In vitro and in vivo high-frequency ultrasound response of microbubbles

Abstract

Recently, there has been a growing interest in utilizing contrast agents with high-frequency ultrasound (HFU > 20 MHz). As a first step to using contrast agents in vivo with HFU, the acoustic response of several agents was examined using a 40-MHz ultrasound biomicroscope (UBM, Vevo 770, VisualSonics). Optison, Definity, and near-micron and submicron agents from Point Biomedical were investigated. A small volume of the agents was injected into a petri dish of saline solution, and radio-frequency (rf) backscatter data from the UBM were digitized with a digital oscilloscope. Agents were also injected into the cardiovascular system of mouse embryos and rf data were acquired in vivo. The echo signals from individual bubbles were windowed in the rf data and the spectra were calculated. The resulting spectra revealed an increasing harmonic content for each of the agents as the acoustic drive amplitude and number of excitation wavelengths increased. Harmonic generation appeared to be due mainly to nonlinear propagation in the water bath and not from a nonlinear response of the contrast agent because reflected spectra without contrast agent showed similar harmonics. Using a modified parameter imaging method, fundamental (40 MHz) and first-harmonic (80 MHz) images were generated.