Comparison between shear wave dispersion magneto motive ultrasound and transient elastography for measuring tissue-mimicking phantom viscoelasticity.

Several methods have been developed over the last several years to analyze the mechanical properties of soft tissue. Elastography, for example, was proposed to evaluate soft tissue stiffness in an attempt to reduce the need for invasive procedures, such as breast biopsies; however, its qualitative nature and the fact that it is operator-dependent have proven to be limitations of the technique. Quantitative shearwave- based techniques have been proposed to obtain information about tissue stiffness independent of the operator. This paper describes shear wave dispersion magnetomotive ultrasound (SDMMUS), a new shear-wave-based method in which a viscoelastic medium labeled with iron oxide nanoparticles is displaced by an external tone burst magnetic field. As in magnetomotive ultrasound (MMUS), SDMMUS uses ultrasound to detect internal mechanical vibrations induced by the interaction between a magnetic field and magnetic nanoparticles. These vibrations generated shear waves that were evaluated to estimate the viscoelastic properties of tissue-mimicking phantoms. These phantoms were manufactured with different concentrations of gelatin and labeled with iron oxide nanoparticles. The elasticity and viscosity obtained with SDMMUS agreed well with the results obtained by traditional ultrasound-based transient elastography.

Magnetic tracking of acoustic radiation force-induced micro-order displacement.

The dynamic behavior of a rigid magnetic sphere induced by an acoustic radiation force was investigated. The sphere was suspended in water in a simple pendulum configuration. The drag force acting on the pendulum during its motion was considered to follow a modified Stokes law for a low Reynolds number, accounting for phenomena related to its oscillatory movement. Steady forces of long (a few seconds) and short (a few milliseconds) durations were used. The movement of the magnetic sphere was tracked using a magnetoresistive sensor. From the new equilibrium position of the sphere in response to the long-duration static radiation force, the amplitude of this force was estimated. To assess the water viscosity, the relaxation movement after the acoustic force had stopped was fitted to a harmonic-motion model. Based on the results for the acoustic force and water viscosity, a theoretical profile of the sphere's micro-order displacement as a function of time caused by short-duration acoustic radiation force agreed well with experimental results.

A new apparatus for analysis of viscoelastic fluids by ultrasound radiation force.

The acoustic radiation force has been used as the method to examine the physical properties of materials in several areas. Vibro-acoustography is an acoustic radiation force technique that is being used to perform analysis of mechanical properties of materials. In this application a focused acoustic modulated force excites target which vibrates at the frequency of modulation. The emitted sound is characteristic of the medium mechanical impedance and it is measured using a dedicated hydrophone. In this paper, we propose a modification of the vibro-acoustography (VAG) technique and apply a technique called vibro-acustomagnetography (VAMG) by replacing the hydrophone by a magnetic sensor with high sensitivity. In this case, the modulated acoustic radiation will be applied on a magnetized target immersed in the fluid under study. With this procedure, static and dynamic displacement of the magnetic target (ball) will be measured when acoustically excited. In this study, we used a magnetoresistive sensor with resolution of about nT for mounting the transducer to detect displacement of the magnetic target. The vibration of the target was induced by a non-contact force, using an ultrasonic beam modulated by two concentric beams generated by confocal piezoelectric elements with equal area and common focus to 7 cm. The target used was a magnetic sphere of NdFeB with a radius of 2.36 mm. The apparatus was evaluated through of measurements in water and oil. Viscoelastic parameters were estimated fitting the nonlinear response of the magnetic transducer function of frequency modulation.