Contrast-enhanced ultrasound imaging in vivo with laser-activated nanodroplets

Laser-activated perfluorocarbon nanodroplets for intracerebral delivery and imaging via blood-brain barrier opening and contrast-enhanced imaging

BACKGROUND

Ultrasound and photoacoustic (US/PA) imaging is a promising tool for in vivo visualization and assessment of drug delivery. However, the acoustic properties of the skull limit the practical application of US/PA imaging in the brain. To address the challenges in targeted drug delivery to the brain and transcranial US/PA imaging, we introduce and evaluate an intracerebral delivery and imaging strategy based on the use of laser-activated perfluorocarbon nanodroplets (PFCnDs).

METHODS

Two specialized PFCnDs were developed to facilitate blood‒brain barrier (BBB) opening and contrast-enhanced US/PA imaging. In mice, PFCnDs were delivered to brain tissue via PFCnD-induced BBB opening to the right side of the brain. In vivo, transcranial US/PA imaging was performed to evaluate the utility of PFCnDs for contrast-enhanced imaging through the skull. Ex vivo, volumetric US/PA imaging was used to characterize the spatial distribution of PFCnDs that entered brain tissue. Immunohistochemical analysis was performed to confirm the spatial extent of BBB opening and the accuracy of the imaging results.

RESULTS

In vivo, transcranial US/PA imaging revealed localized photoacoustic (PA) contrast associated with delivered PFCnDs. In addition, contrast-enhanced ultrasound (CEUS) imaging confirmed the presence of nanodroplets within the same area. Ex vivo, volumetric US/PA imaging revealed PA contrast localized to the area of the brain where PFCnD-induced BBB opening had been performed. Immunohistochemical analysis revealed that the spatial distribution of immunoglobulin (IgG) extravasation into the brain closely matched the imaging results.

CONCLUSIONS

Using our intracerebral delivery and imaging strategy, PFCnDs were successfully delivered to a targeted area of the brain, and they enabled contrast-enhanced US/PA imaging through the skull. Ex vivo imaging, and immunohistochemistry confirmed the accuracy and precision of the approach.

Ultrafast Background-Free Ultrasound Imaging Using Blinking Nanoparticles

Localization-based ultrasound imaging methods that use microbubbles or nanodroplets offer high-resolution imaging with improved sensitivity and reduced background signal. However, these methods require long acquisition times (typically seconds to minutes), preventing their use for real-time imaging and, thus, limiting their clinical translational potential. Here, we present a new ultrafast localization method using blinking ultrasound-responsive nanoparticles (BNPs). When activated with high frame rate (1 kHz) plane wave ultrasound pulses with a mechanical index of 1.5, the BNPs incept growth of micrometer-sized bubbles, which in turn collapse and generate a blinking ultrasound signal. We showed that background-free ultrasound images could be obtained by localizing these blinking events using acquisition times as low as 11 ms. In addition, we demonstrated that BNPs enable in vivo background-free ultrasound imaging in mice. We envision that BNPs will facilitate the clinical translation of localization-based ultrasound imaging for more sensitive detection of cancer and other diseases.

Ultrasound-assisted carbon ion dosimetry and range measurement using injectable polymer-shelled phase-change nanodroplets: in vitro study

Methods allowing for in situ dosimetry and range verification are essential in radiotherapy to reduce the safety margins required to account for uncertainties introduced in the entire treatment workflow. This study suggests a non-invasive dosimetry concept for carbon ion radiotherapy based on phase-change ultrasound contrast agents. Injectable nanodroplets made of a metastable perfluorobutane (PFB) liquid core, stabilized with a crosslinked poly(vinylalcohol) shell, are vaporized at physiological temperature when exposed to carbon ion radiation (C-ions), converting them into echogenic microbubbles. Nanodroplets, embedded in tissue-mimicking phantoms, are exposed at 37 °C to a 312 MeV/u clinical C-ions beam at different doses between 0.1 and 4 Gy. The evaluation of the contrast enhancement from ultrasound imaging of the phantoms, pre- and post-irradiation, reveals a significant radiation-triggered nanodroplets vaporization occurring at the C-ions Bragg peak with sub-millimeter shift reproducibility and dose dependency. The specific response of the nanodroplets to C-ions is further confirmed by varying the phantom position, the beam range, and by performing spread-out Bragg peak irradiation. The nanodroplets' response to C-ions is influenced by their concentration and is dose rate independent. These early findings show the ground-breaking potential of polymer-shelled PFB nanodroplets to enable in vivo carbon ion dosimetry and range verification.

