Ultrasound-mediated Cavitation Research Articles

Cavitation-Mediated Immunomodulation and Its Use with Checkpoint Inhibitors.

The promotion of anti-tumour immune responses can be an effective route to the complete remission of primary and metastatic tumours in a small proportion of patients. Hence, researchers are currently investigating various methods to further characterise and enhance such responses to achieve a beneficial impact across a wider range of patients. Due to its non-invasive, non-ionising, and targetable nature, the application of ultrasound-mediated cavitation has proven to be a popular method to enhance the delivery and activity of immune checkpoint inhibitors. However, to optimise this approach, it is important to understand the biological and physical mechanisms by which cavitation may promote anti-tumour immune responses. Here, the published literature relating to the role that cavitation may play in modulating anti-tumour immunity is therefore assessed.

High-Speed Imaging of Microsphere Transport by Cavitation Activity in a Tissue-Mimicking Phantom.

Ultrasound-mediated cavitation has been harnessed to improve the delivery of various therapeutics, including the extravasation of small molecule drugs and nanoparticles (<1 µm) into soft tissue. This study investigated whether cavitation could also enhance the extravasation of larger (>10 µm) therapeutic particles, representative of radio- or chemo-embolic particles, in a tissue-mimicking phantom. High-speed (103-106 frames/s) optical imaging was used to observe the motion of glass microspheres with diameters of 15-32 or 105-107 µm in an agar phantom under exposure to high-intensity focused ultrasound (0.5 MHz) at a range of peak negative pressures (1.9-2.8 MPa) in the presence of SonoVue microbubbles. In contrast to the microstreaming reported to be responsible for nanoparticle transport, the formation and translation of bubble clouds were found to be primarily responsible for the motion of glass microspheres. The bubble clouds were seen both to create channels in the phantom and to travel along them under the action of primary acoustic radiation force, either propelling or entraining microspheres with them. Collisions between microspheres were also seen to promote cloud formation and cavitation activity. Ultrasound-mediated cavitation can promote the transport of solid microparticles in tissue-mimicking material. Further work is needed to understand the influence of tissue mechanical properties and ultrasound exposure parameters on the extent and uniformity of particle distribution that can be achieved.

Improved Therapeutic Delivery Targeting Clinically Relevant Orthotopic Human Pancreatic Tumors Engrafted in Immunocompromised Pigs Using Ultrasound-Induced Cavitation: A Pilot Study.

Pancreatic tumors can be resistant to drug penetration due to high interstitial fluid pressure, dense stroma, and disarrayed vasculature. Ultrasound-induced cavitation is an emerging technology that may overcome many of these limitations. Low-intensity ultrasound, coupled with co-administered cavitation nuclei consisting of gas-stabilizing sub-micron scale SonoTran Particles, is effective at increasing therapeutic antibody delivery to xenograft flank tumors in mouse models. Here, we sought to evaluate the effectiveness of this approach in situ using a large animal model that mimics human pancreatic cancer patients. Immunocompromised pigs were surgically engrafted with human Panc-1 pancreatic ductal adenocarcinoma (PDAC) tumors in targeted regions of the pancreas. These tumors were found to recapitulate many features of human PDAC tumors. Animals were intravenously injected with the common cancer therapeutics Cetuximab, gemcitabine, and paclitaxel, followed by infusion with SonoTran Particles. Select tumors in each animal were targeted with focused ultrasound to induce cavitation. Cavitation increased the intra-tumor concentrations of Cetuximab, gemcitabine, and paclitaxel by 477%, 148%, and 193%, respectively, compared to tumors that were not targeted with ultrasound in the same animals. Together, these data show that ultrasound-mediated cavitation, when delivered in combination with gas-entrapping particles, improves therapeutic delivery in pancreatic tumors under clinically relevant conditions.

