Influence of microbubble size distribution on postexcitation thresholds for single ultrasound contrast agent using double passive cavitation detection

A recent study has shown that double passive cavitation detection (DPCD) is a valid method for determining cavitation characteristics including collapse thresholds of isolated microbubbles based on the detection of postexcitation signal (PES) occurring 1 to 5 μs after the principle excitation of the bubble. The hypothesis is that PES is associated with the collapse of a free gas bubble released from the ultrasound contrast agent (UCA) after the rupture of its shell. It has been shown that responses of UCAs depend on several parameters such as the shell and gas core as well as the size of the microbubble. However, a detailed study of the influence of size distribution only on single microbubble collapse thresholds is still not available. The aim of this study was to determine whether there was a significant correlation between microbubble size distribution and collapse thresholds. Experiments were performed using serum albumin and dextrose microbubbles. Four size distributions were obtained and we determined microbubble collapse thresholds for each distribution using the DPCD method (3 cycle tone bursts at the central frequency of 4.6 MHz). Statistical analysis was based on comparing the means of collapse threshold for the four size distributions using an analysis of variance. The microbubble collapse threshold was found to be significantly (p<0.05) correlated to the microbubble size distribution: the DPCD experiments demonstrated that, in the configuration of the experiments, PES thresholds were higher for microbubbles exhibiting smaller size distribution.