脉冲回波声成像系统斑纹噪声的一种消减方法

现行脉冲回波声成像系统,由于分辨元内各背向散射波随机相位叠加产生图像斑纹噪声掩盖了图象灰度级间的细微差别,因而降低了图象的分辨率。本文提出一种对接收信号进行处理的DWS方法,它可在保存有用信息的基础上减弱斑纹噪声,并能有效地改善系统的轴向分辨率。     

A simulation model for ultrasonic pulse-echo systems

Echo patterns obtained by using an ultrasonic pulse-echo system are mainly determined by the structure of an object, but are influenced by many other factors such as the directivity and damping characteristics of the ultrasonic transducer, the dynamic range of the display unit and so on. A simulation model has been developed in which if these factors are specified numerically and fed to the simulator, then an A- or B-mode display pattern is obtained as its output. In this paper the principles of the simulator are presented first and some results of the simulation are given and they are compared with echo patterns obtained experimentally.     

Imaging through aberrating media by computational ghost imaging with incoherent light

We demonstrate a series of experiments on imaging through both stationary aberrating media and moving aberrating media by computational ghost imaging(CGI).An incoherent LED light source is used instead of the common pseudothermal light source(laser light passing through a rotating ground glass).A digital micromirror device is used as a simple spatial light modulator to perform CGI.Moreover,a digital filtering method is introduced to improve imaging quality through moving aberrating media.This imaging modality may have potential applications in medicine and astronomy.     

Analogue implementation of analytic signal processing for pulse-echo systems

An alternative to rectification is proposed for detection of an ultrasonic signal. This method is especially useful in medical and non-destructive evaluation (nde) applications. With this method, the magnitude of the complex analytic signal is used to define the envelope of the ultrasonic waveform. The square of this quantity has been shown elsewhere to be equal to the true rate-of-arrival of energy. An earlier study, using digital data processing, has already demonstrated the superior resolvability of closely spaced interfaces obtained with the analytic signal magnitude, as compared to conventional rectification. Here, an analogue implementation is presented which utilizes single-sideband techniques to obtain both quadrature components of the analytic signal and its magnitude. A conventional transducer, pulser, and receiver are used.     

Imaging through aberrating imaging with media by computational ghost incoherent light

we demonstrate a series of experiments on imaging through both stationary aberrating media and moving aberrating media by computational ghost imaging （CGI）. An incoherent LED light source is used instead of the common pseudothermal light source （laser light passing through a rotating ground glass）. A digital micromirror device is used as a simple spatial light modulator to perform CGI. Moreover. a digital filtering method is introduced to improve imaging quality through moving aberrating media. This imaging modality may have potential applications in medicine and astronomy.     

Direct imaging of spatially modulated superfluid phases in atomic fermion systems

It is proposed that the spatially modulated superfluid phase, or the Fulde-Ferrell-Larkin-Ovchinnikov state could be observed in resonant fermion atomic condensates which are realized recently. We examine optimal experimental setups to achieve it by solving the Bogoliubov-de Gennes equation for both idealized one-dimensional and realistic three-dimensional cases. The spontaneous modulation of this superfluid is shown to be directly imaged as the density profiles either by optical absorption or by Stern-Gerlach experiments.     

Scatterer size estimation in pulse-echo ultrasound using focused sources: theoretical approximations and simulation analysis

