Objectives: To characterize the ability of high-intensity focused ultrasound to achieve thrombolysis in vitro and investigate the feasibility of this approach as a means of restoring blood flow in thrombus-occluded arteries in vivo. Materials and Methods: All experiments were approved by the Institutional Animal Care Committee. Thrombolysis was performed with a 1.51-MHz focused ultrasound transducer with pulse lengths of 0.1 to 10 milliseconds and acoustic powers up to 300 W. In vitro experiments were performed with blood clots formed from rabbit arterial blood and situated in 2-mm diameter tubing. Both single location and flow bypass recanalization experiments were conducted. In vitro clot erosion was assessed with 30-MHz ultrasound, with debris size measured with filters and a Coulter counter. In vivo clots were initiated in the femoral arteries of rabbits (n = 26). Cavitation signals from bubbles formed during exposure were monitored. In vivo flow restoration was assessed with 23-MHz Doppler ultrasound. Results: At a single location, in vitro clot erosion volumes increased with exposure power and pulse length, with debris size reducing with increasing pulse length. Flow bypass experiments achieved 99.2% clot erosion with 1.1% of debris above 0.5 mm in size. In vivo, 10 milliseconds pulses were associated with bleeding, but at 1 millisecond, it was feasible to achieve partial flow restoration in 6of the 10 clots with only 1of the 10 showing evidence of bleeding. In all cases, thrombolysis occurred only in the presence of cavitation. Conclusion: High-intensity focused ultrasound thrombolysis is feasible as a means of restoring partial blood flow in thrombus-occluded arteries in the absence of thrombolytic agents. The potential for bleeding with this approach requires further investigation.

Ultrasound stimulated microbubbles (USMB) are being investigated for their potential to promote the uptake of anticancer agents into tumor tissue by exploiting their ability to enhance microvascular permeability. At sufficiently high ultrasound transmit amplitudes it has also recently been shown that USMB treatments can, on their own, induce vascular damage, shutdown blood flow, and inhibit tumor growth. The objective of this study is to examine the antitumor effects of ‘antivascular’ USMB treatments in conjunction with chemotherapy, which differs from previous work which has sought to enhance drug uptake with USMBs by increasing vascular permeability. Conceptually this is a strategy similar to combining vascular disrupting agents with a chemotherapy, and we have selected the taxane docetaxel (Taxotere) for evaluating this approach as it has previously been shown to have potent antitumor effects when combined with small molecule vascular disrupting agents. Experiments were conducted on PC3 tumors implanted in athymic mice. USMB treatments were performed at a frequency of 1 MHz employing sequences of 50 ms bursts (0.00024 duty cycle) at 1.65 MPa. USMB treatments were administered on a weekly basis for 4 weeks with docetaxel (DTX) being given intravenously at a dose level of 5 mg/kg. The USMB treatments, either alone or in combination with DTX, induced an acute reduction in tumor perfusion which was accompanied at the 24 hour point by significantly enhanced necrosis and apoptosis. Longitudinal experiments showed a modest prolongation in survival but no significant growth inhibition occurred in DTX–only and USMB-only treatment groups relative to control tumors. The combined USMB-DTX treatment group produced tumor shrinkage in weeks 4–6, and significant growth inhibition and survival prolongation relative to the control (p<0.001), USMB-only (p<0.01) and DTX-only treatment groups (p<0.01). These results suggest the potential of enhancing the antitumor activity of docetaxel by combining it with antivascular USMB effects.

Considerable effort is being directed toward investigating the use of ultrasound (US) stimulated microbubbles (MB) to promote the uptake of anticancer agents in tumors. In this study we propose and investigate a new method for combining therapeutic ultrasound with anticancer agents, which is to induce antivascular effects and combine these with an antiangiogenic treatment strategy, in this case metronomic chemotherapy. This is effectively a vascular targeting rather than a drug delivery approach. Experiments were conducted on MDA‐MB‐231 breast cancer tumors implanted in athymic mice. Metronomic cyclophosphamide (MCTX) was employed as an antiangiogenic therapy and was administered through the drinking water. Ultrasound stimulated microbubble treatments (USMB) were conducted at 1 MHz employing short bursts (0.00024 duty cycle) at 1.6 MPa in combination with the commercial microbubble agent Definity. USMB treatments were performed on a weekly basis for 4 weeks and MCTX was administered for 10 weeks. The USMB induced an acute reduction of blood flow as confirmed with US contrast imaging and DiOC₇ perfusion staining. Longitudinal experiments demonstrated that significant growth inhibition occurred in MCTX‐only and USMB‐only treatment groups relative to control tumors. The combined USMB and MCTX treatment group showed significant growth inhibition and survival prolongation relative to the USMB‐only (p < 0.01) and MCTX‐only treatment groups (p < 0.01). These results indicate the feasibility of a new approach to combining therapeutic ultrasound with an anticancer agent.

Focused ultrasound with microbubbles is an emerging technique for blood-brain barrier opening. Here, a comprehensive theoretical model of a bubble-fluid-vessel system has been developed which accounts for the bubble's nonspherical oscillations inside a microvessel, and its resulting acoustic emissions. Numerical simulations of unbound and confined encapsulated bubbles were performed to evaluate the effect of the vessel wall on acoustic emissions and vessel wall stresses. Using a Marmottant shell model, the normalized second harmonic to fundamental emissions first decreased as a function of pressure (>50 kPa) until reaching a minima (“transition point”) at which point they increased. The transition point of unbound compared to confined bubble populations occurred at different pressures and was associated with an accompanying increase in shear and circumferential wall stresses. As the wall stresses depend on the bubble to vessel wall distance, the stresses were evaluated for bubbles with their wall at a constant distance to a flat wall. As a result, the wall stresses were bubble size and frequency dependent and the peak stress values induced by bubbles larger than resonance remained constant versus frequency at a constant mechanical index.