聚焦超声的辐射力计算与高强度聚焦超声功率测量实验

本文研究聚焦超声场的辐射力计算，应用几何声学方法，推导了聚焦超声作用于测试靶上的辐射力的通用公式，讨论全反射靶和全吸收靶上的辐射力，最后给出了应用辐射力法测量高强度聚焦超声装置的声功率的实例，其结果与量热法测量的声功率接近，偏差不大于３％。     

辐射力天平法测量相控阵超声换能器的声功率

超声换能器相控阵已用于聚焦超声治疗的研究和应用已有20多年历史,但其声功率P的测量仍缺乏方便有效简易的测量方法。文章旨在提出一种基于凸球面吸收靶的辐射力天平（Radiation Force Balance,RFB）方法,测量其声功率。理论推导了这种RFB在各种阵元组合下的比值r=P/cF（c为水中声速,F为阵的声束轴方向的总辐射力）。论述了测量各种组合的发射声功率的实施方法和测量程序。为实际的测量系统设计和应用建立了良好的工作基础。     

聚焦换能器离散发射阵列的辐射力计算

为了对医学超声中使用的聚焦换能器离散阵列的辐射力进行估算,文章中应用几何声学的方法,推导了聚焦换能器离散发射阵列超声作用于吸收靶上的辐射力公式,并分别给出了活塞阵元与聚焦阵元的辐射力公式。将该几何声学公式与瑞利积分结果比较显示,当声束倾斜角θi<30°,半孔径角α<15°时,误差不超过4.5%。这种结果说明了这种方法可以用来有效地估算离散阵列聚焦换能器的辐射力。     

毫瓦级超声功率副基准

中国计量科学研究院1982年研制建立的毫瓦级超声功率到基准，采用辐射力法获得总声功率值。当超声波通过交界面的不同介质时，在此交界面上产生辐射压力，通过对此压力的测量就可以计算出超声的功率。该装置是由电子微量天平和消声水槽反射靶悬吊系统组成，标准超声源产生的高频激励电压供给超声换能器，以发射超声束，超声束作用在靶上，所产生的辐射力由电子微量天平读取。毫瓦级超声功率副基准1986年被批准为国家副基准，1982年曾荣获广东省科技二等奖。其技术指标为平均声功率测量范围：门一Woo）mw；频率范围：（1－500）MHz；不确定…     

HIFU聚焦探头声场功率测量的凹锥面反射靶设计分析

在HIFU聚焦探头声功率测量中,常用吸收靶来进行声功率测量。锥面反射靶是辐射压力法测量大功率超声功率常用的反射靶面,本文分析了凹锥面反射靶的设计参数与HIFU聚焦探头参数之间的关系。为这种靶面参数设计提供了理论依据。本文分析中未计及靶材及厚度对测量结果的影响。     

瓦级超声功率国家计量副基准

中国计量科学研究院1984年研制建立的瓦级超声功率副基准,于1986年由原国家计量局批准为国家计量副基准。它是采用辐射力法获得总声功率值,这种方法已被IEC工作组推荐使用。该装置由电子微量天平和消声水槽反肘靶悬吊系统等组成,稳定超声源产生的高频激励电压供给超声换能器,以发射超声束;电压由高频数字电压表监控,超声束作用在靶上,所产生的辐射力由电子微量天平读取。1985年曾获得国家技术监督局科技进步三等奖,主要技术指标达到国际先进水平。输出声功率范围：（0.1－20）W;频率范围：1kHz－15MHz;扩展不确定度：5％（k＝2…     

聚焦换能器声强和声功率测量方法研究

针对聚焦声场的特点,以及辐射力天平(RFB)只能获得单一功率指标的缺点,提出一种基于近场测量法的聚焦换能器声强和声功率评价方法。通过声场测量系统对聚焦换能器预聚焦区域中两个平面上的声压扫描测量,运用声强法得到聚焦换能器的声强分布以及辐射声功率。采用活塞换能器的远场测量法与近场测量法进行比对,两种方法得到的声功率误差不超过12%。比较预聚焦区域声功率值和焦点处声功率值,分析声功率评价方法的准确性。发现聚焦声场中不同位置处的声功率值一致性误差5%,同一位置处的声功率值重复性误差2%。结果表明,近场测量法适用于对聚焦换能器声强和声功率的评价,可有效避免直接测量对测量设备的损坏,同时还克服双水听器声强互谱法频率上限低以及测量系统相位不匹配的缺点。     

瓦级超声功率基准装置中反射靶倾斜所引起的误差分析

瓦级超声功率基准装置基于辐射压力原理制成,超声换能器所辐射的总声功率P与作用在全反射靶上的辐射力F之间的关系式为: P=cF/2cos~7θ (1)式中c为超声在液体中传播速度,单位是m/s;θ为靶面法线与入射声束之间的夹角,单     

