Dual-frequency acoustic droplet vaporization detection for medical imaging

Acoustic Detection of Retained Perfluoropropane Droplets Within the Developing Myocardial Infarct Zone

Perfluoropropane droplets (PDs) cross endothelial barriers and can be acoustically activated for selective myocardial extravascular enhancement following intravenous injection (IVI). Our objective was to determine how to optimally activate extravascular PDs for transthoracic ultrasound-enhanced delineation of a developing scar zone (DSZ). Ultrafast-frame-rate microscopy was conducted to determine the effect of pulse sequence on the threshold of bubble formation from PDs. In vitro studies were subsequently performed at different flow rates to determine acoustic activation and inertial cavitation thresholds for a PD infusion using multipulse fundamental non-linear or single-pulse harmonic imaging. IVIs of PDs were given in 9 rats and 10 pigs following prolonged left anterior descending ischemia to detect and quantify PD kinetics within the DSZ. A multipulse sequence had a lower myocardial index threshold for acoustic activation by ultrafast-frame-rate microscopy. Acoustic activation was observed at a myocardial index ≥0.4 below the inertial cavitation threshold for both pulse sequences. In rats, confocal microscopy and serial acoustic activation imaging detected higher droplet presence (relative to remote regions) within the DSZ at 3 min post-IVI. Transthoracic high-mechanical-index impulses with fundamental non-linear imaging in pigs at this time post-IVI resulted in selective contrast enhancement within the DSZ.

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.

Polyvinyl Alcohol Cryogels for Acoustic Characterization of Phase-Change Contrast Agents

Phase-change contrast agents (PCCAs) consisting of lipid-encapsulated low-boiling-point perfluorocarbons can be used in conjunction with ultrasound for diagnostic and therapeutic applications. One benefit of PCCAs is site-specific activation, whereby the liquid core is acoustically vaporized into a bubble detectable via ultrasound imaging. For further evaluation of PCCAs in a variety of applications, it is useful to disperse these nanodroplets into an acoustically compatible stationary matrix. However, many traditional phantom preparations require heating, which causes premature thermal activation of low-boiling-point PCCAs. Polyvinyl alcohol (PVA) cryogels do not require heat to set. Here we propose a simple method for the incorporation of the low-boiling-point PCCAs using octafluoropropane (OFP) and decafluorobutane (DFB) into PVA cryogels for a variety of acoustic characterization applications. We determined the utility of the phantoms by activating droplets with a focused transducer, visualizing the lesions with ultrasound imaging. At 1 MHz, droplet activation was consistently observed at 2.0 and 4.0 MPa for OFP and DFB, respectively.

Exploring the Acoustic and Dynamic Characteristics of Phase-Change Droplets

Acoustic droplet vaporization (ADV) provides the on-demand production of bubbles for use in ultrasound (US)-based diagnostic and therapeutic applications. The droplet-to-bubble transition process has been shown to involve localized internal gas nucleation, followed by a volume expansion of threefold to fivefold and inertial bubble oscillation, all of which take place within a few microseconds. Monitoring these ADV processes is important in gauging the mechanical effects of phase-change droplets in a biological environment, but this is difficult to achieve using regular optical observations. In this study, we utilized acoustic characterization [i.e., simultaneous passive cavitation detection (PCD) and active cavitation detection (ACD)] to investigate the acoustic signatures emitted from phase-change droplets ADV and determined their correlations with the physical behaviors observed using high-speed optical imaging. The experimental results showed that activation with three-cycle 5-MHz US pulse resulted in the droplets (diameter: 3.0- [Formula: see text]) overexpanding and undergoing damped oscillation before settling to bubbles with a final diameter. Meanwhile, a broadband shock wave was observed at the beginning of the PCD signal. The intense fluctuations of the ACD signal revealed that the shock wave arose from the inertial cavitation of nucleated small gas pockets in the droplets. It was particularly interesting that another shock-wave signal with a much lower acoustic frequency (< 2 MHz) was observed at about [Formula: see text] after the first half signal. This signal coincided with the reduction of the ACD signal amplitude that indicated the rebound of the transforming bubble. Since internal gas nucleation is a crucial process of ADV, the first half signal may indicate the occurrence of an ADV event, and the second half signal may further reveal the degrees of expansion and oscillation of the bubble. These acoustic signatures provide opportunities for monitoring ADV dynamics based on the detection of acoustic signals.

