Subharmonic response of lipid-coated monodisperse microbubbles

Subharmonic emissions from lipid-coated microbubbles, which is not radiated by tissues, can be leveraged to improve contrast-enhanced diagnostic ultrasound. In our work, we investigated subharmonic emissions from monodisperse lipid-coated microbubbles under different acoustic pressure and frequencies. First, the resonance frequency of microbubble monodispersion was determined from measured attenuation spectrum. Next, acoustic emissions from the microbubbles were detected by a transducer positioned orthogonal to the excitation transducer. Our study showed that subharmonic emissions were maximized when bubbles were driven at nearly twice the pressure-dependent resonance frequency rather than the linear resonance frequency. We also observed subharmonic emission at low excitation acoustic pressure (< = 30 kPa) for microbubbles coated with densely packed lipid shells, which suggests that minimizing the initial surface tension can enable subharmonic emissions at very low excitation pressures. Further studies we...