Bubble-based acoustic radiation force elasticity imaging

Acoustic radiation force is applied to bubbles generated by laser-induced optical breakdown (LIOB) to study viscoelastic properties of the surrounding medium. In this investigation, femtosecond laser pulses are focused in the volume of gelatin phantoms of different concentrations to form bubbles. A two-element confocal ultrasonic transducer generates acoustic radiation force on individual bubbles while monitoring their displacement within a viscoelastic medium. Tone burst pushes of varying duration have been applied by the outer element at 1.5 MHz. The inner element receives pulse-echo recordings at 7.44 MHz before, during, and after the excitation bursts, and crosscorrelation processing is performed offline to monitor bubble position. Maximum bubble displacements are inversely related to the Young's moduli for different gel phantoms, with a maximum bubble displacement of over 200 microm in a gel phantom with a Young's modulus of 1.7 kPa. Bubble displacements scale with the applied acoustic radiation force and displacements can be normalized to correct for differences in bubble size. Exponential time constants for bubble displacement curves are independent of bubble radius and follow a decreasing trend with the Young's modulus of the surrounding medium. These results demonstrate the potential for bubble-based acoustic radiation force methods to measure tissue viscoelastic properties.     

Rapid method for assessing relative tissue stiffness using MR acoustic radiation force imaging

Acoustic radiation force imaging (ARFI) has been suggested as a tool for remote palpation. In this study an MR‐ARFI sequence, based on echo‐planar‐imaging, is introduced, for remote semi‐quantitative assessment of local tissue stiffness. The focal zone of a high intensity focused ultrasound (HIFU) is positioned at the region of interest and a single HIFU burst is transmitted. The method then measures the entire time integral of the resulting displacement at the focal zone. Combining this measurement with the Kelvin–Voigt viscoelastic tissue model, a local stiffness index is obtained. The method was implemented on gel phantoms, ex‐vivo bovine brain and chicken liver specimens. The results have demonstrated the ability to evaluate the relative local stiffness within 600 ms and to distinguish between different tissues on the basis of their stiffness index. The method may potentially be used for remote palpation of suspicious regions for diagnostic purposes, or for providing a mechanical feedback during therapeutic procedures, such as thermal ablation.     

Assessing and improving acoustic radiation force image quality using a 1.5-D transducer design

A 1.5-D transducer array was proposed to improve acoustic radiation force impulse (ARFI) imaging signal-to-noise ratio (SNRARFI) and image contrast relative to a conventional 1-D array. To predict performance gains from the proposed 1.5-D transducer array, an analytical model for SNRARFI upper bound was derived. The analytical model and 1.5-D ARFI array were validated using a finite element modelbased numerical simulation framework. The analytical model demonstrated good agreement with numerical results (correlation coefficient = 0.995), and simulated lesion images yielded a significant (2.92 dB; p < 0.001) improvement in contrast-tonoise ratio when rendered using the 1.5-D ARFI array.     

A novel motion compensation algorithm for acoustic radiation force elastography

A novel method of physiological motion compensation for use with radiation force elasticity imaging has been developed. The method utilizes a priori information from finite element method models of the response of soft tissue to impulsive radiation force to isolate physiological motion artifacts from radiation force-induced displacement fields. The new algorithmis evaluated in a series of clinically realistic imaging scenarios, and its performance is compared to that achieved with previously described motion compensation algorithms. Though not without limitations, the new model-based motion compensation algorithm performs favorably in many circumstances and may be a logical choice for use with in vivo abdominal imaging.     

