Finite element modeling of acoustic scattering from an encapsulated microbubble near rigid boundary

This article proposes a finite element model (FEM) for predicting the acoustic scattering from an encapsulated microbubble near rigid boundary. The validity of the model is first examined by comparing the acoustic nonlinear response of a free microbubble with that obtained by the Church model. Then this model is used to investigate the effect of the rigid boundary on acoustic scattering signals from microbubble. The results indicate that the resonance frequency decreases while the oscillation amplitude increases as the microbubble approaches the rigid boundary. In addition, the fundamental component of the acoustic scattering signal is enhanced compared with that of the free microbubble.     

Difference-frequency ultrasound generation from microbubbles under dual-frequency excitation

The difference-frequency (DF) ultrasound generated by using parametric effect promises to improve detection depth owing to its low attenuation, which is beneficial for deep tissue imaging. With ultrasound contrast agents infusion, the harmonic components scattered from the microbubbles, including DF, can be generated due to the nonlinear vibration. A theoretical study on the DF generation from microbubbles under the dual-frequency excitation is proposed in formula based on the solution of the RPNNP equation. The optimisation of the DF generation is discussed associated with the applied acoustic pressure, frequency, and the microbubble size. Experiments are performed to validate the theoretical predictions by using a dual-frequency signal to excite microbubbles. Both the numerical and experimental results demonstrate that the optimised DF ultrasound can be achieved as the difference frequency is close to the resonance frequency of the microbubble and improve the contrast-to-tissue ratio in imaging.     

球形集声器在生物组织中形成的组织损伤

球形集声器可在亚波长焦域内形成高强度声压,在高强度聚焦超声治疗中具有潜在应用前景.本文结合非线性声传播理论及生物传热学理论,研究球形集声器在生物组织中形成的组织损伤.实验中采用430 kHz,内径为240 mm的球形集声器对肝组织作用,结果表明:集声器表面声压为53 kPa时作用2 s,可以形成小于波长尺度的组织损伤.理论计算结果与实验结果符合得较好,并且理论模型可优化球形集声器的开口孔径.研究结果表明,球形集声器可应用于肿瘤的精细超声治疗.     

Beam alignments based on the spectrum decomposition of orbital angular momentums for acoustic-vortex communications

Given the enhanced channel capacity of wave chirality, acoustic communications based on the orbital angular momentum (OAM) of acoustic-vortex (AV) beams are of significant interest for underwater data transmissions. However, the stringent beam alignment is required for the coaxial arrangement of transceiver arrays to ensure the accuracy and reliability of OAM decoding. To avoid the required multiple measurements of the traditional orthogonality based algorithm, the beam alignment algorithm based on the OAM spectrum decomposition is proposed for AV communications by using simplified ring-arrays. Numerical studies of the single-OAM and OAM-multiplexed AV beams show that the error of the OAM spectrum increases with the translation distance and the deflection angle of the transceiver arrays. To achieve an ideal arrangement, two methods of the single-array translation alignment and the dual-array deflection alignment are developed based on the least standard deviation of the OAM spectrum (SD-OAM). By decreasing the SD-OAM towards zero using transceiver arrays of 16 transmitters and 16 receivers, accurate beam alignments are accomplished by multiple adjustments in three dimensions. The proposed method is also demonstrated by experimental measurements of the OAM dispersion and the SD-OAM for misaligned beams. The results demonstrate the feasibility of the rapid beam alignment based on the OAM spectrum decomposition by using simplified transceiver ring-arrays, and suggest more application potentials for acoustic communications.     

利用复合换能器测量反射的二次谐波及介质的非线性参量

计算了有限振幅声波在层状媒质中的传播和反射,研究了用二次谐波反射模式的非线性声参量测量方法,设计制作了用于反射法测量非线性声参量的复合结构超声换能器,利用该换能器及有限振幅插入取代法建立了脉冲反射式测量非线性声参量Ｂ／Ａ的实验系统,对若干流体和生物样品进行了非线性声参量的测量,其结果与现有文献上的值吻合的很好。     

In vivo hyperthermia effect induced by high-intensity pulsed ultrasound

Hyperthermia effects (39-44°C) induced by pulsed high-intensity focused ultrasound (HIFU) have been regarded as a promising therapeutic tool for boosting immune responses or enhancing drug delivery into a solid tumor. However, previous studies also reported that the cell death occurs when cells are maintained at 43°C for more than 20 minutes. The aim of this study is to investigate thermal responses inside in vivo rabbit auricular veins exposed to pulsed HIFU (1.17 MHz, 5300 W/cm2 , with relatively low-duty ratios (0.2%-4.3%). The results show that: (1) with constant pulse repetition frequency (PRF) (e.g., 1 Hz), the thermal responses inside the vessel will increase with the increasing duty ratio; (2) a temperature elevation to 43°C can be identified at the duty ratio of 4.3%; (3) with constant duty ratios, the change of PRF will not significantly affect the temperature measurement in the vessel; (4) as the duty ratios lower than 4.3%, the presence of microbubbles will not significantly enhance the thermal responses in the vessel, but will facilitate HIFU-induced inertial cavitation events.     

聚焦超声波在层状生物媒质中的二次谐波声场的理论与实验研究

基于Khokhlov-Zabolotkaya-Kuznetsov(KZK)方程，在频域提出了聚焦超声波在层状生物媒质中传播的理论模型，该模型计及生物媒质的吸收、非线性和边界，同时考虑声源的衍射对声传播的影响.数值研究了聚焦超声波的基波和二次谐波在层状生物媒质中的声传播，并与实验结果相比较.研究结果表明，此方法可以有效地描述聚焦超声波在层状生物媒质中的二次谐波声场. 关键词： 聚焦超声波 层状生物媒质 二次谐波     

非线性声参量层析成像与生物组织定征

结合有限振幅插入取代法和计算机层析成像技术提出非线性参量B／A的成像方法，用卷积滤波法可重建非线性参量层析图像．对正常肝和若干种病变的猪肝脏组织（肝硬化、肝炎和脂肪肝等）进行了实验成像．为了便于比较，文中还给出了衰减系数这一线性参量的层析图像．研究结果指出非线性参量B／A成像在生物组织定征中的有效性及在医学诊断中的应用前景．     

Effects of vapour bubbles on acoustic and temperature distributions of therapeutic ultrasound

This paper describes the evolution of vapour bubbles and its effect on nonlinear ultrasound propagation and temperature rise through tissues for therapeutic ultrasound. An acoustic-thermo coupling algorithm incorporating nonlinearity, diffraction, and temperature-dependent tissue properties, is employed to describe nonlinear ultrasound propagation and thermal effect. Results demonstrate that an obvious migration of peak pressure toward transducer surface is observed while the position of peak temperature changes little in liver tissue before the generation of vapour bubbles, and that the boiling region enlarges towards the surface of transducer in axial direction but increases slowly in radial direction after the generation of vapour bubbles.