Ultrasound-Mediated Cavitation of Magnetic Nanoparticles for Drug Delivery Applications

Abstract Magnetic nanoparticles guided by an external magnetic field can be used to deliver drugs (e.g., chemotherapeutics) to malignant regions (e.g., tumor tissue). Some nonmagnetic, sonosensitive nanoparticles are known to generate cavitation induced by focused ultrasound. This can can be used to monitor spatial/temporal particle distribution and to stimulate local drug release in target regions. Little to nothing is known about the sonosensitivity of magnetic nanoparticles. Therefore, we investigated this physical phenomenon in the present contribution. We showed that certain magnetic nanoparticles have high cavitation potential and are promising for further research in this regard.

Ultrasound improved immune adjuvant delivery to induce DC maturation and T cell activation.

Tumor immunotherapy has emerged as a promising approach to tumor treatment. Currently, immune adjuvant-based therapeutic modalities are rarely curative in solid tumors owing to challenges including the low permeability and extremely poor water solubility of these adjuvants, limiting their ability to effectively promote dendritic cell (DC) maturation. Herein, we employed ultrasound-mediated cavitation (UMC) to promote the delivery of Toll-like receptor agonist (R837)-loaded pH-responsive liposomes (PEOz-Lip@R837) to tumors. The tumor-associated antigens (TAAs) produced by UMC treatment exhibited vaccinal activity, particularly in the presence of immune adjuvants, together promoting the maturation of DC and inducing cytokine production. Importantly, UMC can down-regulate immune checkpoint molecules, like Cd274, Foxp3 and Ctla4, synergistically stimulating the activation and proliferation of T cells in the body to facilitate tumor treatment. This UMC-enhanced PEOz-Lip@R837 approach was able to induce a robust antitumor immune response capable of arresting primary and distant tumor growth, while also developing immunological memory, protecting against tumor rechallenge following initial tumor clearance. Overall, these results highlight a promising UMC- and pH-sensitive immune adjuvant delivery-based treatment for tumors with the potential for clinical application.

Ultrasound combined with nanobubbles promotes systemic anticancer immunity and augments anti-PD1 efficacy

BackgroundThe poor immunogenicity of solid tumors limits the efficacy ofanti-programmed cell death protein 1 (anti-PD1)-based immune checkpoint blockade (ICB); thus, less than 30% of patients with cancer exhibit a response....

Ultrasound-Mediated Cavitation Enhances EGFR-Targeting PLGA-PEG Nano-Micelle Delivery for Triple-Negative Breast Cancer Treatment.

Simple SummaryTriple-negative breast cancer (TNBC) with negative expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor 2 (HER2) is considered to be associated with poorer outcomes and a higher risk of recurrence or metastasis owing to a lack of effective targeted therapeutic drugs. The epidermal growth factor receptor (EGFR) functions is a driver of disease progression in most of TNBC that represents a viable target that can be leveraged to guide the intra-tumoral delivery of chemotherapeutic drugs in TNBC patients. Moreover, ultrasound-mediated cavitation (UMC) strategies increase tissue permeability and extravasation through nuclei-dependent cavitation via sonoporation, thus enabling drugs to better enter target tissues. In this research, a combination of active, targeting nano-micelles and UMC was able to inhibit TNBC tumor growth effectively at lower concentrations while reducing treatment-related toxicity. Thus, this is a very promising treatment strategy in the clinical therapy with TNBC and other cancer types.Triple-negative breast cancer (TNBC) is highly recurring and metastatic breast cancer with overexpressing epidermal growth factor receptor (EGFR). Herein, a series of in vitro and in vivo analyses were used to explore the therapeutic effect of EGFR-targeting nano-micelles (PLGA-PEG/DOX@anti-EGFR) combined with ultrasound-mediated cavitation (UMC). The prepared nano-micelle drug carriers have good biocompatibility and can greatly increase the drug accumulation in tumor regions, thereby reducing off-target toxicity while enhancing anti-tumor efficacy. Moreover, an in vivo analysis of the practical utility of this treatment modality was conducted by using SonoVueTM microbubbles to achieve cavitation under different power intensity levels, with an ultrasonic power intensity of 0.5 W/cm2 maximizing the intra-tumoral blood perfusion. Relative to PLGA-PEG@DOX/anti-EGFR nano-micelles treatment alone, the combination with UMC was better able to suppress tumor growth even at low concentrations. As such, combining actively targeted drug-carrier molecules with UMC represents an effective approach to enhancing therapeutic efficacy while reducing the adverse, systemic effects associated with DOX and other chemotherapeutic drugs, and it can be considered as a promising clinical prospect in the treatment of TNBC.

