Impact of gas composition and incident pressure on the delayed subharmonic emission from phospholipid contrast agents

Subharmonic emission from gas-filled microbubbles is of interest because it can improve the sensitivity of contrast-enhanced ultrasound imaging. However, subharmonic emission exhibits a delay between the insonification and its generation. In this study, we investigated the impact of applied acoustic pressure and microbubbles gas composition on the time dependent emission of subharmonics of five phospholipid contrast agents. Subharmonic generation was first measured at 10 MHz transmit frequency and at two peak negative pressures of 450 kPa, and 700 kPa over a total exposure of 90 minutes. The earlier observed delay for contrast agents is shortened when increasing the incident pressure. To assess the effect of the filling gas on the subharmonic emission, gas content of the vials were substituted between different contrast agents and subharmonic generation was measured. Subharmonic emissions from microbubbles filled with C
4
F
10
, which is a low diffusible gas, exhibited a delayed onset followed with a considerable increase of the subharmonic (of the order 12–18 dB) after 20–40 minutes. On the contrary, for microbubbles filled with SF
6
or air, which are highly diffusible gases, subharmonic emerged almost immediatly after the start of insonation. In conclusion, gas constituting the microbubbles influences significantly the evolution of subharmonics emissions. In particular, when filled whith highly diffusible gas or when the incident pressure is raised, the speed of diffusion is increased. These results suggest that the properties of the filling gas, such as diffusion is the mechanism responsible for the temporal changes of subharmonics.