Acoustic impedance microscopy for biological tissue characterization

A new method for two-dimensional acoustic impedance imaging for biological tissue characterization with micro-scale resolution was proposed. A biological tissue was placed on a plastic substrate with a thickness of 0.5 mm. A focused acoustic pulse with a wide frequency band was irradiated from the “rear side” of the substrate. In order to generate the acoustic wave, an electric pulse with two nanoseconds in width was applied to a PVDF-TrFE type transducer. The component of echo intensity at an appropriate frequency was extracted from the signal received at the same transducer, by performing a time–frequency domain analysis. The spectrum intensity was interpreted into local acoustic impedance of the target tissue. The acoustic impedance of the substrate was carefully assessed prior to the measurement, since it strongly affects the echo intensity. In addition, a calibration was performed using a reference material of which acoustic impedance was known. The reference material was attached on the same substrate at different position in the field of view. An acoustic impedance microscopy with 200 × 200 pixels, its typical field of view being 2 × 2 mm, was obtained by scanning the transducer. The development of parallel fiber in cerebella cultures was clearly observed as the contrast in acoustic impedance, without staining the specimen. The technique is believed to be a powerful tool for biological tissue characterization, as no staining nor slicing is required.     

Correction for transducer influence on sound velocity measurements by the pulse echo method

The accuracy of sound velocity measurements calculated by the pulse echo method is influenced markedly by the transducer used. This influence is the result of multiple internal reflections within the transducer and is dependent both on transducer thickness and the impedance mismatch between transducer and sample. Corrections for this effect have been computed for a wide range of sample acoustic impedances and the results were confirmed by experiment. These corrections improve sound velocity measurements, particularly on low impedance materials.     

球壳换能器电声效率测量方法及声场特性分析

在高强度聚焦超声治疗系统中,换能器的电声效率是治疗剂量控制中的重要参数,其在焦域附近的声场分布同样重要。该研究以球壳聚焦换能器为研究对象,基于辐射力天平法与平面扫描法进行输出声功率的测量,同时测量其激励电功率后计算得到电声效率,并就其声场特性分布以及存在的非线性传播现象进行了简单阐述。两种测量方法测得的电声效率在声学计量的误差允许范围内,实验结果表明两种方法在测量球壳聚焦换能器输出声功率时具备较好的一致性。     

Coherent Brillouin spectroscopy in a strongly scattering liquid by picosecond ultrasonics

In a modification of a picosecond ultrasonic technique, a short acoustic pulse is launched into a liquid sample by a laser pulse absorbed in a semitransparent transducer film and is detected via coherent Brillouin scattering of a time-delayed probe pulse. With both excitation and probing performed from the transducer side, the arrangement is suitable for in vivo study of biological tissues. The signal is collected from a micrometer-thick layer next to the transducer and is not affected by the diffuse scattering of probe light deeper in the sample. The setup, utilizing a 33 nm thick single crystal SrRuO(3) transducer film, is tested on a full fat milk sample, with 11 GHz acoustic frequency recorded.     

Acoustic microcavitation: its active and passive acoustic detection

In this work acoustic microcavitation in water is studied primarily at 0.75 MHz and 1% duty cycle. To detect cavitation, two kinds of acoustic detectors are used. The first one is an unfocused, untuned 1-MHz receiver transducer that serves as a passive detector. The other one is a focused 30-MHz transducer that is used in pulse-echo mode and is called the active detector. Cavitation itself is brought about by a focused PZT-8 crystal driven in pulse mode. The active detector is arranged confocally with respect to the cavitation transducer. Both the interrogating pulse and the cavitation pulse arrive simultaneously at the common focus, which is the region of cavitation. With the test chamber filled with clean water, no cavitation is observed, even when the cavitation transducer is driven to give its peak output of 22 bar peak negative. Cavitation is, however, observed when polystyrene microparticles are added to the host water. Our view of how these smooth, spherical, monodispersed microparticles give rise to cavitation is described with some estimates. An attempt has been made to understand whether the presence of "streaming" affects the thresholds, and it has been found that the active detector field affects the cavitation process.     