A parallel tracking method for acoustic radiation force impulse imaging

Radiation force-based techniques have been developed by several groups for imaging the mechanical properties of tissue. Acoustic Radiation Force Impulse (ARFI) imaging is one such method that uses commercially available scanners to generate localized radiation forces in tissue. The response of the tissue to the radiation force is determined using conventional B-mode imaging pulses to track micron-scale displacements in tissue. Current research in ARFI imaging is focused on producing real-time images of tissue displacements and related mechanical properties. Obstacles to producing a real-time ARFI imaging modality include data acquisition, processing power, data transfer rates, heating of the transducer, and patient safety concerns. We propose a parallel receive beamforming technique to reduce transducer heating and patient acoustic exposure, and to facilitate data acquisition for real-time ARFI imaging. Custom beam sequencing was used with a commercially available scanner to track tissue displacements with parallel-receive beamforming in tissue-mimicking phantoms. Using simulations, the effects of material properties on parallel tracking are observed. Transducer and tissue heating for parallel tracking are compared to standard ARFI beam sequencing. The effects of tracking beam position and size of the tracked region are also discussed in relation to the size and temporal response of the region of applied force, and the impact on ARFI image contrast and signal-to-noise ratio are quantified.     

Assessment of shear modulus of tissue using ultrasound radiation force acting on a spherical acoustic inhomogeneity

An ultrasound-based method to locally assess the shear modulus of a medium is reported. The proposed approach is based on the application of an impulse acoustic radiation force to an inhomogeneity in the medium and subsequent monitoring of the spatio-temporal response. In our experimental studies, a short pulse produced by a 1.5-MHz highly focused ultrasound transducer was used to initiate the motion of a rigid sphere embedded into an elastic medium. Another 25 MHz focused ultrasound transducer operating in pulse-echo mode was used to track the displacement of the sphere. The experiments were performed in gel phantoms with varying shear modulus to demonstrate the relationship between the displacement of the sphere and shear modulus of the surrounding medium. Because the magnitude of acoustic force applied to sphere depends on the acoustic material properties and, therefore, cannot be used to assess the absolute value of shear modulus, the temporal behavior of the displacement of the sphere was analyzed. The results of this study indicate that there is a strong correlation between the shear modulus of a medium and spatio-temporal characteristics of the motion of the rigid sphere embedded in this medium.     

声辐射力脉冲弹性成像技术对大鼠肝脏脂肪变性的诊断研究

研究目的是利用声辐射力脉冲弹性成像技术(Acoustic Radiation Force Impulse,ARFI)评估大鼠肝脏的脂肪变性程度。实验通过对大鼠喂养不同时间高脂食物从而诱导成肝脏脂肪变性不同阶段,进而利用ARFI技术离体测量大鼠肝脏的剪切波速度值(Shear Wave Velocity,SWV)。研究结果显示,对照组[(2.25±0.52)m/s)]与脂肪变性组[(2.83±0.37)m/s]之间存在着显著性差异(P0.05),而脂肪变性不同阶段之间差异却不明显。早期炎症等级与SWV值之间没有显著性相关。因此,ARFI弹性成像技术可以有效地区分大鼠肝脏正常组和脂肪变性组,但是对不同脂肪变性程度之间无法做出有效区分。同时,轻微的炎症活动并不影响ARFI测量。     

Remote detection of acoustic boundaries using radiation imagingoperators

In this paper, we present an acoustic imaging operator. This operator is based upon combining the material boundary conditions at an acoustic boundary and the radiation boundary conditions associated with one way wave propagation. Numerical examples, using the second-order imaging operator, are presented in order to demonstrate the applicability of this method to the detection of two-dimensional boundaries     

Method for microbubble characterization using primary radiation force

Medical ultrasound contrast agents (UCAs) have evolved from straight image enhancers to pathophysiological markers and drug delivery vehicles. However, the exact dynamic behavior of the encapsulated bubbles composing UCAs is still not entirely known. In this article, we propose to characterize full populations of UCAs, by looking at the translational effects of ultrasound radiation force on each bubble in a diluted population. The setup involves a sensitive, fully programmable transmitter/receiver and two unconventional, real-time display modes. Such display modes are used to measure the displacements produced by irradiation at frequencies in the range 2-8 MHz and pressures between 150 kPa and 1.5 MPa. The behavior of individual bubbles freely moving in a water tank is clearly observed, and it is shown that it depends on the bubble physical dimensions as well as on the viscoelastic properties of the encapsulation. A new method also is distilled that estimates the viscoelastic properties of bubble encapsulation by fitting the experimental bubble velocities to values simulated by a numerical model based on the modified Herring equation and the Bjerknes force. The fit results are a shear modulus of 18 MPa and a viscosity of 0.23 Pas for a thermoplastic PVC-AN shell. Phospholipid shell elasticity and friction parameter of the experimental contrast agent are estimated as 0.8 N/m and 1 10(-7) kg/s, respectively (shear modulus of 32 MPa and viscosity of 0.19 Pas, assuming 4-nm shell thickness).     

