Phase-change Nanodroplets Research Articles

A Preliminary Investigation of Radiation-Sensitive Ultrasound Contrast Agents for Photon Dosimetry.

Radiotherapy treatment plans have become highly conformal, posing additional constraints on the accuracy of treatment delivery. Here, we explore the use of radiation-sensitive ultrasound contrast agents (superheated phase-change nanodroplets) as dosimetric radiation sensors. In a series of experiments, we irradiated perfluorobutane nanodroplets dispersed in gel phantoms at various temperatures and assessed the radiation-induced nanodroplet vaporization events using offline or online ultrasound imaging. At 25 °C and 37 °C, the nanodroplet response was only present at higher photon energies (≥10 MV) and limited to <2 vaporization events per cm2 per Gy. A strong response (~2000 vaporizations per cm2 per Gy) was observed at 65 °C, suggesting radiation-induced nucleation of the droplet core at a sufficiently high degree of superheat. These results emphasize the need for alternative nanodroplet formulations, with a more volatile perfluorocarbon core, to enable in vivo photon dosimetry. The current nanodroplet formulation carries potential as an innovative gel dosimeter if an appropriate gel matrix can be found to ensure reproducibility. Eventually, the proposed technology might unlock unprecedented temporal and spatial resolution in image-based dosimetry, thanks to the combination of high-frame-rate ultrasound imaging and the detection of individual vaporization events, thereby addressing some of the burning challenges of new radiotherapy innovations.

Exploring the Postactivation Behavioral Patterns of Intratumorally Injected Theranostic Nanodroplets: An Ultrasound-Only Extravascular Monitoring Technique.

Phase-change nanodroplets (PCNDs) are customizable and controllable theranostic agents of particular interest in extravascular therapies such as drug delivery and histotripsy. High-bulk-boiling-point (HBP) PCNDs are preferred for their enhanced thermal stability under physiological temperature to achieve on-demand therapeutic effects on target sites-mainly in tumor tissue. However, the behavioral patterns of high-concentration, heterogeneously distributed HBP PCNDs in vivo have rarely been explored-the foci of PCND-related therapies mostly fall on the final therapeutic effect rather than the detailed behaviors of PCNDs, which may hamper the development and improvement of in vivo treatments with PCNDs. To fill the gap, we demonstrate an ultrasound-only extravascular monitoring technique to analyze the underlying behavioral patterns of intratumorally injected HBP PCNDs. In our hypothesis, recondensation and coalescence are the two predominant patterns influencing the trend of the postactivation signal of PCNDs. A "blink map" method was, thus, proposed to separate the two parts of the signal by recognizing the unique signal pattern of stochastic recondensation, and four derivative metrics were calculated for further analysis. The results revealed the postactivation patterns of PCNDs at different activation-pulse durations and activation stages throughout the activation-imaging period, and several general trends were observed and explained by existing theories, suggesting the feasibility of our extravascular monitoring technique. Overall, this work enriches the knowledge of the characteristics of HBP PCNDs as extravascular theranostic agents, and the monitoring results have the potential to provide timely feedback on PCND-related treatments underway, which may help adjust the treatment strategy and improve the therapeutic efficacy.

Effect of Phase-Change Nanodroplets and Ultrasound on Blood-Brain Barrier Permeability In Vitro.

Phase-change nanodroplets (PCND;NDs) are emulsions with a perfluorocarbon (PFC) core that undergo acoustic vaporisation as a response to ultrasound (US). Nanodroplets change to microbubbles and cavitate while under the effect of US. This cavitation can apply forces on cell connections in biological barrier membranes, such as the blood-brain barrier (BBB), and trigger a transient and reversible increased permeability to molecules and matter. This study aims to present the preparation of lipid-based NDs and investigate their effects on the brain endothelial cell barrier in vitro. The NDs were prepared using the thin-film hydration method, followed by the PFC addition. They were characterised for size, cavitation (using a high-speed camera), and PFC encapsulation (using FTIR). The bEnd.3 (mouse brain endothelial) cells were seeded onto transwell inserts. Fluorescein with NDs and/or microbubbles were applied on the bEND3 cells and the effect of US on fluorescein permeability was measured. The Live/Dead assay was used to assess the BBB integrity after the treatments. Size and PFC content analysis indicated that the NDs were stable while stored. High-speed camera imaging confirmed that the NDs cavitate after US exposure of 0.12 MPa. The BBB cell model experiments revealed a 4-fold increase in cell membrane permeation after the combined application of US and NDs. The Live/Dead assay results indicated damage to the BBB membrane integrity, but this damage was less when compared to the one caused by microbubbles. This in vitro study shows that nanodroplets have the potential to cause BBB opening in a similar manner to microbubbles. Both cavitation agents caused damage on the endothelial cells. It appears that NDs cause less cell damage compared to microbubbles.

