Microbubble-assisted MRI-guided focused ultrasound for hyperthermia at reduced power levels

Microbubble-enhanced transcranial MR-guided focused ultrasound brain hyperthermia: heating mechanism investigation using finite element method

Recently, our group developed a synergistic brain drug delivery method to achieve simultaneous transcranial hyperthermia and localized blood-brain barrier opening via MR-guided focused ultrasound (MRgFUS). In a rodent model, we demonstrated that the ultrasound power required for transcranial MRgFUS hyperthermia was significantly reduced by injecting microbubbles (MBs). However, the specific mechanisms underlying the power reduction caused by MBs remain unclear. The present study aims to elucidate the mechanisms of MB-enhanced transcranial MRgFUS hyperthermia through numerical studies using the finite element method. The microbubble acoustic emission (MAE) and the viscous dissipation (VD) were hypothesized to be the specific mechanisms. Acoustic wave propagation was used to model the FUS propagation in the brain tissue, and a bubble dynamics equation for describing the dynamics of MBs with small shell thickness was used to model the MB oscillation under FUS exposures. A modified bioheat transfer equation was used to model the temperature in the rodent brain with different heat sources. A theoretical model was used to estimate the bubble shell's surface tension, elasticity, and viscosity losses. The simulation reveals that MAE and VD caused a 40.5% and 52.3% additional temperature rise, respectively. Compared with FUS only, MBs caused a 64.0% temperature increase, which is consistent with our previous animal experiments. Our investigation showed that MAE and VD are the main mechanisms of MB-enhanced transcranial MRgFUS hyperthermia.

Probing the pressure dependence of sound speed and attenuation in bubbly media: Experimental observations, a theoretical model and numerical calculations

The problem of attenuation and sound speed of bubbly media has remained partially unsolved. Comprehensive data regarding pressure-dependent changes of the attenuation and sound speed of a bubbly medium are not available. Our theoretical understanding of the problem is limited to linear or semi-linear theoretical models, which are not accurate in the regime of large amplitude bubble oscillations. Here, by controlling the size of the lipid coated bubbles (mean diameter of ≈5.4μm), we report the first time observation and characterization of the simultaneous pressure dependence of sound speed and attenuation in bubbly water below, at and above microbubbles resonance (frequency range between 1-3 MHz). With increasing acoustic pressure (between 12.5-100 kPa), the frequency of the peak attenuation and sound speed decreases while maximum and minimum amplitudes of the sound speed increase. We propose a nonlinear model for the estimation of the pressure dependent sound speed and attenuation with good agreement with the experiments. The model calculations are validated by comparing with the linear and semi-linear models predictions. One of the major challenges of the previously developed models is the significant overestimation of the attenuation at the bubble resonance at higher void fractions (e.g. 0.005). We addressed this problem by incorporating bubble-bubble interactions and comparing the results to experiments. Influence of the bubble-bubble interactions increases with increasing pressure. Within the examined exposure parameters, we numerically show that, even for low void fractions (e.g. 5.1×10-6) with increasing pressure the sound speed may become 4 times higher than the sound speed in the non-bubbly medium.

An optical and acoustic investigation of microbubble cavitation in small channels under therapeutic ultrasound conditions