聚焦换能器声功率近场互谱测量方法研究

针对辐射力天平法与量热法只能获得单一声功率量值,而高强度聚焦声场中焦平面扫描易损坏水听器的问题,开展基于近场互谱的声功率测量研究。搭建单水听器近场互谱测量系统,对声场预聚焦区域两平行平面进行声场扫描;基于互谱声强原理计算电声强频谱,进而通过水听器声压灵敏度幅频响应计算声强与声功率。通过单边互功率谱极坐标变换水听器幅频响应修正的方法,解决非线性声场中水听器幅频响应不平坦带来测量误差的问题。与基于浮力变化原理的量热法对比,超声功率测量偏差小于10%,验证近场互谱法测量高强度聚焦超声功率的有效性。     

双频高强度聚焦超声换能器应用研究进展

高强度聚焦超声(High Intensity Focused Ultrasound,HIFU)消融实体肿瘤已在临床治疗中展示出良好的应用前景。HIFU消融肿瘤技术由于其使靶区肿瘤组织瞬时升温至60℃以上,产生不可逆性凝固性坏死,同时不影响靶区外正常组织而被广泛应用。目前治疗用超声主要使用单频率高强度聚焦超声,但其临床应用的主要限制是靶区组织消融时间较长,靶区外正常组织损伤风险较大。缩短靶区组织消融时间,对于提高HIFU治疗效率,更好地应用于临床较为关键。在总结HIFU换能器的特性和影响HIFU治疗因素的基础上,综述了应用不同类型的双频HIFU换能器强空化和缩短靶区组织消融时间等方面的研究进展。     

Calibration of a focusing transducer and miniature hydrophone as well as acoustic power measurement based on free-field reciprocity in a spherically focused wave field

The free-field transmitting voltage response at the pressure focus of a spherically focusing transducer was defined and calibrated based on the reciprocity theorem of a free-field spherically focused acoustic wave. The acoustic power, the radiation conductance, and the pressure at the pressure focus were derived and measured accordingly from the transmitting current response on the imaginary mirror symmetric spherical surface of the radiating surface. A miniature hydrophone was calibrated by the self-reciprocity of the spherically focusing source. Comparison results show that the measured acoustic power deviation between the reciprocity method and the radiation force balance method are within +/- 5% for two air-backed focusing transducers at 1.53 MHz and 5.27 MHz, respectively, and the maximum deviation of a hydrophone calibration between the new method and the free-field plane wave reciprocity method is within 1.4 dB in the frequency range from 1 MHz to 2 MHz in experiments.     

A new high intensity focused ultrasound applicator for surgical applications

Improved high-intensity focused ultrasound (HIFU) surgical applicators are required for use in a surgical environment. We report on the performance and characteristics of a new solid-cone HIFU applicator. Previous HIFU devices used a water-filled stand-off to couple the ultrasonic energy from the transducer to the treatment area. The new applicator uses a spherically-focused element and a solid aluminum cone to guide and couple the ultrasound to the tissue. Compared with the water-filled applicators, this new applicator is simpler to set up and manipulate, cannot leak, prevents the possibility of cavitation within the coupling device, and is much easier to sterilize and maintain during surgery. The design minimizes losses caused by shear wave conversion found in tapered solid acoustic velocity transformers operated at high frequencies. Computer simulations predicted good transfer of longitudinal waves. Impedance measurements, beam plots, Schlieren images, and force balance measurements verified strong focusing and suitable transfer of acoustic energy into water. At the focus, the -3 dB beam dimensions are 1.2 mm (axial)×0.3 mm (transverse). Radiation force balance measurements indicate a power transfer efficiency of 40%. In vitro and in vivo tissue experiments confirmed the applicator's ability to produce hemostasis     

Effects of various parameters on ultrasonic separation of inclusions from magnesium alloy melt

The acoustic radiation force generated by ultrasonic standing wave in the flow media can make solid particles suspending in the liquid agglomerate at the nodal planes of the waves and then realize their separation, which is also known as ultrasonic agglomeration in chemical industry. In this paper, ultrasonic waves were employed to promote and accelerate the separation of inclusions from magnesium alloy melt, and the effect of acoustic radiation forces on oxide inclusions removal from magnesium alloy melts were studied by numerical calculation. The agglomeration behavior of the inclusions was also obtained by solving the equations of motion for inclusions. Finally, parametric studies, usually very helpful for continued optimization and design efforts, were carried out to evaluate the effects of various parameters such as ultrasonic power, ultrasonic treating time, particle size and density difference between particle and melt on the inclusions distribution. The results indicate that when a moderate ultrasonic power was applied, most of inclusions could agglomerate at wave nodes in a short time which finally enhanced and accelerated the separation of inclusions from magnesium alloy melt.     

