The stable nonlinear acoustic response of free-floating lipid-coated microbubbles

Abstract

The stability of the microbubbles maintained by the lipid coating is crucial for diagnostic contrast-enhanced ultrasound imaging. We present a study of the stability of the dynamic response of single free-floating microbubbles (DSPC-based homemade microbubbles) with an acoustical camera. A 30 MHz probing wave (800 μs duration) measures the dynamic response of single microbubbles to 42 short sine bursts (1 MHz, 10 μs duration, 3 μs interval between each excitation) at three different peak pressures (25, 100, and 200 kPa). For each microbubble exposed to the 42 consecutive pulses, the following parameters were calculated: the radial strain at the driving frequency (ɛf), at the second/third harmonic frequencies (ɛ2f,ɛ3f), the ratio of radial excursion in expansion over that in compression (EoC) and the dc offset in the time-domain response. Nearly all 1500 individual microbubbles measured showed stable vibrational response. As expected ɛ2f and ɛ3f increase with ɛf, but they reach a plateau when ɛf exceeds about 30%. For ɛf smaller than 15%, we observed compression-dominant behaviors (dc offset < 0 and EoC < 1), while microbubbles show expansion-dominant responses (dc offset > 0 and EoC > 1) for ɛf larger than 15%.