Phase-shift nanodroplets as an emerging sonoresponsive nanomaterial for imaging and drug delivery applications

Nanodroplets - emerging phase-changing sonoresponsive materials - have attracted substantial attention in biomedical applications for both tumour imaging and therapeutic purposes due to their unique response to ultrasound. As ultrasound is applied at different frequencies and powers, nanodroplets have been shown to cavitate by the process of acoustic droplet vapourisation (ADV), causing the development of mechanical forces which promote sonoporation through cellular membranes. This allows drugs to be delivered efficiently into deeper tissues where tumours are located. Recent reviews on nanodroplets are mostly focused on the mechanism of cavitation and their applications in biomedical fields. However, the chemistry of the nanodroplet components has not been discussed or reviewed yet. In this review, the commonly used materials and preparation methods of nanodroplets are summarised. More importantly, this review provides examples of variable chemistry components in nanodroplets which link them to their efficiency as ultrasound-multimodal imaging agents to image and monitor drug delivery. Finally, the drawbacks of current research, future development, and future direction of nanodroplets are discussed.

Effect of Perfluorocarbon Composition on Activation of Phase-Changing Ultrasound Contrast Agents

Background

While microbubble contrast agents (MCAs) are commonly used in ultrasound (US), they are inherently limited to vascular targets due to their size. Alternatively, phase‐changing nanodroplet contrast agents (PNCAs) can be delivered as nanoscale agents (i.e., small enough to extravasate), but when exposed to a US field of sufficient mechanical index (MI), they convert to MCAs, which can be visualized with high contrast using nonlinear US.

Purpose

To investigate the effect of perfluorocarbon (PFC) core composition and presence of cholesterol in particle coatings on stability and image contrast generated from acoustic activation of PNCAs using high‐frequency US suitable for clinical imaging.

Methods

PNCAs with varied core compositions (i.e., mixtures of perfluoropentane [C5] and/or perfluorohexane [C6]) and two coating formulations (i.e., with and without cholesterol) were characterized and investigated for thermal/temporal stability and postactivation, nonlinear US contrast in phantom and in vivo environments. Through hydrophone measurements and nonlinear numerical modeling, MI was estimated for pulse sequences used for PNCA activation.

Results

All PNCA compositions were characterized to have similar diameters (249–267 nm) and polydispersity (0.151–0.185) following fabrication. While PNCAs with majority C5 core composition showed higher levels of spontaneous signal (i.e., not due to US activation) in phantoms than C6‐majority PNCAs, all compositions were stable during imaging experiments. When activating PNCAs with a 12.3‐MHz US pulse (MI = 1.1), C6‐core particles with cholesterol‐free coatings (i.e., CF‐C6‐100 particles) generated a median contrast of 3.1, which was significantly higher (p < 0.001) than other formulations. Further, CF‐C6‐100 particles were activated in a murine model, generating US contrast ≥3.4.

Conclusion

C6‐core PNCAs can provide high‐contrast US imaging with minimal nonspecific activation in phantom and in vivo environments.

Sonoporation based on repeated vaporization of gold nanodroplets

Background

Gold nanodroplets (AuNDs) have been proposed as agents for photothermal therapy and photoacoustic imaging. Previously, we demonstrated that the sonoporation can be more effectively achieved with synchronized optical and acoustic droplet vaporization. By applying a laser pulse at the rarefactional phase of the ultrasound (US) pulse, the vaporization threshold can be reached at a considerably lower laser average power. However, a large loading quantity of the AuNDs may increase the risk of air embolism. The destruction of phase‐shifted AuNDs at the inertial cavitation stage leads to a reduced drug delivery performance. And it also causes instability of echogenicity during therapeutic monitoring.

Purpose

In this study, we propose to further improve the sonoporation effectiveness with repeated vaporization. In other words, the AuNDs repeatedly undergo vaporization and recondensation so that sonoporation effects are accumulated over time at lower energy requirements. Previously, repeated vaporization has been demonstrated as an imaging contrast agent. In this study, we aim to adopt this repeated vaporization scheme for sonoporation.

Methods

Perfluoropentane NDs with a shell made of human serum albumin were used as the US contrast agents. Laser pulses at 808 nm and US pulses of 1 MHz were delivered for triggering vaporization and inertial cavitation of NDs. We detected the vaporization and cavitation effects under different activation firings, US peak negative pressures (PNPs), and laser fluences using 5‐ and 10‐MHz focused US receivers. Numbers of calcein‐AM and propidium iodide signals uptake by BNL hepatocarcinoma cancer cells were used to evaluate the sonoporation and cell death rate of the cells.

Results

We demonstrate that sonoporation can be realized based on repeatable vaporization instead of the commonly adopted inertial cavitation effects. In addition, it is found that the laser fluence and the acoustic pressure can be reduced. As an example, we demonstrate that the acoustic and optical energy for achieving a similar level of sonoporation rate can be as low as 0.44 MPa for the US PNP and 4.01 mJ/cm² for the laser fluence, which are lower than those with our previous approach (0.53 MPa and 4.95 mJ/cm², respectively).