Novel treatment system using endoscopic ultrasound-guided high-intensity focused ultrasound: A proof-of-concept study

AimHigh-intensity focused ultrasound (HIFU) is a novel minimally invasive local treatment of solid tumors. Endoscopic ultrasound-guided HIFU (EUS-HIFU) using mechanical effects would have potential benefits, including precise detection of target lesions and enhance drug delivery. The aim of this study is to develop EUS-HIFU device and to prove our concept in porcine model using a locally injected phase change nano droplet (PCND) as the sensitizer. MethodA phospholipid PCND contained volatile perfluoro-carbon liquids. The prototype HIFU apparatus comprised a small (20 × 20 mm) transducer with center frequency of 2.1 MHz, attachable to a linear EUS transducer. Under general anesthetic, a single porcine received EUS-guided injection of PCND. The HIFU transducer was placed laparotomically in the stomach, and the liver was ablated through the gastric wall. ResultsPCND was injected successfully and a distinct lesion was generated at the HIFU transducer focus only in injected areas that received HIFU exposure at 4.7 kW/cm2 at a duty cycle of 5 % (mean temporal intensity, 0.245 kW/cm2) for 30 s. The generated lesions were mechanically fractionated in macroscopic view. ConclusionThe concept of transluminal HIFU ablation using novel EUS-HIFU system was proved in a porcine animal model. This novel treatment system has great potential for future cancer treatment although further investigation in more animals and different organs are warranted.

Enhanced and spatially controllable neuronal activity induced by transcranial focused ultrasound stimulation combined with phase-change nanodroplets

Non-invasive ultrasound neuromodulation (USNM) is a powerful tool to explore neural circuits and treat neurological disorders. Due to the heterogeneity of the skull and regional variations in modulation and treatment objectives, it is necessary to develop an efficient and spatially controllable neuromodulation approach. Recently, transcranial focused ultrasound (tFUS) combined with external biomicro/nanomaterials for brain stimulation has garnered significant attention. This study focused on tFUS combined with perfluoropentane (PFP) nanodroplets (NDs) to improve the efficacy and spatial controllability of USNM. The developed two-stage variable pulse tFUS sequence that include the acoustic droplet vaporization (ADV) pulse for vaporizing PFP NDs into microbubbles (MBs) and the USNM sequence for inducing mechanical oscillations of the formed MBs to enhance neuronal activity. Further, adjusting the acoustic pressure of the ADV pulse generated the controllable vaporization regions, thereby achieving spatially controllable neuromodulation. The results showed that the mean densities of c-fos+ cells expression in the group of PFP NDs with ADV (109 ± 19 cells/mm2) were significantly higher compared to the group without ADV (37.34 ± 8.24 cells/mm2). The acoustic pressure of the ADV pulse with 1.98 MPa and 2.81 MPa in vitro generated the vaporization regions of 0.146 ± 0.032 cm2 and 0.349 ± 0.056 cm2, respectively. Under the same stimulation conditions, a larger vaporization region was also obtained with higher acoustic pressure in vivo, inducing a broader region of neuronal activation. Therefore, this study will serve as a valuable reference for developing the efficient and spatially controllable tFUS neuromodulation strategy.

Enhanced thermal ablation via an acoustic vortex with a large focal region

Focused ultrasound ablation provides a truly noninvasive tumor treatment option with clinically proven feasibility and safety. However, the frequently required long treatment duration hinders its clinical applicability. In this work, we compared the thermal ablation induced by conventional focused ultrasound (cFUS), split-focus ultrasound (sFUS), and acoustic vortex (AV) in tissue phantoms containing phase-change nanodroplets and mouse tumors. The results indicated that AV could substantially enhance the thermal ablation efficiency compared with cFUS, which was attributed to the larger focal region in the former. Additionally, the ablation region appeared as a unique cylindrical area with a smaller length-to-width ratio in AV than that in cFUS. Though the efficiency could also be improved in sFUS compared with cFUS, the ablation region was irregular and non-ablated tissues were present. Furthermore, in vivo experiments demonstrated that the tumor volume decreased faster and the mice survived longer after AV treatment compared with cFUS. The cavitation activity was also found to be more intense in AV ablation. The proposed method may solve the general issue of low efficiency often observed in cFUS ablation and further promote the development of other ultrasound treatments.