Improved therapeutic antibody delivery to xenograft tumors using cavitation nucleated by gas-entrapping nanoparticles.

Aims: Testing ultrasound-mediated cavitation for enhanced delivery of the therapeutic antibody cetuximab to tumors in a mouse model. Methods: Tumors with strong EGF receptor expression were grown bilaterally. Cetuximab was coadministered intravenously with cavitation nuclei, consisting of either the ultrasound contrast agent Sonovueor gas-stabilizing nanoscale SonoTranParticles. One of the two tumors was exposed to focused ultrasound. Passive acoustic mapping localized and monitored cavitation activity. Both tumors were then excised and cetuximab concentrationwasquantified. Results: Cavitation increased tumoral cetuximab concentration. When nucleated by Sonovue, a 2.1-fold increase (95% CI 1.3- to3.4-fold) was measured, whereas SonoTranParticles gave a 3.6-fold increase (95% CI 2.3- to5.8-fold). Conclusions: Ultrasound-mediated cavitation, especially when nucleated by nanoscale gas-entrapping particles, can noninvasively increase site-specific delivery of therapeutic antibodies to solid tumors.

Flow Augmentation in the Myocardium by Ultrasound Cavitation of Microbubbles: Role of Shear-Mediated Purinergic Signaling

Ultrasound-mediated cavitation of microbubble contrast agents produces high intravascular shear. We hypothesized that microbubble cavitation increases myocardial microvascular perfusion through shear-dependent purinergic pathways downstream from ATP release that is immediate and sustained through cellular ATP channels such as Pannexin-1. Quantitative myocardial contrast echocardiography perfusion imaging and invivo optical imaging of ATP was performed in wild-type and Pannexin-1-deficient (Panx1-/-) mice before and 5 and 30minutes after 10minutes of ultrasound-mediated (1.3MHz, mechanical index 1.3) myocardial microbubble cavitation. Flow augmentation in a preclinical model closer to humans was evaluated in rhesus macaques undergoing myocardial contrast echocardiography perfusion imaging after high-power cavitation in the apical four-chamber plane for 10minutes. Microbubble cavitation in wild-type mice (n=7) increased myocardial perfusion by 64%±25% at 5minutes and 95%±55% at 30minutes compared with baseline (P<.05). In Panx1-/- mice (n=5), perfusion increased by 28%±26% at 5minutes (P=.04) but returned to baseline at 30minutes. Myocardial ATP signal in wild-type (n=7) mice undergoing cavitation compared with sham-treated controls (n=3) was 450-fold higher at 5minutes and 90-fold higher at 30minutes after cavitation (P<.001). The ATP signal in Panx1-/- mice (n=4) was consistently 10-fold lower than that in wild-type mice and was similar to sham controls at 30minutes. In macaques (n=8), myocardial perfusion increased twofold in the cavitation-exposed four-chamber plane, similar in degree to that produced by adenosine, but did not increase in the control two-chamber plane. Cavitation of microbubbles in the myocardial microcirculation produces an immediate release of ATP, likely from cell microporation, as well as sustained release, which is channel dependent and responsible for persistent flow augmentation. These findings provide mechanistic insight by which cavitation improves perfusion and reduces infarct size in patients with myocardial infarction.

A method for the transection of the liver parenchyma using ultrasound-based energy devices: do we still need the CUSA?