Eigenfunction analysis of stochastic backscatter for characterization of acoustic aberration in medical ultrasound imaging

Presented here is a characterization of aberration in medical ultrasound imaging. The characterization is optimal in the sense of maximizing the expected energy in a modified beamformer output of the received acoustic backscatter. Aberration correction based on this characterization takes the form of an aberration correction filter. The situation considered is frequently found in applications when imaging organs through a body wall: aberration is introduced in a layer close to the transducer, and acoustic backscatter from a scattering region behind the body wall is measured at the transducer surface. The scattering region consists of scatterers randomly distributed with very short correlation length compared to the acoustic wavelength of the transmit pulse. The scatterer distribution is therefore assumed to be delta correlated. This paper shows how maximizing the expected energy in a modified beamformer output signal naturally leads to eigenfunctions of a Fredholm integral operator, where the associated kernel function is a spatial correlation function of the received stochastic signal. Aberration characterization and aberration correction are presented for simulated data constructed to mimic aberration introduced by the abdominal wall. The results compare well with what is obtainable using data from a simulated point source.     

Application of piezoelectric transient response in transducer acoustic power measurement

To explore a more convenient method for measuring the focused ultrasound power,a piezoelectric ceramic plate was used to receive ultrasonic signal directly.Due to an acoustic force acts on the surface of piezoelectric ceramic plate,the piezoelectric response was obtained by means of electromechanical analogy,which was composed of voltage response caused by forced vibration and high frequency attenuation response caused by natural vibration.The conversion relationship between output signal of piezoelectric ceramic plate and acoustic power of transducer was analyzed.The envelope of output piezoelectric signal was extracted in twice,and a voltage amplitude curve with sinusoidal distribution that could describe the changes of acoustic power was obtained.Under different drive voltage of transducer,the maximum peak voltage of envelope curve was found respectively.Their squared values were made a linear fitting with acoustic power measured by acoustic power meter,and then the proportional coefficient of theoretical relational expression was calibrated.The experimental results are in good agreement with the theory.The relative error between calibrated theoretical acoustic power and that measured by acoustic power meter was less than 8.7%.The paper can provide a guideline for measuring acoustic power of transducer by using piezoelectric signal.     

基于时间反转法的相控换能器声场的仿真研究

相控换能器具有焦距可调的优势。本文以82阵元相控换能器建立的3D数值仿真模型为例,基于时间反转法提取阵元的激励信号,利用时域有限差分(FDTD)法对Westervelt声波非线性传播方程进行声场数值仿真,研究不同阵元分布、偏离声轴的距离、设定焦距大小对形成声场的影响,可调控范围及其消除旁瓣方法。研究结果表明,随机分布相控阵可明显降低声场中的旁瓣;随着偏离声轴距离的增加,主瓣声压幅值逐渐减小,旁瓣与主瓣的最大声强比值r逐渐增大,且沿声轴的可调控范围逐渐减小;随声轴方向上设定焦距的增加,主瓣声压幅值先增大后减小,r值先减小后增大;基于时间反转法的高声压旁瓣消除法可在一定程度上扩大相控阵声场的可调控范围。     

压电瞬态响应在换能器声功率测量中的应用

探索一种简便的聚焦超声功率测量方法,利用压电陶瓷片直接接收超声信号,通过机电类比得到压电瞬态响应由压电片在声波作用力下引起受迫振动产生的电压响应与固有振动产生的高频衰减响应叠加而成,分析输出压电信号与换能器声功率之间的换算关系。对输出压电信号进行二次包络提取,获得表征声功率变化的电压幅度曲线,分别找出不同换能器驱动电压下包络曲线的最大峰值电压,将其平方值与声功率计所测声功率进行线性拟合,并对理论关系式中的比例系数进行标定。实验结果所得线性拟合度较高,且标定后所得声功率与声功率计所测值相对误差低于8.7%,证明了通过压电瞬态响应测量换能器声功率具有可行性。      

Generation and reception of short acoustic pulses by multilayer piezoelectric transducers

By means of the Fourier transformation, a formula is obtained for calculating the pulse response of a transducer when a square pulse of a given duration is supplied to its input and the excited “sound” is reflected from the free flat end of the acoustic line and returns to the transducer. For a frequency of 100 MHz, real lithium niobate transducers that are connected through intermediate layers to the acoustic line made of fused quartz and to a rear load with a given acoustic impedance are considered. The pulse response is calculated for the transducers with different values of the sublayer thickness, rear acoustic load, transmission line impedance, and original pulse duration. The results of the calculations are compared with the experiment.     