Characterising the chemical and physical properties of phase-change nanodroplets

Phase-change nanodroplets have attracted increasing interest in recent years as ultrasound theranostic nanoparticles. They are smaller compared to microbubbles and they may distribute better in tissues (e.g. in tumours). They are composed of a stabilising shell and a perfluorocarbon core. Nanodroplets can vaporise into echogenic microbubbles forming cavitation nuclei when exposed to ultrasound. Their perfluorocarbon core phase-change is responsible for the acoustic droplet vaporisation. However, methods to quantify the perfluorocarbon core in nanodroplets are lacking. This is an important feature that can help explain nanodroplet phase change characteristics. In this study, we fabricated nanodroplets using lipids shell and perfluorocarbons. To assess the amount of perfluorocarbon in the core we used two methods, 19F NMR and FTIR. To assess the cavitation after vaporisation we used an ultrasound transducer (1.1 MHz) and a high-speed camera. The 19F NMR based method showed that the fluorine signal correlated accurately with the perfluorocarbon concentration. Using this correlation, we were able to quantify the perfluorocarbon core of nanodroplets. This method was used to assess the content of the perfluorocarbon of the nanodroplets in solutions over time. It was found that perfluoropentane nanodroplets lost their content faster and at higher ratio compared to perfluorohexane nanodroplets. The high-speed imaging indicates that the nanodroplets generate cavitation comparable to that from commercial contrast agent microbubbles. Nanodroplet characterisation should include perfluorocarbon concentration assessment as critical information for their development.

Acoustic super-resolved spatiotemporal monitoring of theranostic nanodroplets with tuned post-activation dynamics

Phase-change nanodroplets (PCNDs) have been used as controllable theranostic agents in diverse therapeutic and diagnostic scenarios over the years. However, there is still a pressing need to monitor the accurate distribution of non-flowing extravascular PCNDs by clinical ultrasound for further quantitative analysis. Here, we propose a spatiotemporally super-resolved ultrasound monitoring technique based on tuned post-activation dynamics of high-bulk-boiling-point PCNDs with customized perfluorocarbon cores. The underlying idea was to make the stochastic recondensation-induced contrast signal dominate the total post-activation signal, thus obtaining detectable and resolvable recondensation signals at physiological temperature. The recondensation signal was extracted by inter-frame subtraction and then processed by a deep learning-based detection algorithm tailored to the recondensation signal patterns. Experiments in tissue-mimicking phantoms demonstrate that co-restricting the concentration of PCNDs and the focus-wave activation pulse duration could help generate recondensation signals with ideal detectability and sparsity for accurate monitoring. Meanwhile, the quantitative analysis of the super-resolved results shows the spatiotemporal sensitivity of the proposed technique under varying concentrations and activation pulse durations, which was consistent with the patterns drawn from the total post-activation signal and existing theories on post-activation dynamics of PCNDs. This technique may be suitable for in-depth extravascular monitoring and dose analysis for PCNDs-involved therapies.

Extending the Iterative Nonlinear Contrast Source method to simulate mutual interaction in large populations of microbubbles

There is an ongoing development of ultrasound contrast agents (UCAs) for specific medical diagnostic and therapeutic applications. Recent researches involve all kinds of targeted and loaded microbubbles, monodisperse microbubbles, and phase-change nanodroplets. To design and improve applications that use UCAs, it is necessary to have a thorough understanding of the mutual interaction of ultrasound fields and UCAs. The individual response of microbubbles and nanodroplets is extensively studied and, in the case of bubbles, multiple variants of the Rayleigh-Plesset equation are available to describe their behavior. However, novel applications, such as therapeutic proton beam localization, may strongly depend on the mutual interaction in a population of UCAs. In this presentation, a numerical approach for the analysis of ultrasound-bubble interaction in a population with many (order one million) microbubbles will be discussed. This approach is based on the earlier developed Iterative Nonlinear Contrast Source (INCS) method for simulating nonlinear ultrasound waves. In the current case, the population of microbubbles will be represented by a set of pressure-dependent contrast point sources that are iteratively updated by the INCS method, where each iteration adds an order of multiple scattering. Results will be presented for the linear scattering in populations with different concentrations of microbubbles.