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.     

Dynamic mechanical response of elastic spherical inclusions to impulsive acoustic radiation force excitation

Acoustic radiation force impulse imaging has been used clinically to study the dynamic response of lesions relative to their background material to focused, impulsive acoustic radiation force excitations through the generation of dynamic displacement field images. Dynamic displacement data are typically displayed as a set of parametric images, including displacement immediately after excitation, maximum displacement, time to peak displacement, and recovery time from peak displacement. To date, however, no definitive trends have been established between these parametric images and the tissues' mechanical properties. This work demonstrates that displacement magnitude, time to peak displacement, and recovery time are all inversely related to the Young's modulus in homogeneous elastic media. Experimentally, pulse repetition frequency during displacement tracking limits stiffness resolution using the time to peak displacement parameter. The excitation pulse duration also impacts the time to peak parameter, with longer pulses reducing the inertial effects present during impulsive excitations. Material density affects tissue dynamics, but is not expected to play a significant role in biological tissues. The presence of an elastic spherical inclusion in the imaged medium significantly alters the tissue dynamics in response to impulsive, focused acoustic radiation force excitations. Times to peak displacement for excitations within and outside an elastic inclusion are still indicative of local material stiffness; however, recovery times are altered due to the reflection and transmission of shear waves at the inclusion boundaries. These shear wave interactions cause stiffer inclusions to appear to be displaced longer than the more compliant background material. The magnitude of shear waves reflected at elastic lesion boundaries is dependent on the stiffness contrast between the inclusion and the background material, and the stiffness and size of the inclusion dictate when shear wave reflections within the lesion will interfere with one another. Jitter and bias associated with the ultrasonic displacement tracking also impact the estimation of a tissue's dynamic response to acoustic radiation force excitation.     

Langevin acoustic radiation force of a high-order bessel beam on a rigid sphere

The acoustic radiation force of Langevin type resulting from the interaction of a high-order Bessel beam with a rigid immovable sphere in an ideal fluid is theoretically investigated. The analysis is based on applying the generalized Rayleigh series used in the near-field acoustic scattering problem to calculate the force. With appropriate selection of specific Bessel beam parameters, results for the rigid sphere unexpectedly reveal a negative radiation force caused by the Lagrangean energy density. Specifically, the negative force on the rigid sphere arises when the kinematic energy density is larger than the potential energy density. This condition provides an impetus for further designing acoustic tweezers operating with high-order Bessel beams of progressive waves for potential applications in particle entrapment and manipulation.     

二维阵列换能器声辐射力分布的计算分析

利用角谱理论得到了圆形活塞换能器阵元组阵后作用在平面悬浮物体上的声辐射力分布公式。通过数值仿真,分析了换能器频率、阵元间距以及阵元数目对声辐射力分布的影响。计算结果表明,换能器组阵使得声辐射力分布的指向性变窄,强度增强;随着换能器频率的提高、阵元间距的增大以及阵元数目的增多,声辐射力分布的主瓣更尖锐,但阵元间距的增大会使声辐射力分布的旁瓣增高。为了改善声辐射力的空间分布,采用伪逆矩阵算法,以能量增益为目标函数,通过调节换能器阵元表面振动速度的幅值和相位来形成多焦点的声辐射力分布,为阵列换能器声辐射力分布的调控和声悬浮稳定性的研究提供帮助。     

