Acoustic vaporization threshold of lipid-coated perfluoropentane droplets

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing micro-bubbles as contrast agents in situ as well as higher stability and the possibility of achieving smaller sizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with a perfluoropentane (PFP) core (diameter 400-3000 nm) is acoustically measured as a function of the excitation frequency in a tubeless setup at room temperature. The changes in scattered responses-fundamental, sub-, and second harmonic-are investigated, a quantitative criterion is used to determine the ADV phenomenon, and findings are discussed. The average threshold obtained using three different scattered components increases with frequency-1.05 +/- 0.28 MPa at 2.25 MHz, 1.89 +/- 0.57 MPa at 5 MHz, and 2.34 +/- 0.014 MPa at 10 MHz. The scattered response from vaporized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV threshold value. (C) 2018 Acoustical Society of America.