Novel Magnetic Elastic Phase-Change Nanodroplets as Dual Mode Contrast Agent for Ultrasound and Magnetic Resonance Imaging.

Recently, dual-mode imaging systems merging magnetic resonance imaging (MRI) and ultrasound (US) have been developed. Designing a dual-mode contrast agent is complex due to different mechanisms of enhancement. Herein, we describe novel phase change nanodroplets (PCNDs) with perfluoropentane encapsulated in a pre-polyglycerol sebacate (pre-PGS) shell loaded with polyethylene glycol (PEG)-coated iron oxide nanoparticles as having a dual-mode contrast agent effect. Iron oxide nanoparticles were prepared via the chemical co-precipitation method and PCNDs were prepared via the solvent displacement technique. PCNDs showed excellent enhancement in the in vitro US much more than Sonovue® microbubbles. Furthermore, they caused a susceptibility effect resulting in a reduction of signal intensity on MRI. An increase in the concentration of nanoparticles caused an increase in the MR contrast effect but a reduction in US intensity. The concentration of nanoparticles in a shell of PCNDs was optimized to obtain a dual-mode contrast effect. Biocompatibility, hemocompatibility, and immunogenicity assays showed that PCNDs were safe and non-immunogenic. Another finding was the dual-mode potential of unloaded PCNDs as T1 MR and US contrast agents. Results suggest the excellent potential of these PCNDs for use as dual-mode contrast agents for both MRI and US.

Ultrasound-Activated Nanodroplet Disruption of the Enterococcus faecalis Biofilm in Dental Root Canal.

Conventional methods used to control bacterial biofilm infection in root canals have poor efficacy, causing repeated and chronic infections, which pose a great challenge to clinical treatment. Microbubbles, due to their small size and ultrasound (US)-enhanced cavitation effects, have attracted considerable clinical attention. They possess the potential for therapeutic application in restricted spaces. We address the above problem with a strategy for the restricted space of root canals. Herein, phase-change nanodroplets (P-NDs) exposed to US are combined with common antibacterial drugs to disrupt a 7 day Enterococcus faecalis biofilm in an in vitro human tooth model. Specifically, the preparation of P-NDs is based on secondary cavitation. Their average particle size is ∼144 nm, and the stability is favorable. The clearance effect for the biofilm is notable (the disruption rate of P-NDs + US is 63.1%, P < 0.01), while the effect of an antibacterial in conjunction with 2% chlorhexidine (Chx) is significant (the antibiofilm rate of P-NDs@2% Chx + US is 96.2%, P < 0.001). Furthermore, biocompatibility testing on human periodontal ligament fibroblasts demonstrated that P-NDs are safe. In summary, the strategy that we have proposed is suitable for the removal of biofilms in root canals. Notably, it also has great potential for application in the treatment of bacterial infections in restricted spaces.

Super-resolution Imaging with a multifrequency large-aperture hemispherical phased array

High-resolution imaging of microvasculature is important for diagnostic and therapeutic applications. Here, the utility of a multi-frequency large-aperture hemispherical phased array in the context of vasculature imaging via passive beamforming and super-resolution techniques was explored at various receive frequencies, to investigate the capability of mapping the emissions of lipid coated phase-change nanodroplets flowing through tube phantoms as well as rabbit renal and cerebral vasculatures in-vivo. With super-resolution techniques, the mean lateral (axial) full-width-at-half-maximum image intensity was estimated to be 16 ± 3 (32 ± 6) μm and 7 ± 1 (15 ± 2) μm, corresponding to approximately 1/85 of normal resolution when received with the subarrays at 612 and 1224 kHz, respectively. The mean positional uncertainties were approximately 1/350 (lateral) and 1/180 (axial) of the receive wavelength in water. The renal and cerebral vasculatures of rabbits were partially visualized. A correlation between transmit pulse and nanodroplet vaporization is observed, demonstrating potentials for nanodroplet vaporization modulation and signal-to-noise ratio enhancement. This study demonstrates the feasibility of mapping vaporized nanodroplets with passive beamforming and super-resolution imaging techniques. It may provide means to map complete vasculature pre- or post-treatment with therapeutic focused ultrasound in organs that can be surrounded by the hemispherical array.

