An in vitro study of the correlation between bubble distribution, acoustic emission, and cell damage by contrast ultrasound

Closed-loop cavitation control for focused ultrasound-mediated blood-brain barrier opening by long-circulating microbubbles

Focused ultrasound (FUS) exposure in the presence of microbubbles (MBs) has been successfully used in the delivery of various sizes of therapeutic molecules across the blood-brain barrier (BBB). While acoustic pressure is correlated with the BBB opening size, real-time control of BBB opening to avoid vascular and neural damage is still a challenge. This arises mainly from the variability of FUS-MB interactions due to the variations of animal-specific metabolic environment and specific experimental setup. In this study, we demonstrate a closed-loop cavitation control framework to induce BBB opening for delivering large therapeutic molecules without causing macro tissue damages. To this end, we performed in mice long-term (5 min) cavitation monitoring facilitated by using long-circulating MBs. Monitoring the long-term temporal kinetics of the MBs under varying level of FUS pressure allowed to identify in situ, animal specific activity regimes forming pressure-dependent activity bands. This enables to determine the boundaries of each activity band (i.e. steady oscillation, transition, inertial cavitation) independent from the physical and physiological dynamics of the experiment. However, such a calibration approach is time consuming and to speed up characterization of the in situ, animal specific FUS-MB dynamics, we tested a novel method called 'pre-calibration' that closely reproduces the results of long-term monitoring but with a much shorter duration. Once the activity bands are determined from the pre-calibration method, an operation band can be selected around the desired cavitation dose. To drive cavitation in the selected operation band, we developed an adaptive, closed-loop controller that updates the acoustic pressure between each sonication based on measured cavitation dose. Finally, we quantitatively assessed the safety of different activity bands and validated the proposed methods and controller framework. The proposed framework serves to optimize the FUS pressure instantly to maintain the targeted cavitation level while improving safety control.

Sonoporation-induced depolarization of plasma membrane potential: analysis of heterogeneous impact

Disrupting plasma membrane integrity would inevitably promote anomalous ion fluxes across the membrane and thereby upset the trans-membranous potential. In this article, we report new findings on how sonoporation as a physical membrane perforation strategy would lead to different forms of plasma membrane potential disruption. Our investigation was conducted with a customized fluorescence imaging platform that enabled live monitoring of plasma membrane potential in relation to individual sonoporation events triggered on HeLa cervical cancer cells. Sonovue microbubbles were used as sonoporation agents (added at a 4:3 cell-to-bubble ratio), and they were activated by 1-MHz pulsed ultrasound with 0.35-MPa peak negative pressure, 20-cycle pulse duration, 20-Hz pulse repetition frequency and 1-s total exposure duration. Results indicate that the plasma membrane potential response was heterogeneous among sonoporated cells: (i) membrane potential of irreversibly sonoporated cells was permanently depolarized; (ii) reversibly sonoporated cells exhibited either transient or sustained membrane depolarization; (iii) intact cells adjacent to sonoporated ones underwent transitory membrane depolarization. These findings effectively serve to substantiate the causal relationship between sonoporation and plasma membrane potential.

Spatiotemporal evolution of cavitation dynamics exhibited by flowing microbubbles during ultrasound exposure

Ultrasound and microbubble-based therapies utilize cavitation to generate bioeffects, yet cavitation dynamics during individual pulses and across consecutive pulses remain poorly understood under physiologically relevant flow conditions. SonoVue(®) microbubbles were made to flow (fluid velocity: 10-40 mm/s) through a vessel in a tissue-mimicking material and were exposed to ultrasound [frequency: 0.5 MHz, peak-rarefactional pressure (PRP): 150-1200 kPa, pulse length: 1-100,000 cycles, pulse repetition frequency (PRF): 1-50 Hz, number of pulses: 10-250]. Radiated emissions were captured on a linear array, and passive acoustic mapping was used to spatiotemporally resolve cavitation events. At low PRPs, stable cavitation was maintained throughout several pulses, thus generating a steady rise in energy with low upstream spatial bias within the focal volume. At high PRPs, inertial cavitation was concentrated in the first 6.3 ± 1.3 ms of a pulse, followed by an energy reduction and high upstream bias. Multiple pulses at PRFs below a flow-dependent critical rate (PRF(crit)) produced predictable and consistent cavitation dynamics. Above the PRF(crit), energy generated was unpredictable and spatially biased. In conclusion, key parameters in microbubble-seeded flow conditions were matched with specific types, magnitudes, distributions, and durations of cavitation; this may help in understanding empirically observed in vivo phenomena and guide future pulse sequence designs.