Background: Laparoscopic liver resection is generally accepted for the treatment of patients with primary or secondary liver tumors. CUSA is one of the techniques for the parenchymal dissection of the liver and has been adopted in laparoscopic setting for precise preparation of intrahepatic structure. However this technique requires sophisticated instruments and the learning curve is long. Methods: Here we demonstrate a method by using the ultrasound-based energy device and taking the advantages of ultrasound-mediated cavitation for the dissection of the liver parenchyma. The intrahepatic structure could be dissected precisely by the transducer of the device under the continuously dripping of saline to the surface of transection plane. The tissue debris would be flashed away by the saline. The vessels could be sealed and divided directly by using the same device with or without clipping. The technique achieves the similar effect as by using CUSA. The advantages of the technique consists of the less instrument demand, the simplicity in the handling and less frequently in the changing of instruments during the transection of the liver. Results: A parallel demonstration of a laparoscopic right hepatectomy using anterior approach and a left hepatectomy in patients with malignant liver tumors will be showed in the videos. Conclusion: The method might replace CUSA as a simple, efficient and cost-effective surgical technique for laparoscopic liver resection.

Ultrasound-mediated cavitation does not decrease the activity of small molecule, antibody or viral-based medicines.

The treatment of cancer using nanomedicines is limited by the poor penetration of these potentially powerful agents into and throughout solid tumors. Externally controlled mechanical stimuli, such as the generation of cavitation-induced microstreaming using ultrasound (US), can provide a means of improving nanomedicine delivery. Notably, it has been demonstrated that by focusing, monitoring and controlling the US exposure, delivery can be achieved without damage to surrounding tissue or vasculature. However, there is a risk that such stimuli may disrupt the structure and thereby diminish the activity of the delivered drugs, especially complex antibody and viral-based nanomedicines. In this study, we characterize the impact of cavitation on four different agents, doxorubicin (Dox), cetuximab, adenovirus (Ad) and vaccinia virus (VV), representing a scale of sophistication from a simple small-molecule drug to complex biological agents. To achieve tight regulation of the level and duration of cavitation exposure, a “cavitation test rig” was designed and built. The activity of each agent was assessed with and without exposure to a defined cavitation regime which has previously been shown to provide effective and safe delivery of agents to tumors in preclinical studies. The fluorescence profile of Dox remained unchanged after exposure to cavitation, and the efficacy of this drug in killing a cancer cell line remained the same. Similarly, the ability of cetuximab to bind its epidermal growth factor receptor target was not diminished following exposure to cavitation. The encoding of the reporter gene luciferase within the Ad and VV constructs tested here allowed the infectivity of these viruses to be easily quantified. Exposure to cavitation did not impact on the activity of either virus. These data provide compelling evidence that the US parameters used to safely and successfully delivery nanomedicines to tumors in preclinical models do not detrimentally impact on the structure or activity of these nanomedicines.

Augmentation of Muscle Blood Flow by Ultrasound Cavitation Is Mediated by ATP and Purinergic Signaling.

