Excitation of polymer-shelled contrast agents with high-frequency ultrasound

Abstract

Few experimental and complementary theoretical studies have investigated high-frequency (>20 MHz) nonlinear responses from polymer-shelled ultrasound contrast agents. Three polymer agents with different shell properties were examined for their single-bubble backscatter when excited with a 40 MHz tone burst. Higher-order harmonic responses were observed for the three agents; however, their occurrence was at least partly due to nonlinear propagation. Only one of the agents (1.1 microm mean diameter) showed a subharmonic response for longer excitations (approximately 10-15 cycles) and midlevel pressure excitations ( 2.5 MPa). Theoretical calculations of the backscattered spectrum revealed behavior similar to the experimental results in specific parameter regimes.

Fig. 1

(Color online)(a) Wire reflector spectra for 1 cycle and 15 cycle tone bursts. (b) Spectra for agents P1 and P3 reveal harmonic components while only (c) P2 reveals any subharmonic activity.

Fig. 2

(Color online) Selected spectrograms for the P2 agent, 15 cycle excitation at 40 MHz with (a) 1.5 MPa and (b) 5.9 MPa negative peak pressure excitations.

Fig. 3

(Color online) Theoretical (a) R(t)/R₀ curve, (b) spectrum of the pressure calculated from the R(t) curve, and (c) spectrogram of pressure curve for P2 excited by a 40 MHz, 15 cycle, 5.9 MPa excitation.