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

Long-pulsed ultrasound-mediated microbubble thrombolysis in a rat model of microvascular obstruction

In up to 30% patients who experience acute myocardial infarction, successful recanalization of the epicardial coronary artery cannot provide adequate microvascular reperfusion. In this study, we sought to determine whether long-pulsed ultrasound (US)-mediated microbubble (MB) cavitation was useful for the treatment of microvascular obstruction, and the therapeutic effects were compared within different long-pulse-length and short-pulsed US. Microvascular obstruction model was established by injecting micro-thrombi into common iliac artery of a rat's hind limb. About 1 MHz US with different long pulse lengths (ranging from 100 to 50,000 cycles) was delivered, compared to short pulse (5 cycles). The control group was given MB only without therapeutic US. Contrast perfusion images were performed at baseline, emboli, and 1, 5, 10 min post-embolization, and peak plateau video intensity (A) was obtained to evaluate the therapeutic effects. Long-tone-burst US showed better thrombolytic effects than short-pulsed US (1,000, 5,000 cycles >500 cycles, >5 cycles, and control) (P < 0.01). 1,000 cycles group showed the optimal thrombolytic effect, but microvascular hemorrhage was observed in 50,000 cycles group. In conclusion, long-tone-burst US-enhanced MB therapy mediated successful thrombolysis and may offer a powerful approach for the treatment for microvascular obstruction within a certain pulse length.

Mediation of long-pulsed ultrasound enhanced microbubble recombinant tissue plasminogen activator thrombolysis in a rat model of platelet-rich thrombus

Background

Ultrasound (US)-enhanced microbubble (MB) therapy has been investigated as a therapeutic technique to facilitate the thrombolysis for the treatment of pericardial and microvascular obstruction. This study sought to assess the therapeutic effects of long-pulsed US-assisted MB-mediated recombinant tissue plasminogen activator (rt-PA) thrombolysis in a rat model of platelet-rich thrombus.

Methods

Ferric chloride (10%) was used to induce total arterial occlusion before formation of platelet-rich thrombi. Therapeutic long-tone-burst US (1 MHz, 0.6 MPa, 1,000-µs pulse length) was used, and 2.9×109/mL of lipid MBs and 1 mg/mL of rt-PA were infused. Subsequently, 42 Sprague-Dawley (SD) male rats were randomly divided into seven groups: (I) control; (II) rt-PA; (III) high duty cycle US + MB; (IV) low duty cycle US + rt-PA; (V) high duty cycle US + rt-PA; (VI) low duty cycle US + rt-PA + MB; and (VII) high duty cycle US + rt-PA + MB. The recanalization grades were evaluated after 20 minutes' treatment.

Results

Compared to the control, there was significant improvement in recanalization in the US + rt-PA groups (P=0.01 vs. control), US (low duty cycle) + rt-PA + MB (P=0.003 vs. control) and US (high duty cycle) + rt-PA + MB (P<0.001 vs. control) groups, in which recanalization was successfully achieved in all rats.

Conclusions

Long-pulsed US-enhanced MB-mediated rt-PA thrombolysis offered a powerful approach in the treatment of platelet-rich thrombus.

Quantifying the Effect of Acoustic Parameters on Temporal and Spatial Cavitation Activity: Gauging Cavitation Dose

OBJECTIVE

Cavitation-enhanced delivery of therapeutic agents is under development for the treatment of cancer and neurodegenerative and cardiovascular diseases, including sonothrombolysis for deep vein thrombosis. The objective of this study was to quantify the spatial and temporal distribution of cavitation activity nucleated by Definity infused through the EKOS catheter over a range of acoustic parameters controlled by the EKOS endovascular system.

METHODS

Three insonation protocols were compared in an in vitro phantom mimicking venous flow to measure the effect of peak rarefactional pressure, pulse duration and pulse repetition frequency on cavitation activity energy, location and duration. Inertial and stable cavitation activity was quantified using passive cavitation imaging, and a metric of cavitation dose based on energy density was defined.

RESULTS

For all three insonation protocols, cavitation was sustained for the entire 30 min Definity infusion. The evolution of cavitation energy during each pulse duration was similar for all three protocols. For insonation protocols with higher peak rarefactional acoustic pressures, inertial and stable cavitation doses also increased. A complex relationship between the temporal behavior of cavitation energy within each pulse and the pulse repetition frequency affected the cavitation dose for the three insonation protocols. The relative predominance of stable or inertial cavitation dose varied according to insonation schemes. Passive cavitation images revealed the spatial distribution of cavitation activity.

CONCLUSION

Our cavitation dose metric based on energy density enabled the impact of different acoustic parameters on cavitation activity to be measured. Depending on the type of cavitation to be promoted or suppressed, particular pulsing schemes could be employed in future studies, for example, to correlate cavitation dose with sonothrombolytic efficacy.