Internal deformation of a uniform elastic solid by acoustic radiation force

Tissue elasticity estimation is a growing area of ultrasound research. One proposed approach would apply acoustic radiation force to displace tissue and use ultrasonic motion tracking techniques to measure the resultant displacement. Such a technique might allow noninvasive imaging of tissue elastic properties. The potential of this method will be limited by the magnitude of displacements which can be generated at reasonable acoustic intensity levels. This paper presents methods for estimating the internal displacements induced in an elastic solid by acoustic radiation force. These methods predict displacements on the order of 400 microns in the human vitreous body, 0.008 micron in human breast, and 0.020 micron in human liver at an acoustic intensity of 1.0 W/cm2 (in water) and an operating frequency of 10 MHz. While the displacement generated in the vitreous should be readily detectable using ultrasonic methods, the displacements generated in the breast and liver will be much more difficult to detect. Methods are also developed for predicting the time dependent temperature increases associated with attenuated acoustic fields in the absence of perfusion. These results indicate promise for radiation force imaging in the vitreous, but potential difficulties in applying these techniques in other parts of the body.     

大张角共焦换能器对振动声成像的影响*

振动声成像是超声成像的一种重要形式,它可以得到包含共焦区组织的弹性信息和声衰减信息的信号,将接收到的信号用于成像即可获得反映组织特性的图像。该文对大张角共焦换能器作用下振动声成像中声辐射力和切变位移进行了理论计算和数值模拟,并通过改变张角变化及频率大小研究其对声辐射力和切变位移的影响。这项工作为大张角共焦换能器在振动声成像中的应用提供了理论支持。     

Intense acoustic bursts as a signal-enhancement mechanism in ultrasound-modulated optical tomography

Biophotonic imaging with ultrasound-modulated optical tomography (UOT) promises ultrasonically resolved imaging in biological tissues. A key challenge in this imaging technique is a low signal-to-noise ratio (SNR). We show significant UOT signal enhancement by using intense time-gated acoustic bursts. A CCD camera captured the speckle pattern from a laser-illuminated tissue phantom. Differences in speckle contrast were observed when ultrasonic bursts were applied, compared with when no ultrasound was applied. When CCD triggering was synchronized with burst initiation, acoustic-radiation-force-induced displacements were detected. To avoid mechanical contrast in UOT images, the CCD camera acquisition was delayed several milliseconds until transient effects of acoustic radiation force attenuated to a satisfactory level. The SNR of our system was sufficiently high to provide an image pixel per acoustic burst without signal averaging. Because of the substantially improved SNR, the use of intense acoustic bursts is a promising signal enhancement strategy for UOT.     

Doppler ultrasound detection of shear waves remotely induced in tissue phantoms and tissue in vitro

In shear wave elasticity imaging (SWEI), mechanical excitation within the tissue is remotely generated using radiation force of focused ultrasound. The induced shear strain is subsequently detected to estimate visco-elastic properties of tissue and thus aid diagnostics. In this paper, the mechanical response of tissue to radiation force was detected using a modified ultrasound Doppler technique. The experiments were performed on tissue mimicking and tissue containing phantoms using a commercial diagnostic scanner. This scanner was modified to control both the pushing and probing beams. The pushing beam was fired repetitively along a single direction while interlaced probing beams swept the surrounding region of interest to detect the induced motion. The detectability of inhomogeneous inclusions using ultrasonic Doppler SWEI method has been demonstrated in this study. The displacement fields measured in elastic phantoms clearly reveal the oscillatory nature of the mechanical relaxation processes in response to impulsive load due to the boundary effects. This relaxation dynamics was also present in cooked muscle tissue, but was not detected in more viscous and less elastic phantom and raw muscles. Presence of a local heterogeneity in the vicinity of the focal region of the pushing beam results in generation of a standing wave field pattern which is manifested in the oscillatory response of the excited region of the tissue. There has been made an assumption that dynamic characteristics of the relaxation process may be used for visualization of inhomogeneities.     

