Cavitation detection during and following HIFU exposure in vitro

Is the existence of a sustained hyperechogenic region on B-scan images following HIFU treatment a necessary and sufficient condition for cavitation to have occurred during HIFU exposure? Three means of cavitation monitoring were used synchronously, before, during, and after continuous-wave HIFU exposure of an agar-graphite tissue phantom. A 1.1-MHz HIFU transducer was confocally aligned with a 15-MHz passive cavitation detection (PCD) transducer and a 5-MHz scan head. The HIFU pressure amplitude was increased in steps of 0.26 MPa every 5 s. A peak detector recorded the peak PCD signal level and a dynamic signal analyzer monitored broadband noise emissions (5–10 MHz). The sudden onset of a PCD output signal occurred at a peak-negative focal pressure of 1.25 MPa; no post-HIFU hyperechogenic region was visible on the B-scan images. A hyperechogenic region did eventually appear, but only for focal pressures in excess of 1.8 MPa. Inertial cavitation can therefore occur during HIFU exposure in the absence of a post-HIFU hyperechogenic region. The focal pressure for which such a region is observed can be as much as 50% higher than the threshold pressure for inertial cavitation inception. [Work supported by the ASA and the NSF Center for Subsurface Sensing and Imaging Systems.]