High frequency nonlinear scattering from a micrometer to submicrometer sized lipid encapsulated contrast agent.

An experimental lipid encapsulated contrast agent comprised substantially of micrometer to submicrometer diameter bubbles was evaluated for its capacity to produce nonlinear scattering in response to high transmit frequencies. Agent characterization experiments were conducted at transmit frequencies of 20 and 30 MHz with bandwidths of 5, 15 and 25% using a broadband focused PVDF transducer. The presence of subharmonic energy was observed for all bandwidths at a wide range of pressures (0.49 to 5.7 MPa and 0.45 to 4.5 MPa for the 20 and 30 MHz cases, respectively). Distinct ultraharmonics were observed only in the 5% bandwidth cases. Second harmonic energy was also present, but this was at least partly due to nonlinear propagation, as indicated by linear scatterer signals. Evidence of destruction was found only at higher peak negative pressures (e.g., >2 MPa for 30 MHz 5% bandwidth pulse). The results suggest that small lipid bubble formulations may be useful for the purposes of high frequency nonlinear contrast imaging.