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

The study of single microbubble (MB) acoustics has relevance to signal andimage processing of ultrasound (US) contrast imaging. The aim of this work is tostudy the effect of consecutive pulses on MB response. RF backscatter data from223 single lipid shelled Definity MBs were collected using a flow phantom and amodified Philips Sonos 5500 scanner. The results from 2 consecutive imagingpulses (1.6MHz, 550kPa, 6 cycles, 1kHz PRF) are presented and compared tosimilar results from a theoretical model of Definity (Keller-Miksis equation,Mooney-Rivlin shell model). Bubble survival in the 2nd pulse response wasobserved for 64% of scatterers. Surviving signals were observed to decay inamplitude and increase in relative harmonic content with each incident pulse,with the rate of decay being dependant on the first pulse response. Signals fromMBs at resonance and above, are more resilient to the incident pulse comparedto off-resonant MBs. Resonant MBs have been observed to migrate from resonantscatter in the first pulse response to off-resonant scatter in subsequentpulses, suggesting a change in size which takes them across the resonance peak.Theoretical results confirm that realistic changes in size can account for suchdifferences, and will lead to an increase in the relative harmonic components asmeasured here. In light of these findings, the larger decay of off-resonantscatter compared to resonant scatter may offer increased performance of pulsesequences such as pulse amplitude modulation, and the order of incident pulsesin the sequence then becomes crucial. Mechanisms including acoustically-drivendiffusion and lipid shedding could be incorporated in future theoretical modelsto account for these effects.