Delayed Echo Enhancement Imaging to Quantify Myocardial Infarct Size

BACKGROUND

Perfluoropropane droplets formulated from commercial microbubbles exhibit different acoustic characteristics than their parent microbubbles, most likely from enhanced endothelial permeability. This enhanced permeability may permit delayed echo-enhancement imaging (DEEI) similar to delayed enhancement magnetic resonance imaging (DE-MRI). We hypothesized this would allow detection and quantification of myocardial scar.

METHODS

In 15 pigs undergoing 90 minutes of left anterior descending ischemia by either balloon (n = 13) or thrombotic occlusion (n = 2), DE-MRI was performed at 2-24 days postocclusion. Delayed echo-enhancement imaging was performed at 2-4 minutes following an intravenous injection of 1 mL of 50% Definity (Lantheus Medical) compressed into 180 nm droplets; DEEI was attempted in all pigs with single-pulse harmonic imaging at 1.7 transmit/3.4 MHz receive. Myocardial defects observed with DEEI were quantified (percentage of infarct area) and compared with DE-MRI as well as postmortem staining. In six pigs, multipulse low-mechanical index (MI) fundamental nonlinear imaging (FNLI) with intermittent high-MI impulses was performed to determine whether droplet activation within the infarct zone was achievable with a longer pulse duration.

RESULTS

The range of infarct size area by DE-MRI ranged from 0% to 46% of total left ventricular area. Single-pulse harmonic imaging detected a contrast defect that correlated closely with infarct area by DE-MRI (r = 0.81, P = .0001). The FNLI high-MI impulses resulted in droplet activation in both the infarct and normal zones. Harmonic subtraction of the FNLI images resulted in infarct zone enhancement that also correlated closely with infarct size (r = 0.83; P = .04). Droplets were observed on postmortem transmission electron microscopy within myocytes of the infarct and remote normal zone.

CONCLUSION

Intravenously Definity nanodroplets can be utilized to detect and quantify infarct zone at the bedside using DEEI techniques.

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.

Transcranial activation and imaging of low boiling point phase-change contrast agents through the temporal bone using an ultrafast interframe activation ultrasound sequence

PURPOSE

As a cavitation enhancer, low boiling point phase-change contrast agents (PCCA) offer potential for ultrasound-mediated drug delivery in applications including intracerebral hemorrhage or brain tumors. In addition to introducing cavitation, ultrasound imaging also has the ability to provide guidance and monitoring of the therapeutic process by localizing delivery events. However, the highly attenuating skull poses a challenge for achieving an image with useful contrast. In this study, the feasibility of transcranial activation and imaging of low boiling point PCCAs through the human temporal bone was investigated by using a low frequency ultrafast interframe activation ultrasound (UIAU) imaging sequence with singular value decomposition-based denoising.

METHODS

Lipid-shelled PCCAs filled with decafluorobutane were activated and imaged at 37°C in tissue-mimicking phantoms both without and with an ex vivo human skull using the new UIAU sequence and a low frequency diagnostic transducer array at frequencies from 1.5 to 3.5 MHz. A singular value decomposition-based denoising filter was developed and used to further enhance transcranial image contrast. The contrast-to-tissue ratio (CTR) and contrast enhancement (CE) of UIAU was quantitatively evaluated and compared with the amplitude modulation pulse inversion (AMPI) and vaporization detection imaging (VDI) approaches.

