调频激励增强微气泡的次谐波理论和实验研究

超声造影剂的次谐波成像可以提高造影组织比，提供更好的图像质量. 提出一种利用调频脉冲激励以增强造影剂微气泡产生的次谐波新方法. 基于修正的Church方程，从理论上讨论了次谐波的产生与调频激励声压的关系及产生阈值，并且实验证实了优化调频信号的带宽及调频时间可以提高次谐波信号幅度及改善主瓣和旁瓣特性. 理论与实验表明，与传统脉冲信号激励相比，调频信号激励产生的次谐波幅度可提高约22dB. 关键词： 调频激励 超声造影剂 微气泡 次谐波     

The secondary Bjerknes force between two gas bubbles under dual-frequency acoustic excitation

The secondary Bjerknes force is one of the essential mechanisms of mutual interactions between bubbles oscillating in a sound field. The dual-frequency acoustic excitation has been applied in several fields such as sonochemistry, biomedicine and material engineering. In this paper, the secondary Bjerknes force under dual-frequency excitation is investigated both analytically and numerically within a large parameter zone. The unique characteristics (i.e., the complicated patterns of the parameter zone for sign change and the combination resonances) of the secondary Bjerknes force under dual-frequency excitation are revealed. Moreover, the influence of several parameters (e.g., the pressure amplitude, the bubble distance and the phase difference between sound waves) on the secondary Bjerknes force is also investigated numerically.     

Phantom investigation of phase-inversion-based dual-frequency excitation imaging for improved contrast display

Objective

The goal of this work is to examine the effects of pulse-inversion (PI) technique in combination with dual-frequency (DF) excitation method to separate the high-order nonlinear responses from microbubble contrast agents for improvement of image contrast. DF excitation method has been previously developed to induce the low-frequency ultrasound nonlinear responses from bubbles by using the composition of two high-frequency sinusoids (f₁ and f₂).

Motivation

Although the simple filtering was conventionally utilized to provide signal separation, the PI approach is better in the sense that it minimizes the mutual interferences among these high-order nonlinear responses in the presence of spectral overlap. The novelty of the work is that, in addition to the common PI summation, the PI subtraction was also applied in DF excitation method.

Methods

DF excitation pulses having an envelope frequency of 3 MHz (i.e., f₁ = 8.5 MHz and f₂ = 11.5 MHz) with pulse lengths of 3-10 μs and the pressure amplitudes from 0.5 to 1.5 MPa were used to interrogate the nonlinear responses of SonoVue™ microbubbles in the phantom experiments. The high-order nonlinear responses in the DF excitation were extracted for contrast imaging using PI summation for even-order nonlinear components or PI subtraction for odd-order nonlinear ones.

Results

Our results indicated that, as compared to the conventional filtering technique, the PI processing effectively increases the contrast-to-tissue ratio (CTR) of the third-order nonlinear response at 5.5 MHz and the fourth-order nonlinear response at 6 MHz by 2-5 dB. For these high-order nonlinear components, the CTR increase varies with the transmission pressures from 0.5 to 1.5 MPa due to the microbubbles’ displacement induced by the radiation force of DF excitation.

Conclusions

For DF excitation technique, the PI processing can help to extract either the odd-order or the even-order nonlinear components for higher CTR estimates.     

超声联合微泡增强细胞内药物递送的研究进展

超声联合微泡介导的细胞内药物递送是通过微泡的声空化与细胞的相互作用而实现的.作为一种非侵入式的、非病毒的、具有靶向性的、可在成像技术引导下的药物递送技术,超声联合微泡在临床应用上具有独特的优势.该文围绕超声联合微泡实现药物递送的发生机理,从微泡的声学动态响应、细胞响应、细胞外物质进入细胞的动态过程及临床试验进展4方面对...     

用于光生毫米波的双频微片激光器热致频差特性研究

研究了激光二极管端面抽运的双纵模双频微片激光器的热效应对其输出频谱的影响. 通过对端面抽运Nd:YVO₄微片的热传导方程求解得出晶体内部的温度场分布; 并进一步对微片晶体热致折射率变化进行分析, 由此计算出双频微片激光器的热致频谱变化. 实验结果表明: 当抽运功率较小时得到稳定的双纵模双频激光输出; 当抽运功率增大时, 晶体热效应使双频激光频谱双峰之间的频差减小, 各纵模波长频谱宽度加宽, 实验结果与理论分析相符合. 关键词： 光载无线通信 双频微片激光器 热效应 频率差     

Chaos and chaotic control in a relative rotation nonlinear dynamical system under parametric excitation

This paper studies the chaotic behaviours of a relative rotation nonlinear dynamical system under parametric excitation and its control. The dynamical equation of relative rotation nonlinear dynamical system under parametric excitation is deduced by using the dissipation Lagrange equation. The criterion of existence of chaos under parametric excitation is given by using the Melnikov theory. The chaotic behaviours are detected by numerical simulations including bifurcation diagrams, Poincar map and maximal Lyapunov exponent. Furthermore, it implements chaotic control using non-feedback method. It obtains the parameter condition of chaotic control by the Melnikov theory. Numerical simulation results show the consistence with the theoretical analysis. The chaotic motions can be controlled to period-motions by adding an excitation term.     

