Subharmonic response from single, polymer‐shell contrast agents subjected to high‐frequency excitation.

Abstract

Three polymer‐shelled agents (nominal mean diameters of 0.56, 1.1, and 3.4 μm) from POINT Biomedical were investigated to determine the optimal parameter space for generating a subharmonic response from single agents when subjected to high‐frequency (above 20 MHz) excitation. A flow phantom was constructed to restrict the flow of a dilute contrast agent solution to a small volume. Two single‐element transducers, with nominal center frequencies of 40 and 20 MHz, were aligned such that they were confocal and orthogonal to each other, and their mutual focus was positioned within the flow phantom. The 40 MHz transducer was used in transmit/receive mode while the 20 MHz transducer was used passively in the receive mode. The radio‐frequency backscatter signals from individual contrast agents were digitized simultaneously from both transducers under a variety of pulse durations (5–20 cycles), acoustic pressure levels (1.5–6.0 MPa), and driving frequencies (35–45 MHz). Echo signals from individual contrast agents were windowed and spectra were calculated. For each contrast agent, the magnitude of the subharmonic (20–MHz) response was normalized with respect to the magnitude of the fundamental (40–MHz) backscatter. Experimental results agreed closely with theoretical calculations. [Work supported by NIH EB006372.]