The speckle in ultrasound images has long been thought to contain information related to the tissue microstructure. Many different investigators have analyzed the frequency characteristics of the backscattered signals to estimate the scatterer acoustic concentration and size. Previous work has been mostly restricted to unfocused or weakly focused ultrasound sources, thus limiting its implementation with diagnostically relevant fields. Herein, we derive equations capable of estimating the size of a scatterer for any reasonably focused source provided that the velocity potential field in the focal region can be approximated as a three-dimensional Gaussian beam, scatterers are a sufficient distance from the source, and the field is approximately constant across the scatterer. The calculations show that, when estimating the scatterer size, correcting for focusing requires a generalized attenuation-compensation function that includes both attenuation and focusing along the beam axis. The Gaussian approximation is validated by comparing the ideal velocity potential field for three spherically focused sources with f-numbers of 1, 2, and 4 to the Gaussian approximation for frequencies from 2 to 14 MHz. The theoretical derivations are evaluated by simulating the backscatter by using spherically focused sources (f-numbers of 1, 2, and 4) adjacent to attenuating media (0.05 to 1 dB/cm/MHz) that contain scatterers with Gaussian impedance distributions. The generalized attenuation-compensation function yielded results accurate to 7.2% while the traditional attenuation-compensation functions that neglected focusing had errors as high as 103%.     

Scatterer size estimation in pulse-echo ultrasound using focused sources: calibration measurements and phantom experiments

In a companion paper [T. A. Bigelow and W. D. O'Brien Jr., J. Acoust. Soc. Am. 116, 578 (2004)], theory, supported by simulations, showed that accurate scatterer size estimates could be obtained using highly focused sources provided that the derived generalized attenuation-compensation function was used and the velocity potential field near the focus could be approximated as a three-dimensional Gaussian. Herein, the theory is further evaluated via experimental studies. A calibration technique is developed to find the necessary equivalent Gaussian dimensions for a focused source using reflections obtained from a rigid plane scanned through the focus. Then, the theoretical analysis of focused sources is validated experimentally using three spherically focused ultrasound transducers to estimate the radius of glass beads imbedded in tissue mimicking phantoms. Both the impact of focusing (f/1, f/2, and f/4) and the effect of scatterer type (comparing glass bead results to simulation results that used scatterers with Gaussian impedance distributions) were tested. The simulated differences agree with the measured differences to within 2.5% provided that the comparison is made between the same scatterer type and sources with the same equivalent Gaussian dimensions. The improvement provided by the generalized attenuation-compensation function is greatly influenced by the type of scatterer whose size is being estimated and decreases as the wavelength dependence of the Gaussian depth of focus is reduced.     

In vitro comparative study of vibro-acoustography versus pulse-echo ultrasound in imaging permanent prostate brachytherapy seeds

Background

Permanent prostate brachytherapy (PPB) is a common treatment for early stage prostate cancer. While the modern approach using trans-rectal ultrasound guidance has demonstrated excellent outcome, the efficacy of PPB depends on achieving complete radiation dose coverage of the prostate by obtaining a proper radiation source (seed) distribution. Currently, brachytherapy seed placement is guided by trans-rectal ultrasound imaging and fluoroscopy. A significant percentage of seeds are not detected by trans-rectal ultrasound because certain seed orientations are invisible making accurate intra-operative feedback of radiation dosimetry very difficult, if not impossible. Therefore, intra-operative correction of suboptimal seed distributions cannot easily be done with current methods. Vibro-acoustography (VA) is an imaging modality that is capable of imaging solids at any orientation, and the resulting images are speckle free.

Objective and methods

The purpose of this study is to compare the capabilities of VA and pulse-echo ultrasound in imaging PPB seeds at various angles and show the sensitivity of detection to seed orientation. In the VA experiment, two intersecting ultrasound beams driven at f₁ = 3.00 MHz and f₂ = 3.020 MHz respectively were focused on the seeds attached to a latex membrane while the amplitude of the acoustic emission produced at the difference frequency 20 kHz was detected by a low frequency hydrophone.

Results

Finite element simulations and results of experiments conducted under well-controlled conditions in a water tank on a series of seeds indicate that the seeds can be detected at any orientation with VA, whereas pulse-echo ultrasound is very sensitive to the seed orientation.

Conclusion

It is concluded that vibro-acoustography is superior to pulse-echo ultrasound for detection of PPB seeds.     

