A primary purpose of elasticity imaging, commonly known as elastography, is to extract the viscoelastic properties of a medium (including soft tissue) from the displacement caused by a stress field. Dynamic elastography methods that use the acoustic radiation force of ultrasound have several advantages, such as, non-invasiveness, low cost, and ability to produce a highly localized force field. A method for remotely generating localized low frequency shear waves in soft tissue is investigated, by using the modulated radiation force resulting from two intersecting quasi-CW confocal ultrasound beams of slightly different frequencies. In contrast to most radiation force-based methods previously presented, such shear waves are narrowband rather than broadband. As they propagate within a viscoelastic medium, different frequency-dependent effects will not significantly affect their spectrum, thereby providing a means for measuring the shear attenuation and speed as a function of frequency. Furthermore, to improve the detection signal-to-noise-ratio (SNR), increased acoustic pressure conditions may be needed, causing higher harmonics to be generated due to nonlinear propagation effects. Shear-wave propagation at harmonic modulation frequencies does not appear to have been previously discussed in the elastography literature. The properties of the narrowband shear wave propagation in soft tissue are studied by using the Voigt viscoelastic model and Green’s functions. In particular, the manner in which the characteristics of the viscoelastic medium affect their evolution under both low-amplitude (linear) and high-amplitude (nonlinear) source excitation and conditions that conform to human safety standards. It is shown that an exact solution of the viscoelastic Green’s function is needed to properly represent the propagation in higher-viscosity media, such as soft tissue, at frequencies much beyond a few hundred hertz. Methods for estimating the shear modulus and viscosity in viscoelastic media are developed based on both the fundamental and harmonic shear components.

A primary purpose of elasticity imaging, commonly known as elastography, is to extract the viscoelastic properties of a medium (including soft tissue) from the displacement caused by a stress field. Dynamic elastography methods that use the acoustic radiation force of ultrasound have several advantages, such as, non-invasiveness, low cost, and ability to produce a highly localized force field. A method for remotely generating localized low frequency shear waves in soft tissue is investigated, by using the modulated radiation force resulting from two intersecting quasi-CW confocal ultrasound beams of slightly different frequencies. In contrast to most radiation force-based methods previously presented, such shear waves are narrowband rather than broadband. As they propagate within a viscoelastic medium, different frequency-dependent effects will not significantly affect their spectrum, thereby providing a means for measuring the shear attenuation and speed as a function of frequency. Furthermore, to improve the detection signal-to-noise-ratio (SNR), increased acoustic pressure conditions may be needed, causing higher harmonics to be generated due to nonlinear propagation effects. Shear-wave propagation at harmonic modulation frequencies does not appear to have been previously discussed in the elastography literature. The properties of the narrowband shear wave propagation in soft tissue are studied by using the Voigt viscoelastic model and Green’s functions. In particular, the manner in which the characteristics of the viscoelastic medium affect their evolution under both low-amplitude (linear) and high-amplitude (nonlinear) source excitation and conditions that conform to human safety standards. It is shown that an exact solution of the viscoelastic Green’s function is needed to properly represent the propagation in higher-viscosity media, such as soft tissue, at frequencies much beyond a few hundred hertz. Methods for estimating the shear modulus and viscosity in viscoelastic media are developed based on both the fundamental and harmonic shear components.