Microbubble Concentration Research Articles

Microbubble Size and Dose Effects on Pharmacokinetics.

Optimization of contrast-enhanced imaging and focused ultrasound therapy requires a comprehensive understanding of in vivo microbubble (MB) pharmacokinetics. Prior studies have focused pharmacokinetic analysis on indirect techniques, such as ultrasound imaging of the blood pool and gas chromatography of exhaled gases. The goal of this work was to measure the MB concentration directly in blood and correlate the pharmacokinetic parameters with the MB size and dose. MB volume dose (MVD) was chosen to combine the size distribution and number into a single-dose parameter. Different MB sizes (2, 3, and 5 μm diameter) at 5-40 μL/kg MVD were intravenously injected. Blood samples were withdrawn at different times (1-10 min) and analyzed by image processing. We found that for an MVD threshold < 40 μL/kg for 2 and 3 μm and <10 μL/kg for 5 μm, MB clearance followed first-order kinetics. When matching MVD, MBs of different sizes had comparable half-lives, indicating that gas dissolution and elimination by the lungs are the primary mechanisms for elimination. Above the MVD threshold, MB clearance followed biexponential kinetics, suggesting a second elimination mechanism mediated by organ retention, possibly in the lung, liver, and spleen. In conclusion, we present the first direct MB pharmacokinetic study, demonstrate the utility of MVD as a unified dose metric, and provide insights into the mechanisms of MB clearance from circulation.

Deep Learning-Based Microbubble Localization for Ultrasound Localization Microscopy.

Ultrasound localization microscopy (ULM) is an emerging vascular imaging technique that overcomes the resolution-penetration compromise of ultrasound imaging. Accurate and robust microbubble (MB) localization is essential for successful ULM. In this study, we present a deep learning (DL)-based localization technique that uses both Field-II simulation and in vivo chickenembryo chorioallantoic membrane (CAM) data for training. Both radio frequency (RF) and in-phase and quadrature (IQ) data were tested in this study. The simulation experiment shows that the proposed DL-based localization was able to reduce both missing MB localization rate and MB localization error. In general, RF data showed better performance than IQ. For the in vivo CAM study with high MB concentration, DL-based localization was able to reduce the vessel MB saturation time by more than 50% compared to conventional localization. In addition, we propose a DL-based framework for real-time visualization of the high-resolution microvasculature. The findings of this article support the use of DL for more robust and faster MB localization, especially under high MB concentrations. The results indicate that further improvement could be achieved by incorporating temporal information of the MB data.

1Pa5-10 Dependence of release of liposome included in giant cluster vesicles on microbubble concentration

1Pa5-10 Dependence of release of liposome included in giant cluster vesicles on microbubble concentration

Enhanced visibility through microbubble-induced photoacoustic fluctuation imaging

A photoacoustic contrast mechanism is presented based on the photoacoustic fluctuations induced by microbubbles flowing inside a micro-vessel filled with a continuous absorber. It is demonstrated that the standard deviation of a homogeneous absorber mixed with microbubbles increases non-linearly as the microbubble concentration and microbubble size is increased. This effect is then utilized to perform photoacoustic fluctuation imaging with increased visibility and contrast of a blood flow phantom.

Ultrafast ultrasound localization microscopy method for spinal cord mircovasculature imaging

