Thresholds for inertial cavitation in Albunex suspensions under pulsed ultrasound conditions

Stabilized microbubbles used as echo-contrast agents can be destroyed by ul trasonic irradiation. We have identified two pressure thresholds at which t hese microbubbles undergo inertial cavitation there, defined as the collaps e of gas bubbles followed by emission of an acoustic broadband noise). The first threshold (P1) corresponds to the pressure at which all the microbubb les in a cavitation field lose their property as an effective scatterer bec ause of fragmentation or deflation. The second threshold (P2) is associated with the acoustic reactivation of the remnants of the contrast agents and is related to the onset of more violent inertial cavitation. P1 and P2 were measured as a function of the concentration of Albunex(R) (Molecular Biosy stems Inc., San Diego, CA) contrast agent, the number of transmitting acous tic cycles, and the purse repetition frequency (PRF). The ultrasound freque ncy used was 1.1 R MHz, and the peak negative acoustic pressures ranged fro m 0 to 8 MPa, Our results, measured in Isoton(R) II (Coulter Diagnostics, M iami, FL) and whole blood solutions, showed that P1 increased with increasi ng Albunex(R) concentration and decreased with increasing PRF, whereas P2 d ecreased with increasing Albunex(R) concentration and was independent of th e PRF. Both P1 and Pa decreased with increasing number of acoustic cycles N for N < 10 and were independent of the number of cycles for N > 10. Ultras ound images of Albunex(R) acquired by a commercial scanner showed echo enha ncement not only at pressure levels below PI but also at levels above P2. T he threshold P2 was achieved at ultrasound energies above the diagnostic le vel. Inertial cavitation produced at P2 was associated with a higher level of hemolysis compared with P1. The results of this investigation have poten tial significance for both diagnostic and therapeutic ultrasound applicatio ns.