Augmentation of tissue blood flow by therapeutic ultrasound is thought to rely on convective shear. Microbubble contrast agents that undergo ultrasound-mediated cavitation markedly amplify these effects. We hypothesized that purinergic signaling is responsible for shear-dependent increases in muscle perfusion during therapeutic cavitation. Unilateral exposure of the proximal hindlimb of mice (with or without ischemia produced by iliac ligation) to therapeutic ultrasound (1.3 MHz, mechanical index 1.3) was performed for 10 minutes after intravenous injection of 2×108 lipid microbubbles. Microvascular perfusion was evaluated by low-power contrast ultrasound perfusion imaging. In vivo muscle ATP release and in vitro ATP release from endothelial cells or erythrocytes were assessed by a luciferin-luciferase assay. Purinergic signaling pathways were assessed by studying interventions that (1) accelerated ATP degradation; (2) inhibited P2Y receptors, adenosine receptors, or KATP channels; or (3) inhibited downstream signaling pathways involving endothelial nitric oxide synthase or prostanoid production (indomethacin). Augmentation in muscle perfusion by ultrasound cavitation was assessed in a proof-of-concept clinical trial in 12 subjects with stable sickle cell disease. Therapeutic ultrasound cavitation increased muscle perfusion by 7-fold in normal mice, reversed tissue ischemia for up to 24 hours in the murine model of peripheral artery disease, and doubled muscle perfusion in patients with sickle cell disease. Augmentation in flow extended well beyond the region of ultrasound exposure. Ultrasound cavitation produced an ≈40-fold focal and sustained increase in ATP, the source of which included both endothelial cells and erythrocytes. Inhibitory studies indicated that ATP was a critical mediator of flow augmentation that acts primarily through either P2Y receptors or adenosine produced by ectonucleotidase activity. Combined indomethacin and inhibition of endothelial nitric oxide synthase abolished the effects of therapeutic ultrasound, indicating downstream signaling through both nitric oxide and prostaglandins. Therapeutic ultrasound using microbubble cavitation to increase muscle perfusion relies on shear-dependent increases in ATP, which can act through a diverse portfolio of purinergic signaling pathways. These events can reverse hindlimb ischemia in mice for >24 hours and increase muscle blood flow in patients with sickle cell disease. URL: http://clinicaltrials.gov. Unique identifier: NCT01566890.

Augmentation of limb perfusion and reversal of tissue ischemia produced by ultrasound-mediated microbubble cavitation.

Ultrasound can increase tissue blood flow, in part, through the intravascular shear produced by oscillatory pressure fluctuations. We hypothesized that ultrasound-mediated increases in perfusion can be augmented by microbubble contrast agents that undergo ultrasound-mediated cavitation and sought to characterize the biological mediators. Contrast ultrasound perfusion imaging of hindlimb skeletal muscle and femoral artery diameter measurement were performed in nonischemic mice after unilateral 10-minute exposure to intermittent ultrasound alone (mechanical index, 0.6 or 1.3) or ultrasound with lipid microbubbles (2×10(8) IV). Studies were also performed after inhibiting shear- or pressure-dependent vasodilator pathways, and in mice with hindlimb ischemia. Ultrasound alone produced a 2-fold increase (P<0.05) in muscle perfusion regardless of ultrasound power. Ultrasound-mediated augmentation in flow was greater with microbubbles (3- and 10-fold higher than control for mechanical index 0.6 and 1.3, respectively; P<0.05), as was femoral artery dilation. Inhibition of endothelial nitric oxide synthase attenuated flow augmentation produced by ultrasound and microbubbles by 70% (P<0.01), whereas inhibition of adenosine-A2a receptors and epoxyeicosatrienoic acids had minimal effect. Limb nitric oxide production and muscle phospho-endothelial nitric oxide synthase increased in a stepwise fashion by ultrasound and ultrasound with microbubbles. In mice with unilateral hindlimb ischemia (40%-50% reduction in flow), ultrasound (mechanical index, 1.3) with microbubbles increased perfusion by 2-fold to a degree that was greater than the control nonischemic limb. Increases in muscle blood flow during high-power ultrasound are markedly amplified by the intravascular presence of microbubbles and can reverse tissue ischemia. These effects are most likely mediated by cavitation-related increases in shear and activation of endothelial nitric oxide synthase.

