Influence of shell properties on rupture of polymeric contrast agents in response to overpressure

The response of polymer-shelled ultrasound contrast agents (UCAs) to static overpressure was investigated. UCAs manufactured by Point Biomedical and Philips Research were studied. The nominal diameters of Point and Philips UCAs were 3 and 2 μm, respectively. A test chamber with a microscope-reticule lid was filled with glycerol. UCAs were reconstituted in 0.1 mL of water and added onto glycerol surface that was in contact with the reticule. Additional glycerol was injected (5 mL/hr) into the test chamber to induce an overpressure ranging from 2 to 180 kPa in one hour. A video-microscope synchronously imaged the UCAs. Individual UCAs in each frame were sized using a semi-automated post-processing scheme composed of four steps: 1) a 0.5 × 0.5 μm median filter, 2) binarization, 3) Sobel based edge detection, and 4) Hough transform estimation of diameter and centroid. The rupture data for both populations suggested subclasses that exhibited different rupture behavior although their mean diameters were not statistically different. The rupture pressures provided a measure of UCA fragility; the Philips UCAs were more resilient to rupture than the Point UCAs. Observed variations in rupture pressures were attributed to shell imperfections.