Therapeutic focused ultrasound in combination with encapsulated microbubbles is being widely investigated for its ability to elicit bioeffects in the microvasculature, such as transient permeabilization for drug delivery or at higher pressures to achieve 'antivascular' effects. While it is well established that the behaviors of microbubbles are altered when they are situated within sufficiently small vessels, there is a paucity of data examining how the bubble population dynamics and emissions change as a function of channel (vessel) diameter over a size range relevant to therapeutic ultrasound, particularly at pressures relevant to antivascular ultrasound. Here we use acoustic emissions detection and high-speed microscopy (10 kframes/s) to examine the behavior of a polydisperse clinically employed agent (Definity®) in wall-less channels as their diameters are scaled from 1200 to 15 µm. Pressures are varied from 0.1 to 3 MPa using either a 5 ms pulse or a sequence of 0.1 ms pulses spaced at 1 ms, both of which have been previously employed in an in vivo context. With increasing pressure, the 1200 µm channel - on the order of small arteries and veins - exhibited inertial cavitation, 1/2 subharmonics and 3/2 ultraharmonics, consistent with numerous previous reports. The 200 and 100 µm channels - in the size range of larger microvessels less affected by therapeutic focused ultrasound - exhibited a distinctly different behavior, having muted development of 1/2 subharmonics and 3/2 ultraharmonics and reduced persistence. These were associated with radiation forces displacing bubbles to the distal wall and inducing clusters that then rapidly dissipated along with emissions. As the diameter transitioned to 50 and then 15 µm - a size regime that is most relevant to therapeutic focused ultrasound - there was a higher threshold for the onset of inertial cavitation as well as subharmonics and ultraharmonics, which importantly had more complex orders that are not normally reported. Clusters also occurred in these channels (e.g. at 3 MPa, the mean lateral and axial sizes were 23 and 72 µm in the 15 µm channel; 50 and 90 µm in the 50 µm channel), however in this case they occupied the entire lumens and displaced the wall boundaries. Damage to the 15 µm channel was observed for both pulse types, but at a lower pressure for the long pulse. Experiments conducted with a 'nanobubble' (<0.45 µm) subpopulation of Definity followed broadly similar features to 'native' Definity, albeit at a higher pressure threshold for inertial cavitation. These results provide new insights into the behavior of microbubbles in small vessels at higher pressures and have implications for therapeutic focused ultrasound cavitation monitoring and control.

Preclinical robotic device for magnetic resonance imaging guided focussed ultrasound

BACKGROUND

A robotic device featuring three motion axes was manufactured for preclinical research on focussed ultrasound (FUS). The device comprises a 2.75 MHz single element ultrasonic transducer and is guided by Magnetic Resonance Imaging (MRI).

METHODS

The compatibility of the device with the MRI was evaluated by estimating the influence on the signal-to-noise ratio (SNR). The efficacy of the transducer in generating ablative temperatures was evaluated in phantoms and excised porcine tissue.

RESULTS

System's activation in the MRI scanner reduced the SNR to an acceptable level without compromising the image quality. The transducer demonstrated efficient heating ability as proved by MR thermometry. Discrete and overlapping thermal lesions were inflicted in excised tissue.

CONCLUSIONS

The FUS system was proven effective for FUS thermal applications in the MRI setting. It can thus be used for multiple preclinical applications of the emerging MRI-guided FUS technology. The device can be scaled-up for human use with minor modifications.

Simultaneous Localized Brain Mild Hyperthermia and Blood-Brain Barrier Opening via Feedback-Controlled Transcranial MR-guided Focused Ultrasound and Microbubbles

OBJECTIVE

Non-invasive methods to enhance drug delivery and efficacy in the brain have been pursued for decades. Focused ultrasound hyperthermia (HT) combined with thermosensitive therapeutics have been demonstrated promising in enhancing local drug delivery to solid tumors. We hypothesized that the presence of microbubbles (MBs) combined with transcranial MR-guided focused ultrasound (MRgFUS) could be used to reduce the ultrasound power required for HT while simultaneously increasing drug delivery by locally opening the blood-brain barrier (BBB).

METHODS

Transcranial HT (42 C, 10 min) was performed in wild-type mice using a small animal MRgFUS system incorporated into a 9.4T Bruker MR scanner, with infusions of saline or Definity MBs with doses of 20 or 100 l/kg/min (denoted as MB-20 and MB-100). MR thermometry data was continuously acquired as feedback for the ultrasound controller during the procedure.

RESULTS

Spatiotemporally precise transcranial HT was achieved in both saline and MB groups. A significant ultrasound power reduction (-45.7%, p = 0.006) was observed in the MB-20 group compared to saline. Localized BBB opening was achieved in MB groups confirmed by CE-T1w MR images. There were no structural abnormalities, edema, hemorrhage, or acute microglial activation in all groups, confirmed by T2w MR imaging and histology.

CONCLUSION

Our investigations showed that it is feasible and safe to achieve spatiotemporally precise brain HT at significantly reduced power and simultaneous localized BBB opening via transcranial MRgFUS and MBs.

SIGNIFICANCE

This study provides a new synergistic brain drug delivery method with clinical translation potential.