Conclusion

We demonstrated the feasibility of vaporization‐based sonoporation at a lower optical and acoustic energy. It is an advantageous method that can enhance drug delivery efficiency, therapeutic safety and potentially deliver an upgraded gene therapy strategy for improved theragnosis.

A comprehensive update of micro- and nanobubbles as theranostics in oncology

Advances in diagnostic and imaging capabilities have allowed cancers to be detected earlier and characterized more robustly. These strategies have recently branched into theranostics whereby contrast agents traditionally used for imaging have been co-loaded with therapeutics to simultaneously diagnose and treat cancers in a patient-specific manner. Microbubbles (MB) and nanobubbles (NB) are contrast agents which can be modulated to meet the theranostic needs particularly in the realm of oncology. The current review focuses on the ultrasound-responsive MB/NB platforms used as a theranostic tool in oncology. We discuss in detail the key parameters that influence the utility of MB/NB formulations and implications of such treatment modalities. Recent advances in composition strategies, latest works in the pre-clinical stages and multiple paradigm-shifting innovations in the field of MB/NB are discussed in-depth in this review. The clinical application of MB/NB is currently limited to diagnostic imaging. Surface chemistry modification strategies will help tune the formulations toward therapeutic applications. It is also anticipated that MB/NB will see increased use to deliver gas therapeutics. Scalability and stability considerations will be at the forefront as these particles get introduced into the clinical theranostic toolbox.

Ultrasound and Photoacoustic Imaging of Laser-Activated Phase-Change Perfluorocarbon Nanodroplets

Laser-activated perfluorocarbon nanodroplets (PFCnDs) are emerging phase-change contrast agents that showed promising potential in ultrasound and photoacoustic (US/PA) imaging. Unlike monophase gaseous microbubbles, PFCnDs shift their state from liquid to gas via optical activation and can provide high US/PA contrast on demand. Depending on the choice of perfluorocarbon core, the vaporization and condensation dynamics of the PFCnDs are controllable. Therefore, these configurable properties of activation and deactivation of PFCnDs are employed to enable various imaging approaches, including contrast-enhanced imaging and super-resolution imaging. In addition, synchronous application of both acoustic and optical pulses showed a promising outcome vaporizing PFCnDs with lower activation thresholds. Furthermore, due to their sub-micrometer size, PFCnDs can be used for molecular imaging of extravascular tissue. PFCnDs can also be an effective therapeutic tool. As PFCnDs can carry therapeutic drugs or other particles, they can be used for drug delivery, as well as photothermal and photodynamic therapies. Blood barrier opening for neurological applications was recently demonstrated with optically-triggered PFCnDs. This paper specifically focuses on the activation and deactivation properties of laser-activated PFCnDs and associated US/PA imaging approaches, and briefly discusses their theranostic potential and future directions.

Advantages of CT nano-contrast agent in tumor diagnosis: A retrospective study

The purpose of this retrospective study was to explore the advantages of computed tomography (CT) nano-contrast agent in tumor diagnosis.A total of 100 patients with malignant tumor who were diagnosed in Shaanxi Province Public Hospital between January 2018 and January 2019 were included in this retrospective study. They were randomly divided into observation and control groups with 50 patients in each group. The patients in the observation group used new type of nano-contrast agent for examination, and the patients in the control group used traditional iohexol contrast agent for examination. The detection rate, misdiagnosis rate, and incidence of adverse reactions were observed. In addition, single photon emission computed tomography or CT scan was performed on patients to observe the radioactive concentration.The detection rate was 100% in the observation group and 84% in the control group, and the difference between the 2 groups was statistically significant (χ2 = 8.763, P = .001). The incidence of adverse reactions was 2% in the observation group and 30% in the control group, and the difference between the 2 groups was significantly different (χ2 = 12.683, P = .000). The radioactive concentration in the observation group was markedly higher than that in the control group (t = 19.692, P = .001).The use of CT nano-contrast agent in tumor diagnosis had higher detection rate of tumor and radioactive concentration, and it had lower misdiagnosis rate and adverse reaction rate than traditional iohexol contrast agent.