一类准周期参激非线性相对转动动力系统的稳定性与时滞反馈控制

建立了一类含准周期参数激励和时滞反馈的相对转动非线性系统的动力学方程. 采用多尺度法求解1/2亚谐波主参数共振下的分岔响应方程,并分析了系统的稳定性. 在求解非受控系统的定常解的基础上,通过讨论系统的动力学特性,研究了准周期参数激励对系统响应的影响. 采用时滞反馈控制的方法对系统分岔和极限环(域)进行控制,数值模拟的结果表明通过改变时滞参数可以实现对系统分岔的控制,并能有效地控制极限环(域)的幅值和稳定性. 关键词： 相对转动 准周期参激 时滞反馈 极限环     

Dual-high-frequency ultrasound excitation on microbubble destruction volume

Objective and motivation

The goal of this work was to test experimentally that exposing air bubbles or ultrasound contrast agents in water to amplitude modulated wave allows control of inertial cavitation affected volume and hence could limit the undesirable bioeffects.

Methods

Focused transducer operating at the center frequency of 10 MHz and having about 65% fractional bandwidth was excited by 3 μs 8.5 and 11.5 MHz tone-bursts to produce 3 MHz envelope signal. The 3 MHz frequency was selected because it corresponds to the resonance frequency of the microbubbles used in the experiment. Another 5 MHz transducer was used as a receiver to produce B-mode image. Peak negative acoustic pressure was adjusted in the range from 0.5 to 3.5 MPa. The spectrum amplitudes obtained from the imaging of SonoVue^TM contrast agent when using the envelope and a separate 3 MHz transducer were compared to determine their cross-section at the - 6 dB level.

Results

The conventional 3 MHz tone-burst excitation resulted in the region of interest (ROI) cross-section of 2.47 mm while amplitude modulated, dual-frequency excitation with difference frequency of 3 MHz produced cross-section equal to 1.2 mm.

Conclusion

These results corroborate our hypothesis that, in addition to the considerably higher penetration depth of dual-frequency excitation due to the lower attenuation at 3 MHz than that at 8.5 and 11.5 MHz, the sample volume of dual-frequency excitation is also smaller than that of linear 3-MHz method for more spatially confined destruction of microbubbles.     

Numerical investigation of the inertial cavitation threshold by dual-frequency excitation in the fluid and tissue

Inertial cavitation thresholds, which are defined as bubble growth by 2-fold from the equilibrium radius, by two types of ultrasonic excitation (at the classical single-frequency mode and dual-frequency mode) were calculated. The effect of the dual-frequency excitation on the inertial cavitation threshold in the different surrounding media (fluid and tissue) was studied, and the paramount parameters (driving frequency, amplitude ratio, phase difference, and frequency ratio) were also optimized to maximize the inertial cavitation. The numerical prediction confirms the previous experimental results that the dual-frequency excitation is capable of reducing the inertial cavitation threshold in comparison to the single-frequency one at the same output power. The dual-frequency excitation at the high frequency (i.e., 3.1 + 3.5 MHz vs. 1.1 + 1.3 MHz) is preferred in this study. The simulation results suggest that the same amplitudes of individual components, zero phase difference, and large frequency difference are beneficial for enhancing the bubble cavitation. Overall, this work may provide a theoretical model for further investigation of dual-frequency excitation and guidance of its applications for a better outcome.     

