Single-element focused transducer method for harmonic motion imaging

The feasibility of the harmonic motion imaging (HMI) technique for simultaneous monitoring and generation of focused ultrasound therapy using two separate focused ultrasound transducer elements was previously shown (1). In this study, a new HMI technique is described that images tissue displacement induced by harmonic radiation force excitation using a single focused ultrasound element. First, wave propagation simulation models were used to compare the use of one Amplitude- Modulated (AM) focused beam versus two overlapping focused beams as previously implemented for HMI (2). Simulation results indicated that, unlike the two-beam configuration, the AM beam produced a consistent, stable focus for the applied harmonic radiation force. The AM beam thus offered the unique advantage of sustaining the application of the spatially-invariant radiation force. Experiments were then performed on gelatin gel phantoms and in-vitro tissues. The radiation force was generated by a 4.68 MHz focused transducer using a low-frequency Amplitude- Modulated (AM) RF-signal. A 7.5 MHz single-element, imaging transducer was placed through the center of the focused transducer so that the diagnostic and focused beams were aligned. Consecutive RF signals were acquired with a PRF of 5 kHz and the displacements were estimated using 1D cross- correlation. Finally, taking advantage of the real-time capability of our method, the change in the elastic properties was monitored during focused ultrasound (FUS) ablation of in-vitro tissues. Based on the harmonic displacements, their temperature- dependence, and the calculated acoustic radiation force, the change in the regional elastic modulus was monitored during heating. In conclusion, the feasibility of using an AM radiation force for HMI for simultaneous monitoring and treatment during ultrasound therapy was demonstrated in phantoms and tissues in-vitro. Further study of this method will include stiffness and temperature estimation, ex-vivo and in-vivo.