Monitoring of temperature variation in the focal region of an ultrasonic transducer

A method for remote monitoring of temperature in the focal region of a high-intensity ultrasonic transducer is described. Results of measurements and theoretical simulation are presented. The measurements were conducted on a polymer sample with thermophysical and acoustic parameters close to the properties of a soft biological tissue. The sample was heated by a focused piezoelectric transducer with different values of radiation power. The delay of a probe pulse transmitted through the heated region perpendicularly to the axis of the intense ultrasonic beam was detected. The local character of temperature measurements was provided by focusing the probe pulse at the heated region. The application of an additional transducer installed confocally with the probing one provided an opportunity to enhance the precision of measurements. An analysis was conducted on the basis of a numerical solution of the heat conduction equation. The function of thermal sources in the heat conduction equation was calculated according to the results of measuring the pressure distribution in the focal region of the heating transducer. The experimental data obtained agree well with the results of simulation and demonstrate a fundamental possibility of using the proposed ultrasonic technique for remote temperature measurements.     

Temporal analysis of tissue displacement induced by a transient ultrasound radiation force

One of the stress sources that can be used in dynamic elastography imaging methods is the acoustic radiation force. However, displacements of the medium induced by this stress field are generally not fully understood in terms of spatial distribution and temporal evolution. A model has been developed based on the elastodynamic Green's function describing the different acoustic waves generated by focused ultrasound. The function is composed of three terms: two far-field terms, which correspond to a purely longitudinal compression wave and a purely transverse shear wave, and a coupling near-field term which has a longitudinal component and a transverse component. For propagation distances in the shear wavelength range, the predominant term is the near field term. The displacement duration corresponds to the propagation duration of the shear wave between the farthest source point and the observation point. This time therefore depends on the source size and the local shear modulus of the tissue. Evolution of the displacement/time curve profile, which is directly linked to spatial and temporal source profiles, is computed at different radial distances, for different durations of force applications and different shear elastic coefficients. Experimental results performed with an optical interferometric method in a homogeneous tissue-mimicking phantom agreed with the theoretical profiles.     

利用k-Wave计算超声在腹壁组织的声辐射力*

随着科学技术的发展,声辐射力在生物医学领域得到了更为广泛的应用,尤其是在弹性成像领域。为了使弹性成像技术更加精准,对声辐射力的预测至关重要。该文基于腹壁组织图像,利用k-Wave对超声波在腹壁组织区域传播时的声场进行数值模拟,获得了其声场分布,进而求得了组织中声辐射力分布情况,同时对面阵换能器的阵元宽度、间距、阵元个数以及工作频率等参量对声辐射力的影响进行了计算与分析。结果表明,腹壁组织的声场分布受其非均匀性的影响,声辐射力的分布情况依赖于换能器参量的选择。该研究为声辐射力在弹性成像技术中的应用奠定了基础,为其技术的改进提供了重要依据。     

Simulations of lesion detection using a combined phased array LHMI-technique

Ultrasound based elasticity imaging techniques have been developed during the past decades. Some of these techniques are based on an internal radiation force stimulation in which a transient or dynamic radiation force is produced by using a single or dual-frequency sonication. In addition, sonication and data acquisition can be implemented using combined or separate transducers. In this simulation study of lesion detection using localized harmonic motion imaging (LHMI), we used a combined phased array designed for simultaneous thermal ablation and lesion detection. In the sonication mode, a focused single-frequency amplitude-modulated sonication is used to induce harmonic motion and in the tracking mode, some of the array elements are used for pulse-echo tracking of the induced displacements. The results showed that the size of the lesion affected the induced displacement around the sonication point. The displacement tracking simulations demonstrated that these changes in the displacement distributions can be detected using only a few of the array elements in the tracking mode but the exact size of the lesion can not be detected accurately. The simulations also showed that two lesions having the radius of 2.5mm can be distinguished if distance between these lesions is at least 2.5mm.     

