Quantitative measurement of ultrasound disruption of polymer-shelled microbubbles.

The goal of this study was to assess the threshold of disruption and subsequent time-course of acoustic response of four experimental nitrogen-filled polymer-shelled microbubbles. Using an in vitro measurement system, a sequence of low-amplitude detection pulses measured the change in echo caused by a high-amplitude disruption pulse on a dilute suspension of bubbles. Detection pulses were transmitted 0.5 ms before disruption and between 1 and 200 ms after disruption. Separate transducers, aligned confocally and orthogonally, were used to transmit and receive bubble echoes. After disruption, all agents demonstrated a transient increase in scattered power. Above the disruption threshold, highly echogenic, nonlinear scatterers were observed. Their echoes slowly disappeared after disruption with median decay times from 7.4 to 13.6 ms, calculated by fitting to a mono-exponential decay. This is consistent with a process wherein the shell is disrupted, releasing the gas and generating free gas bubbles, which cause high-amplitude nonlinear scattering followed by relatively slow diffusion of the gas into solution. This picture has been observed optically with single bubbles and differs from the concept of "stimulated acoustic emission" from disrupted bubbles.

Characterization of the in vitro adherence behavior of ultrasound responsive double-shelled microspheres targeted to cellular adhesion molecules.

We have developed novel adhesion molecule-targeted double-shelled microspheres which encapsulate nitrogen. We report in vitro targeting studies utilizing these microspheres conjugated to target-specific antibodies directed towards ICAM-1 and VCAM-1. In static adherence experiments, the adherence patterns of microspheres conjugated to three different monoclonal antibodies (two targeted to ICAM-1 and one to VCAM-1) to their target surfaces were very different. Maximum microsphere adherence at the lowest target and/or ligand densities was observed with the VCAM-1 system. Differences in target-specific adherence were also observed between anti-ICAM-1 and anti-VCAM-1 microsphere conjugates in flow adherence studies. Equilibrium binding studies of the target proteins in solution to the microsphere-bound ligands showed that the affinity constants of two microsphere-bound monoclonal antibodies for their target proteins are similar. Thus, ligand-target affinity is not the only determinant of microsphere adherence to the target surface in our systems. Shear stress was found to have an effect on the mean diameter of adhered microspheres; a decrease in the mean diameter with increasing shear was observed. The magnitude of this effect was dependent on both microsphere-bound ligand and target surface densities, with a more pronounced change at lower densities. Adhered microspheres were readily detectable using ultrasound at the lowest tested surface density of 40 mm(-2).