Optimizing Acoustic Activation of Phase Change Contrast Agents With the Activation Pressure Matching Method: A Review

Influence of Phase Change Droplet Activation and Microbubble Cavitation on the Microenvironment of Hepatocellular Carcinoma

OBJECTIVE

Both microbubble ultrasound contrast agents and acoustic phase change droplets (APCD) have been explored in hepatocellular carcinoma (HCC). This work aimed to evaluate changes to the HCC microenvironment following either microbubble or APCD destruction in a syngeneic pre-clinical model.

METHODS

Mouse RIL-175 HCC tumors were grown in the right flank of 64 immunocompetent mice. Pre-treatment, photoacoustic volumetric tumor oxygenation, and power Doppler measurements were obtained using a Vevo 3100 system (VisualSonics, Toronto, Canada). The experimental groups received a 0.1 mL bolus injection of either Definity ultrasound contrast agent (Lantheus Medical Imaging) or APCD fabricated by condensing Definity. Following injection, ultrasound destruction was performed using flash-replenishment sequences on a Sequoia with a 10L4 probe (Siemens) for the duration of enhancement. Tumor oxygenation and power Doppler measurements were then repeated immediately post-ultrasound treatment. Twenty-four hours post-treatment, animals were euthanized, and tumors were harvested and stained for CD31, Cleaved Caspase 3 and CD45.

RESULTS

Imaging biomarkers demonstrated a significant reduction in percent vascularity following either microbubble or APCD destruction in the tumor microenvironment ( p < 0.022) but no significant changes in tumor oxygenation (p = 0.12). Similarly, immunohistochemistry data demonstrated a significant decrease in CD31 expression (p < 0.042) and an increase in apoptosis (p < 0.014) in tumors treated with destroyed microbubbles or APCD relative to controls. Finally, a significant increase in CD45 expression was observed in tumors treated with APCD (p = 0.046), indicating an increase in tumor immune response.

CONCLUSION

Ultrasound-triggered destruction of both microbubbles and APCD reduces vascularity, increases apoptosis, and may also increase immune response in this HCC model.

Synchronous Intravital Imaging and Cavitation Monitoring of Antivascular Focused Ultrasound in Tumor Microvasculature Using Monodisperse Low Boiling Point Nanodroplets

Focused ultrasound-stimulated microbubbles can induce blood flow shutdown and ischemic necrosis at higher pressures in an approach termed antivascular ultrasound. Combined with conventional therapies of chemotherapy, immunotherapy, and radiation therapy, this approach has demonstrated tumor growth inhibition and profound synergistic antitumor effects. However, the lower cavitation threshold of microbubbles can potentially yield off-target damage that the polydispersity of clinical agent may further exacerbate. Here we investigate the use of a monodisperse nanodroplet formulation for achieving antivascular effects in tumors. We first develop stable low boiling point monodisperse lipid nanodroplets and examine them as an alternative agent to mediate antivascular ultrasound. With synchronous intravital imaging and ultrasound monitoring of focused ultrasound-stimulated nanodroplets in tumor microvasculature, we show that nanodroplets can trigger blood flow shutdown and do so with a sharper pressure threshold and with fewer additional events than conventionally used microbubbles. We further leverage the smaller size and prolonged pharmacokinetic profile of nanodroplets to allow for potential passive accumulation in tumor tissue prior to antivascular ultrasound, which may be a means by which to promote selective tumor targeting. We find that vascular shutdown is accompanied by inertial cavitation and complex-order sub- and ultraharmonic acoustic signatures, presenting an opportunity for effective feedback control of antivascular ultrasound.

Aberration correction in abdominal histotripsy

In transabdominal histotripsy, ultrasound pulses are focused on the body to noninvasively destroy soft tissues via cavitation. However, the ability to focus is limited by phase aberration, or decorrelation of the ultrasound pulses due to spatial variation in the speed of sound throughout heterogeneous tissue. Phase aberration shifts, broadens, and weakens the focus, thereby reducing the safety and efficacy of histotripsy therapy. This paper reviews and discusses aberration effects in histotripsy and in related therapeutic ultrasound techniques (e.g., high intensity focused ultrasound), with an emphasis on aberration by soft tissues. Methods for aberration correction are reviewed and can be classified into two groups: model-based methods, which use segmented images of the tissue as input to an acoustic propagation model to predict and compensate phase differences, and signal-based methods, which use a receive-capable therapy array to detect phase differences by sensing acoustic signals backpropagating from the focus. The relative advantages and disadvantages of both groups of methods are discussed. Importantly, model-based methods can correct focal shift, while signal-based methods can restore substantial focal pressure, suggesting that both methods should be combined in a 2-step approach. Aberration correction will be critical to improving histotripsy treatments and expanding the histotripsy treatment envelope to enable non-invasive, non-thermal histotripsy therapy for more patients.