Applying ultrasound phase-change contrast agents to guide therapeutic intervention in type 1 diabetes

Ultrasound is a widely deployable and cost-effective clinical imaging modality. However, conventional microbubble contrast agents are restricted to the vasculature. Sub-micron nanodroplet (ND) phase-change agents can be vaporized into micron-sized bubbles, serving as a microbubble precursor. In principle, the enhanced permeabiltiy and retention effect can be used with sub-micron NDs to detect injured tissues. Methods to detect type1 diabetes (T1D) progression prior to clinical diagnosis are needed. T1D results from autoreactive T-cells infiltrating the islets of Langerhans, destroying insulin-producing beta-cells. Overt disease takes years to present and at diagnosis there is substantial beta-cells loss. Therapeutic intervention to preserve beta-cell mass is hampered by an inability to follow pre-symptomatic T1D progression and tracking whether therapeutic interventions are impacting disease progression. We will present ultrasound imaging of phase-change nanodroplet (ND) contrast-agent accumulation within the islet. ND accumulation is dependent on immune infiltration, therefore tracks pre-symptomatic T1D development and progression to diabetes. Measurement of ND accumulation detected pre-symptomatic T1D earlier and with greater sensitivity compared to existing measurements of circulating autoantibodies. This provides an opportunity to guide early therapeutic treatments to prevent T1D.

An Analysis of Sonothrombolysis and Cavitation for Retracted and Unretracted Clots Using Microbubbles Versus Low-Boiling-Point Nanodroplets.

The thrombolysis potential of low-boiling-point (-2 °C) perfluorocarbon phase-change nanodroplets (NDs) has previously been demonstrated on aged clots, and we hypothesized that this efficacy would extend to retracted clots. We tested this hypothesis by comparing sonothrombolysis of both unretracted and retracted clots using ND-mediated ultrasound (US+ND) and microbubble-mediated ultrasound (US+MB), respectively. Assessment data included clot mass reduction, cavitation detection, and cavitation cloud imaging in vitro. Acoustic parameters included a 7.9-MPa peak negative pressure and 180-cycle bursts with 5-Hz repetition (the corresponding duty cycle and time-averaged intensity of 0.09% and 1.87 W/cm2, respectively) based on prior studies. With these parameters, we observed a significantly reduced efficacy of US+MB in the retracted versus unretracted model (the averaged mass reduction rate from 1.83%/min to 0.54%/min). Unlike US+MB, US+ND exhibited less reduction of efficacy in the retracted model (from 2.15%/min to 1.04%/min on average). The cavitation detection results correlate with the sonothrombolysis efficacy results showing that both stable and inertial cavitation generated in a retracted clot by US+ND is higher than that by US+MB. We observed that ND-mediated cavitation shows a tendency to occur inside a clot, whereas MB-mediated cavitation occurs near the surface of a retracted clot, and this difference is more significant with retracted clots compared to unretracted clots. We conclude that ND-mediated sonothrombolysis outperforms MB-mediated therapy regardless of clot retraction, and this advantage of ND-mediated cavitation is emphasized for retracted clots. The primary mechanisms are hypothesized to be sustained cavitation level and cavitation clouds in the proximity of a retracted clot by US+ND.

Contrast enhanced sonothrombolysis using streptokinase loaded phase change nano-droplets for potential treatment of deep venous thrombosis.