Functional ultrasound-triggered phase-shift perfluorocarbon nanodroplets for cancer therapy

In recent years, because of their unique properties, the use of perfluorocarbon nanodroplets (PFC NDs) in ultrasound-mediated tumor theranostics has attracted increasing interest. PFC is one of the most stable organic compounds with high hydrophobicity. Phase-shift PFC NDs can be transformed into highly echogenic microbubbles for ultrasound and photoacoustic imaging by ultrasound and laser light. In addition, in the process of acoustic droplet vaporization, PFC NDs with cavitation nuclei can be combined with a variety of ultrasound technologies to produce cavitation effects for tumor ablation, antivascular therapy and release of therapeutic agents loaded in nanodroplets. Moreover, they can also be used to overcome tumor hypoxia by virtue of high oxygen solubility. In this review, first the preparation and stabilization of PFC NDs are summarized and then the issues and outlook are discussed. More importantly, multifunctional platforms based on PFC NDs for cancer diagnostics, therapy and theranostics are reviewed in detail.

Therapeutic oxygen delivery by perfluorocarbon-based colloids

After the protocol-related indecisive clinical trial of Oxygent, a perfluorooctylbromide/phospholipid nanoemulsion, in cardiac surgery, that often unduly assigned the observed untoward effects to the product, the development of perfluorocarbon (PFC)-based O₂ nanoemulsions ("blood substitutes") has come to a low. Yet, significant further demonstrations of PFC O₂-delivery efficacy have continuously been reported, such as relief of hypoxia after myocardial infarction or stroke; protection of vital organs during surgery; potentiation of O₂-dependent cancer therapies, including radio-, photodynamic-, chemo- and immunotherapies; regeneration of damaged nerve, bone or cartilage; preservation of organ grafts destined for transplantation; and control of gas supply in tissue engineering and biotechnological productions. PFC colloids capable of augmenting O₂ delivery include primarily injectable PFC nanoemulsions, microbubbles and phase-shift nanoemulsions. Careful selection of PFC and other colloid components is critical. The basics of O₂ delivery by PFC nanoemulsions will be briefly reminded. Improved knowledge of O₂ delivery mechanisms has been acquired. Advanced, size-adjustable O₂-delivering nanoemulsions have been designed that have extended room-temperature shelf-stability. Alternate O₂ delivery options are being investigated that rely on injectable PFC-stabilized microbubbles or phase-shift PFC nanoemulsions. The latter combine prolonged circulation in the vasculature, capacity for penetrating tumor tissues, and acute responsiveness to ultrasound and other external stimuli. Progress in microbubble and phase-shift emulsion engineering, control of phase-shift activation (vaporization), understanding and control of bubble/ultrasound/tissue interactions is discussed. Control of the phase-shift event and of microbubble size require utmost attention. Further PFC-based colloidal systems, including polymeric micelles, PFC-loaded organic or inorganic nanoparticles and scaffolds, have been devised that also carry substantial amounts of O₂. Local, on-demand O₂ delivery can be triggered by external stimuli, including focused ultrasound irradiation or tumor microenvironment. PFC colloid functionalization and targeting can help adjust their properties for specific indications, augment their efficacy, improve safety profiles, and expand the range of their indications. Many new medical and biotechnological applications involving fluorinated colloids are being assessed, including in the clinic. Further uses of PFC-based colloidal nanotherapeutics will be briefly mentioned that concern contrast diagnostic imaging, including molecular imaging and immune cell tracking; controlled delivery of therapeutic energy, as for noninvasive surgical ablation and sonothrombolysis; and delivery of drugs and genes, including across the blood-brain barrier. Even when the fluorinated colloids investigated are designed for other purposes than O₂ supply, they will inevitably also carry and deliver a certain amount of O₂, and may thus be considered for O₂ delivery or co-delivery applications. Conversely, O₂-carrying PFC nanoemulsions possess by nature a unique aptitude for ¹⁹F MR imaging, and hence, cell tracking, while PFC-stabilized microbubbles are ideal resonators for ultrasound contrast imaging and can undergo precise manipulation and on-demand destruction by ultrasound waves, thereby opening multiple theranostic opportunities.

Folic acid-functionalized gadolinium-loaded phase transition nanodroplets for dual-modal ultrasound/magnetic resonance imaging of hepatocellular carcinoma