Acoustic energy output of a transducer unit

The acoustic energy output of a magnetostrictive transducer unit has been observed by the calorimetric method. The object was to observe the energy output of the unit from low to high ultrasonic intensities under identical conditions of transducer frequency and acoustic load. The design and construction of the calorimeter and its mode of operation are discussed in some detail.     

Transient acoustic field radiated by a thickness—shear mode transducer

The acoustic field generated by a thickness—shear mode transducer is theoretically analysed by an approximate model to find the origin of the longitudinal pulse in the signal received by the same transducer. It is believed to arise from the edge longitudinal wave.     

Wideband high-frequency line-focus PVDF transducer for materials characterization

This paper presents the design, fabrication, operating characteristics and applications of a wideband, high-frequency, line-focus beam transducer we constructed using a 9 microm thick piezoelectric polyvinylidene fluoride (PVDF) film. This transducer possesses a focal length of 2.38 mm and an aperture angle of 84 degrees. The frequency spectrum of the signal measured at the focal point indicates that the transducer has a wide frequency response which extends from 10 MHz to over 100 MHz. When compensated for the frequency-dependent attenuation of the coupling medium, the operational frequency exceeds 150 MHz. The transducer can be operated in a time-resolved pulse mode or in a radio-frequency (rf) tone burst mode. An application of the transducer to determine the anisotropic elastic property of a silicon wafer is demonstrated. The phase velocities of surface acoustic waves (SAW) propagating along various directions on the (001) surface of cubic silicon are measured and compared to computed values.     

Dispersion of an acoustic pulse passing through a large-grained polycrystalline film

Propagation of a short acoustic pulse through a polycrystalline film comprised of large randomly oriented elastically anisotropic grains is analyzed theoretically. For average grain size much larger than the film thickness, a short acoustic pulse launched normally into the film will traverse each grain in a time determined by the acoustic slowness in the direction normal to the film, which will depend on the local grain orientation. A typical measurement averages over a large number of grains resulting in the broadening of the composite output pulse. The resulting pulse shape is characterized by distinct features related to stationary values of the directionally dependent acoustic slowness of the crystalline material. Maxima and minima in the slowness yield discontinuities in the pulse shape, while saddle points yield logarithmic singularities. For cubic and hexagonal crystals, power law singularities result from cones of directions in which the slowness is a maximum or minimum. Numerical results, taking into account Gaussian broadening of the input pulse, are presented for thin film materials commonly encountered in picosecond ultrasonic experiments, such as copper, gold, and aluminum.     

Generation and detection of acoustic solitons in crystalline slabs by laser ultrasonics

Acoustic solitons have been recently observed in different systems (Si, Sapphire, MgO, alpha-quartz). Such acoustic waves could lead to sub-picosecond acoustic pulses. In this paper, we report on the formation of acoustic solitons in a GaAs crystalline slab. A short picosecond acoustic pulse is generated by absorption of a femtosecond laser pulse in an aluminum thin film deposited on one side of the slab. This strain pulse travels through the sample up to the opposite side where it is detected by a time delayed laser pulse reflected by an aluminum transducer. We use interferometric detection to measure independently the real and imaginary parts of the relative change in optical reflectivity induced by the acoustic pulse. We find that, at low temperature and with a laser pump pulse energy of 10 nJ, an acoustic soliton clearly separates from the acoustic pulse in GaAs slab. The soliton shape is compared with numerical simulations for different excitation conditions. From the very unique properties of solitons, we infer a soliton pulse duration of about 2.3 ps which corresponds to a spatial extent of only 12 nm.     

Characteristics of thermal acoustic radiation as a source of acoustic signals

A linear dependence of the output voltage of an acoustic thermometer on the temperature difference between the source and the piezoelectric transducer is demonstrated experimentally. The constant component of the output voltage is determined by the noise temperature of the receiving device. The main feature of the thermal acoustic radiation as a source of acoustic signals is that the signal is represented not by the total thermal radiation of the object, which is proportional to the absolute temperature of the latter, but by the part of this radiation that is proportional to the temperature difference between the object and the transducer.     