Radiation force and shear motions in inhomogeneous media

An action of radiation force induced by ultrasonic beam in waterlike media such as biological tissues (where the shear modulus is small as compared to the bulk compressibility) is considered. A new, nondissipative mechanism of generation of shear displacement due to a smooth (nonreflecting) medium inhomogeneity is suggested, and the corresponding medium displacement is evaluated. It is shown that a linear primary acoustic field in nondissipative, isotropic elastic medium cannot excite a nonpotential radiation force and, hence, a shear motion, whereas even smooth inhomogeneity makes this effect possible. An example is considered showing that the generated displacement pulse can be significantly longer than the primary ultrasound pulse. It is noted that, unlike the dissipative effect, the nondissipative action on a localized inhomogeneity (such as a lesion in a tissue) changes its sign along the beam axis, thus stretching or compressing the focus area.     

Feasibility of using Nakagami distribution in evaluating the formation of ultrasound-induced thermal lesions

The acoustic posterior shadowing effects of bubbles influence the accuracy for defining the location and range of ablated thermal lesions during focused ultrasound surgery when using ultrasonic monitoring imaging. This paper explored the feasibility of using Nakagami distribution to evaluate the ablated region induced by focused ultrasound exposures at different acoustic power levels in transparent tissue-mimicking phantoms. The mean value of the Nakagami parameter m was about 0.5 in the cavitation region and increased to around 1 in the ablated region. Nakagami images were not subject to significant shadowing effects of bubbles. Ultrasound-induced thermal lesions observed in the photos and Nakagami images were overshadowed by bubbles in the B-mode images. The lesion size predicted in the Nakagami images was smaller than that predicted in the photos due to the sub resolvable effect of Nakagami imaging at the interface. This preliminary study on tissue-mimicking phantom suggested that the Nakagami parameter m may have the potential use in evaluating the formation of ultrasound-induced thermal lesion when the shadowing effect of bubbles is strong while the thermal lesion was small. Further studies in vivo and in vitro will be needed to evaluate the potential application.     

Harmonic pulsed excitation and motion detection of a vibrating reflective target

Elasticity imaging is an emerging medical imaging modality. Methods involving acoustic radiation force excitation and pulse-echo ultrasound motion detection have been investigated to assess the mechanical response of tissue. In this work new methods for dynamic radiation force excitation and motion detection are presented. The theory and model for harmonic motion detection of a vibrating reflective target are presented. The model incorporates processing of radio frequency data acquired using pulse-echo ultrasound to measure harmonic motion with amplitudes ranging from 100 to 10,000 nm. A numerical study was performed to assess the effects of different parameters on the accuracy and precision of displacement amplitude and phase estimation and showed how estimation errors could be minimized. Harmonic pulsed excitation is introduced as a multifrequency radiation force excitation method that utilizes ultrasound tonebursts repeated at a rate f(r). The radiation force, consisting of frequency components at multiples of f(r), is generated using 3.0 MHz ultrasound, and motion detection is performed simultaneously with 9.0 MHz pulse-echo ultrasound. A parameterized experimental analysis showed that displacement can be measured with small errors for motion with amplitudes as low as 100 nm. The parameterized numerical and experimental analyses provide insight into how to optimize acquisition parameters to minimize measurement errors.     

Comparison of laser interferometry and radiation force method of measuring ultrasonic power

The aim of this paper is to compare two different methods for the calculation of the ultrasonic output power of underwater transducers: the radiation force balance, which is the standard method, and the laser heterodyne interferometry, which is rather used to depict displacement or velocity distributions of the acoustic field. Here it is shown that the latter can also be used to calculate the acoustic time-average power with an uncertainty of about 22%, the radiation force balance giving an uncertainty of 12% (with 95% confidence). The interferometry experiments performed with two transducers working at 2.25 MHz and 8.25 MHz showed that they produce different acoustic fields (respectively Gaussian and Lorentz-sigmoidal distributions). Taking into account the acoustic field profiles, the acoustic time-average power from interferometry was calculated. It was found very similar to the time-average power measured with the radiation force balance in the plane-wave assumption.     