Dual-modal molecular imaging by combining two imaging techniques can provide complementary information for early cancer diagnosis and therapeutic monitoring. In the present manuscript, folic acid (FA)-functionalized gadolinium-loaded nanodroplets (NDs) are introduced as dual-modal ultrasound (US)/magnetic resonance (MR) imaging contrast agents. These phase-change contrast agents (PCCAs) with alginate (Alg) stabilizing shell and a liquid perfluorohexane (PFH) core were successfully synthesized via the nano-emulsion method and characterized. In this regard, mouse hepatocellular carcinoma (Hepa1-6) as target cancer cells and mouse fibroblast (L929) as control cells were used. The in vitro and in vivo cytotoxicity assessments indicated that Gd/PFH@Alg and Gd/PFH@Alg-FA nanodroplets are highly biocompatible. Gd-loaded NDs do not induce organ toxicity, and no significant hemolytic activity in human red blood cells is observed. Additionally, nanodroplets exhibited strong ultrasound signal intensities as well as T₁-weighted MRI signal enhancement with a high relaxivity value of 6.40 mM^-1 s^-1, which is significantly higher than that of the clinical Gadovist contrast agent (r₁ = 4.01 mM^-1 s^-1). Cellular uptake of Gd-NDs-FA by Hepa1-6 cancer cells was approximately 2.5-fold higher than that of Gd-NDs after 12 h incubation. Furthermore, in vivo results confirmed that the Gd-NDs-FA bound selectively to cancer cells and were accumulated in the tumor region. In conclusion, Gd/PFH@Alg-FA nanodroplets have great potential as US/MR dual-modal imaging nanoprobes for the early diagnosis of cancer.

Ultrasound-Responsive Carriers for Therapeutic Applications

Ultrasound (US)-responsive carriers have emerged as promising theranostic candidates because of their ability to enhance US-contrast, promote image-guided drug delivery, cause on-demand pulsatile release of drugs in response to ultrasound stimuli, as well as to enhance the permeability of physiological barriers such as the stratum corneum, the vascular endothelium, and the blood-brain barrier (BBB). US-responsive carriers include microbubbles MBs, liposomes, droplets, hydrogels, and nanobubble-nanoparticle complexes and have been explored for cavitation-mediated US-responsive drug delivery. Recently, a transient increase in the permeability of the BBB by microbubble (MB)-assisted low-frequency US has shown promise in enhancing the delivery of therapeutic agents in the case of neurological disorders. Further, the periodic mechanical stimulus generated by US-responsive MBs have also been explored in tissue engineering and has directly influenced the differentiation of mesenchymal stem cells into cartilage. This Review discusses the various types of US-responsive carriers and explores their emerging roles in therapeutics ranging from drug delivery to tissue engineering.

Imaging the Activation of Low-Boiling-Point Phase-Change Contrast Agents in the Presence of Tissue Motion Using Ultrafast Inter-frame Activation Ultrasound Imaging

Nanoscale phase-change contrast agents (PCCAs) have been found to have great potential in non-invasive extravascular imaging and therapeutic delivery. However, the contrast-to-tissue ratio (CTR) of PCCA images is usually limited because of either physiological motion or incomplete cancelation of tissue signal. Therefore, to improve the CTR of PCCA images in the presence of physiological motion, a new imaging technique, ultrafast inter-frame activation ultrasound (UIAU) imaging, is proposed and validated. Results of studies with controlled motion in tissue-mimicking phantoms indicate UIAU could provide significantly higher CTRs (maximum: 17.3 ± 0.9 dB) relative to conventional pulse inversion imaging (maximum CTR: 3.4 ± 1.4 dB). UIAU has CTRs up to 16.1 ± 1.0 dB relative to 3.9 ± 2.3 dB for differential imaging in the presence of physiological motion at 20 mm/s. In vivo imaging of PCCAs in the rat liver also reveals the ability of UIAU to enhance PCCA image contrast in the presence of physiological motion.

Gas-generating nanoparticles for contrast-enhanced ultrasound imaging

We present gas-generating solid nanoparticles as a new concept of an ultrasound contrast agent. The developed nanoparticles are sufficiently small (less than 100 nm in diameter) to escape vasculature and yet, upon external pulsed laser light activation, release nitrogen gas for enhanced contrast in ultrasound imaging. The gas-generating nanoconstructs combine the photocatalytic function of gold nanoparticles and photolysis of azide compounds. Using ultrasound imaging, we demonstrate the controlled, on-demand generation of nitrogen gas from nanoparticles due to the decomposition of azide groups triggered by pulsed laser irradiation. The resulting gas forms bubbles that cause backscattered ultrasound signals and, therefore, modulate the contrast in ultrasound imaging.

Toward optimization of blood brain barrier opening induced by laser-activated perfluorocarbon nanodroplets

The blood brain barrier (BBB), a component of the brain's natural defense system, is often a roadblock for the monitoring and treatment of neurological disorders. Recently, we introduced a technique to open the blood brain barrier through the use of laser-activated perfluorohexane nanodroplets (PFHnDs), a phase-change nanoagent that undergoes repeated vaporization and recondensation when excited by a pulsed laser. Laser-activated PFHnDs were shown to enable noninvasive and localized opening of the BBB, allowing extravasation of various sized agents into the brain tissue. In this current work, the laser-activated PFHnD-induced BBB opening is further explored. In particular, laser fluence and the number of laser pulses used for the PFHnD-induced BBB opening are examined and evaluated both qualitatively and quantitatively to determine the effect of these parameters on BBB opening. The results of these studies show trends between increased laser fluence and an increased BBB opening as well as between an increased number of laser pulses and an increased BBB opening, however, with limitations on the extent of the BBB opening after a certain number of pulses. Overall, the results of these studies serve as a guideline to choosing suitable laser parameters for safe and effective BBB opening.