Development of an accelerometer-based underwater acoustic intensity sensor

An underwater acoustic intensity sensor is described. This sensor derives acoustic intensity from simultaneous, co-located measurement of the acoustic pressure and one component of the acoustic particle acceleration vector. The sensor consists of a pressure transducer in the form of a hollow piezoceramic cylinder and a pair of miniature accelerometers mounted inside the cylinder. Since this sensor derives acoustic intensity from measurement of acoustic pressure and acoustic particle acceleration, it is called a p-a intensity probe. The sensor is ballasted to be nearly neutrally buoyant. It is desirable for the accelerometers to measure only the rigid body motion of the assembled probe and for the effective centers of the pressure sensor and accelerometer to be coincident. This is achieved by symmetric disposition of a pair of accelerometers inside the ceramic cylinder. The response of the intensity probe is determined by comparison with a reference hydrophone in a predominantly reactive acoustic field.     

New acoustooptic effect: Constant high diffraction efficiency in wide range of acoustic power

A new effect, viz., acoustooptic Bragg diffraction without the overmodulation mode, in which the efficiency of the Bragg order attains its maximal value (close to 100%) upon an increase in the intensity of an acoustic wave and then remains practically unchanged, is predicted theoretically and observed experimentally. The effect takes place in the case of considerable bending of phase fronts of the acoustic field in the acoustooptic diffraction plane and attains its maximal value at a relatively low frequency of sound, a small width of a piezoelectric transducer, strong acoustic anisotropy of the medium, and a large distance between the light beam and the piezoelectric transducer.     

Improving temporal stability of stable cavitation activity of circulating microbubbles using a closed-loop controller based on pulse-length regulation

Stable cavitation (SC) has shown great potential for novel therapeutic applications. The spatiotemporal distribution of the SC activity of microbubbles circulating in a target region is not only correlated with the uniformity of treatment, but also with some undesirable effects. Therefore, it is important to achieve controllable and desirable SC activity in target regions for improved therapeutic efficiency and biosafety. This study proposes a closed-loop feedback controller based on pulse length (PL) regulation to improve the temporal stability of SC activity. Microbubbles circulating in a physiological flowing phantom were exposed to a 1 MHz focused transducer. The SC signals produced were initially received by another 7.5 MHz plane transducer, followed by high-speed signal acquisition and real-time processing. Based on the real-time-measured SC intensity excited by the current acoustic pulse, the proposed closed-loop feedback controller used three proportional coefficients to regulate the peak negative pressure (PNP) and PL of the next acoustic pulse during the acceleration and stable stages, respectively. The results show that the rise time and the temporal stability of the SC intensity of the microbubbles circulating in these two stages were improved significantly by the optimized proportional coefficients used in the proposed controller. Importantly, when compared with the traditional closed-loop feedback controller based on PNP regulation, the proposed closed-loop feedback controller based on PL regulation reduced the probability of a transition between stable and inertial cavitation, thus avoiding the risk of disadvantageous bioeffects in practical applications. These results demonstrate the effectiveness of the proposed PL-based closed-loop feedback controller and provide a feasible strategy for realization of controllable cavitation activity in applications.     

Experimental study of underwater transmission characteristics of high-frequency 30 MHz polyurea ultrasonic transducer

In this paper, we present the transmission characteristics of a polyurea ultrasonic transducer operating in water. In this study, we used a polyurea transducer with fundamental resonance at approximately 30 MHz. Firstly, acoustic pressure radiated from the transducer was measured using a hydrophone, which has a diameter of 0.2 mm. The transmission characteristics such as relative bandwidth, pulse width, and acoustic sensitivity were calculated from the experimental results. The results of the experiment showed a relative bandwidth of 50% and a pulse width of 0.061 μs. The acoustic sensitivity was 0.60 kPa/V with good linearity, where the correlation coefficient R in the fitting calculation was 0.996. A maximum pressure of 13.1 kPa was observed when the transducer was excited at a zero-to-peak voltage of 21 V. Moreover, we experimentally verified the results. The results of the pulse/echo experiment showed that the estimated diameters of the copper wires were 458 and 726 μm, where the differences between the actual and measured values were 15% and 4%, respectively. Acoustic streaming was also observed so that a particle velocity map was estimated by particle image velocimetry (PIV). The sound pressure calculated from the particle velocity obtained by PIV showed good agreement with the acoustic pressure measured using the hydrophone, where the differences between the calculated and measured values were 12–19%.