Behavior of ultrasound contrast agents near the fragmentation threshold

Abstract

Understanding the destruction process of ultrasound contrast agents is important in therapeutic ultrasound applications (such as ultrasound-enhanced drug delivery), as well as in certain imaging applications (such as ‘‘flash echo’’ imaging of the myocardium). In the destruction of OptisonTM microbubbles, our observations suggest that there are two pressure thresholds: a lower threshold which leads to shell rupture and the production of daughter (derivative) bubbles, and a higher threshold leading to the onset of the inertial cavitation (IC) activity. Slightly above the shell-disruption (SD) threshold, the acoustic scattering signal decreases, presumably due in part to the partial dissolution of the derivative bubbles. The extent of the quiescent region, the IC pressure threshold, and the strength of the subsequent cavitation activity are all highly dependent on the acoustic pulse parameters. It is found that for pressure amplitudes in excess of the IC threshold, a longer pulse length or a higher pressure level will decrease the duration of this quiescent period. By controlling the bubbles’ response through fine-tuning the acoustic parameters near the SD and IC threshold, one could design an ultrasound system which would optimize the desirable effects of contrast agents for imaging and therapy applications. [Work supported in part by ONR and WTC.]