Ambient pressure sensitivity of microbubbles investigated through a parameter study.

Measurements on microbubbles clearly indicate a relation between the ambient pressure and the acoustic behavior of the bubble. The purpose of this study was to optimize the sensitivity of ambient pressure measurements, using the subharmonic component, through microbubble response simulations. The behavior of two microbubbles corresponding to two different contrast agents was investigated as a function of driving pulse and ambient overpressure p(ov). Simulations of Levovist using a rectangular driving pulse show an almost linear reduction in the subharmonic component as p(ov) is increased. For a 20 cycle driving pulse, a reduction of 4.6 dB is observed when changing p(ov) from 0 to 25 kPa. Increasing the pulse duration makes the reduction even clearer. For a pulse with 64 cycles, the reduction is 9.9 dB. This simulation is in good correspondence with measurement results presented in the literature. Further simulations of Levovist show that also the shape and the acoustic pressure of the driving pulse are very important factors. The best pressure sensitivity of Levovist was found to be 0.88 dB/kPa. For Sonazoid, a sensitivity of 1.14 dB/kPa has been found, although the reduction is not completely linear as a function of the ambient pressure.