VEGF165 gene delivery to uterine arteries by ultrasound-mediated cavitation of plasmid-conjugated microbubbles prevents fetal growth restriction in transgenic mouse model of placental insufficiency

The effect of ultrasound exposure on human lens epithelial cells (HLE-B3) was investigated in vitro, specifically on the generation of oxidative stress upon ultrasound application using various clinically-relevant settings. In addition to ultrasound-induced heat effects, oxidative stress has been recently proposed as one of the main mechanisms for ultrasound-induced effects on human cells. In this work, the levels of biocompatibility and generation of oxidative stress by exposure of ultrasound to HLE-B3 were evaluated quantitatively and qualitatively by the MTT assay, Live/Dead assay, reactive oxygen species (ROS) and intracellular calcium level. Oxidative stress induction is traditionally achieved through administrations of H2O2 and thus the administration of H2O2 was used as the positive control group for comparison herein. Concerning the administrations of H2O2 are considered invasive and may potentially have side effects, ultrasound as physical stimulation could be a safer and non-invasive method to induce similar oxidative stress environments. The effect of ultrasound on cell viability and induction of oxidative stress increases with ultrasound intensity. The result reveals that the continuous ultrasound has a positive impact on the oxidative stress levels but does negatively on the cell viability, as compared to the pulsed ultrasound. Furthermore, our work demonstrates that the exposure of 58 kPa continuous ultrasound without microbubbles can maintain acceptable cell viability and produce oxidative stress effects similar to the traditional administrations of H2O2. In summary, exposure of ultrasound can generate oxidative stress comparable to traditional administrations of H2O2. The effect of generating oxidative stress is adjustable through ultrasound parameters, including the pulsed or continuous wave, the intensity of ultrasound and addition of microbubbles.

IUGR in rats is associated with changes in placental expression of MTAs

IUGR in rats is associated with changes in placental expression of MTAs

Improving the cardio protective effect of aFGF in ischemic myocardium with ultrasound-mediated cavitation of heparin modified microbubbles: preliminary experiment

Ultrasound (US)-mediated cavitation of microbubbles has evolved into a new tool for organ-specific gene and drug delivery. This paper was to investigate the feasibility of acidic fibroblast growth factor (aFGF) intravenous delivery to the ischemic myocardium of rats by ultrasonic microbubbles modified with heparin. Heparin modified microbubbles (HMB) were prepared by the freeze-dried method. Acute myocardial infarction (AMI) model was established and the cardio protective effect of the aFGF combing with HMB (aFGF-HMB) under US-mediated cavitation technique was investigated. aFGF-HMB combined with US-mediated cavitation technique was examined by ECG. Ejection fraction (EF), fractional shortening (FS) and left ventricular diastolic diameter (LVDd) were measured to monitor the improvement of global myocardial contractile function. Myocardial tissue was stained with hematoxylin and eosine (HE) to evaluate the elaborate general morphology of the ischemic myocardium. From morphologic observation and echocardiography in rat heart, aFGF-HMB had suitable size distribution, physical stability and good acoustic resonance function. From AMI rat experiments, aFGF-HMB under US-mediated cavitation technique exerted aFGF cardio protective effect in ischemic myocardium. From histological evaluation, US-mediated cavitation of aFGF-HMB showed improvement of myocardial ischemia. With the visual imaging and US-triggered drug release advantages, US-mediated cavitation of aFGF-HMB might be developed as a novel technique for targeting delivery of aFGF into ischemic myocardium.

Transfection of Ang-1 gene via ultrasound-mediated SonoVue microbubble destruction in vitro and in vivo