Function of spinal cord is crucial to nerve conduction pathway. Traumatic spinal cord injury often results in a vasculature disruption after primary insult and further leads to abnormal responses of the intact vessels in neighboring tissue during secondary injury. Therefore, the vasculature and blood supply play significant roles in evaluating the spinal cord function . Ultrasound localization microscopy (ULM) overcomes the shortcomings of extensively used angiography, such as computed tomography angiography (CTA) and magnetic resonance angiography (MRA), in terms of limited resolution, radiation and poor-portability, which meets the needs of comprehensive intraoperative examination and prognosis tracking. In this study, an L22-14vX probe with a transmission frequency of 15.625 MHz is utilized, yielding an imaging wavelength of 100 μm. The ULM is conducted based on ultrafast ultrasound technology with multiple tilted plane-wave illuminations. Robust principal component analysis (RPCA) based spatial-temporal clutter filtering method is used for separating the microbubble signals from tissue signals and high frequency noise. Through microbubble localization, trajectory tracking and mapping, subwavelength super-resolution ultrasound imaging is finally achieved. The whole process of microbubble localization and vessel reconstruction are monitored through measuring the time dependent microbubble detections and saturation. Saturation curve corresponds to the time dependent total area covered by microbubble detections on the image. Quantification analysis is carried out for evaluating the imaging results including resolution measurements based on the Fourier ring correlation (FRC) and full-width at half-maximum (FWHM). The &lt;i&gt;i&lt;/i&gt;&lt;i&gt;n-vivo&lt;/i&gt; experimental results show that ULM can be used to obtain super-resolution vasculature imaging in rat spinal cord. The velocity distributed from 1 mm/s to 50 mm/s can be detected. Within the same vessel, the velocity of a point is inversely correlated with the distance from the point to the center of the vessel. The velocity in the center of the vessel is larger than that at the wall of the vessel. The larger vessels support higher flow in the center of the vessel. The FWHM results indicate that ultrafast Doppler displays vessels in diameters between 135 μm and 270 μm while ULM displays them in diameters between 28 μm and 35 μm. The FRC-based resolution evaluation shows that the ULM achieves a super resolution of 16 μm, much less than the imaging wavelength of 100 μm. Yet, long acquisition time is required to detect microbubbles in the smallest vessels, leading to long reconstruction of the microvasculature, which is still a problem worth studying . Compromise between saturation and acquisition time needs considering. Generally speaking, microbubbles are more likely to flow in large vessels, leading to relatively short reconstruction time of large vessels. When saturation curve almost converges, the imaging improvement with new vessels is not so significant that the detail sacrifice of some small microvessels can reduce acquisition time (i.e. most of microvasculature can still be gained when the saturation curve does not converge). Besides, the increase of microbubble concentration and advanced track identification and extraction may also accelerate the saturation rate of convergence with acquisition time decreasing. In conclusion, ULM can be used to obtain a super-resolution imaging of spinal cord microvasculature, giving a 10-fold improvement in resolution in comparison with ultrafast Doppler imaging. Relevant results can facilitate the super-resolution ULM imaging of spinal cord which may promote the function diagnosis, treatment intervention, disability prevention, and prognosis recovery of spinal cord injury.

Contrast-enhanced ultrasound imaging using long-circulating cationic magnetic microbubbles in vitro and in vivo validations

Traditional encapsulated microbubbles are recently used as delivery carriers for drugs and genes, but they have low efficiency. If the local microbubble concentration could be increased, this might be able to improve the therapeutic efficacy of diseases. In this study, we developed novel cationic magnetic microbubbles (MBM), which could simultaneously realize targeted aggregation under a magnetic field as well as ultrasonographic real-time visualization. Their physicochemical properties, biocompatibility, ultrasonography, magnetic response characteristics, and biodistribution were systematically evaluated. Here, the MBM were 2.55 ± 0.14 µm in size with a positive zeta potential, and had a good biocompatibility. They were able to enhance ultrasonographic contrast both in vitro and in vivo. MBM could be attracted by an external magnet for directional movement and aggregation in vitro. We confirmed that MBM also had a great magnetic response in vivo, by means of fluorescence imaging and contrast-enhanced ultrasound imaging. Following intravenous injection into tumor-bearing mice, MBM showed excellent stability in the internal circulation, and could accumulate in the tumor vasculature through magnetic targeting. With the excellent combination of magnetic response and acoustic properties, cationic magnetic microbubbles (MBM) have promising potential for use as a new kind of drug/gene carrier for theranostics in the future.

Acoustofluidic-mediated molecular delivery to human T cells with a three-dimensional-printed flow chamber.