Current thrombolytic therapies for deep venous thrombosis are limited due to the wide side effect profile. Contrast mediated sonothrombolysis is a promising approach for thrombus treatment. The current study examines the effectiveness of in vitro streptokinase (SK) loaded phase-change nanodroplet (PCND) mediated sonothrombolysis at 7 MHz for the diagnosis of deep venous thrombosis. Lecithin shell and perfluorohexane core nanodroplets were prepared via the thin-film hydration method and morphologically characterized. Sonothrombolysis was performed at 7 MHz at different mechanical indexes of samples i.e., only sonothrombolysis, PCND mediated sonothrombolysis, sonothrombolysis with SK and SK loaded PCND mediated sonothrombolysis. Thrombolysis efficacy was assessed by measuring clot weight changes during 30 min US exposure, recording the mean gray intensity from the US images of the clot by computer software ImageJ, and spectrophotometric quantification of the hemoglobin in the clot lysate. In 15 minutes of sonothrombolysis performed at high mechanical index (0.9 and 1.2), SK loaded PCNDs showed a 48.61% and 74.29% reduction of mean gray intensity. At 0.9 and 1.2 MI, 86% and 92% weight loss was noted for SK-loaded PCNDs in confidence with spectrophotometric results. A significant difference (P < 0.05) was noted for SK-loaded PCND mediated sonothrombolysis compared to other groups. Loading of SK inside the PCNDs enhanced the efficacy of sonothrombolysis. An increase in MI and time also increased the efficacy of sonothrombolysis. This in vitro study showed the potential use of SK-loaded perfluorohexane core PCNDs as sonothrombolytic agents for deep venous thrombosis.

Ultrasound-assisted investigation of photon triggered vaporization of poly(vinylalcohol) phase-change nanodroplets: A preliminary concept study with dosimetry perspective.

We investigate the vaporization of phase-change ultrasound contrast agents using photon radiation for dosimetry perspectives in radiotherapy. We studied superheated perfluorobutane nanodroplets with a crosslinked poly(vinylalcohol) shell. The nanodroplets' physico-chemical properties, and their acoustic transition have been assessed firstly. Then, poly(vinylalcohol)-perfluorobutane nanodroplets were dispersed in poly(acrylamide) hydrogel phantoms and exposed to a photon beam. We addressed the effect of several parameters influencing the nanodroplets radiation sensitivity (energy/delivered dose/dose rate/temperature). The nanodroplets-vaporization post-photon exposure was evaluated using ultrasound imaging at a low mechanical index. Poly(vinylalcohol)-perfluorobutane nanodroplets show a good colloidal stability over four weeks and remain highly stable at temperatures up to 78°C. Nanodroplets acoustically-triggered phase transition leads to microbubbles with diameters <10μm and an activation threshold of mechanical index=0.4, at 7.5MHz. A small number of vaporization events occur post-photon exposure (6MV/15MV), at doses between 2 and 10Gy, leading to ultrasound contrast increase up to 60% at RT. The nanodroplets become efficiently sensitive to photons when heated to a temperature of 65°C (while remaining below the superheat limit temperature) during irradiation. Nanodroplets' core is linked to the degree of superheat in the metastable state and plays a critical role in determining nanodroplet' stability and sensitivity to ionizing radiation, requiring higher or lower linear energy transfer vaporization thresholds. While poly(vinylalcohol)-perfluorobutane nanodroplets could be slightly activated by photons at ambient conditions, a good balance between the degree of superheat and stability will aim at optimizing the design of nanodroplets to reach high sensitivity to photons at physiological conditions.

Towards real-time super-resolution imaging with fastAWSALM

Localization-based ultrasound super-resolution imaging can visualize the microvascular structure beyond the diffraction limit but is sensitive to contrast concentration and requires acquisition for seconds. Low-boiling-point phase-change nanodroplets achieve super-resolved images with subsecond temporal resolution through activation and destruction with high frame rate plane waves in real-time.

Modulating ultrasound contrast generation from injectable nanodroplets for proton range verification by varying the degree of superheat.

