Optimising phase and amplitude modulation schemes for imaging microbubble contrast agents at low acoustic power

A series of in vitro experiments were performed to determine the efficacy of generalised phase- and amplitude-modulated sequences for low-power nonlinear microbubble contrast imaging. The microbubble agent Definity (Dupont, Boston, MA) was exposed to sequences in which the phase and amplitude were changed from one pulse to the next. Echoes from these pulses were combined to suppress or enhance particular linear or nonlinear components. The results show that established two-pulse pulse-inversion and amplitude-modulation approaches perform similarly, providing 14 +/- 1 dB of enhancement, compared with the echoes from the linear scatterer. A two-pulse combined phase and amplitude sequence achieved an additional 4 +/- 1 dB of enhancement. This improvement is due to improved preservation of second and third order harmonic signals, while maintaining the suppression of the linear signals. These results were obtained at low power, below the threshold of microbubble destruction, and are applicable to real-time perfusion imaging.