Some non-linear aspects of the electronic stages in time-domain modelling of NDE pulse-echo ultrasonic systems

Electronics interfacing with NDE probes frequently include non-linear switching devices and semiconductor networks, which influence the excitation pulses and detected echo signals. Classical approaches to modelling a pulse-echo process use ideal assumptions for the electronics and do not consider these influences on the echoes, which can be very relevant in HF cases. This paper proposes new ways to consider these non-linear effects in a time-domain simulation process, extending previous approaches by including new elements in the modelling. Specific electrical models covering the pulse-echo process are applied in the evaluation of echo-graphic signals. They include semiconductor devices and other non-ideal elements. From these models, and using SPICE as a simulation tool, strong non-linear effects on pulsed responses, computed for both E/R stages of typical NDE transceivers, are analysed.     

Distributed delays stabilize ecological feedback systems

We consider the effect of distributed delays in predator-prey models and ecological food webs. Whereas the occurrence of delays in population dynamics is usually regarded a destabilizing factor leading to the extinction of species, we here demonstrate complementarily that delay distributions yield larger stability regimes than single delays. Food webs with distributed delays closely resemble nondelayed systems in terms of ecological stability measures. Thus, we state that dependence of dynamics on multiple instances in the past is an important, but so far underestimated, factor for stability in dynamical systems.     

Accuracy of a pulse-echo boundary tomogram

An ultrasonic tomogram was developed with a view to providing the outline of a cross section of the ulna in vivo. The construction and design of the instrument was undertaken to obtain a resolution as close as possible to theoretical limits. The apparatus was constructed to be used in conjunction with a system for assessing bone mineral content using a conventional clinical x-ray set. It had been expected that the information from the two instruments would make it possible to obtain a precise spatial plot of bone density. It was thought that the tomograph would respond only to reflections at the bone--soft tissue interface, where there is a large change in acoustic impedance, but in vivo trials have shown that other soft tissue interfaces give rise to reflection, which makes the performance unsatisfactory. However, some interesting results have been obtained from solid objects in water, which have shown that there is a limitation on the accuracy of the plot depending on the angle of incidence of the ultrasonic beam and not dependent on errors of mechanical or electronic origin. Some theory is put forward to account for irregularities in plots for these objects.     

Consensus of compound-order multi-agent systems with communication delays

In complex environments, many distributed networked systems can only be illustrated with fractional-order dynamics. When multi-agent systems show individual diversity with difference agents, heterogeneous (integer-order and fractional-order) dynamics are used to illustrate the agent systems and compose integerfractional compounded-order systems. Applying Laplace transform and frequency domain theory of the fractional-order operator, the consensus of delayed multi-agent systems with directed weighted topologies is studied. Since an integer-order model is the special case of a fractional-order model, the results in this paper can be extended to systems with integer-order models. Finally, numerical examples are used to verify our results.     

时延多智能体系统分组一致性分析

研究了拓扑结构为连通无向图与连通二分图的一阶时延多智能系统分别在两类控制协议下的分组一致性问题．基于广义Nyquist准则与频域控制理论的方法,分析并得到了多智能体系统渐进分组收敛一致的充分条件．通过该条件发现系统分组一致性的达到与系统的时延以及智能体间的耦合强度相关．同时,时延的大小影响着系统的收敛速度与动态性能．仿真实验的结果进一步验证了理论分析所得结论的正确性．     

Impulsive control of nonlinear systems with time-varying delays

A whole impulsive control scheme of nonlinear systems with time-varying delays, which is an extension for impulsive control of nonlinear systems without time delay, is presented in this paper. Utilizing the Lyapunov functions and the impulsive-type comparison principles, we establish a series of different conditions under which impulsively controlled nonlinear systems with time-varying delays are asymptotically stable. Then we estimate upper bounds of impulse interval and time-varying delays for asymptotically stable control. Finally a numerical example is given to illustrate the effectiveness of the method.