Activation of Phase Change Contrast Agents Using Ionizing Radiation

The feasibility of activating phase change contrast agents (PCCA) made from Definity (Lantheus Medical Imaging) using X-rays was investigated. A 10 mL of Definity PCCA (0.1 mL PCCA/mL) were injected into gelatin phantoms and irradiated using doses of 0, 30, 50, and 100 Gy. Size distribution and PCCA activation were measured after irradiation. Definity PCCAs were activated at a threshold of 50 Gy. Changes were visible with microscopy, visual inspection of T-flasks, and ultrasound imaging of gelatin phantoms. Moreover, increasing the radiation dose above 50 Gy appeared to further activate PCCA. These results indicate Definity PCCAs may be useful for ultrasound-based radiation dosimetry.

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.

Tuning the Interfacial Properties of Fluorous Colloids Toward Ultrasound Programmable Bioactivity

Liquid-in-liquid emulsions are kinetically stable colloids that undergo liquid-to-gas phase transitions in response to thermal or acoustic stimuli. Perfluorocarbons (PFCs) are preferred species as their highly fluorinated nature imparts unique properties that are unparalleled by nonfluorinated counterparts. However, traditional methods to prepare PFC emulsions lack the ability to precisely tune the thermodynamic stability of the fluorous-water interphase and consequently control their vaporization behavior. Here, we report a privileged fluoroalkanoic acid that undergoes concentration-dependent assembly on the surfaces of fluorous droplets to modulate interfacial tension. This allows for the rational formulation of orthogonal PFC droplets that can be programmed to vaporize at specified ultrasound powers. We exploit this behavior in two exemplary biomedical settings by developing emulsions that aid ultrasound-mediated hemostasis and enable bioorthogonal delivery of molecular sensors to mammalian cells. Mechanistic insights gained from these studies can be generalized to tune the thermodynamic interfacial equilibria of PFC emulsions toward designing controllable tools for precision medicine.

The influence of droplet concentration on phase change and inertial cavitation thresholds associated with acoustic droplet vaporization

Acoustic droplet vaporization (ADV) is an important process that enables the theragnostic application of acoustically activated droplets, where the nucleation of inertial cavitation (IC) activity must be precisely controlled. This Letter describes threshold pressure measurements for ADV and acoustic emissions consistent with IC activity of lipid-shelled non-superheated perfluoropentane nanodroplets over a range of physiologically relevant concentrations at 1.1-MHz. Under the frequency investigated, results show that the thresholds were relatively independent of concentration for intermediate concentrations (10⁵, 10⁶, and 10⁷ droplets/ml), thus indicating an optimal range of droplet concentrations for conducting threshold studies. For the highest concentration, the difference between the threshold for IC and the threshold for ADV was greatly reduced, suggesting that it might prove difficult to induce ADV without concomitant IC in applications that employ higher concentrations.

Transient acoustic vaporization signatures unique to low boiling point phase change contrast agents enable super-resolution ultrasound imaging without spatiotemporal filtering

The unique activation signal of phase-change contrast agents (PCCAs or droplets) can be separated from the tissue signal and localized to generate super-resolution (SR) ultrasound (US) images. Lipid-shelled, perfluorocarbon PCCAs can be stochastically vaporized (activated) by a plane wave US transmission thereby enabling them to be used as separable targets for ultrasound localization microscopy. The unique signature of droplet vaporization imaging and the transient inherent nature of this signature increases signal contrast and therefore localization confidence, while the poor resolution of the low-frequency vaporization signal is overcome by the super-resolution result. Furthermore, our proposed PCCA SR technique does not require the use of user-dependent and flow-dependent spatio-temporal filtering via singular-value decomposition. Rather, matched filters selected by Fourier-domain analysis are able to identify and localize PCCA activations. Droplet SR was demonstrated in a crossed-microtube water phantom by localizing the activation signals of octafluoropropane nanodroplets (OFP, C₃F₈, -37 °C boiling point) to resolve 100 µm diameter fluorinated ethylene propylene tubes, which are ordinarily 35% smaller than the native diffraction-limited resolution of the imaging system utilized.

