Relative contributions of various modes of acoustic radiation force to creating mechanical stress in soft tissues

There are several physical effects responsible for the generation of acoustic radiation force (ARF) in soft tissue. The first effect, which is widely used in elastographic applications of ARF, is the transfer of momentum from ultrasonic wave in an attenuating medium due to absorption and scattering. Two other effects include reflection of sound wave from the boundaries between tissues differing in the acoustic impedance and second, spatial variations of the energy density of the propagating wave as a result of variations in the sound wave velocity in a medium. The latter mode of ARF can be directed both outward and toward the source of the propagating wave. In this study, relative contributions of various modes of acoustic radiation force are investigated both theoretically and experimentally. Using Green function for the acoustic potential near the boundaries between tissue structures, the expression for the motion of tissue interfaces under the action of short ultrasonic pulses is derived. Theoretical estimates of dynamics of the soft tissue response to various modes ARF induced by the microsecond range ultrasonic pulses are in agreement with experimental data. Findings of this study may serve as the basis of new approaches in soft tissue elastography.