参数激励与晶体摆动场辐射的稳定性

寻找新光源, 特别是短波长相干光源备受关注. 本文讨论了晶体摆动场辐射作为短波长激光的可能性和必须满足的基本条件; 指出了至今尚未获得可利用的短波长激光可能不只是技术原因, 而且还有物理原因. 利用参数激励方法对这个问题进行了分析. 在经典力学框架内和偶极近似下, 引入正弦平方势, 把粒子运动方程化为具有阻尼项和参数激励项的摆方程. 利用Melnikov方法讨论了系统的稳定性, 并对系统的临界条件进行了分析. 结果表明: 系统的稳定性与其参数有关, 只需适当调节这些参数, 系统的稳定性就可以原则上得到保证. 关键词： 晶体摆动场辐射 沟道辐射 参数激励 稳定性     

Investigation on the relationship between overpressure and sub-harmonic response from encapsulated microbubbles

Sub-harmonic component generated from microbubbles is proven to be potentially used in noninvasive blood pressure measurement. Both theoretical and experimental studies are performed in the present work to investigate the dependence of the sub-harmonic generation on the overpressure with different excitation pressure amplitudes and pulse lengths. With4-MHz ultrasound excitation at an applied acoustic pressure amplitude of 0.24 MPa, the measured sub-harmonic amplitude exhibits a decreasing change as overpressure increases; while non-monotonic change is observed for the applied acoustic pressures of 0.36 MPa and 0.48 MPa, and the peak position in the curve of the sub-harmonic response versus the overpressure shifts toward higher overpressure as the excitation pressure amplitude increases. Furthermore, the exciting pulse with long duration could lead to a better sensitivity of the sub-harmonic response to overpressure. The measured results are explained by the numerical simulations based on the Marmottant model. The numerical simulations qualitatively accord with the measured results. This work might provide a preliminary proof for the optimization of the noninvasive blood pressure measurement through using sub-harmonic generation from microbubbles.     

Nonlinear vibration of carbon nanotube embedded in viscous elastic matrix under parametric excitation by nonlocal continuum theory

In the present work, the nonlinear vibration of a carbon nanotube which is subjected to the external parametric excitation is studied. By the nonlocal continuum theory and nonlinear von Kármán beam theory, the governing equation of the carbon nanotube is derived with the consideration of the large deformation. The principle parametric resonance of the nanotube is discussed and the approximation explicit solution is presented by the multiple scale method. Numerical calculations are performed. It can be observed that when the mode number is 1, the stable region can be significantly changed by the parametric excitation, length-to-diameter ratio and matrix stiffness. This phenomenon becomes different to appear if the mode number increases. Moreover, the small scale effects have great influences on the positive bifurcation point for the short carbon nanotube, and the nonlocal continuum theory can present the proper model.     

Coded excitation for ultrasound tissue harmonic imaging

Coded excitation can improve the signal-to-noise ratio (SNR) in ultrasound tissue harmonic imaging (THI). However, it could suffer from the increased sidelobe artifact caused by incomplete pulse compression due to the spectral overlap between the fundamental and harmonic components of ultrasound signal after nonlinear propagation in tissues. In this paper, three coded tissue harmonic imaging (CTHI) techniques based on bandpass filtering, power modulation and pulse inversion (i.e., CTHI-BF, CTHI-PM, and CTHI-PI) were evaluated by measuring the peak range sidelobe level (PRSL) with varying frequency bandwidths. From simulation and in vitro studies, the CTHI-PI outperforms the CTHI-BF and CTHI-PM methods in terms of the PRSL, e.g., −43.5 dB vs. −24.8 dB and −23.0 dB, respectively.     

Parameter optimization of pulse compression in ultrasound imaging systems with coded excitation

A linear array imaging system with coded excitation is considered, where the proposed excitation/compression scheme maximizes the signal-to-noise ratio (SNR) and minimizes sidelobes at the output of the compression filter. A pulse with linear frequency modulation (LFM) is used for coded excitation. The excitation/compression scheme is based on the fast digital mismatched filtering. The parameter optimization of the excitation/compression scheme includes (i) choice of an optimal filtering function for the mismatched filtering; (ii) choice of an optimal window function for tapering of the chirp amplitude; (iii) optimization of a chirp-to-transducer bandwidth ratio; (iv) choice of an appropriate n-bit quantizer. The simulation results show that the excitation/compression scheme can be implemented as a Dolph–Chebyshev filter including amplitude tapering of the chirp with a Lanczos window. An example of such an optimized system is given where the chirp bandwidth is chosen to be 2.5 times the transducer bandwidth and equals 6 MHz: The sidelobes are suppressed to −80 dB, for a central frequency of 4 MHz, and to −94 dB, for a central frequency of 8 MHz. The corresponding improvement of the SNR is 18 and 21 dB, respectively, when compared to a conventional short pulse imaging system. Simulation of B-mode images demonstrates the advantage of coded excitation systems of detecting regions with low contrast.     

