Characterization of nonlinear viscoelastic properties of Ultrasound Contrast Agents

Ultrasound Contrast Agents (UCAs) composed of gaseous microbubbles exhibit acoustic properties that differ from those of surrounding tissue. In order to increase the contrast to tissue ratio, it is necessary to propose methods that characterize optimally the ultrasonic properties of UCAs. The aim of this study is to measure the viscoelastic properties of UCAs using a Dynamic Acousto-Elastic (DAE) method. The approach, based on the interaction of two acoustic waves, provides non-contact and localized measurements of viscoelastic and dissipative nonlinearities of UCAs. Sonovue and BR14 UCAs (BraccoResearch, GE) were thus characterized at two different probing frequencies (1 MHZ, 2 MHz) and two dilutions (1/100 and 1/1000). As expected very high viscoelastic and dissipative nonlinearities were obtained. Hysteresis and asymmetries between compression and expansion phases are observed for both viscoelastic and dissipative parameters. Same trends were seen for BR14 and SonoVue. The probing pulse frequencies and the dilutions provide different results for viscoelastic nonlinearities, whereas only dilutions impact the dissipative nonlinearities.