patterns vary spatiotemporally, and unlike shallow bed patterns, are harmonic rather than subharmonic. Received: 17 May 1999