Lipid Shell Retention and Selective Binding Capability Following Repeated Transient Acoustic Microdroplet Vaporization

Targeted therapy and molecular imaging using ultrasound have been widely explored using microbubble contrast agents, and more recently, activatable droplet contrast agents that vaporize when exposed to focused ultrasound have been explored. These droplets are coated with a stabilizing, functionalizable shell, typically comprised of fully saturated phospholipids. While the shedding of the lipid shell under ultrasound exposure is a well-studied phenomenon in microbubbles, it has not been fully explored in droplet-based contrast agents, particularly in those that undergo a reversible phase change and recondense following vaporization. Here, we investigate the retention of the lipid shell following repeated transient vaporization events. Two separate fluorescent markers were used to track individual lipid subpopulations: PEGylated lipids, to which targeting ligands are typically bound, and non-PEGylated lipids, which primarily contribute to droplet stability. Following confirmation of the homogeneous surface distribution of each subpopulation of shell lipids using confocal microscopy, high-speed optical imaging provided visual evidence of the ability to repeatedly induce vaporization and recondensation in micron-scale droplets using 5.208 MHz, 3.17 MPa focused ultrasound pulses transmitted from an imaging transducer. Flow cytometry analysis indicated that while PEGylated lipids were fully retained following repeated transient phase change events, 20% of the bulk lipids were shed. While this likely contributed to an observed significant reduction in the average droplet diameter, the selective binding capabilities of droplets functionalized with an RGD peptide, targeted to the integrin α_vβ₃, were not affected. These results indicate that repeated droplet activation may promote shifts in the droplet size distribution but will not influence the accuracy of targeting for therapy or molecular imaging.

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.

Effect of Hydrostatic Pressure, Boundary Constraints and Viscosity on the Vaporization Threshold of Low-Boiling-Point Phase-Change Contrast Agents

The vaporization of low-boiling-point phase-change contrast agents (PCCAs) using ultrasound has been explored in vitro and in vivo. However, it has been reported that the pressure required for activation is higher in vivo, even after attenuation is accounted for. In this study, the effect of boundary constraints, hydrostatic pressure and viscosity on PCCA vaporization pressure threshold are evaluated to explore possible mechanisms for variations in in vivo vaporization behavior. Vaporization was measured in microtubes of varying inner diameter and a pressurized chamber under different hydrostatic pressures using a range of ultrasound pressures. Furthermore, the activation threshold was evaluated in the kidneys of rats. The results confirm that the vaporization threshold is higher in vivo and reveal an increasing activation threshold inversely proportional to constraining tube size and inversely proportional to surrounding viscosity in constrained environments. Counterintuitively, increased hydrostatic pressure had no significant effect experimentally on the PCCA vaporization threshold, although it was confirmed that this result was supported by homogeneous nucleation theory for liquid perfluorocarbon vaporization. These factors suggest that constraints caused by the surrounding tissue and capillary walls, as well as increased viscosity in vivo, contribute to the increased vaporization threshold compared with in vitro experiments, although more work is required to confirm all relevant factors.

Vaporization Detection Imaging: A Technique for Imaging Low-Boiling-Point Phase-Change Contrast Agents with a High Depth of Penetration and Contrast-to-Tissue Ratio

