Detection of contrast microbubble shell rupture

The goal of this work is to detect experimentally and quantitatively the destruction of Optison TM ultrasound contrast agent (UCA) microbubbles. Two experimental systems were used. The first system consisted of a passive cavitation detector (PCD) with transmitted pulses at one of three frequencies (0.9, 2.8 and 4.6 MHz), one of three pulse durations (3, 5 or 7 cycles) and peak rarefactional pressures from 0.07 to 5.4 MPa. UCAs in solution were diluted so that the transmitted pulse interacted, on average, with a single UCA. The UCA response and acoustic emissions were detected with a 13-MHz center-frequency receiver. The second system consisted of a PCD setup (transmit 7 cycles at 0.99 MHz at peak rarefactional pressures of 0.3 or 0.8 MPa, receive at a center-frequency of 9.8 MHz) coupled to an optical microscope. UCAs were isolated in a 200-μmdiameter cellulose fiber aligned at the focal positions of the transducers and the microscope. Optical data were acquired with a digital camera synchronized to the transmit acoustic pulse. Post-excitation acoustic emissions with broadband spectral content from the PCD time traces were used to characterize UCA destruction thresholds. Microscopic data provided independent confirmation that only a single microbubble was interrogated and whether that was destroyed by the incident acoustic pulse. Results characterize Optison TM destruction thresholds. The PCD technique presented in this work provides a straight-forward evaluation of UCA destruction.