Coded excitation using biphase-coded pulse with mismatched filters for high-frequency ultrasound imaging

A scheme of using phase-coded excitation and mismatched filter compression for high-frequency ultrasound imaging is presented in this paper. Biphase-coded pulses were constructed to excite the transducer. Received signals were compressed with mismatched filters optimized by minimizing peak-sidelobe-level (PSL). Both simulation and experiments were carried out to demonstrate the advantage of this technique. The simulation results demonstrated a possible sidelobe reduction (<-90 dB) with a slightly decrease of the signal-to-noise ratio of less than 1 dB compared with the compression using matched filters alone. The experimental results showed about 14 dB SNR improvement as well as -40 dB sidelobe level when the Barker-13 code excitation with 3-cycle sinusoidal wave carrier was used.     

The strontium-vapor laser pumping efficiency under running-wave excitation

The effect of the design features of the gas-discharge tube on the processes proceeding in the discharge circuit of a metal-vapor laser has been investigated. The position of electrodes in the gas-discharge tube has been found to significantly affect the processes at work in the discharge circuit and the energy characteristics of the laser. For instance, gas-discharge tubes with electrodes placed in the hot region of the discharge channel are typified by high metastable-population rates at the leading edge of the excitation pulse, whereas with electrodes positioned in cold buffer regions, the leading edge of the voltage pulse across the resistive component of the tube impedance is seen to peak. Conditions for running-wave generation in the active media of lasers on self-terminated transitions of metal atoms and the running-wave use efficiency for laser pumping are discussed, considering a strontium-vapor laser as an illustration. It is shown that the running wave is generated as the result of the breakdown in the anode end of the gas-discharge tube and is maintained by the energy stored in the capacitive component of the impedance of the gas-discharge tube. The lasing pulse duration under running-wave excitation corresponds to the time it takes an ionization wave to propagate from the anode to the cathode of the gas-discharge tube, with pumping efficiency being ~6–8% for a strontium-vapor laser. The average lasing power varies within 10–15% depending on whether the totally reflecting cavity mirror is placed near the anode or the cathode of the tube. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 79–87, December, 2008.     

随机参激和外激联合作用下非线性动力系统的路径积分解

将基于Gauss Legendre公式的路径积分法推广到随机参激和外激联合作用的非线性动力系统.研究了受高斯白噪声参激和外激联合作用的非线性振子，将所得路径积分解与其精确解（满足一定条件时该系统存在精确解）或Monte Carlo随机模拟结果相比较，充分验证了路径积分的准确性.并借助路径积分数值解，捕捉到该随机系统的一维P分岔. 关键词： 路径积分 P分岔 随机参激 随机外激     

锁模激光相干激发原子的超窄共振效应

本文提出一种相干激发理论模型,解释了文献[1～3]用锁模激光脉冲序列激发原子产生超窄共振效应的实质.推导了原子密度算符ρA(t)的表示式和光谱信号表示式,给出了有关曲线.而且预言了越窄共振效应的新结果.这一理论模型可推广应用于多光子过程.     

A fundamental study on the degradation of paracetamol under single- and dual-frequency ultrasound

The degradation of paracetamol, a widely found emerging pharmaceutical contaminant, was investigated under a wide range of single-frequency and dual-frequency ultrasonic irradiations. For single-frequency ultrasonic irradiation, plate transducers of 22, 98, 200, 300, 400, 500, 760, 850, 1000, and 2000 kHz were employed and for dual-frequency ultrasonic irradiation, the plate transducers were coupled with a 20 kHz ultrasonic horn in opposing configuration. The sonochemical activity was quantified using two dosimetry methods to measure the yield of HO• and H₂O₂ separately, as well as sonochemiluminescence measurement. Moreover, the severity of the bubble collapses as well as the spatial and size distribution of the cavitation bubbles were evaluated via sonoluminescence measurement. The paracetamol degradation rate was maximised at 850 kHz, in both single and dual-frequency ultrasonic irradiation. A synergistic index higher than 1 was observed for all degrading frequencies (200 – 1000 kHz) under dual-frequency ultrasound irradiation, showing the capability of dual-frequency system for enhancing pollutant degradation. A comparison of the results of degradation, dosimetry, and sonoluminescence intensity measurement revealed the stronger dependency of the degradation on the yield of HO• for both single and dual-frequency systems, which confirms degradation by HO• as the main removal mechanism. However, an enhanced degradation for frequencies higher than 500 kHz was observed despite a lower HO• yield, which could be attributed to the improved mass transfer of hydrophilic compounds at higher frequencies. The sonoluminescence intensity measurements showed that applying dual-frequency ultrasonic irradiation for 200 and 400 kHz made the bubbles larger and less uniform in size, with a portion of which not contributing to the yield of reactive oxidant species, whereas for the rest of the frequencies, dual-frequency ultrasound irradiation made the cavitation bubbles smaller and more uniform, resulting in a linear correlation between the overall sonoluminescence intensity and the yield of reactive oxidant species.