Transmit Frequency Adaptation for Contrast to Tissue Ratio Optimization in Second Harmonic Imaging

Conventional ultrasound contrast imaging systems use sequences of sine waves at a fixed transmit frequency. The excitation of the system is thus a sine wave whose frequency is often fixed around two-thirds of the central frequency of the transducer in the second harmonic imaging. However, this choice requires a knowledge of the transducer and does not take into account the specificities of microbubbles and medium. The main purpose of our study was to propose an automatic technique providing the best contrast-to-tissue ratio by selecting the optimal transmit frequency. The simulations and the experiment showed that the optimal transmit frequency value did not correspond to the usual value. By using the optimized value, the contrast was improved by around 6 dB. Finally, by providing a closed loop system, the system automatically proposed the optimal control without any a priori knowledge of the system or the medium explored.