Using the Doppler effect to characterize microbubble shell viscoelasticity

The experimental testing and quality control of microbubble formulations for ultrasound applications requires high-throughput measurement of physicochemical properties, such as shell elasticity and viscosity. Single-bubble optical experiments are slow and expensive. Ultrasound attenuation experiments on microbubble populations can be done quickly, but the measurement is averaged over the ensemble, and it is often difficult to separate effects of microbubble size from shell viscoelasticity. Here, we discuss a relatively simple alternative method of using the Doppler effect to measure ultrasound radiation force-induced displacements of many individual microbubbles within a population. Our method involves insonifying a diluted suspension of freely floating microbubbles with a linear-array ultrasound probe driven by an open scanner. The microbubble displacements along the axis of the insonified region were acquired from the frequency shifts in the measured echo signals using the multi-gate spectral Doppler approach. These measurements were compared with theoretical peak microbubble displacements computed by combining a modified Rayleigh-Plesset equation and a balance of translational forces. The shell elasticity and viscosity producing the measured peak microbubble displacement was determined by assuming a resonant bubble size at each driving frequency. Our initial results are encouraging, and demonstrate a step toward high-throughput microbubble characterization using the Doppler effect.