Temporal evolution of subharmonic emissions from a lipid-encapsulated contrast agent

A large body of work has investigated the influence of the excitation pulse, agent size distribution and the ambient pressure on the subharmonic response of microbubble contrast agents (MCA). The purpose of this study was to investigate the temporal evolution of the subharmonic emissions, i.e., whether the subharmonic response is influenced by the time elapsed since agent constitution. We measured subharmonic emissions from a commercial lipid-encapsulated contrast agent (Targestar-P®, Targeson Inc, San Diego, CA,USA) over the time span of 60 minutes. The excitation parameters were: 10-MHz frequency, 30 - 290 kPa pressures, 60 cycles, and 1-kHz pulse repetition frequency. The subharmonic emissions were observed to increase by 11 dB over 60 minutes relative to those measured immediately after reconstitution. The most striking increase (> 8 dB) was observed in the first 15 minutes. Although we did not observe a change in the agent size distribution, the pressure threshold for subharmonic emissions reduced by...