Compression-only behavior and subharmonic scattering of phospholipid-shell microbubbles

Subharmonic scattering of phospholipid-shell microbubbles excited at very low acoustic pressure amplitudes (¿30 kPa) has been associated with echo responses from compression-only bubbles, having initial surface tension values close to zero. In this work, the relation between subharmonics and compression-only behavior of phospholipid-shell microbubbles was investigated as a function of the initial surface tension by applying an ambient overpressure ranging from 0 to 140 mmHg. The microbubbles were excited using a 64-cycle Hanning-apodized transmit burst with a center frequency of 4 MHz and peak-negative pressure amplitude of 50 kPa. An increase in subharmonic response of 10-20 dB was measured after applying an overpressure of 140 mmHg. Simulations using the Marmottant model, taking into account the effect of ambient overpressure on bubble size and initial surface tension, confirmed the relation between subharmonics observed in the pressure-time curves and compression-only behavior observed in the radius-time curves. The increase in subharmonic response as a function of ambient overpressure, i.e. as a function of the initial surface tension, was correctly predicted by the model. Subharmonics present in the echo responses of phospholipid-shell microbubbles excited at very low acoustic pressure amplitudes are indeed related to the echo responses from compression-only bubbles. The increase in subharmonics as a function of ambient overpressure may be exploited for improving methods for noninvasive pressure measurement.