In vitro characterization of the subharmonic ultrasound signal from Definity microbubbles at high frequencies

Ultrasound microbubble contrast agents have been demonstrated to scatter subharmonic energy at one-half the driving frequency. At ultrasound frequencies in the 20-40 MHz range, the subharmonic offers the potential to differentiate the blood in the microcirculation from the surrounding tissue. It is unknown whether current contrast agents, manufactured to be resonant between 2 and 12 MHz, are ideal for subharmonic imaging at higher frequencies. We performed numerical simulations of the Keller-Miksis model for the behavior of a single bubble and experimental investigations of Definity microbubbles in water. The results supported the hypothesis that off-resonant bubbles, excited at their second harmonic, may be primarily responsible for the observed subharmonic energy. For frequencies between 20 and 32 MHz and 32 and 40 MHz, the optimal bubble diameters for the generation of subharmonics in vitro were determined experimentally to be 1.2-5 microm and less than 1.2 microm, respectively. Definity may be a suitable ultrasound contrast agent for subharmonic imaging at 20 MHz with peak-negative pressures between 380 and 590 kPa and pulses greater than or equal to four cycles in duration.