A novel matrix-array-based MR-conditional ultrasound system for local hyperthermia of small animals

OBJECTIVE

The goal of this work was to develop a novel modular focused ultrasound hyperthermia (FUS-HT) system for preclinical applications with the following characteristics: MR-compatible, compact probe for integration into a PET/MR small animal scanner, 3D-beam steering capabilities, high resolution focusing for generation of spatially confined FUS-HT effects.

METHODS

For 3D-beam steering capabilities, a matrix array approach with 11 11 elements was chosen. For reaching the required level of integration, the array was mounted with a conductive backing directly on the interconnection PCB. The array is driven by a modified version of our 128 channel ultrasound research platform DiPhAS. The system was characterized using sound field measurements and validated using tissue-mimicking phantoms. Preliminary MR-compatibility tests were performed using a 7T Bruker MRI scanner.

RESULTS

Four 11 11 arrays between 0.5 and 2 MHz were developed and characterized with respect to sound field properties and HT generation. Focus sizes between 1 and 4 mm were reached depending on depth and frequency. We showed heating by 4C within 60 s in phantoms. The integration concept allows a probe thickness of less than 12 mm.

CONCLUSION

We demonstrated FUS-HT capabilities of our modular system based on matrix arrays and a 128 channel electronics system within a 3D-steering range of up to 30. The suitability for integration into a small animal MR could be demonstrated in basic MR-compatibility tests.

SIGNIFICANCE

The developed system presents a new generation of FUS-HT for preclinical and translational work providing safe, reversible, localized, and controlled HT.

Functional micro/nanobubbles for ultrasound medicine and visualizable guidance

Chemically functionalized gas-filled bubbles with a versatile micro/nano-sized scale have witnessed a long history of developments and emerging applications in disease diagnosis and treatments. In combination with ultrasound and image-guidance, micro/nanobubbles have been endowed with the capabilities of biomedical imaging, drug delivery, gene transfection and disease-oriented therapy. As an external stimulus, ultrasound (US)-mediated targeting treatments have been achieving unprecedented efficiency. Nowadays, US is playing a crucial role in visualizing biological/pathological changes in lives as a reliable imaging technique and a powerful therapeutic tool. This review retrospects the history of ultrasound, the chemistry of functionalized agents and summarizes recent advancements of functional micro/nanobubbles as US contrast agents in preclinical and transclinical research. Latest ultrasound-based treatment modalities in association with functional micro/nanobubbles have been highlighted as their great potentials for disease precision therapy. It is believed that these state-of-the-art micro/nanobubbles will become a booster for ultrasound medicine and visualizable guidance to serve future human healthcare in a more comprehensive and practical manner.

Focused Ultrasound-Induced Cavitation Sensitizes Cancer Cells to Radiation Therapy and Hyperthermia

Focused ultrasound (FUS) has become an important non-invasive therapy for solid tumor ablation via thermal effects. The cavitation effect induced by FUS is thereby avoided but applied for lithotripsy, support drug delivery and the induction of blood vessel destruction for cancer therapy. In this study, head and neck cancer (FaDu), glioblastoma (T98G), and prostate cancer (PC-3) cells were exposed to FUS by using an in vitro FUS system followed by single-dose X-ray radiation therapy (RT) or water bath hyperthermia (HT). Sensitization effects of short FUS shots with cavitation (FUS-Cav) or without cavitation (FUS) to RT or HT (45 °C, 30 min) were evaluated. FUS-Cav significantly increases the sensitivity of cancer cells to RT and HT by reducing long-term clonogenic survival, short-term cell metabolic activity, cell invasion, and induction of sonoporation. Our results demonstrated that short FUS treatment with cavitation has good potential to sensitize cancer cells to RT and HT non-invasively.

