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.

The influence of viscosity on the shear strain remotely induced by focused ultrasound in viscoelastic media

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.

The estimation of elasticity and viscosity of soft tissues in vitro using the data of remote acoustic palpation

The presented study revealed the significant dependence of displacement magnitude and strain relaxation on phantom elasticity and viscosity. It has been shown that simultaneous analysis of temporal behavior and magnitude of shear strain induced by the radiation force of focused ultrasonic beam gives the necessary data for quantitative estimation of tissue shear modulus because of the known functional dependencies of displacement on local viscosity and elasticity. As a result, the simplest calibration procedure of acoustic radiation force-based methods is performed and algorithm for separate reconstruction of tissue elasticity and viscosity is proposed. These findings were tested, in particular, using the data obtained for specially prepared phantoms containing calf liver and muscle tissue in vitro. The observed complex character of shear strain relaxation and noise in some tissue phantoms and tissues in vitro reduces the preciseness of viscoelastic properties estimation.

Pulsed Doppler flow-line spectrum for focused transducers with apodized apertures

The properties of the single flow-line Doppler spectrum using pulsed wave (PW) ultrasound is studied on the basis of previously developed spectral theory for transducers with apodized apertures. It has been shown previously that the spectral width of Doppler signals from a sample volume in low velocity-shear flow is independent of the sample volume depth but that is not true for the spectra from the individual streamlines. The work presented here on the Doppler flow-line spectrum shows that its width should be invariant with flow-line location, if the sample volume depth is fixed. At the same time, for a transducer operating in PW mode not only the Doppler spectral width depends on the sample volume depth, but also the modal Doppler frequency shift changes with flow-line displacement in the illuminating field except if the sample volume centre and the beam focus coincide. The variation of modal Doppler frequency shift is the more explicit manifestation of the effect of wavefront curvature increasing for lines and sounding depths distant from the focal point. The values of the Doppler shift and spectral bandwidth are reported taking account of beam diffraction and variations in its geometry due to focusing.

Ultrasound Doppler monitoring of soft tissues in vitro and tissue phantoms heating and thermal destruction induced by acoustic remote palpation

The rise of shear strain value under temperature increase in biological tissue samples in vitro and tissue phantoms was studied and the range of shear modulus and viscosity calculated. It has been shown that the acoustic radiation force-based methods with the usage of ultrasound Doppler probing provides the potential ability of noninvasive real-time monitoring of tissues' ultrasound thermal destruction process. At that, the thermal destruction is possible under action of wave beam that creates the radiation force and local tissue displacements so that tissue ablation and acoustic remote palpation could be realized by means of the same ultrasound transducer. The experiments were performed using gelatin-based tissue-mimicking phantoms and freshly excised samples of bovine muscle tissue. It was determined also that fluctuating pattern of detected displacement amplitude variation is the indicator of the phase transitions beginning in the heated field of soft tissue or tissue phantom.

Correlation functions and power spectra of Doppler response signals in ultrasonic medical applications

Ultrasound Doppler methods are widely used in clinical practice as prospective investigational tool to study the vascular system and soft biological tissues. Meanwhile, the most general relationship between the power Doppler spectra, spectral characteristics of the scattering fluctuations and the probing ultrasound field parameters for some clinical implementations are still unexplored. Based upon the continuum model of scattering inhomogeneities, a set of the closed-form expressions for the correlation functions and the spectra of Doppler response of soft tissues and blood have been derived. The influence of the correlation among inhomogeneities and the diffusion processes on the Doppler power spectra formed by stationary flows have been examined. Computer simulations of Doppler spectra were performed for different values of correlation radius and diffusion coefficient. With simulation results the effects of the correlation among inhomogeneities and the diffusion processes on the spectral width and mean frequency are established and discussed in respect to turbulent flows. Closed-form expressions for correlation functions and Doppler spectra for the vibrational sonoelastography technique for visualizing malignant tumors in tissues have been derived. Based on the peculiarities of the obtained Doppler spectra, it is shown that the differentiation of soft tissues with respect to the amplitude value of constrained oscillations is feasible. The expressions were derived for the cases of non-stationary accelerated blood movement. It has been found that the frequency dependence reveals solely at a finite time of observation and depends on the initial phase of the accelerated movement.

[Study of bromthymol blue binding with plasma membranes]

It has been established that sorption of bromthymol blue by membranes is due to interaction with integral proteins. The value of the bond is ascribed both to the structural state and electrochemical property of membranes.

[Effect of intramolecular energetic barriers on conformation-non-equilibrium states of protein globules in ensemble]

Within the Blumenfeld's model interpretation theoretical investigations of conformation relaxation of the protein globule were carried out. Analytical expressions describing the relaxation time and cyclic frequency of enzymes synchronization by arbitrary number of metastable states and unequal probabilities of transitions between them were found. It was shown that inequality of activation energies affects the frequency and number of cycles of enzymes synchronous work to a considerably greater degree, than the relaxation time of initial disturbance. The number of macromolecules in the definite conformation state was evaluated and conditions of experimental observation of synchronization were discussed.

[Non-linear real-time adaptive filtration of ultrasound TI628A echotomoscope images]

The statistical uncertainty caused by speckle noise artifacts is the reason for the great importance of the problem which is the optimum choice between the medical diagnostic systems resolution and the statistical accuracy of histological tissue identification. The way of speckle noise suppression, which is closely associated with the well-known idea of adaptive filtration and based on the physical analysis of the origin of true and false signals, is very promising. The testing results of the nonlinear real-time adaptive filter which has been designed for a TI628A echotomoscope are presented. The filter has been shown to have a rather high contrast and space resolution and reduces the speckle noise and other artifacts of the images.