Contrast agent response to chirp reversal: simulations, optical observations, and acoustical verification

Active response of a microbubble is characterized by its resonance behavior where the microbubble might oscillate after the excitation waveform has been turned off. We investigate in this paper an excitation approach based on this resonance phenomenon using chirps. The technique, called chirp reversal, consists in transmitting a first excitation signal, the up-sweep chirp (UPF) of increasing frequency with time, and a second excitation signal, the down-sweep (DNF) that is a replica of the first signal, but time reversed with a sweep of decreasing frequency with time. Simulations using a modified Rayleigh-Plesset equation were carried out to determine bubble response to chirp reversal. In addition, optical observations and acoustical measurements were carried out to corroborate the theoretical findings. Results of simulations show differences between bubbles' oscillations in response to up-sweep and down-sweep chirps mainly for transmitted center frequencies above the bubble's resonance frequency. Bubbles that are at resonance or far away from resonance engender identical responses. From the optical data, the larger bubbles showed different dynamics when up-sweep or down-sweep chirps were transmitted. Smaller bubbles (< 2μm diameter) appear to be less sensitive to frequency sweep at 1.7 MHz center frequency. However, driven at a higher center frequency, smaller bubbles tend to be more sensitive. These results were confirmed through the acoustical measurements. We concluded that simulations and experimental data show that significant differences might be observed between bubbles' responses to UPF and DNF chirps. We demonstrate in this study that, for an optimal use of chirp reversal, the transmit frequency should be higher than the resonance frequency of the contrast microbubbles.