RESULTS

Image results demonstrate enhanced contrast in the phantom channel with suppressed background when the low boiling point PCCA was activated both without and with an ex vivo human skull using the UIAU sequence. Quantitative results show that without the skull, low frequency UIAU imaging provided significantly higher image contrast (CTR ≥ 18.56 dB and CE ≥ 18.66 dB) than that of AMPI and VDI (P < 0.05). Transcranial imaging results indicated that the CE of UIAU (≥18.80 dB) was significantly higher than AMPI for free-field activation pressures of 5 and 6 MPa. The CE of UIAU is also significantly higher than that of VDI when PCCAs were activated at 2.5 MHz and 3 MHz (P < 0.05). The CTR (23.30 [20.07-25.56] dB) of denoised UIAU increased by 12.58 dB relative to the non-denoised case and was significantly higher than that of AMPI at an activation pressure of 4 MPa (P < 0.05).

CONCLUSIONS

Results indicate that low boiling point PCCAs can be activated and imaged at low frequencies including imaging through the temporal bone using the UIAU sequence. The UIAU imaging approach provides higher contrast than AMPI and VDI, especially at lower activation pressures with additional removal of electronic noise.

Acoustic Droplet Vaporization in Acoustically Responsive Scaffolds: Effects of Frequency of Excitation, Volume Fraction and Threshold Determination Method

Ultrasound-induced vaporization of liquid perfluorocarbon (PFC) droplets into microbubbles, termed acoustic droplet vaporization (ADV), has potential therapeutic and diagnostic applications. Recently, we demonstrated how ADV-a threshold-based phenomenon-can modulate the release of biomolecules from composite hydrogels, thereby stimulating regenerative processes, such as angiogenesis. These composite hydrogels, called acoustically responsive scaffolds (ARSs), consist of monodispersed, micron size PFC emulsions embedded within a fibrin matrix. This study investigated the effects of frequency of excitation (2.25, 5, 7.5 and 10 MHz) and volume fraction (0.05%, 0.2% and 1% [v/v]) of monodispersed, double emulsions in the ARSs on the ADV threshold. We determined and compared the ADV thresholds via acoustic methods, including active detection, passive detection and attenuation, as well as an echogenicity-based method using B-mode imaging. The ADV threshold determined via these four techniques showed an increasing trend with frequency of excitation. Further analysis of the wave propagation showed that the amplitudes of high frequency harmonics were diminished in ARSs with high volume fractions of emulsion. The ADV threshold inversely correlated with the volume fraction of emulsion at the lowest excitation frequency. However, at higher frequencies, possibly due to the high acoustic reflectivity of the PFC emulsions, the ADV threshold correlated directly with the volume fraction of the emulsion. Additionally, the ADV efficiency correlated with the supra-threshold acoustic pressure. Overall, these results elucidate fundamental acoustic properties of the ARSs, which can be used in future applications.

Unravelling the Acoustic and Thermal Responses of Perfluorocarbon Liquid Droplets Stabilized with Cellulose Nanofibers

The attractive colloidal and physicochemical properties of cellulose nanofibers (CNFs) at interfaces have recently been exploited in the facile production of a number of environmentally benign materials, e.g. foams, emulsions, and capsules. Herein, these unique properties are exploited in a new type of CNF-stabilized perfluoropentane droplets produced via a straightforward and simple mixing protocol. Droplets with a comparatively narrow size distribution (ca. 1-5 μm in diameter) were fabricated, and their potential in the acoustic droplet vaporization process was evaluated. For this, the particle-stabilized droplets were assessed in three independent experimental examinations, namely temperature, acoustic, and ultrasonic standing wave tests. During the acoustic droplet vaporization (ADV) process, droplets were converted to gas-filled microbubbles, offering enhanced visualization by ultrasound. The acoustic pressure threshold of about 0.62 MPa was identified for the cellulose-stabilized droplets. A phase transition temperature of about 22 °C was observed, at which a significant fraction of larger droplets (above ca. 3 μm in diameter) were converted into bubbles, whereas a large part of the population of smaller droplets were stable up to higher temperatures (temperatures up to 45 °C tested). Moreover, under ultrasound standing wave conditions, droplets were relocated to antinodes demonstrating the behavior associated with the negative contrast particles. The combined results make the CNF-stabilized droplets interesting in cell-droplet interaction experiments and ultrasound imaging.

