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Shear wave elasticity imaging (SWEI), an emerging acoustic technology for medical diagnostics, is based on remote generation of shear waves in tissue by radiation force in the focal region of an ultrasonic beam. In this study, the feasibility of Doppler ultrasonic technique to visualize the remotely induced shear waves was demonstrated. The generation of shear displacement in the focal region of a pulsed 1-MHz ultrasound beam with pulse duration of approximately about 2 ms and intensity levels on the order of 145 W/cm2, and consequent propagation of shear wave in tissue-mimicking and muscle tissue in vitro, were measured. The analysis of temporal behavior of shear displacement within the focal plane allowed estimation of shear wave velocities. The velocities were 4 and 7 m/s in hard phantom and tissue containing phantom, respectively. The measured shear displacements on the order of micrometers in gel-based phantoms are in reasonable agreement with theoretical estimates derived from an earlier developed model of shear wave generation by radiation force of focused ultrasound. The study revealed significant dependence of shear strain on the medium viscosity. The complex oscillatory character of shear strain relaxation in viscoelastic phantom and muscle tissue in vitro was observed.

In this work the spatial resolution and measurement accuracy of the ultrasound diagnostic system at acoustic remote palpation (ARP) using high-intensity focusing ultrasound (HIFU) are studied theoretically and experimentally. A physical model is proposed, which describes the specific features of ARP taking into account the remote nature of ultrasound Doppler probing of the soft tissues local movements, which are caused by the radiation pressure of HIFU pulse. Taking into account the accepted simplifying assumptions it is shown that the model conclusions are in a good agreement with the results of the experiments on measuring the value of displacements under the influence of HIFU. In particular, the nontrivial dependence of the value of displacements, measured by the Doppler method, on the probing depth and focusing degree of the incident and scattered wave beams, is proved. An experimental study was performed on the transverse resolution at ARP in the case of probing of the medium with Young's modulus irregularity, as well as on the influence of noise and interference on the measurement accuracy and resolution. It is concluded, that the transverse resolution at ARP is determined by the parameters of the local area of the movement, and can be significantly higher than the transverse intrinsic resolution of the ultrasound system at B-mode of diagnostics. The obtained results indicate that ARP is a promising method for monitoring the process of the soft tissues thermal ablation, when HIFU is used.

Virtually all modem ultrasonic imaging devices are based on the principle of B-scanning [1]. To obtain high lateral resolution of B-scanning, modem medical diagnostic echo-pulse scanners are equipped with coherent sources of ultrasonic radiation. However, this is accompanied by speckle noise, which spoils the high resolution advantages of coherent radiation [10]. This is explained by the fact that the signal reflected from a continuous medium contains both a coherent component proportional to the local coefficient of reverse scattering and a component determined by a quasi-random distribution of reflectors within the measuring volume formed by the probing pulse. The speckle structure of the image (gain size) appears when individual scattering structures are not resolved, and resulting interference of scattered waves (with random phase) causes fluctuations of resulting high-contrast image intensity. In ultrasonic tomography, such high-contrast noise masks useful structural information of images. This is particularly true in case of small-sized objects.There is now a large number of approaches to the elimination of speckle noise which are based on sophisticated methods of scanning (multi-aspect or multi-frequency) [1, 2, 10]. The physical basis of these methods of processing the echosignals is to obtain several uncorrelated copies of the image of interest and their further averaging intended to form the sought image. The specific digital algorithm is either arithmetic or geometric averaging, an algorithm of minimization, etc., is selected taking into account the efficiency of reduction of the coherence of imaging [6]. However, practical implementation of these algorithms is often difficult because it requires sophisticated scanning and processing equipment [5, 6, 8, 9]. This is particularly true in the case of real-time imaging (RTI) of moving objects, because ff the time of each measurement is large, the resulting image is blurred, lnterframe filtration (averaging by two or three sequential frames) is a compromise solution, because speckle signals are poorly correlated with changes in the measuring volume.The problem of optimal selection between high spatial resolution of diagnostic system and statistical accuracy of attribution of the tissue tested to a given histological class is very important because of statistical uncertainty introduced by the speckle noise artifacts. The importance of this issue has been discussed in the literature [1, 2, 13]. The human visual system responds mainly to the statistical characteristics of an image of the ftrst and second order [2], whereas the use of these orders of the reflected signal statistics allows the relative contributions of both interference and noninterference factors of the object of interest to be determined [13].The approach to elimination of speckle noise of echo-images described in [2, 3] appears very promising. This approach is based on the physical interpretation of the causes of true and false signals. This approach is close to the method of adaptive filtrat...