Delayed subharmonic response of a phospholipid-shelled ultrasound contrast agent: effect of temperature and protein concentration

Abstract

The subharmonic response of ultrasound contrast agents (UCAs) is important for imaging and therapy. Therefore, factors influencing the temporal evolution of subharmonic signals should be elucidated. We evaluated the influence of temperature (25°C and 37°C) and protein concentration in the fluid (0.5% w/v at 25°C and 37°C) on the subharmonic response of Definity, a phospholipid-shelled UCA. Definity was diluted to 106 microbubbles/ml and sonicated at 2 MHz frequency, 50 cycles, and 470 kPa peak rarefactional pressure. The subharmonic response demonstrated striking qualitative and quantitative changes with time. At 25°C, a delayed subharmonic onset was observed. The subharmonic signal appeared after 28 min, peaking at 18dB above the baseline. At 37°C, the subharmonic signal appeared immediately but it peaked (5 dB increase) after 52 min. In the presence of dissolved protein, the subharmonic signal appeared at 16 min at 25°C and at 4min at 37°C. The signal peaked at 24 dB and 10 dB above the baseline at 25°C and 37°C, respectively. These observations underscore the need to standardize acoustic measurements and may have implications for subharmonic imaging, non-invasive pressure sensing, and cavitation detection.