Ultrasound-Stimulated Phase-Change Contrast Agents for Transepithelial Delivery of Macromolecules, Toward Gastrointestinal Drug Delivery

The gastrointestinal (GI) tract presents a notoriously difficult barrier for macromolecular drug delivery, especially for biologics. Herein, we demonstrate that ultrasound-stimulated phase change contrast agents (PCCAs) can transiently disrupt confluent colorectal adenocarcinoma monolayers and improve the transepithelial transport of a macromolecular model drug. With ultrasound treatment in the presence of PCCAs, we achieved a maximum of 44 ± 15% transepithelial delivery of 70-kDa fluorescein isothiocyanate-dextran, compared with negligible delivery through sham control monolayers. Among all tested rarefactional pressures (300-600 kPa), dextran delivery efficiency was consistently greatest at 300 kPa. To explore this unexpected finding, we quantified stable and inertial cavitation energy generated by various ultrasound exposure conditions. In general, lower pressures resulted in more persistent cavitation activity during the 30-s ultrasound exposures, which may explain the enhanced dextran delivery efficiency. Thus, a unique advantage of using low boiling point PCCAs for this application is that the same low-pressure pulses can be used to induce vaporization and provide maximal delivery.

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.

Low intensity focused ultrasound (LIFU) triggered drug release from cetuximab-conjugated phase-changeable nanoparticles for precision theranostics against anaplastic thyroid carcinoma

This study provides an efficient theranostic strategy for concurrent targeted ultrasound molecular imaging and effective synergistic antitumor therapy.

Selective infarct zone imaging with intravenous acoustically activated droplets

Background

Microbubbles (MB) can be compressed to nanometer-sized droplets and reactivated with diagnostic ultrasound; these reactivated MB possess unique imaging characteristics.

Objective

We hypothesized that droplets formed from compressing Definity MB may be used for infarct-enhancement imaging.

Methods

Fourteen rats underwent ligation of their left anterior descending (LAD) artery, and five pigs underwent 90 minute balloon occlusions of their mid LAD. At 48 hours in rats, transthoracic ultrasound was performed at two and four minutes following 200 μL intravenous injections (IVI) of Definity droplets (DD), at which point the MI was increased from 0.5 to 1.5 to assess for a transient contrast enhancement zone (TEZ) within akinetic segments. In pigs, 1.0 mL injections of DD were administered and low frame rate (triggered end systolic or 10 Hz) imaging 2–4 minutes post iVI to selectively activate and image the infarct zone (IZ). Infarct size was defined by delayed enhancement magnetic resonance imaging (DE-MRI) and post-mortem staining (TTC).

Results

Increasing MI to 1.5 (at two or four minutes after IVI) resulted in a TEZ in rats, which correlated with infarct size (r = 0.94, p<0.001). A TEZ was not seen at 2–4 minutes in any rat (n = 8) following Definity MB injections. Fluorescent staining confirmed DD presence within the infarct zone 10 minutes after intravenous injection. In pigs, selective enhancement within the IZ was achieved by using a low frame rate single pulse harmonic mode; IZ size matched the location seen with DE-MRI and correlated with TTC defect size (r = 0.90, p<0.05).

Conclusion

DD formulated from commercially available MB can be acoustically activated for selective infarct enhancement imaging.

Drug Release from Phase-Changeable Nanodroplets Triggered by Low-Intensity Focused Ultrasound