Amplitude-dependent losses in ultrasound exposure measurement

Energy losses resulting from the nonlinear propagation of ultrasonic pulses in water have been measured using a polyvinylidene difluoride membrane hydrophone and a radiation-force balance. The focused ultrasonic transducers used were of low focal gain operating at source intensities and frequencies typical of those used in medical diagnostic applications. Energy transfer into harmonic components has been quantified by hydrophone measurements at the focus. At values of shock parameter sigma>pi/2, total loss of intensity was observed, with the greatest loss reaching 2.75 dB of the intensity predicted by linear extrapolation from low-amplitude measurements. A similar but smaller-magnitude reduction in the radiation force measured by a force balance was observed. These results are related to ranges of acoustic parameters obtained from surveys on clinical equipment. It is concluded that a significant majority of contemporary clinical scanners can generate ultrasonic pulses which will lose energy during transmission through water due to amplitude-dependent nonlinear losses, and that it is necessary to consider these, and other nonlinear phenomena, when predicting exposure conditions in vivo.     

Magnetic tracking of acoustic radiation force-induced micro-order displacement

The dynamic behavior of a rigid magnetic sphere induced by an acoustic radiation force was investigated. The sphere was suspended in water in a simple pendulum configuration. The drag force acting on the pendulum during its motion was considered to follow a modified Stokes law for a low Reynolds number, accounting for phenomena related to its oscillatory movement. Steady forces of long (a few seconds) and short (a few milliseconds) durations were used. The movement of the magnetic sphere was tracked using a magnetoresistive sensor. From the new equilibrium position of the sphere in response to the long-duration static radiation force, the amplitude of this force was estimated. To assess the water viscosity, the relaxation movement after the acoustic force had stopped was fitted to a harmonic-motion model. Based on the results for the acoustic force and water viscosity, a theoretical profile of the sphere's micro-order displacement as a function of time caused by short-duration acoustic radiation force agreed well with experimental results.     

The Effect of Frequency Sweeping and Fluid Flow on Particle Trajectories in Ultrasonic Standing Waves

Particle concentration and separation in ultrasonic standing waves through the action of the acoustic radiation force on suspended particles are discussed. The acoustic radiation force is a function of the density and compressibility of the fluid and the suspended particles. A two-dimensional theoretical model is developed for particle trajectory calculations. An electroacoustic model is used to predict the acoustic field in a resonator, driven by a piezoelectric transducer. Second, the results of the linear acoustic model are used to calculate the acoustic radiation force acting on a particle suspended in the resonator. Third, a particle trajectory model is developed that integrates the equation of motion of a particle subjected to a buoyancy force, a fluid drag force, and the acoustic radiation force. Computational fluid dynamics calculations are performed to calculate the velocity field that is subsequently used to calculate fluid drag. For a fixed frequency excitation, the particles are concentrated along the stable node locations of the acoustic radiation force. Through a periodic sweeping of the excitation frequency particle translation is achieved. Two types of frequency sweeps are considered, a ramp approach and a step-change method. Numerical results of particle trajectory calculations are presented for two configurations of flow-through resonators and for two types of frequency sweeping. It is shown that most effective particle separation occurs when the fluid drag force is orthogonal to the acoustic radiation force.     

A finite-element method model of soft tissue response to impulsive acoustic radiation force

Several groups are studying acoustic radiation force and its ability to image the mechanical properties of tissue. Acoustic radiation force impulse (ARFI) imaging is one modality using standard diagnostic ultrasound scanners to generate localized, impulsive, acoustic radiation forces in tissue. The dynamic response of tissue is measured via conventional ultrasonic speckle-tracking methods and provides information about the mechanical properties of tissue. A finite-element method (FEM) model has been developed that simulates the dynamic response of tissues, with and without spherical inclusions, to an impulsive acoustic radiation force excitation from a linear array transducer. These FEM models were validated with calibrated phantoms. Shear wave speed, and therefore elasticity, dictates tissue relaxation following ARFI excitation, but Poisson's ratio and density do not significantly alter tissue relaxation rates. Increased acoustic attenuation in tissue increases the relative amount of tissue displacement in the near field compared with the focal depth, but relaxation rates are not altered. Applications of this model include improving image quality, and distilling material and structural information from tissue's dynamic response to ARFI excitation. Future work on these models includes incorporation of viscous material properties and modeling the ultrasonic tracking of displaced scatterers.     

A preliminary evaluation of the effects of primary and secondaryradiation forces on acoustic contrast agents

Primary and secondary radiation forces result from pressure gradients in the incident and scattered ultrasonic fields. These forces and their dependence on experimental parameters are described, and the theory for primary radiation force is extended to consider a pulsed traveling wave. Both primary and secondary radiation forces are shown to have a significant effect on the flow of microbubbles through a small vessel during insonation. The primary radiation force produces displacement of microspheres across a 100 micron vessel radius for a small transmitted acoustic pressure. The displacement produced by primary radiation force is shown to display the expected linear dependence on the pulse repetition frequency and a nonlinear dependence on transmitted pressure. The secondary radiation force produces a reversible attraction and aggregation of microspheres with a significant attraction over a distance of approximately 100 microns. The magnitude of the secondary radiation force is proportional to the inverse of the squared separation distance, and thus two aggregates accelerate as they approach one another. We show that this force is sufficient to produce aggregates that remain intact for a physiologically appropriate shear rate. Brief interruption of acoustic transmission allows an immediate disruption of the aggregate