Harmonic motion imaging: a new technique for the detection of the local mechanical properties of tissues

Abstract

Several techniques have been developed in an effort to estimate mechanical properties of tissues. In this paper, we will discuss the advantages of utilizing a new technique that performs RF signal tracking in order to estimate the localized oscillatory motion resulting from a harmonic radiation force produced by two focused ultrasound transducer elements with overlapping beams oscillating at distinct frequencies. Finite-element and Monte-Carlo simulations were performed in order to characterize the range of oscillatory displacements produced by a harmonic radiation force. The frequencies investigated ranged from 200 Hz to 800 Hz and the stiffness between 20 and 80 kPa. In the experimental verification, three transducers were utilized: two focused transducers at 3.75 MHz and a diagnostic transducer 1.1 MHz operating at pulse/receive mode. Agar gels of 7 - 95 kPa stiffness were utilized. Displacement estimates were obtained during the application of the radiation force oscillating at frequencies of 200 - 800 Hz. In experiments, the estimated oscillatory displacement spanned from -800 to 600 microns depending on the magnitude of the applied radiation force. A frequency upshift and an exponential displacement decrease were obtained with stiffness increase in experiments and simulations. These preliminary results demonstrate the feasibility of imaging localized harmonic motion as induced by an oscillatory ultrasound radiation force.