Estimates of echo correlation and measurement bias in acoustic radiation force impulse imaging

Acoustic radiation force impulse (ARFI) imaging is a novel imaging modality in which pulses from a diagnostic ultrasound scanner are used to displace tissue and track its motion. The region displaced has lateral and elevational dimensions of similar scale to the ultrasound beams used to track the motion. Therefore, there is a range of tissue displacements present within the tracking beam, leading to decorrelation of the echo signal. Expressions are derived for the expected value of the displacement estimate and the cross-correlation at the expected displacement. Numerical simulations confirm the analytical model.     

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。     

Efficient evaluation of the acoustic radiation using multipole expansion

The variational boundary element method (VBEM) is widely used to compute the acoustic radiation of structures. The classical numerical implementation of the VBEM suffers from the computational cost associated with double surface integration. In a previous paper [1], the authors proposed a novel method, based on multipole expansions, to accelerate the double layer potential calculation for structures having a periodic mesh. This technique, while efficient, is still limited by the cost of computing the surface pressure from the double surface potential. This paper presents an acceleration technique, based on multipole expansion, that allies both efficiency and accuracy. Copyright © 1999 John Wiley & Sons, Ltd.     

Lower-limb vascular imaging with acoustic radiation force elastography: Demonstration of in vivo feasibility

Acoustic radiation force impulse (ARFI) imaging characterizes the mechanical properties of tissue by measuring displacement resulting from applied ultrasonic radiation force. In this paper, we describe the current status of ARFI imaging for lower-limb vascular applications and present results from both tissue-mimicking phantoms and in vivo experiments. Initial experiments were performed on vascular phantoms constructed with polyvinyl alcohol for basic evaluation of the modality. Multilayer vessels and vessels with compliant occlusions of varying plaque load were evaluated with ARFI imaging techniques. Phantom layers and plaque are well resolved in the ARFI images, with higher contrast than B-mode, demonstrating the ability of ARFI imaging to identify regions of different mechanical properties. Healthy human subjects and those with diagnosed lower-limb peripheral arterial disease were imaged. Proximal and distal vascular walls are well visualized in ARFI images, with higher mean contrast than corresponding B-mode images. ARFI images reveal information not observed by conventional ultrasound and lend confidence to the feasibility of using ARFI imaging during lower-limb vascular workup.     

Three-dimensional acoustic radiation force of a eukaryotic cell arbitrarily positioned in a Gaussian beam

Expressions are derived for calculating the three-dimensional acoustic radiation force(ARF) on a multilayer microsphere positioned arbitrarily in a Gaussian beam. A theoretical model of a three-layer microsphere with a cell membrane, cytoplasm, and nucleus is established to study how particle geometry and position affect the three-dimensional ARF, and its results agree well with finite-elemen numerical results.The microsphere can be moved relative to the beam axis by changing its structure and p...     

Full-circular surface acoustic wave excitation for high resolution acoustic microscopy using spherical lens and time gate technology

With a fixed gate width under the condition where the focus of an acoustic lens was set inside the sample, we varied signal taking-in time. Discrimination was made between differences in time required for an ultrasonic signal reflected from the sample to reach the acoustic lens. This process also enabled three types of images to be obtained separately: the surface reflection wave image, a combination of images based on the interference of the surface reflection wave with surface acoustic waves, and the surface acoustic wave image. Thus it was presumed that this process also would reveal the causes of image contrast and allow an easy interpretation of images. Furthermore, the image resolution was improved, because the surface acoustic wave image was drawn by an ultrasonic beam produced by full-circular surface acoustic wave excitation propagating toward the center converging concentrically; the theoretical resolution was 0.4 times the value of the surface acoustic wave wavelength lambda(R) and independent of the defocus value of the acoustic lens. Several kinds of samples were observed with this method. The results showed that the new method permitted observation of the internal structures of samples while offering new knowledge through the data reflecting the ultrasonic wave damping and scatter drawn on the display.