FEASIBILITY STUDY OF A CLINICAL BLOOD-BRAIN BARRIER OPENING ULTRASOUND SYSTEM

In this paper, we investigate the focalization properties of single-element transducers at intermediate frequencies (500 kHz) through primate and human skulls. The study addresses the transcranial targeting involved in ultrasound-induced blood brain barrier (BBB) opening with clinically relevant targets such as the hippocampus and the basal ganglia, which are typically affected by early Alzheimer's and Parkinson's disease, respectively. The targeted brain structures were extracted from three-dimensional (3D) brain atlases registered with the skulls and used to virtually position and orient the transducers. The frequency dependence is first investigated and the capability of targeting of different structures is explored. Preliminary in vivo feasibility is investigated in mice at this frequency. A simple, affordable and convenient system is found to be feasible for BBB opening in primates and humans capable of successfully targeting the hippocampus, putamen and substantia nigra and could thus allow for its broader impact and applications.

Ultrasound molecular imaging of tumor angiogenesis with an integrin targeted microbubble contrast agent

RATIONALE AND OBJECTIVES

Ultrasound molecular imaging is an emerging technique for sensitive detection of intravascular targets. Molecular imaging of angiogenesis has strong potential for both clinical use and as a research tool in tumor biology and the development of antiangiogenic therapies. Our objectives are to develop a robust ultrasound contrast agent platform using microbubbles (MB) to which targeting ligands can be conjugated by biocompatible, covalent conjugation chemistry, and to develop a pure low mechanical index (MI) imaging processing method and corresponding quantification method. The MB and the imaging methods were evaluated in a mouse model of breast cancer in vivo.

MATERIALS AND METHODS

We used a cyclic arginine-glycine-aspartic acid (cRGD) pentapeptide containing a terminal cysteine group conjugated to the surface of MB bearing pyridyldithio-propionate (PDP) for targeting αvβ3 integrins. As negative controls, MB without a ligand or MB bearing a scrambled sequence (cRAD) were prepared. To enable characterization of peptides bound to MB surfaces, the cRGD peptide was labeled with FITC and detected by plate fluorometry, flow cytometry, and fluorescence microscopy. Targeted adhesion of cRGD-MB was demonstrated in an in vitro flow adhesion assay against recombinant murine αvβ3 integrin protein and αvβ3 integrin-expressing endothelial cells (bEnd.3). The specificity of cRGD-MB for αvβ3 integrin was demonstrated by treating bEnd.3 EC with a blocking antibody. A murine model of mammary carcinoma was used to assess targeted adhesion and ultrasound molecular imaging in vivo. The targeted MB were visualized using a low MI contrast imaging pulse sequence, and quantified by intensity normalization and 2-dimensional Fourier transform analysis.

RESULTS

The cRGD ligand concentration on the MB surface was ∼8.2 × 10(6) molecules per MB. At a wall shear stress of 1.0 dynes/cm, cRGD-MB exhibited 5-fold higher adhesion to immobilized recombinant αvβ3 integrin relative to nontargeted MB and cRAD-MB controls. Similarly, cRGD-MB showed significantly greater adhesion to bEnd.3 EC compared with nontargeted MB and cRAD-MB. In addition, cRGD-MB, but not nontargeted MB or cRAD-MB, showed significantly enhanced contrast signals with a high tumor-to-background ratio. The adhesion of cRGD-MB to bEnd.3 was reduced by 80% after using anti-αv monoclonal antibody to treat bEnd.3. The normalized image intensity amplitude was ∼0.8, 7 minutes after the administration of cRGD-MB relative to the intensity amplitude at the time of injection, while the spatial variance in image intensity improved the detection of bound agents. The accumulation of cRGD-MB was blocked by preadministration with an anti-αv blocking antibody.