Phase-change contrast agents (PCCAs) possess advantages over microbubble contrast agents, such as the ability to extravasate and circulate longer in the vasculature that could enhance the diagnostic capabilities of contrast-enhanced ultrasound. PCCAs typically have a liquid perfluorocarbon (PFC) core that can be vaporized into echogenic microbubbles. Vaporization of submicron agents filled with liquid PFCs at body temperature usually requires therapeutic pressures higher than typically used for diagnostic imaging, but low-boiling-point PCCAs using decafluorobutane or octafluoropropane can be vaporized using pressures in the diagnostic imaging regime. Low-boiling-point PCCAs produce a unique acoustic signature that can be separated from tissue and bubble signals to make images with high contrast-to-tissue ratios. In this work, we explore the effect of pulse length and concentration on the vaporization signal of PCCAs and a new technique to capture and use the signals to make high contrast-to-tissue ratio images in vivo. The results indicate that using a short pulse may be ideal for imaging because it does not interact with created bubbles but still produces strong signals for making images. Furthermore, it was found that capturing PCCA vaporization signals produced higher contrast-to-tissue ratio values and better depth of penetration than imaging the bubbles generated by droplet activation using conventional contrast imaging techniques. The resolution of the vaporization signal images is poor because of the low frequency of the signals, but their high sensitivity may be used for applications such as molecular imaging, where the detection of small numbers of contrast agents is important.

In Vivo Molecular Imaging Using Low-Boiling-Point Phase-Change Contrast Agents: A Proof of Concept Study

Sub-micron phase-change contrast agents (PCCAs) have been proposed as a tool for ultrasound molecular imaging based on their potential to extravasate and target extravascular markers and also because of the potential to image these contrast agents with a high contrast-to-tissue ratio. We compare in vivo ultrasound molecular imaging with targeted low-boiling-point PCCAs and targeted microbubble contrast agents. Both agents were targeted to the intravascular (endothelial) integrin α_vß₃via a cyclic RGD peptide (cyclo-Arg-Gly-Asp-D-Tyr-Cys) mechanism and imaged in vivo in a rodent fibrosarcoma model, which exhibits angiogenic microvasculature. Signal intensity was measured using two different techniques, conventional contrast-specific imaging (amplitude/phase modulation) and a droplet vaporization imaging sequence, which detects the unique signature of vaporizing PCCAs. Data indicate that PCCA-specific imaging is more sensitive to small numbers of bound agents than conventional contrast imaging. However, data also revealed that contrast from targeted microbubbles was greater than that provided by PCCAs. Both control and targeted PCCAs were observed to be retained in tissue post-vaporization, which was expected for targeted agents but not expected for control agents. The exact mechanism underlying this observation remains unknown.

In Vivo Assessment of the Potential for Renal Bio-Effects from the Vaporization of Perfluorocarbon Phase-Change Contrast Agents

Low-boiling-point perfluorocarbon phase-change contrast agents (PCCAs) provide an alternative to microbubble contrast agents. Although parameter ranges related to in vivo bio-effects of microbubbles are fairly well characterized, few studies have been done to evaluate the potential of bio-effects related to PCCAs. To bridge this gap, we present an assessment of biological effects (e.g., hemorrhage) related to acoustically excited PCCAs in the rodent kidney. The presence or absence of bio-effects was observed after sonication with various perfluorocarbon core PCCAs (decafluorobutane, octafluoropropane or a 1:1 mixture) and as a function of activation pulse mechanical index (MI; minimum activation threshold, which was a moderate MI of 0.81-1.35 vs. a clinical maximum of 1.9). Bio-effects on renal tissue were assessed through hematology and histology including measurement of blood creatinine levels and the quantity of red blood cell (RBC) casts present in hematoxylin and eosin-stained kidney tissue sections after sonication. Short-term (24 h) and long-term (2 and 4 wk) analyses were performed after treatment. Results indicated that bio-effects from PCCA vaporization were not observed at lower mechanical indices. At higher mechanical indices, bio-effects were observed at 24 h, although these were not observable 2 wk after treatment.

From Micro- to Nano-Multifunctional Theranostic Platform: Effective Ultrasound Imaging Is Not Just a Matter of Scale

Ultrasound Contrast Agents (UCAs) consisting of gas-filled-coated Microbubbles (MBs) with diameters between 1 and 10 µm have been used for a number of decades in diagnostic imaging. In recent years, submicron contrast agents have proven to be a viable alternative to MBs for ultrasound (US)-based applications for their capability to extravasate and accumulate in the tumor tissue via the enhanced permeability and retention effect. After a short overview of the more recent approaches to ultrasound-mediated imaging and therapeutics at the nanoscale, phase-change contrast agents (PCCAs), which can be phase-transitioned into highly echogenic MBs by means of US, are here presented. The phenomenon of acoustic droplet vaporization (ADV) to produce bubbles is widely investigated for both imaging and therapeutic applications to develop promising theranostic platforms.