Objective To investigate the transfection efficacy and expression of Ang-1 gene and proangiogenesis in vitro and vivo by ultrasound-mediated microbubble destruction.Methods 293T cells were divided into three groups:group A was given hAng-1 plasmid and microbubbles plus ultrasonic irradiation,group B was given hAng-1 plasmid and ultrasound,group C was given hAng-1 plasmid only (without ultrasound).Forty-eight hours after transfection,the transient expression rate was observed under fluorescence microscopy and flow cytometry.RT-PCR and Western blot analysis were taken to evaluate the mRNA and protein expression of Ang-1 respectively.Twenty-seven rabbit models of ligated left circumflex branch coronary artery were divided into 3 groups randomly as follow:group Ⅰ (accepted intravenous injection of SonoVue microbubble and Ang-1 plus ultrasonic irradiation),group Ⅱ (accepted intravenous injection of Ang-1 with ultrasound),group Ⅲ (control group).Myocardial contrast echocardiography (MCE) was executed on all animals before and after the treatment.Two weeks after gene delivery,RT-PCR and Western blot analysis were taken to evaluate mRNA and protein expression of Ang-1 respectively.Microvessel density (MVD) counting of infracted myocardium,observed by Factor Ⅷ immunochemical staining,was performed to value the proangiogenesis effect of Ang-1 delivered by ultrasound mediated cavitation of microbubble.Results Green fluorescence was observed in group A and B by fluorescence microscopy,which was negative in group C.The transfection expression rate was significantly improved in group A ( P ＜ 0.01).In vivo,Microbubbles could be observed in former ischemic myocardium in MCEexamination and the Ang-1 mRNA and protein could be detected in group Ⅰ.On the other hand,the contrast agent was defected obviously and none of the animals showed Ang-1 mRNA and protein expression in other two groups.The MVD counting showed significant improvement in group Ⅰ whereas other two groups didn't.Conclusions Microbubble-enhanced ultrasound exposure can improve the Ang-1 gene transfection expression rate observably both in vitro and in vivo.This strategy for delivering has great proangiogenesis effect in vivo. Key words: Ultrasonography; Microbubbles; Transfection; Angiopoietin-1

Assay of hydroxyl radicals generated by focused ultrasound

Water sonolysis leads to the formation of hydroxyl radicals (OH ). Various techniques are used to detect the OH production and thus to assess the level of ultrasound-mediated cavitation generated in vitro. In this study, we used terephthalic acid (TA) as an OH trap. This method is based on the fluorescent properties of hydroxyterephthalic acid (HTA) formed by the reaction of TA with OH and used as an indicator of the degree of inertial cavitation caused. The experimental system is comprised mainly of a focused piezoelectric ultrasound transmitter and a measurement cell containing 1X PBS/TA diluted solution. In the first part, we aimed to characterize the most appropriate experimental conditions (TA dosimeter solution, irradiation time) in order to optimize the resulting HTA fluorescence values. Then, we could determine that the HTA production increased with the level of the cavitation phenomenon caused by the acoustic power from which OH production may be estimated.

Ultrasound-mediated cavitation thresholds of liquid perfluorocarbon droplets in vitro

This study was undertaken to measure the ultrasound (US)-mediated cavitation threshold of microdroplets as a function of its content and US parameters (frequency, amplitude and burst length). Albumin-coated droplets were prepared with perfluoropropane, perfluorohexane or perfluoromethylcyclohexane contents. The filtered suspensions were diluted to 1:1000 (v) and compared with Optison“. The formulations were injected into an acoustically transparent vessel and sonicated with a single focused transducer. The frequencies employed were 0.74, 1.1, 2.18 and 3.3 MHz and the burst length and acoustic pressure were varied. The inertial cavitation threshold for each experiment was monitored through passive acoustic detection. The formation of droplet emulsion of the perfluorocarbon increased the natural boiling point of the perfluorocarbon. However, perfluorocarbon droplets having contents with higher molecular weights and boiling points did not have detectably higher inertial cavitation thresholds and, thus, the droplets do not need to be in a superheated state to be cavitated by US bursts. Therefore, higher boiling point perfluorocarbons should be investigated for this purpose and may prove to be useful for both imaging and therapy. The inertial cavitation threshold of perfluorocarbon droplets increases with frequency, and was approximately 0.7 MPa at 0.74 MHz and 1.75 MPa at 3.3 MHz. Optison”, already in a gaseous state, has the lowest cavitation threshold of all formulations studied. Results show that, for the frequencies tested, there is no dependence between inertial cavitation threshold and burst lengths between 20 and 100 ms. As a conclusion, the inertial cavitation threshold of albumin-coated microdroplets of several perfluorocarbons was determined in vitro. The results indicate that the physical properties of these droplets are such that they may be useful for localized US therapies. (E-mail: kullervo@bwh.harvard.edu)