Design and Demonstration of a Configurable Imaging Platform for Combined Laser, Ultrasound, and Elasticity Imaging

This paper introduces a configurable combined laser, ultrasound, and elasticity (CLUE) imaging platform. The CLUE platform enables imaging sequences capable of simultaneously providing quantitative acoustic, optical, and mechanical contrast for comprehensive diagnosis and monitoring of complex diseases, such as cancer. The CLUE imaging platform was developed on a Verasonics ultrasound scanner integrated with a pulsed laser, and it was designed to be modular and scalable to allow researchers to create their own specific imaging sequences efficiently. The CLUE imaging platform and sequence were demonstrated in a tissue-mimicking phantom containing a stiff inclusion labeled with optically-activated nanodroplets and in an ex vivo mouse spleen. We have shown that CLUE imaging can simultaneously capture multi-functional imaging signals providing quantitative information on tissue.

Lipid Shell Composition Plays a Critical Role in the Stable Size Reduction of Perfluorocarbon Nanodroplets

Perfluorocarbon nanodroplets (PFCnDs) are phase-change contrast agents that have the potential to enable extravascular contrast-enhanced ultrasound and photoacoustic (US/PA) imaging. Producing consistently small, monodisperse PFCnDs remains a challenge without resorting to technically challenging methods. We investigated the impact of variable shell composition on PFCnD size and US/PA image properties. Our results suggest that increasing the molar percentage of PEGylated lipid reduces the size and size variance of PFCnDs. Furthermore, our imaging studies revealed that nanodroplets with more PEGylated lipids produce increased US/PA signal compared with those with the standard formulation. Finally, we highlight the ability of this approach to facilitate US/PA imaging in a murine model of breast cancer. These data indicate that, through a facile synthesis process, it is possible to produce monodisperse, small-sized PFCnDs. Novel in their simplicity, these methods may promote the use of PFCnDs among a broader user base to study a variety of extravascular phenomena.

Color-coded perfluorocarbon nanodroplets for multiplexed ultrasound and Photoacoustic imaging

Laser-activated perfluorocarbon nanodroplets are an emerging class of phase change, dual-contrast agents that can be utilized in ultrasound and photoacoustic imaging. Through the ability to differentiate subpopulations of nanodroplets via laser activation at different wavelengths of near-infrared light, optically-triggered color-coded perfluorocarbon nanodroplets present themselves as an attractive tool for multiplexed ultrasound and photoacoustic imaging. In particular, laser-activated droplets can be used to provide quantitative spatiotemporal information regarding distinct biological targets, allowing for their potential use in a wide range of diagnos tic and therapeutic applications. In the work presented, laser-activated color-coded perfluorocarbon nanodroplets are synthesized to selectively respond to laser irradiation at corresponding wavelengths. The dynamic ultrasound and photoacoustic signals produced by laser-activated perfluorocarbon nanodroplets are evaluated in situ prior to implementation in a murine model. In vivo, these particles are used to distinguish unique particle trafficking mechanisms and are shown to provide ultrasound and photoacoustic contrast for up to 72 hours within lymphatics. Overall, the conducted studies show that laser-activated color-coded perfluorocarbon nanodroplets are a promising agent for multiplexed ultrasound and photoacoustic imaging.

Leveraging the Imaging Transmit Pulse to Manipulate Phase-Change Nanodroplets for Contrast-Enhanced Ultrasound

Phase-change perfluorohexane nanodroplets (PFHnDs) are a new class of recondensable submicrometer-sized contrast agents that have potential for contrast-enhanced and super-resolution ultrasound imaging with an ability to reach extravascular targets. The PFHnDs can be optically triggered to undergo vaporization, resulting in spatially stationary, temporally transient microbubbles. The vaporized PFHnDs are hyperechoic in ultrasound imaging for several to hundreds of milliseconds before recondensing to their native, hypoechoic, liquid nanodroplet state. The decay of echogenicity, i.e., the dynamic behavior of the ultrasound signal from optically triggered PFHnDs in ultrasound imaging, can be captured using high-frame-rate ultrasound imaging. We explore the possibility to manipulate the echogenicity dynamics of optically triggered PFHnDs in ultrasound imaging by changing the phase of the ultrasound imaging pulse. Specifically, the ultrasound imaging system was programmed to transmit two imaging pulses with inverse polarities. We show that the imaging pulse phase can affect the amplitude and the temporal behavior of PFHnD echogenicity in ultrasound imaging. The results of this study demonstrate that the ultrasound echogenicity is significantly increased (about 78% improvement) and the hyperechoic timespan of optically triggered PFHnDs is significantly longer (about four times) if the nanodroplets are imaged by an ultrasound pulse starting with rarefactional pressure versus a pulse starting with compressional pressure. Our finding has direct and significant implications for contrast-enhanced ultrasound imaging of droplets in applications such as super-resolution imaging and molecular imaging where detection of individual or low-concentration PFHnDs is required.

