9C-5 2D Simulation of the Harmonic Motion Imaging (HMI) with Experimental Validation

Amplitude-modulated (AM) harmonic motion imaging (HMI) is one of the radiation-force-based techniques, which has the capability of imaging tissue mechanical properties during the application of the acoustic radiation force. Since only displacement images have been presented until now, the theory between tissue displacement and modulus in AM-HMI has not been validated. Here, a finite-element model (FEM) is used to accurately model the dynamic response in tissue mimicking phantoms to evaluate HMI performance. The FEM and experimental results of phantoms with the same stiffness variations are compared and used to describe the behavior of tissue during the application of the stimulus. A harmonic force was generated by a 4.68 MHz single-element focused ultrasound (FUS) transducer. Since the focus is highly localized and has a harmonic response from AM beam, the motion characteristics can be directly related to the regional tissue modulus. The resulting motion was imaged simultaneously using a diagnostic (pulse-echo) transducer at center frequency of 7.5 MHz. RF signals were acquired using a standard pulse-echo technique with a PRF of 5.4 kHz. ID cross-correlation technique was performed to estimate the resulting axial displacement. In the FEM simulation, a rectangular mesh with dimensions of 35 mm in the axial and 30 mm in the lateral directions was generated. There were a total of 2771 nodes and 5424 triangular elements in the mesh. For simplicity, the mesh was assumed to be a purely elastic medium with Young's modulus of 10 kPa. The acoustic pressure field was simulated in Field II using the same transducer parameters as in the experiment. This pressure field was used as the excitation force to generate displacements. The imaging field was simulated in Matlab 7.2 using a 2D convolutional image formation model with 128 acoustic elements, a center frequency of 7.5 MHz and 40 MHz sampling frequency. Only the displacement estimation at the center RF lines was considered in the validation with the single-element pulse-echo transducer used in the experiments. A good agreement between the results from the FEM and experiment findings was observed in phantom study. Finally, in vivo application is presented.