Dynamic Behavior of Polymer Microbubbles During Long Ultrasound Tone-Burst Excitation and Its Application for Sonoreperfusion Therapy

Abstract

Objective: Ultrasound (US)-targeted microbubble (MB) cavitation (UTMC)-mediated therapies have been found to restore perfusion and enhance drug/gene delivery. Because of the potentially longer circulation time and relative ease of storage and reconstitution of polymer-shelled MBs compared with lipid MBs, we investigated the dynamic behavior of polymer microbubbles and their therapeutic potential for sonoreperfusion (SRP) therapy.

Methods: The fate of polymer MBs during a single long tone-burst exposure (1 MHz, 5 ms) at various acoustic pressures and MB concentrations was recorded via high-speed microscopy and passive cavitation detection (PCD). SRP efficacy of the polymer MBs was investigated in an in vitro flow system and compared with that of lipid MBs.

Discussion: Microscopy videos indicated that polymer MBs formed gas-filled clusters that continued to oscillate, fragment and form new gas-filled clusters during the single US burst. PCD confirmed continued acoustic activity throughout the 5-ms US excitation. SRP efficacy with polymer MBs increased with pulse duration and acoustic pressure similarly to that with lipid MBs but no significant differences were found between polymer and lipid MBs.

Conclusion: These data suggest that persistent cavitation activity from polymer MBs during long tone-burst US excitation confers excellent reperfusion efficacy.

Keywords: High-speed microscopy; Microbubble dynamics; Sonoreperfusion; Ultrasound contrast agents; Ultrasound therapy; Ultrasound-targeted microbubble cavitation.

Figure 1.

(a) A simplified schematic diagram of the experimental apparatus for high-speed microscopy and passive cavitation detection. The system comprised a multi-frame high speed camera, a custom microscope, an US and optical imaging chamber, an US delivery system, and a passive cavitation detector. (b) A schematic diagram of the US and optical imaging chamber. (c) A schematic diagram of the sonoreperfusion therapy.

Figure 2.

The fate of polymer MBs during US excitation at different time points of a 5 ms treatment at various acoustic pressures at 2×10⁶ MB/mL. Only one frame of each movie is displayed, chosen at approximately the maximum expansion of the MB or MB cluster. The frame size is 325×599 μm.

Figure 3.

The fate of polymer MBs during US excitation at different time points of a 5 ms treatment at various acoustic pressures at 2×10⁷ MB/mL. The frame size is 325×599 μm. Also see online supplemental movies 1–5.

Figure 4.

The fate of polymer MB during US excitation at different time points of a 5 ms treatment at various acoustic pressures at 2×10⁸ MB/mL. The frame size is 325×599 μm.

Figure 5.

Sequential still frames of high-speed movie showing the detailed acoustic activity of a cluster of polymer MBs 500 acoustic cycles into a long tone-burst (1 MHz, 1.5 MPa). For this example, the movie was taken at 5 Mfps with a 60× objective and the movie duration was 25.6 μs. The frame size is 100×100 μm. Also see supplemental movie 6.

Figure 6.

Mean joint time-frequency spectra of the acoustic activity of polymer MBs during US excitation of 5 ms treatment at various acoustic pressures and concentrations (n=10 for each setting). (a) 0.25 MPa; (b) 0.50 MPa; (c) 1.0 MPa; (d) 1.5 MPa. Column 1: No MB; Column 2: 2×10⁶ MB/mL; Column 3: 2×10⁷ MB/mL; Column 4: 2×10⁸ MB/mL. Display dynamic range is 80 dB.

Figure 7.

Inertial cavitation strength (a-c) and cumulative inertial cavitation dose (d-f) of the acoustic activity of polymer MBs during 5 ms US excitation at various acoustic pressures at 2×10⁶ MB/mL (a, d), 2×10⁷ MB/mL (b, e), and 2×10⁸ MB/mL (c, f), respectively (n=10 for each setting).

Figure 8.

Comparison of the cumulative inertial cavitation dose (mean ± standard deviation, n=10 for each setting) of the acoustic activity of lipid MBs during US excitation of a 5 ms treatment at various acoustic pressures and concentrations. Total ICD increased with acoustic pressure for each MB concentration used (p<0.05, ANOVA) and increased with MB concentration for acoustic pressures at 0.5 MPa, 1.0 MPa and 1.5 MPa (p<0.05, ANOVA).

Figure 9.

Time course of pressure gradient across the mesh during UTMC treatment with polymer and lipid MBs. (a) 0.6 MPa, (b) 1.0 MPa, (c) 1.5 MPa.

Figure 10.

Therapeutic efficacy during UTMC treatment with polymer and lipid MBs. (a) Terminal pressure drop; (b) Lytic rate. Both parameters increased with acoustic pressure (p<0.05, ANOVA) and pulse length (p<0.05, ANOVA), but no significant differences were found between polymer and lipid microbubbles.