Elasticity imaging is a burgeoning medical imaging field. Many methods have been proposed that impart a force to tissue and measure the mechanical response. One method, vibro-acoustography, uses the ultrasound radiation force to harmonically vibrate tissue and measure the resulting acoustic emission field with a nearby hydrophone. Another method, vibrometry, uses the ultrasound radiation force accompanied with a measurement of the resulting velocity or displacement of the vibrating tissue or object has also been used for different applications. An extension of the vibro-acoustography method using a multifrequency stress field to vibrate an object is described. The objective of this paper is to present the image formation theory for multifrequency vibro-acoustography. We show that the number of low-frequency components created by this multifrequency method scales with the square of the number of ultrasound sources used. We provide experimental validation of the point-spread function of the multifrequency stress field and show examples of both vibrometry and vibro-acoustography imaging applications. This method holds the potential for a large gain of information with no increase in scanning time compared to conventional vibro-acoustography systems.

We provide a detailed analysis on the acoustic radiation force and torque exerted on a homogeneous viscoelastic particle in the long-wave limit (i.e. the particle radius is much smaller than the incident wavelength) by an arbitrary wave. We assume that the particle behaves as a linear viscoelastic solid, which obeys the fractional Kelvin-Voigt model. Simple analytical expressions for the radiation force and torque are obtained. The developed theory is used to describe the interaction of acoustic waves (traveling and standing plane waves, and zero- and first-order Bessel beams) in the MHz-range with polymeric particles, namely lexan, low-density (LDPE) and high-density (HDPE) polyethylene. We found that particle absorption is chiefly the cause of the radiation force due to a traveling plane wave and zero-order Bessel beam when the frequency is smaller than 5MHz (HDPE), 3.9MHz (LDPE), and 0.9MHz (lexan). Whereas in a standing wave field, the radiation force is mildly changed due to dispersion inside the particle. We also show that the radiation torque caused by a first-order Bessel beam varies nearly quadratic with frequency. These findings may enable new possibilities of particle handling in acoustophoretic techniques.

In this article, we introduce the theory for vibro-magnetometry (VM) with computational simulations for an idealized experimental setup. A method based on acoustic radiation force and magnetic measurement has been proposed for interrogating the mechanical vibrations of a target immersed in fluid medium. In this method, ultrasound radiation is used to exert a low-frequency (in kHz range) force on a rigid magnetized target immersed in a viscoelastic medium. In response, the target vibrates sinusoidally in a pattern determined by viscoelastic properties of the medium. The magnetic field resulting from target vibration is related to both the ultrasonic and low-frequency (kHz range) mechanical characteristics of the medium. We report the relationship between the magnetic field signal and the incident ultrasonic pressure field in terms of the mechanical parameters of the medium. Simulations were conducted to demonstrate a simple approach based on using amplitude-modulated ultrasound to generate a dynamic acoustic radiation force on a magnetic target. The magnetic field generated by vibration of this target is then obtained and used to estimate the radiation-force-induced displacement as a function of time. It was observed that the intensity of the dynamic component of the magnetic field caused by the acoustic excitation is high enough to be registered by a conventional magnetic sensor. When a low stress is applied on a reactive magnetic target embedded in the medium, the subsequent oscillating magnetic field is measured by a dedicated magnetic sensor, yielding the applicable mechanical information of the host medium. The proposed methodology presents a powerful tool for evaluation of acoustic radiation force as well as the mechanical properties of soft materials.

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a b s t r a c tMaking use of the addition theorem for the cylindrical wave functions and the complexsource-point method in cylindrical coordinates, an exact solution to the Helmholtz equation is derived, which corresponds to a tightly focused (or collimated) cylindrical quasi-Gaussian beam with arbitrary waist. The solution is termed "quasi-Gaussian" to make a distinction from the standard Gaussian beam solution obtained in the paraxial approximation. The advantage of introducing this new solution is the efficient and fast computational modeling of tightly focused or quasi-collimated cylindrical wave-fronts depending on the dimensionless waist parameter kw 0 , where k is the wavenumber of the acoustical radiation. Moreover, a closed-form partial-wave series expansion is obtained for the incident field, which has the property that the axial scattering (i.e. along the direction of wave propagation) and the axial acoustic radiation force (which is a time-averaged quantity) on a cylinder, can be calculated without any approximations in the limit of linear acoustical waves in a nonviscous fluid. Examples are found where the extinction in the radiation force function plot is found to be correlated with conditions giving reduction of the backscattering from an elastic cylinder. Those results are useful in beam-forming design, particle manipulation in acoustic tweezers operating with focused cylindrical beams, and the prediction of the scattering and radiation forces on a cylindrical particle or liquid bridges.