Despite the physical benefits of protons over conventional photon radiation in cancer treatment, range uncertainties impede the ability to harness the full potential of proton therapy. While monitoring the proton range in vivo could reduce the currently adopted safety margins, a routinely applicable range verification technique is still lacking. Recently, phase-change nanodroplets were proposed for proton range verification, demonstrating a reproducible relationship between the proton range and generated ultrasound contrast after radiation-induced vaporization at 25°C. In this study, previous findings are extended with proton irradiations at different temperatures, including the physiological temperature of 37°C, for a novel nanodroplet formulation. Moreover, the potential to modulate the linear energy transfer (LET) threshold for vaporization by varying the degree of superheat is investigated, where the aim is to demonstrate vaporization of nanodroplets directly by primary protons. Perfluorobutane nanodroplets with a shell made of polyvinyl alcohol (PVA-PFB) or 10,12-pentacosadyinoic acid (PCDA-PFB) were dispersed in polyacrylamide hydrogels and irradiated with 62MeV passively scattered protons at temperatures of 37°C and 50°C. Nanodroplet transition into echogenic microbubbles was assessed using ultrasound imaging (gray value and attenuation analysis) and optical images. The proton range was measured independently and compared to the generated contrast. Nanodroplet design proved crucial to ensure thermal stability, as PVA-shelled nanodroplets dramatically outperformed their PCDA-shelled counterpart. At body temperature, a uniform radiation response proximal to the Bragg peak is attributed to nuclear reaction products interacting with PVA-PFB nanodroplets, with the 50% drop in ultrasound contrast being 0.17mm±0.20mm (mean±standard deviation) in front of the proton range. Also at 50°C, highly reproducible ultrasound contrast profiles were obtained with shifts of -0.74mm±0.09mm (gray value analysis), -0.86mm±0.04mm (attenuation analysis) and -0.64mm±0.29mm (optical analysis). Moreover, a strong contrast enhancement was observed near the Bragg peak, suggesting that nanodroplets were sensitive to primary protons. By varying the degree of superheat of the nanodroplets' core, one can modulate the intensity of the generated ultrasound contrast. Moreover, a submillimeter reproducible relationship between the ultrasound contrast and the proton range was obtained, either indirectly via the visualization of secondary reaction products or directly through the detection of primary protons, depending on the degree of superheat. The potential of PVA-PFB nanodroplets for in vivo proton range verification was confirmed by observing a reproducible radiation response at physiological temperature, and further studies aim to assess the nanodroplets' performance in a physiological environment. Ultimately, cost-effective online or offline ultrasound imaging of radiation-induced nanodroplet vaporization could facilitate the reduction of safety margins in treatment planning and enable adaptive proton therapy.

Nanodroplet-mediated catheter-directed sonothrombolysis of retracted blood clots

One major challenge in current microbubble (MB) and tissue plasminogen activator (tPA)-mediated sonothrombolysis techniques is effectively treating retracted blood clots, owing to the high density and low porosity of retracted clots. Nanodroplets (NDs) have the potential to enhance retracted clot lysis owing to their small size and ability to penetrate into retracted clots to enhance drug delivery. For the first time, we demonstrate that a sub-megahertz, forward-viewing intravascular (FVI) transducer can be used for ND-mediated sonothrombolysis, in vitro. In this study, we determined the minimum peak negative pressure to induce cavitation with low-boiling point phase change nanodroplets and clot lysis. We then compared nanodroplet mediated sonothrombolysis to MB and tPA mediate techniques. The clot lysis as a percent mass decrease in retracted clots was 9 ± 8%, 9 ± 5%, 16 ± 5%, 14 ± 9%, 17 ± 9%, 30 ± 8%, and 40 ± 9% for the control group, tPA alone, tPA + US, MB + US, MB + tPA + US, ND + US, and ND + tPA + US groups, respectively. In retracted blood clots, combined ND- and tPA-mediated sonothrombolysis was able to significantly enhance retracted clot lysis compared with traditional MB and tPA-mediated sonothrombolysis techniques. Combined nanodroplet with tPA-mediated sonothrombolysis may provide a feasible strategy for safely treating retracted clots.

Improving the heating efficiency of high intensity focused ultrasound ablation through the use of phase change nanodroplets and multifocus sonication

Thermal ablation by ultrasound is being explored as a local therapy for cancers of soft tissue, such as the liver or breast. One challenge for these treatments are off-target effects, including heating outside of the intended region or skin burns. Improvements in heating efficiency can mitigate these undesired outcomes, and here, we describe methods for using phase-shift nanodroplets (PSNDs) with multi-focus sonications to enhance volumetric ablation and ablation efficiency at constant powers while increasing the pre-focal temperature by less than 6 C. Multi-focus ablation with 4 foci performed the best and achieved a mean ablation volume of 120.2 ± 22.4 mm3 and ablation efficiency of 0.04 mm3 J−1 versus an ablation volume of 61.2 ± 21.16 mm3 and ablation efficiency of 0.02 mm3 J−1 in single focus case. The combined use of PSNDs with multi-focal ultrasound presented here is a new approach to increasing ablation efficiency while reducing off-target effects and could be generally applied in various focused ultrasound thermal ablation treatments.