Localized harmonic motion imaging: theory, simulations and experiments

Several techniques have been developed in an effort to estimate mechanical properties of tissues. These techniques typically estimate static or harmonic motion resulting from an externally or internally applied mechanical stimulus. In this paper, we discuss the advantages of utilizing a new technique that performs RF signal tracking in order to estimate the localized oscillatory motion resulting from the harmonic radiation force produced by two focused ultrasound transducer elements with overlapping beams oscillating at distinct frequencies. Four agar gels were utilized in order to determine the effect of stiffness on the motion amplitude. Estimates of the displacement relative to the initial position (i.e., at the onset of the application of the radiation force) were obtained during the application of the radiation force that oscillated at frequencies ranging between 200 Hz and 800 Hz. In the simulations, an exponential decrease of the displacement amplitude with stiffness was observed at all frequencies investigated. An M-mode version to depict both the spatial and temporal variation of the locally induced displacement was used. In experiments with gels of different stiffness, the resulting amplitude of the harmonic displacement estimated oscillated at the same frequencies and an exponential decrease of the displacement amplitude with get stiffness was also observed. In tissue experiments, the results showed that the method is feasible in tissues and that FUS ablation can be detected. These preliminary results demonstrate the feasibility of imaging localized harmonic motion as induced by an oscillatory ultrasound radiation force. Due to the highly localized and harmonic nature of the estimated response, this technique may be proven highly suitable for accurate estimation of the elastic modulus variation in tissues due to disease.