Background: As one of the most effective triggers with high tissue-penetrating capability and non-invasive feature, ultrasound shows great potential for controlling the drug release and enhancing the chemotherapeutic efficacy. In this study, we report, for the first time, construction of a phase-changeable drug-delivery nanosystem with programmable low-intensity focused ultrasound (LIFU) that could trigger drug-release and significantly enhance anticancer drug delivery. Methods: Liquid-gas phase-changeable perfluorocarbon (perfluoropentane) and an anticancer drug (doxorubicin) were simultaneously encapsulated in two kinds of nanodroplets. By triggering LIFU, the nanodroplets could be converted into microbubbles locally in tumor tissues for acoustic imaging and the loaded anticancer drug (doxorubicin) was released after the microbubble collapse. Based on the acoustic property of shell materials, such as shell stiffness, two types of nanodroplets (lipid-based nanodroplets and PLGA-based nanodroplets) were activated by different acoustic pressure levels. Ultrasound irradiation duration and power of LIFU were tested and selected to monitor and control the drug release from nanodroplets. Various ultrasound energies were introduced to induce the phase transition and microbubble collapse of nanodroplets in vitro (3 W/3 min for lipid nanodroplets; 8 W/3 min for PLGA nanodroplets). Results: We detected three steps in the drug-releasing profiles exhibiting the programmable patterns. Importantly, the intratumoral accumulation and distribution of the drug with LIFU exposure were significantly enhanced, and tumor proliferation was substantially inhibited. Co-delivery of two drug-loaded nanodroplets could overcome the physical barriers of tumor tissues during chemotherapy. Conclusion: Our study provides a new strategy for the efficient ultrasound-triggered chemotherapy by nanocarriers with programmable LIFU capable of achieving the on-demand drug release.

Acoustic Behavior of a Reactivated, Commercially Available Ultrasound Contrast Agent

BACKGROUND

Commercially available microbubbles such as Definity contain octafluoropropane encapsulated in a lipid shell. This perfluorocarbon can be compressed into liquid nanodroplets at room temperatures and activated with transthoracic diagnostic ultrasound. The aim of this study was to determine the size range and acoustic characteristics of Definity nanodroplets (DNDs) compared with Definity microbubbles (DMBs).

METHODS

An in vitro flow system was used with a diagnostic ultrasound transducer (S5-1, iE33). DMBs were prepared using package insert instructions. DNDs were prepared by cooling DMBs in a -10°C to -15°C isopropyl alcohol bath before hand-pressurizing the solution. The formed DNDs were sized, diluted to 1% solutions, and infused continuously into a phosphate-buffered saline solution running within Silastic tubing. Acoustic intensity (AI) was compared with equivalent dilutions of DMBs at different mechanical indices (MIs) ranging from 0.2 to 1.4 (n = 6 comparisons at each MI) using real-time 56-Hz and triggered 2-Hz frame rates (FRs). A 3-cm-thick tissue-mimicking phantom was used to simulate transthoracic attenuation. In vivo transthoracic studies were performed in four normal pigs infused with 10% intravenous infusions of DMBs or DNDs at real-time and triggered end-systolic FRs to compare differences in myocardial and left ventricular cavity AI.

RESULTS

DNDs were smaller than DMBs and ranged in size from 50 to 1,000 nm. In vitro studies revealed that at an MI of 0.2 and an FR of 56 Hz, DMBs had high AI (37 ± 2 dB), but AI dropped to 25 ± 2 dB at an MI of 1.0 (P < .001, analysis of variance). In comparison, DNDs had virtually no AI at MIs of 0.2 to 0.6 at both triggered and 56-Hz FRs (1 ± 0 dB), but AI increased to 34 ± 2 dB at an MI of 1.4 using an FR of 56 Hz (P < .001 vs MI of 0.2). AI also persisted longer at 56 Hz with DNDs when using higher MIs. In vivo studies demonstrated higher myocardial AI for DNDs at higher MIs when using real-time FR, most likely from microvascular nanodroplet activation.

CONCLUSION

These data indicate significant differences in acoustic responses of the commercially available DMBs when administered as an equivalent number of DNDs. The DND formulation may render them more useful for high-MI real-time imaging and other targeted transthoracic diagnostic applications.

Wideband acoustic activation and detection of droplet vaporization events using a capacitive micromachined ultrasonic transducer

An ongoing challenge exists in understanding and optimizing the acoustic droplet vaporization (ADV) process to enhance contrast agent effectiveness for biomedical applications. Acoustic signatures from vaporization events can be identified and differentiated from microbubble or tissue signals based on their frequency content. The present study exploited the wide bandwidth of a 128-element capacitive micromachined ultrasonic transducer (CMUT) array for activation (8 MHz) and real-time imaging (1 MHz) of ADV events from droplets circulating in a tube. Compared to a commercial piezoelectric probe, the CMUT array provides a substantial increase of the contrast-to-noise ratio.