Precise micro-particle and bubble manipulation by tunable ultrasonic bottle beams

This paper reports a method to generate tunable bottle beams using an ultrasonic lens, by which the bottle position can be precisely adjusted with the change of the acoustic frequency. Therefore, the position of a single particle or bubble in liquid can be manipulated without using phased array which is costly and huge with complex circuits. Furthermore, we introduced this method to multiple bubble manipulation using acoustic holography. The bottle properties against frequency are theoretically and experimentally analyzed. It is shown that the bottle position depends almost linearly on the operating frequency, which provides a basis for the precise manipulation of bubbles and particles. In addition, the relationship between the acoustic radiation force and the drag force under different incident acoustic pressures is considered, establishing a limit on the moving velocity of the trapped particles. The ultrasonic field observation is further demonstrated by Schlieren imaging system. The proposed method has potential biomedical applications, such as more flexible cell manipulation and targeted drug delivery in vivo, as well as potential applications in the study of chemical reactions between micro objects.     

Theoretical Analysis of Shear Wave Interference Patterns by Means of Dynamic Acoustic Radiation Forces

Acoustic radiation forces associated with high intensity focused ultrasound stimulate shear wave propagation allowing shear wave speed and shear viscosity estimation of tissue structures. As wave speeds are meters per second, real time displacement tracking over an extend field-of-view using ultrasound is problematic due to very high frame rate requirements. However, two spatially separated dynamic external sources can stimulate shear wave motion leading to shear wave interference patterns. Advantages are shear waves can be imaged at lower frame rates and local interference pattern spatial properties reflect tissue's viscoelastic properties. Here a theoretical analysis of shear wave interference patterns by means of dynamic acoustic radiation forces is detailed. Using a viscoelastic Green's function analysis, tissue motion due to a pair of focused ultrasound beams and associated radiation forces are presented. Overall, this paper theoretically demonstrates shear wave interference patterns can be stimulated using dynamic acoustic radiation forces and tracked using conventional ultrasound imaging.     

Application of pulse acoustic microscopy technique for 3D imaging bulk microstructure of carbon fiber-reinforced composites

Impulse acoustic microscopy technique is applied for 3D imaging of bulk microstructure of composite materials. Short pulses of focused high-frequency ultrasound have been employed for layer-by-layer imaging of internal microstructure of carbon fiber-reinforced composite (CFRC) laminates. The method provides spatial resolution of 60 microm and in-depth resolution of 80 microm, approximately. Echo signals reflected from structural units--plies, fiber bundles; and microflaws form acoustic images of microstructure at different depth inside samples. The images make it possible to see ply arrays, packing of bundles in plies; binding material distribution over the specimen body. They reveal failure of interply adhesion, buckling of single plies and fiber bundles, internal defoliations and disbonds, voids in the specimen body. The series of successive images offer outstanding possibilities to reconstruct the bulk structure, to estimate local variations of properties, topological and geometrical characteristics of structural components. The imaging technique has been applied to study different types of fiber packing--unidirectional, cross-ply and woven laminates. Mechanisms of ultrasonic contrast for diverse elements in acoustic images of CFRC laminate bulk microstructure and structural defects are discussed.     

Ultrasonic strain imaging and reconstructive elastography for biological tissue

Mechanical properties of biological tissue represent important diagnostic information and are of histological and pathological relevance. In order to obtain non-invasively mechanical properties of tissue, we developed a real-time strain imaging system for clinical applications. The output data of this system also allow an inverse elastography approach leading to the spatial distribution of the relative elastic modulus of tissue. The internal displacement field of biological tissue is determined using the above mentioned strain imaging system by applying quasi-static compression to the considered tissue. Axial displacements are calculated by comparing echo signal sets obtained prior to and immediately following less than 0.1% compression, using the fast root seeking technique. Strain images representing mechanical tissue properties in a non-quantitative manner are displayed in real-time mode. For additional quantitative imaging, the stiffness distribution is calculated from the displacement field assuming the investigated material to be elastic, isotropic, and nearly incompressible. Different inverse problem approaches for calculating the shear modulus distribution using the internal displacement field have been implemented and compared. The results of an ongoing clinical study with more than 200 patients show, that our real-time strain imaging system is able to differentiate malignant and benign tissue areas in the prostate with a high degree of accuracy (sensitivity=76% and specificity=89%). The reconstruction approaches applied to the strain image data deliver quantitative tissue information and seem promising for an additional differential diagnosis of lesions in biological tissue. Our real-time system has the potential of improving diagnosis of prostate and breast cancer.     