Cell-based therapies have garnered significant interest to treat cancer and other diseases. Acoustofluidic technologies are in development to improve cell therapy manufacturing by facilitating rapid molecular delivery across the plasma membrane via ultrasound and microbubbles (MBs). In this study, a three-dimensional (3D) printed acoustofluidic device was used to deliver a fluorescent molecule, calcein, to human T cells. Intracellular delivery of calcein was assessed after varying parameters such as MB face charge, MB concentration, flow channel geometry, ultrasound pressure, and delivery time point after ultrasound treatment. MBs with a cationic surface charge caused statistically significant increases in calcein delivery during acoustofluidic treatment compared to MBs with a neutral surface charge (p < 0.001). Calcein delivery was significantly higher with a concentric spiral channel geometry compared to a rectilinear channel geometry (p < 0.001). Additionally, calcein delivery was significantly enhanced at increased ultrasound pressures of 5.1 MPa compared to lower ultrasound pressures between 0-3.8 MPa (p < 0.001). These results demonstrate that a 3D-printed acoustofluidic device can significantly enhance intracellular delivery of biomolecules to T cells, which may be a viable approach to advance cell-based therapies.

Subharmonic Scattering of SonoVue Microbubbles Within 10-40-mmHg Overpressures In Vitro.

Ultrasound contrast agent microbubbles are considered promising sensors to measure portal vein pressure noninvasively. In this study, we investigated the subharmonic scattering power and optimal incident acoustic pressure of SonoVue microbubbles (concentration: [Formula: see text]/mL 0.9% NaCl solution) in the ambient pressure range of 10-40 mmHg with 10-mmHg increments at a temperature of 25 °C. The results demonstrated that the subharmonic response of the SonoVue microbubbles existed in three stages: the first growth stage (40-300 kPa), saturation (300-400 kPa), and the second growth stage (400-540 kPa). In the first growth stage, the subharmonic amplitude increased with ambient pressure. However, while the ambient pressure increased, the subharmonic amplitude decreased in the second growth stage. The best correlation of the subharmonic amplitudes with the ambient pressures was obtained at a high incident acoustic pressure of 520 kPa (sensitivity: 0.15 dB/mmHg, r2 = 0.99 , and root-mean-square error = 0.49 mmHg), which indicated that the subharmonic signals in the second growth stage might be suitable for estimating low ambient pressures. The results presented in our study may pave the way for portal vein pressure estimation using SonoVue microbubbles as sensors in clinical applications.

3-D Ultrafast Ultrasound Imaging of Microbubbles Trapped Using an Acoustic Vortex.

Increasing the local concentration of microbubbles (MBs) within the blood flow plays a crucial role in several medical applications, but there are few imaging modalities available for volumetric tracking of the aggregated MBs in real time. Here we describe a device integrating acoustic vortex tweezers (AVTs) and ultrasound plane-wave imaging (PWI) to achieve the goal of controlling the spatial distribution of MBs in blood vessels and simultaneously monitoring this process using the same probe. Experiments were conducted using a 5-MHz 2-D array ultrasound probe (with three cycles of excitation at an acoustic pressure of 2000 kPa) and 1.2- [Formula: see text]-diameter MBs at a flow rate of 20 mm/s. The AVT waveform was produced by modulating the repetition frequency of the transmitted pulse asymmetrically (4 and 8 kHz at the inflow and outflow ends, respectively). In order to simultaneously capture MBs and carry out imaging with the same probe, the asymmetric AVT pulse signal and the ultrasound-imaging pulse signal were arranged in a staggered series, and the imaging was carried out using plane-wave pulses at nine angles (-7° to 7°) in compounded PWI (volume rate: 200 Hz). Microscopy observations showed that freely suspended MBs could indeed be gathered by the asymmetric AVT in the flow field to form an MBs cluster with a spot size of about [Formula: see text], which could resist the flow to remain at a fixed location for about 22 s. After the asymmetric AVT signal and the ultrasound-imaging pulse signal were turned on for 1 s, the ultrasound 3-D image showed that the signal intensity of the MB clusters increased by 13.1 dB ± 2.9 dB in relation to the background area. These results show that the proposed strategy can be used to accumulate flowing MBs at a desired location and to simultaneously observe this phenomenon. This tool could be used in the future to improve the outcomes of MB-related treatments for various diseases.

