Threshold of microbubble agents collapse by ultrasound irradiation

Echo enhancers consist of microballoons and the authors aimed to investigate the thresholds of microballoon collapse of two different shell types. PVC-AN microballoon is a mimicking echo enhancer with a hard shell and Albunex(R) that is a commercially available albumin shelled echo enhancer. The thresholds of the microballoon collapse were analyzed by detecting subharmonic wave production. Scattered waveforms by PVC-AN that was diluted to 1/1000 with normal saline. The scattered waves from the microballoon were detected by a hydrophone and analyzed by a digital oscilloscope with a 10 bit A/D conversion rate. The spectrum was calculated with a fast Fourier transform applied to the leading 5 microseconds of the waveform. The authors concluded that the threshold of PVC-AN collapse exists between 1.5 to 2.25 W/cm² of ultrasound intensity. They however, could not determine the threshold of Albunex(R) that was diluted to 1/100 with normal saline, since subharmonics were recognized even by 0.1 W/cm² irradiation which was the lowest intensity in the authors´ system