Nonlinear resonance behaviour and shell property estimates from individual microbubbles in the 4-13 MHz range assessed with low amplitude acoustic spectroscopy

There is an increasing interest in contrast applications in the 5-15 MHz range. In order to improve image quality and agent design, a greater understanding of microbubble behaviour in this frequency range is required. In this study, individual lipid encapsulated microbubbles (Definity and Target-Ready MicroMarker) were insonified at low pressures (<; 25 kPa) using an “acoustic spectroscopy” approach; namely insonicated with a sequence of tone bursts from 4 to 13.5 MHz in order to investigate frequency dependent scattering. The fundamental radial excursion amplitude was calculated from the frequency dependent scattering in order to produce a resonance curve for a given bubble. Over the size range of 2.5 to 4 μm, 69% of Target-Ready MicroMarker exhibited an asymmetric resonance, characterized by a skewing of the resonance curve and indicative of nonlinear behaviour. For Definity, these responses were observed for 8% over the size range 1.7 to 3.1 μm. For the subset of bubbles exhibiting linear, symmetric resonance curves, shell elasticity and viscosity values were estimated. Target-Ready MicroMarker is characterized by a stiffer shell (3<;χ₀<;5) N/m than Definity (0.5<;χ₀<;2.5) N/m, and distinct strain-softening and shear-thinning rheological behaviour. For Definity, no clear strain or shear rate dependence of the shell properties is evident.