Angular dependence of the acoustic signal of a microbubble cloud

Microbubble-mediated ultrasound therapies have a common need for methods that can noninvasively monitor the treatment. One approach is to use the bubbles' acoustic emissions as feedback to the operator or a control unit. Current methods interpret the emissions' frequency content to infer the microbubble activities and predict therapeutic outcomes. However, different studies placed their sensors at different angles relative to the emitter and bubble cloud. Here, it is evaluated whether such angles influence the captured emissions such as the frequency content. In computer simulations, 128 coupled bubbles were sonicated with a 0.5-MHz, 0.35-MPa pulse, and the acoustic emissions generated by the bubbles were captured with two sensors placed at different angles. The simulation was replicated in experiments using a microbubble-filled gel channel (0.5-MHz, 0.19-0.75-MPa pulses). A hydrophone captured the emissions at two different angles. In both the simulation and the experiments, one angle captured periodic time-domain signals, which had high contributions from the first three harmonics. In contrast, the other angle captured visually aperiodic time-domain features, which had much higher harmonic and broadband content. Thus, by placing acoustic sensors at different positions, substantially different acoustic emissions were captured, potentially leading to very different conclusions about the treatment outcome.

Localized anesthesia of a specific brain region using ultrasound-responsive barbiturate nanodroplets

Background: Targeted neuromodulation is a valuable technique for the study and treatment of the brain. Using focused ultrasound to target the local delivery of anesthetics in the brain offers a safe and reproducible option for suppressing neuronal activity. Objective: To develop a potential new tool for localized neuromodulation through the triggered release of pentobarbital from ultrasound-responsive nanodroplets. Method: The commercial microbubble contrast agent, Definity, was filled with decafluorobutane gas and loaded with a lipophilic anesthetic drug, before being condensed into liquid-filled nanodroplets of 210 ± 80 nm. Focused ultrasound at 0.58 MHz was found to convert nanodroplets into microbubbles, simultaneously releasing the drug and inducing local anesthesia in the motor cortex of rats (n=8). Results: Behavioral analysis indicated a 19.1 ± 13% motor deficit on the contralateral side of treated animals, assessed through the cylinder test and gait analysis, illustrating successful local anesthesia, without compromising the blood-brain barrier. Conclusion: Pentobarbital-loaded decafluorobutane-core Definity-based nanodroplets are a potential agent for ultrasound-triggered and targeted neuromodulation.

Hyperthermia can alter tumor physiology and improve chemo- and radio-therapy efficacy

Hyperthermia has demonstrated clinical success in improving the efficacy of both chemo- and radio-therapy in solid tumors. Pre-clinical and clinical research studies have demonstrated that targeted hyperthermia can increase tumor blood flow and increase the perfused fraction of the tumor in a temperature and time dependent manner. Changes in tumor blood circulation can produce significant physiological changes including enhanced vascular permeability, increased oxygenation, decreased interstitial fluid pressure, and reestablishment of normal physiological pH conditions. These alterations in tumor physiology can positively impact both small molecule and nanomedicine chemotherapy accumulation and distribution within the tumor, as well as the fraction of the tumor susceptible to radiation therapy. Hyperthermia can trigger drug release from thermosensitive formulations and further improve the accumulation, distribution, and efficacy of chemotherapy.

Applications of Ultrasound to Stimulate Therapeutic Revascularization

Many pathological conditions are characterized or caused by the presence of an insufficient or aberrant local vasculature. Thus, therapeutic approaches aimed at modulating the caliber and/or density of the vasculature by controlling angiogenesis and arteriogenesis have been under development for many years. As our understanding of the underlying cellular and molecular mechanisms of these vascular growth processes continues to grow, so too do the available targets for therapeutic intervention. Nonetheless, the tools needed to implement such therapies have often had inherent weaknesses (i.e., invasiveness, expense, poor targeting, and control) that preclude successful outcomes. Approximately 20 years ago, the potential for using ultrasound as a new tool for therapeutically manipulating angiogenesis and arteriogenesis began to emerge. Indeed, the ability of ultrasound, especially when used in combination with contrast agent microbubbles, to mechanically manipulate the microvasculature has opened several doors for exploration. In turn, multiple studies on the influence of ultrasound-mediated bioeffects on vascular growth and the use of ultrasound for the targeted stimulation of blood vessel growth via drug and gene delivery have been performed and published over the years. In this review article, we first discuss the basic principles of therapeutic ultrasound for stimulating angiogenesis and arteriogenesis. We then follow this with a comprehensive cataloging of studies that have used ultrasound for stimulating revascularization to date. Finally, we offer a brief perspective on the future of such approaches, in the context of both further research development and possible clinical translation.