Abstract 12472: Development of an in vitro System to Study Mechanisms of Ultrasound-Targeted Microbubble Cavitation-Mediated Blood Brain Barrier Opening

Introduction: Ultrasound-targeted microbubble cavitation ( UTMC ) is being explored as a method to transiently open the blood brain barrier ( BBB ). While UTMC-mediated BBB opening is a promising avenue for increasing drug delivery to the central nervous system, its underlying mechanisms are incompletely understood. We sought to develop an in vitro system simulating cerebral endothelial barrier function, to facilitate studies aimed at exploring mechanisms of UTMC-mediated BBB opening. Methods: We developed a transwell model with murine brain microvasculature endothelial cells (bEnd.3) and rat C6 glioma cells on opposite sides of a support membrance. Three concentrations of lipid microbubbles ( MB ) were added before applying pulsed ultrasound (1 MHz, 10 μs pulse duration, 10 ms pulse interval) at 3 peak negative pressures ( PNP ) for 20 s. Endothelial barrier function was assessed using transendothelial electrical resistance ( TEER ) and permeability using 10 kDa Texas Red dextran. Cells were stained with Hoechst (nucleus), CellMask Deep Red (cell coverage), calcein-AM (viability), and propidium iodide ( PI ; sonoporation) Differences between experimental conditions were compared using unpaired 2-tailed t-test. Significance was defined as p &lt;0.05. Results: At a given MB:cell ratio, higher PNPs increased the percent of viably sonoporated cells ( Fig 1A ), but caused increasing cell loss from the transwell (decrease in % area coverage) at PNP greater than 250kPa and MB:cell ratio higher than 3:1 ( Fig 1B ). Therefore, 250kPa ultrasound and 3:1 MB ratio were used in the permeability studies: BBB permeability increased by 15 min after UTMC as measured by TEER ( Fig 1C ) and dextran flux ( Fig 1D ), but recovered 1 hour later. Conclusions: We developed a co-culture transwell model of the BBB to simulate UTMC-induced reversible endothelial barrier hyperpermeability. This model will uniquely allow systematic investigation of mechanisms of UTMC-mediated BBB opening.

Ultrasound microvasculature imaging with entropy-based radiality super-resolution (ERSR)

Objective: Microvasculature is highly relevant to the occurrence and development of pathologies such as cancer and diabetes. Ultrasound localization microscopy (ULM) has bypassed the diffraction limit and demonstrated its great potential to provide new imaging modality and establish new diagnostic criteria in clinical application. However, sparse microbubble distribution can be a significant bottleneck for improving temporal resolution, even for further clinical translation. Other important challenges for ULM to tackle in clinic also include high microbubble concentration and low frame rate. Approach: As part of the efforts to facilitate clinical translation, this paper focused on the low frame rate and the high microbubble distribution issue and proposed a new super-resolution imaging strategy called entropy-based radiality super-resolution (ERSR). The feasibility of ERSR is validated with simulations, phantom experiment and contrast-enhanced ultrasound scan of rabbit sciatic nerve with clinical accessible ultrasound system. Main results: ERSR can achieve 10 times improvement in spatial resolution compared to conventional ultrasound imaging, higher temporal resolution (∼10 times higher) and contrast-to-noise ratio under high-density microbubbles, compared with ULM under low-density microbubbles. Significance: We conclude ERSR could be a valuable imaging tool with high spatio-temporal resolution for clinical diagnosis and assessment of diseases potentially.

Scaleable production of microbubbles using an ultrasound-modulated microfluidic device

Surfactant-coated gas microbubbles are widely used as contrast agents in ultrasound imaging and increasingly in therapeutic applications. The response of microbubbles to ultrasound can be strongly influenced by their size and coating properties, and hence the production method. Ultrasonic emulsification (sonication) is the most commonly employed method and can generate high concentrations of microbubbles rapidly, but with a broad size distribution, and there is a risk of contamination and/or degradation of sensitive components. Microfluidic devices provide excellent control over microbubble size, but are often challenging or costly to manufacture, offer low production rates (<106s-1), and are prone to clogging. In this study, a hybrid sonication-microfluidic or "sonofluidic" device was developed. Bubbles of ∼180 μm diameter were produced rapidly in a T-junction and subsequently exposed to ultrasound (71-73 kHz) within a microchannel, generating microbubbles (mean diameter: 1-2 μm) at a rate of >108s-1 using a single device. Microbubbles were prepared using either the sonofluidic device or conventional sonication, and their size, concentration, and stability were comparable. The mean diameter, concentration, and stability were found to be comparable between techniques, but the microbubbles produced by the sonofluidic device were all <5 μm in diameter and thus did not require any post-production fractionation.

