Detection of Microbubble Ultrasound Contrast Agent Destruction Applied to Definity

For different applications such as imaging, drug delivery and tissue perfusion measurement, it is necessary to know the inertial cavitation (IC) threshold of ultrasonic contrast agent (UCA) microbubbles. The IC threshold for Definity R © was determined using a passive cavitation detection (PCD) system. The IC threshold criterion was rebound. The incident ultrasonic field at a constant pulse repetition frequency of 10 Hz was varied with frequency (0.9, 2.8, 4.6 and 7.1 MHz), pulse duration (3, 5, and 7 cycles), and pressure level (ranging from 0.4 to 8.7 MPa). The transmit transducer excited the contrast agent while the 13-MHz receive transducer (mounted confocal to the transmitter) simultaneously received acoustic emissions from the microbubbles. The concentration of Definity R © was chosen so that statistically only one microbubble was in this focal volume. Simulation results showed that a collapsing microbubble emits a characteristic broadband signal that was used to determine IC events. In order to classify the data, five different classes were created: Noise, Oscillation, Collapse, Multiple Bubbles and Not Classified. The peak rarefactional pressure thresholds of IC events were determined for three of the four frequencies (e.g., 1.6, 0.8, and 2.6 MPa for 0.9, 2.8, and 4.6 MHz, respectively). It was not possible to determine a threshold pressure at 7.1 MHz.