CONCLUSIONS

The results demonstrate the functionality of a novel MB contrast agent covalently coupled to an RGD peptide for ultrasound molecular imaging of αvβ3 integrin and the feasibility of quantitative molecular ultrasound imaging with a low MI.

In vivo transcranial cavitation threshold detection during ultrasound-induced blood-brain barrier opening in mice

The in vivo cavitation response associated with blood-brain barrier (BBB) opening as induced by transcranial focused ultrasound (FUS) in conjunction with microbubbles was studied in order to better identify the underlying mechanism in its noninvasive application. A cylindrically focused hydrophone, confocal with the FUS transducer, was used as a passive cavitation detector (PCD) to identify the threshold of inertial cavitation (IC) in the presence of Definity® microbubbles (mean diameter range: 1.1-3.3 µm, Lantheus Medical Imaging, MA, USA). A vessel phantom was first used to determine the reliability of the PCD prior to in vivo use. A cerebral blood vessel was simulated by generating a cylindrical channel of 610 µm in diameter inside a polyacrylamide gel and by saturating its volume with microbubbles. The microbubbles were sonicated through an excised mouse skull. Second, the same PCD setup was employed for in vivo noninvasive (i.e. transdermal and transcranial) cavitation detection during BBB opening. After the intravenous administration of Definity® microbubbles, pulsed FUS was applied (frequency: 1.525 or 1.5 MHz, peak-rarefactional pressure: 0.15-0.60 MPa, duty cycle: 20%, PRF: 10 Hz, duration: 1 min with a 30 s interval) to the right hippocampus of twenty-six (n = 26) mice in vivo through intact scalp and skull. T1 and T2-weighted MR images were used to verify the BBB opening. A spectrogram was generated at each pressure in order to detect the IC onset and duration. The threshold of BBB opening was found to be at a 0.30 MPa peak-rarefactional pressure in vivo. Both the phantom and in vivo studies indicated that the IC pressure threshold had a peak-rarefactional amplitude of 0.45 MPa. This indicated that BBB opening may not require IC at or near the threshold. Histological analysis showed that BBB opening could be induced without any cellular damage at 0.30 and 0.45 MPa. In conclusion, the cavitation response could be detected without craniotomy in mice and IC may not be required for BBB opening at relatively low pressures.

An ex vivo study of the correlation between acoustic emission and microvascular damage

The objective of this study was to conduct an ex vivo examination of correlation between acoustic emission and tissue damage. Intravital microscopy was employed in conjunction with ultrasound exposure in cremaster muscle of male Wistar rats. Definity microbubbles were administered intravenously through the tail vein (80microL.kg(-1).min(-1)infusion rate) with the aid of a syringe pump. For the pulse repetition frequency (PRF) study, exposures were performed at four locations of the cremaster at a PRF of 1000, 500, 100 and 10Hz (one location per PRF per rat). The 100-pulse exposures were implemented at a peak rarefactional pressure (P(r)) of 2MPa, frequency of 2.25MHz with 46 cycle pulses. For the pressure amplitude threshold study, 100-pulse exposures (46 cycle pulses) were conducted at various peak rarefactional pressures from 0.5MPa to 2MPa at a frequency of 2.25MHz and PRF of 100Hz. Photomicrographs were captured before and 2-min postexposure. On a pulse-to-pulse basis, the 10Hz acoustic emission was considerably higher and more sustained than those at other PRFs (1000, 500, and 100Hz) (p<0.05). Damage, measured as area of extravasation of red blood cells (RBCs), was also significantly higher at 10Hz PRF than at 1000, 500 and 100Hz (p<0.01). The correlation of acoustic emission to tissue damage showed a trend of increasing damage with increasing cumulative function of the relative integrated power spectrum (CRIPS; R(2)=0.75). No visible damage was present at P(r)< or =0.85MPa. Damage, however, was observed at P(r)> or =1.0MPa and it increased with increasing acoustic pressure.