Effect of Acoustic Parameters and Microbubble Concentration on the Likelihood of Encapsulated Microbubble Coalescence

Microbubble contrast agents are commonly used for therapeutic and diagnostic imaging applications. Under certain conditions, these contrast agents can coalesce on ultrasound application and form larger bubbles than the initial population. The formation of large microbubbles potentially influences therapeutic outcomes and imaging quality. We studied clinically relevant ultrasound parameters related to low-pressure therapy and contrast-enhanced ultrasound imaging to determine their effect on microbubble coalescence and subsequent changes in microbubble size distributions in vitro. Results indicate that therapeutic ultrasound at low frequencies, moderate pressures and high duty cycles are capable of forming bubbles greater than two times larger than the initial bubble distribution. Furthermore, acoustic parameters related to contrast-enhanced ultrasound imaging that are at higher frequency, low-pressure and low-duty cycle exhibit no statistically significant changes in bubble diameter, suggesting that standard contrast ultrasound imaging does not cause coalescence. Overall, this work suggests that the microbubble coalescence phenomenon can readily occur at acoustic parameters used in therapeutic ultrasound, generating bubbles much larger than those found in commercial contrast agents, although coalescence is unlikely to be significant in diagnostic contrast-enhanced ultrasound imaging. This observation warrants further expansion of parameter ranges and investigation of resulting effects.

Collective Dynamics of Bulk Nanobubbles with Size-Dependent Surface Tension.

It has been suggested that irreversible adsorption at the gas/liquid interface of bulk nanobubbles will reduce the Laplace pressure, leading to their stability. However, most previous studies have focused on the stability of individual nanobubbles. Bulk nanobubbles are polydispersed suspensions, and gas molecules can diffuse between bubbles, leading to their collective dynamics, which may be crucial to understanding their formation process and stability. In this study, we proposed a mean-field theory for computing the evolution of the size-distribution function of bulk nanobubbles with size-dependent surface tension. We applied this theory to investigate the evolution of bulk nanobubbles with insoluble surfactants pinned at their gas/water interface. The results show that Ostwald ripening can be suppressed when enough surfactants are adsorbed. Bulk nanobubbles can be produced by the shrinkage of microbubbles in an air-saturated solution. The mean stable size is controlled by the amount of surfactants and the initial microbubble concentration; these predictions are qualitatively consistent with the experimental results of micro/nanobubbles produced using the microfluidic method.

Multivariable Dependence of Acoustic Contrast of Fluorocarbon and Xenon Microbubbles under Flow

Microbubbles (MBs) are 1 to 10 µm gas particles stabilized by an amphiphilic shell capable of responding to biomedical ultrasound with strong acoustic signals, allowing them to be commonly used in ultrasound imaging and therapy. The composition of both the shell and the core determines their stability and acoustic properties. While there has been extensive characterization of the dissolution, oscillation, cavitation, collapse and therefore, ultrasound contrast of MBs under static conditions, few reports have examined such behavior under hydrodynamic flow. In this study, we evaluate the interplay of ultrasound parameters (five different mechanical indices [MIs]), MB shell parameter (shell stiffness), type of gas (perfluorocarbon for diagnostic imaging and xenon as a therapeutic gas), and a flow parameter (flow rate) on the ultrasound signal of phospholipid-stabilized MBs flowing through a latex tube embedded in a tissue-mimicking phantom. We find that the contrast gradient (CG), a metric of the rate of decay of contrast along the length of the tube, and the contrast peak (CP), the location where the maximum contrast is reached, depend on the conditions of flow, imaging, and MB material. For instance, while the contrast near the flow inlet of the field of view is highest for a softer shell (dipalmitoylphosphatidylcholine [DPPC], C16) than for stiffer shells (distearoylphosphatidylcholine [DSPC], C18, and dibehenoylphosphatidylcholine [DBPC], C22), the contrast decay is also faster; stiffer shells provide more resistance and hence lead to slower MB dissolution/destruction. At higher flow rates, the CG is low for a fixed length of time because each MB is exposed to ultrasound for a shorter period. The CG becomes high for low flow rates, especially at high incident pressures (high MI), causing more MB destruction closer to the inlet of the field of view. Also, the CP shifts toward the inlet at low flow rates, high MIs, and low shell stiffness. We also report the first demonstration of sustained ultrasound flow imaging of a water-soluble, therapeutic gas MB (xenon). We find that an increased MB concentration is necessary for obtaining the same signal magnitude for xenon MBs. In summary, this study builds a framework depicting how multiple variables simultaneously affect the evolution of MB ultrasound contrast under flow. Depending on the MB composition, imaging conditions, transducer positioning, and image processing, building on such a framework could potentially allow for extraction of additional diagnostic information than is commonly analyzed for physiological flow.

