Subharmonic spectroscopy of ultrasound contrast agents

We have investigated the subharmonic vibration of BR14 (Bracco Research S.A., Geneva, Switzerland) contrast agent microbubbles between 8-11 MHz. The response of the bubbles is recorded optically with the ultrafast recording camera (Brandaris 128) at three acoustic pressures 50, 100 and 120 kPa. The vibration of the microbubbles as a function of excitation frequency is measured and frequency content is determined. 40% of studied bubbles show subharmonic oscillations. For bubbles smaller than 3μm in diameter the driving frequency of maximum subharmonic response increases for increasing pressures opposite to what has been reported for larger bubbles (>; 3μm).