The fate of resonant and off-resonant microbubble signals in response to consecutive imaging pulses

The study of single microbubble (MB) acoustics has relevance to signal and image processing of ultrasound (US) contrast imaging. The aim of this work is to study the effect of consecutive pulses on MB response. RF backscatter data from 223 single lipid shelled Definity® MBs were collected using a flow phantom and a modified Philips Sonos 5500 scanner. The results from 2 consecutive imaging pulses (1.6MHz, 550kPa, 6 cycles, 1kHz PRF) are presented and compared to similar results from a theoretical model of Definity (Keller-Miksis equation, Mooney-Rivlin shell model). Bubble survival in the 2nd pulse response was observed for 64% of scatterers. Surviving signals were observed to decay in amplitude and increase in relative harmonic content with each incident pulse, with the rate of decay being dependant on the first pulse response. Signals from MBs at resonance and above, are more resilient to the incident pulse compared to off-resonant MBs. Resonant MBs have been observed to migrate from resonant scatter in the first pulse response to off-resonant scatter in subsequent pulses, suggesting a change in size which takes them across the resonance peak. Theoretical results confirm that realistic changes in size can account for such differences, and will lead to an increase in the relative harmonic components as measured here. In light of these findings, the larger decay of off-resonant scatter compared to resonant scatter may offer increased performance of pulse sequences such as pulse amplitude modulation, and the order of incident pulses in the sequence then becomes crucial. Mechanisms including acoustically driven diffusion and lipid shedding could be incorporated in future theoretical models to account for these effects.