Gemcitabine-loaded microbubble system for ultrasound imaging and therapy

Ultrasound imaging presents many positive attributes, including safety, real-time imaging, universal accessibility, and cost. However, inherent difficulties in discrimination between soft tissues and tumors prompted development of stabilized microbubble contrast agents. This presents the opportunity to develop agents in which drug is entrapped in the microbubble shell. We describe preparation and characterization of theranostic poly(lactide) (PLA) and pegylated PLA (PEG-PLA) shelled microbubbles that entrap gemcitabine, a commonly used drug for pancreatic cancer (PDAC). Entrapping 6 wt% gemcitabine did not significantly affect drug activity, microbubble morphology, or ultrasound contrast activity compared with unmodified microbubbles. In vitro microbubble concentrations yielding ≥ 500nM entrapped gemcitabine were needed for complete cell death in MIA PaCa-2 PDAC drug sensitivity assays, compared with 62.5 nM free gemcitabine. In vivo administration of gemcitabine-loaded microbubbles to xenograft MIA PaCa-2 PDAC tumors in athymic mice was well tolerated and provided substantial tumoral image enhancement before and after destructive ultrasound pulses. However, no significant differences in tumor growth were observed among treatment groups, in keeping with the in vitro observation that much higher doses of gemcitabine are required to mirror free gemcitabine activity. Statement of significanceThe preliminary results shown here are encouraging and support further investigation into increased gemcitabine loading. Encapsulation of gemcitabine within polylactic acid (PLA) microbubbles does not damage its activity towards pancreatic cancer (pancreatic ductal adenocarcinoma, PDAC) cells. Excellent imaging and evidence of penetration into the highly desmoplastic PDAC tumors is demonstrated. Microbubble destruction was confirmed in vivo, showing that elevated mechanical index shatters the microbubbles for enhanced delivery. The potential to slow PDAC growth in vivo is shown, but higher gemcitabine concentrations are required. Current efforts are directed at increasing drug loading by inclusion of drug-carrying nanoparticles for effective in vivo treatment.

Efficacy Estimation of Microbubble-Assisted Local Sonothrombolysis Using a Catheter with a Series of Miniature Transducers.

Intravascular ultrasound has good prospects for clinical applications in sonothrombolysis. The catheter-based side-looking intravascular ultrasound thrombolysis (e.g., Ekosonic catheters) used in clinical studies has a high frequency (2 MHz). The lower-frequency ultrasound requires a larger-diameter transducer. In our study, we designed and manufactured a small ultrasound-based prototype catheter that can emit a lower frequency ultrasound (1.1 MHz). In order to evaluate the safety and efficacy of local low-frequency ultrasound-enhanced thrombolysis, a microbubble (MB) was introduced to augment thrombolysis effect of locally delivered low-intensity ultrasound. The results demonstrated that combination of ultrasound and MB realized higher clot lysis than urokinase-only treatment (17.0% ± 1.2% vs. 14.9% ± 2.7%) under optimal ultrasound settings of 1.1 MHz, 0.414 MPa, 4.89 W/cm2, 5% duty cycle and MB concentration of 60 μg/mL. When urokinase was added, the fibrinolysis accelerated by MB and ultrasound resulted in a further increased thrombolysis rate that was more than two times than that of urokinase alone (36.7% ± 5.5% vs. 14.9% ± 2.7%). However, a great quantity of ultrasound energy was required to achieve substantial clot lysis without MB, leading to the situation that temperature accumulated inside the clot became harmful. We suggest that MB-assisted local sonothrombolysis be considered as adjuvant therapy of thrombolytic agents.

