A dual-frequency excitation technique for enhancing the sub-harmonic emission from encapsulated microbubbles

Sub-harmonic imaging using encapsulated microbubbles (EMs) improves the contrast of ultrasound imaging by taking advantage of increased contrast to the tissue signal. A dual-frequency excitation technique (DFET) is proposed for enhancing the sub-harmonic emission from EMs as compared with the conventional single frequency sinusoidal excitation technique (SFSET). This study includes theoretical simulation and in vitro experimental verification. A dual-frequency signal (2 and 4 MHz) is used to insonate EMs developed in our laboratory. Both theoretical and experimental studies indicate that the DFET may be able to improve the amplitude of the sub-harmonic component up to 13 dB over the SFSET. Increasing the value of the pulse repetition frequency or the number of cycles of ultrasound tone burst in the application of the DFET may increase the sub-harmonic emission. Furthermore, it is confirmed that the amplitude ratio of the second frequency (4 MHz) to the first frequency (2 MHz) and phase shift of the second frequency with respect to the first frequency also play an important role in sub-harmonic emission. A ratio of 0.5 and a phase shift around 180 degrees are found to be the optimum values.