Comparison of the acoustic response of liquid-PFOB and solid-core nanoparticles between 20 and 40 MHz

Liquid-core nanoparticles are promising candidates for targeted ultrasound-controlled therapy but their acoustic detection remains challenging. The current work characterizes high frequency (20-40 MHz) acoustic response from liquid-core nanoparticles with a thick PLGA shell. A programmable, single channel, high frequency, broad bandwidth (2-80 MHz) prototype was used to implement conventional and pulse inversion sequences. A 32-MHz center-frequency lithium niobate (LNO) transducer was focused within the 0.4-mm, lumen of a wall-less flow phantom while matched-concentration solutions of four types (liquid and solid core) of nanoparticles passed through. Sets of radiofrequency signals (N = 100) were acquired from solutions with concentrations from 25 mg/mL to 0.025 mg/mL. Gaussian-modulated sinusoidal tone bursts (4 KHz PRF) were transmitted at 20-MHz with peak negative pressures of 700kPa-2.01 MPa and pulse duration between 5 and 23 cycles. Results shows that the liquid-core nanoparticles with the least shell thickness provided the strongest acoustic response (10 dB higher than filled NP and 15 dB higher than LN). All particles presented second harmonic responses for higher pressure and higher pulse durations but second harmonic response from liquid core with the least shell thickness increased more rapidly with increasing incident pressure than for the other particles tested. High-frequency pulse inversion sequences implemented in this work decreased the fundamental intensity for all types of particles and reinforced the second harmonic response of tested particles.