Magnetic Resonance Detection of Gas Microbubbles via HyperCEST: A Path Toward Dual Modality Contrast Agent.

Gas microbubbles are an established clinical ultrasound contrast agent. They could also become a powerful magnetic resonance (MR) intravascular contrast agent, but their low susceptibility-induced contrast requires high circulating concentrations or the addition of exogenous paramagnetic nanoparticles for MR detection. In order to detect clinical in vivo concentrations of raw microbubbles via MR, an alternative detection scheme must be used. HyperCEST is an NMR technique capable of indirectly detecting signals from very dilute molecules (concentrations well below the NMR detection threshold) that exchange hyperpolarized 129 Xe. Here, we use quantitative hyperCEST to show that microbubbles are very efficient hyperCEST agents. They can accommodate and saturate millions of 129 Xe atoms at a time, allowing for their indirect detection at concentrations as low as 10 femtomolar. The increased MR sensitivity to microbubbles achieved via hyperCEST can bridge the gap for microbubbles to become a dual modality contrast agent.

Microbubble-Enhanced Heating: Exploring the Effect of Microbubble Concentration and Pressure Amplitude on High-Intensity Focused Ultrasound Treatments

High-intensity focused ultrasound (HIFU) is a non-invasive tool that can be used for targeted thermal ablation treatments. Currently, HIFU is clinically approved for treatment of uterine fibroids, various cancers, and certain brain applications. However, for brain applications such as essential tremors, HIFU can only be used to treat limited areas confined to the center of the brain because of geometrical limitations (shape of the transducer and skull). A major obstacle to advancing this technology is the inability to treat non-central brain locations without causing damage to the skin and/or skull. Previous research has indicated that cavitation-induced bubbles or microbubble contrast agents can be used to enhance HIFU treatments by increasing ablation regions and shortening acoustic exposures at lower acoustic pressures. However, little research has been done to explore the interplay between microbubble concentration and pressure amplitude on HIFU treatments. We developed an in vitro experimental setup to study lesion formation at three different acoustic pressures and three microbubble concentrations. Real-time ultrasound imaging was integrated to monitor initial microbubble concentration and subsequent behavior during the HIFU treatments. Depending on the pressure used for the HIFU treatment, there was an optimal concentration of microbubbles that led to enhanced heating in the focal area. If the concentration of microbubbles was too high, the treatment was detrimentally affected because of non-linear attenuation by the pre-focal microbubbles. Additionally, the real-time ultrasound imaging provided a reliable method to monitor microbubble activity during the HIFU treatments, which is important for translation to in vivo HIFU applications with microbubbles.

A Deep Learning Framework for Spatiotemporal Ultrasound Localization Microscopy.

Ultrasound Localization Microscopy (ULM) can resolve the microvascular bed down to a few micrometers. To achieve such performance, microbubble contrast agents must perfuse the entire microvascular network. Microbubbles are then located individually and tracked over time to sample individual vessels, typically over hundreds of thousands of images. To overcome the fundamental limit of diffraction and achieve a dense reconstruction of the network, low microbubble concentrations must be used, which leads to acquisitions lasting several minutes. Conventional processing pipelines are currently unable to deal with interference from multiple nearby microbubbles, further reducing achievable concentrations. This work overcomes this problem by proposing a Deep Learning approach to recover dense vascular networks from ultrasound acquisitions with high microbubble concentrations. A realistic mouse brain microvascular network, segmented from 2-photon microscopy, was used to train a three-dimensional convolutional neural network (CNN) based on a V-net architecture. Ultrasound data sets from multiple microbubbles flowing through the microvascular network were simulated and used as ground truth to train the 3D CNN to track microbubbles. The 3D-CNN approach was validated in silico using a subset of the data and in vivo in a rat brain. In silico, the CNN reconstructed vascular networks with higher precision (81%) than a conventional ULM framework (70%). In vivo, the CNN could resolve micro vessels as small as 10 μ m with an improvement in resolution when compared against a conventional approach.