Super-Resolution Imaging With Ultrafast Ultrasound Imaging of Optically Triggered Perfluorohexane Nanodroplets

Super-resolution imaging with moving microbubbles has shown potential in identifying fine details of deep-lying vascular compartments. To image the extravascular targets, this paper has employed nanometer-sized, optically triggered perfluorohexane nanodroplets (PFHnDs). In response to pulsed laser irradiation, the PFHnDs repeatedly vaporize and stochastically recondense, resulting in random changes of ultrasound signals. Our previous study has shown that the stochastic recondensation of the PFHnDs can be used to isolate individual PFHnDs for super-resolution imaging. This paper introduces an improved method for super-resolution imaging with ultrafast ultrasound imaging of PFHnDs. The previous method was based on subtraction of two consecutive ultrasound images to detect signals from recondensed, isolated droplets, whereas our current method compounds respective multiple pre- and post-recondensation ultrafast ultrasound images prior to subtraction to improve the spatial resolution further. To evaluate the axial and lateral resolutions of our method, we repeatedly imaged a phantom containing PFHnDs using a programmable ultrasound system synchronized with a pulsed laser system. As a result, our method improved the lateral and axial resolutions by 54% and 68%, respectively, over the previous super-resolution imaging approach, indicating that it can be used for localizing extravascular molecular targets with superior accuracy.

Laser-activated perfluorocarbon nanodroplets: a new tool for blood brain barrier opening

A major obstacle in the monitoring and treatment of neurological diseases is the blood brain barrier (BBB), a semipermeable barrier that prevents the delivery of many therapeutics and imaging contrast agents to the brain. In this work, we explored the possibility of laser-activated perfluorocarbon nanodroplets (PFCnDs) to open the BBB and deliver agents to the brain tissue. Specifically, near infrared (NIR) dye-loaded PFCnDs comprised of a perfluorocarbon (PFC) core with a boiling point above physiological temperature were repeatedly vaporized and recondensed from liquid droplet to gas bubble under pulsed laser excitation. As a result, this pulse-to-pulse repeated behavior enabled the recurring interaction of PFCnDs with the endothelial lining of the BBB, allowing for a BBB opening and extravasation of dye into the brain tissue. The blood brain barrier opening and delivery of agents to tissue was confirmed on the macro and the molecular level by evaluating Evans Blue staining, ultrasound-guided photoacoustic (USPA) imaging, and histological tissue analysis. The demonstrated PFCnD-assisted pulsed laser method for BBB opening, therefore, represents a tool that has the potential to enable non-invasive, cost-effective, and efficient image-guided delivery of contrast and therapeutic agents to the brain.

Combined Multiwavelength Photoacoustic and Plane-Wave Ultrasound Imaging for Probing Dynamic Phase-Change Contrast Agents

OBJECTIVE

The purpose of this study was to introduce combined multiwavelength photoacoustic (PA) and plane-wave ultrasound (US) imaging referred to as mwPA/pwUS imaging capable of probing the rapid dynamic behavior of optically activated phase-change contrast agents.

METHODS

A dedicated mwPA/pwUS imaging sequence was developed based on a programmable US system synchronized with a tunable laser to irradiate tissue with laser pulses at desired optical wavelengths and to acquire post laser pulse PA images followed by ultrafast plane-wave US images. To evaluate the mwPA/pwUS imaging, a capillary filled with optically responsive perfluorohexane nanodroplets (PFHnDs) containing a dye with the peak absorption at 760 nm was imaged with optical wavelengths ranging from 700 to 940 nm. The differences between post-laser ultrafast US images [i.e., differential US (ΔUS)] were taken to visualize the recondensation dynamics of PFHnDs at each wavelength.

RESULTS

The PA images of PFHnDs showed higher contrast near 760 nm wavelength, corresponding to the peak absorption of the dye encapsulated in the PFHnDs. Moreover, the ΔUS signals immediately after 760-nm pulsed laser irradiation were also high due to the increased US contrast associated with vaporized PFHnDs.

