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

Abstract

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.

Displacement Spectra Under Isometric Muscle Contraction

“…Compared to displacements, the tissue velocities for all volunteers are predictably characterized by a wider spectral band in a range of up to 70 to 80 Hz, as follows from Figure 4. Figure 6 shows the dependence of the signal power (23) on the upper-bound frequency f.…”

“…The differences between the spectra of normal vibrational movements of the cardiac muscle and those under cardiac infarction were determined. New diagnostic ultrasound methods, which are aimed at studying the elasticity of soft tissues, are now under development, [16][17][18][19] including acoustic radiation force-based methods, [20][21][22][23][24][25][26][27] for which precision submicrometer measurements are required.…”

Spatial Resolution and Measurement Accuracy of the Ultrasound Diagnostic System at Acoustic Remote Palpation Using High Intensity Focusing Ultrasound

“…Due to the developed specialized software, the Angiodin diagnostic scanner control system allowed to fine-adjust all the parameters, necessary for performing ARP, and to measure the displacements at the chosen point of phantom. Like in [23][24][25][26], the Doppler probing was synchronized with the HIFU pulses, whose parameters, for example pulse duration, could also vary.…”

“…Recently, in ultrasonic medical diagnostics the methods of ultrasound elastography of soft tissues, such as acoustic radiation force impulse (ARFI) imaging [20][21][22] and shear wave elastography (SWE) [22][23][24], have been widely used. These methods are based on ultrasound registration of the soft tissue response to the radiation force of an ultrasound pulse, and in the both methods the same ultrasound transducer of the diagnostic system is used for both creation of the radiation pressure force and registration of the response [20].…”

Acoustic radiation force in tissue-like solids due to modulated sound field

“…In this class of diagnostic ultrasound techniques, the carrier signal (MHz rate) is often modulated by a low-frequency envelope (kHz rate) (Fatemi and Greenleaf, 1998;Bercoff et al, 2004;Barannik et al, 2004;Guzina et al, 2009) both to facilitate the motion measurements and to confine their support to a neighborhood of the focal point which simplifies the interpretation. In the landmark study of the effects of ultrasound modulation on the ARF (Sarvazyan et al, 1998), the germane radiation force in a tissue-like solid was approximated as a product between the non-modulated expression for the ARF (computed for a viscous fluid) and the squared modulation envelope.…”