Quantitative measurement of local elasticity of SiOx film by atomic force acoustic microscopy

<正>In this paper the elastic properties of SiO_x film are investigated quantitatively for local fixed point and qualitatively for overall area by atomic force acoustic microscopy(AFAM) in which the sample is vibrated at the ultrasonic frequency while the sample surface is touched and scanned with the tip contacting the sample respectively for fixed point and continuous measurements.The SiO_x films on the silicon wafers are prepared by the plasma enhanced chemical vapour deposition(PECVD).The local contact stiffness of the tip-SiO_x film is calculated from the contact resonance spectrum measured with the atomic force acoustic microscopy.Using the reference approach,indentation modulus of SiO_x film for fixed point is obtained.The images of cantilever amplitude are also visualized and analysed when the SiO_x surface is excited at a fixed frequency.The results show that the acoustic amplitude images can reflect the elastic properties of the sample.     

Transient displacement induced in shear wave elastography: comparison between analytical results and ultrasound measurements

It is now accepted that an effective way to investigate the elastic properties of soft tissues is to generate a localized transient acoustic radiation force and to follow the associated displacements in the time/space domain. Shear waves induced by this stress field are particularly interesting in this kind of medium because they are governed by the shear elastic modulus mu, which is directly linked to the Young modulus, and spatial distribution and temporal evolution of the transient motion induced must therefore be obtained in detail. We report here a model based on the elastodynamic Green's function formalism to describe these displacements. 3D simulation of radiation force in homogenous elastic media was performed and the displacement curves computed at different radial distances for different temporal force profiles. Amplitude and duration of displacement were found to be reliable parameters to characterize the elastic properties of the medium. Experimental measurements were performed in a homogeneous agar-gelatin tissue-mimicking phantom, and two transducers were used to generate the radiation force and follow the induced displacements. Displacements obtained from different lateral locations around the applied force axis were then used to reconstruct the shear-wave propagation in a scan plane as a function of time. The experimental displacements/curves agreed with the theoretical profiles obtained by the elastodynamic Green's function formalism.     

Estimating localized oscillatory tissue motion for assessment of the underlying mechanical modulus

The technique of harmonic motion imaging (HMI) uses the localized stimulus of the oscillatory ultrasonic radiation force as produced by two overlapping beams of distinct frequencies, and estimates the resulting harmonic displacement in the tissue in order to assess its underlying mechanical properties. In this paper, we studied the relationship between measured displacement and stiffness in gels and tissues in vitro. Two focused ultrasound transducers with a 100 mm focal length were used at frequencies of 3.7500 MHz and either 3.7502 (or 3.7508 MHz), respectively, in order to produce an oscillatory motion at 200 Hz in the gel or tissue. A 1.1 MHz diagnostic transducer (Imasonics, Inc.) was also focused at 100 mm and acquired 5 ms RF signals (pulse repetition frequency (PRF)=3.5 kHz) at 100 MHz sampling frequency during radiation force application. First, three 50x50 mm(2) acrylamide gels were prepared at concentrations of 4%, 8% and 16%. The resulting displacement was estimated using crosscorrelation techniques between successively acquired RF signals with a 2 mm window and 80% window overlap at 1260 W/cm(2). A normal 1-D indentation instrument (TeMPeST) applied oscillatory loads at 0.1-200 Hz with a 5 mm-diameter flat indenter. Then, 12 displacement measurements in 6 porcine muscle specimens (two measurements/case, as above) were made in vitro, before and after ablation which was performed for 10 s at 1260 W/cm(2). In all gel cases, the harmonic displacement was found to linearly increase with intensity and exponentially decrease with gel concentration. The TeMPeST measurements showed that the elastic moduli for the 4%, 8% and 16% gels equaled 3.93+/-0.06, 17.1+/-0.2 and 75+/-2 kPa, respectively, demonstrating that the HMI displacement estimate depends directly on the gel stiffness. Finally, in the tissues samples, the mean displacement amplitude showed a twofold decrease between non-ablated and ablated tissue, demonstrating a correspondence between the HMI response and an increase in stiffness measured with the TeMPeST instrument.