CONCLUSION

The mwPA/pwUS imaging allowed for the US-based optical spectroscopic characterization of PFHnDs and their dynamics.

SIGNIFICANCE

The introduced mwPA/pwUS imaging sequence can be used in various clinical applications where both spectroscopic PA imaging of endogenous and/or exogenous chromophores and ultrafast US imaging of phase-change nanodroplets are desired.

Recent Development of Technology and Application of Photoacoustic Molecular Imaging Toward Clinical Translation

The deep imaging capability and optical absorption contrast offered by photoacoustic imaging promote the use of this technology in clinical applications. By exploiting the optical absorption properties of endogenous chromophores such as hemoglobin and lipid, molecular information at a depth of a few centimeters can be unveiled. This information shows promise to reveal lesions indicating early stage of various human diseases, such as cancer and atherosclerosis. In addition, the use of exogenous contrast agents can further extend the capability of photoacoustic imaging in clinical diagnosis and treatment. In this review, the current state of the art and applications of photoacoustic molecular probes will be critically reviewed, as well as some spearheading translational efforts that have taken place over the past 5 years. Some of the most critical barriers to clinical translation of this novel technology will be discussed, and some thoughts will be given on future endeavors and pathways.

Review: optically-triggered phase-transition droplets for photoacoustic imaging

Optically-triggered phase-transition droplets have been introduced as a promising contrast agent for photoacoustic and ultrasound imaging that not only provide significantly enhanced contrast but also have potential as photoacoustic theranostic molecular probes incorporated with targeting molecules and therapeutics. For further understanding the dynamics of optical droplet vaporization process, an innovative, methodical analysis by concurrent acoustical and ultrafast optical recordings, comparing with a theoretical model has been employed. In addition, the repeatability of the droplet vaporization-recondensation process, which enables continuous photoacoustic imaging has been studied through the same approach. Further understanding the underlying physics of the optical droplet vaporization and associated dynamics may guide the optimal design of the droplets. Some innovative approaches in preclinical studies have been recently demonstrated, including sono-photoacoustic imaging, dual-modality of photoacoustic and ultrasound imaging, and super-resolution photoacoustic imaging. In this review, current development of optically triggered phase-transition droplets and understanding on the vaporization dynamics, their applications are introduced and future directions are discussed.

A model for acoustic vaporization dynamics of a bubble/droplet system encapsulated within a hyperelastic shell

Nanodroplets have great, promising medical applications such as contrast imaging, embolotherapy, or targeted drug delivery. Their functions can be mechanically activated by means of focused ultrasound inducing a phase change of the inner liquid known as the acoustic droplet vaporization (ADV) process. In this context, a four-phases (vapor + liquid + shell + surrounding environment) model of ADV is proposed. Attention is especially devoted to the mechanical properties of the encapsulating shell, incorporating the well-known strain-softening behavior of Mooney-Rivlin material adapted to very large deformations of soft, nearly incompressible materials. Various responses to ultrasound excitation are illustrated, depending on linear and nonlinear mechanical shell properties and acoustical excitation parameters. Different classes of ADV outcomes are exhibited, and a relevant threshold ensuring complete vaporization of the inner liquid layer is defined. The dependence of this threshold with acoustical, geometrical, and mechanical parameters is also provided.

Copper Sulfide Perfluorocarbon Nanodroplets as Clinically Relevant Photoacoustic/Ultrasound Imaging Agents

We have developed laser-activated perfluorocarbon nanodroplets containing copper sulfide nanoparticles (CuS NPs) for contrast-enhanced ultrasound and photoacoustic imaging. As potential clinical contrast agents, CuS NPs have favorable properties including biocompatibility, biodegradability, and enhance contrast in photoacoustic images at clinically relevant depths. However, CuS NPs are not efficient optical absorbers when compared to plasmonic nanoparticles and therefore, contrast enhancement with CuS NPs is limited, requiring high concentrations to generate images with sufficient signal-to-noise ratio. We have combined CuS NPs with laser-activated perfluorocarbon nanodroplets (PFCnDs) to achieve enhanced photoacoustic contrast and, more importantly, ultrasound contrast while retaining the favorable clinical characteristics of CuS NPs. The imaging characteristics of synthesized CuS-PFCnD constructs were first tested in tissue-mimicking phantoms and then in in vivo murine models. The results demonstrate that CuS-PFCnDs enhance contrast in photoacoustic (PA) and ultrasound (US) imaging. Upon systemic administration in vivo, CuS-PFCnDs remain stable and their unique vaporization provides sufficient PA/US contrast that can be further exploited for contrast-enhanced background-free imaging. The conducted studies provide a solid foundation for further development of CuS-PFCnDs as PA/US diagnostic and eventually therapeutic agents for clinical applications.