Anin vitrostudy of a microbubble contrast agent using a clinical ultrasound imaging system

Target tumor therapy in human gastric cancer cells through the combination of docetaxel-loaded cationic lipid microbubbles and ultrasound-triggered microbubble destruction

It is well accepted that ultrasound-induced microbubble (USMB) cavitation is a promising method for drug delivery. Ultrasound-targeted destruction of cytotoxic drug-loaded lipid microbubbles (LMs) is used to promote the treatment of cancer. This study aimed to investigate the antitumor effects from a combination of docetaxel-loaded cationic lipid microbubbles (DLLM⁺) and ultrasound (US)-triggered microbubble destruction (UTMD) on gastric cancer (GC). It was found that the functional dose of DOC in this study was 1 × 10^-9 mol/L. We found that DLLM combined with the UTMD group showed greater growth inhibition of the cultured human gastric cancer cells (HGCCs) when compared with the other five groups by arresting the G₂/M phase in the cell cycle. However, DLLM⁺ combined with UTMD showed a higher inhibition rate of tumor growth than DLLM combined with UTMD and that of the RC/CMV-p16 combined with UTMD in vitro and in vivo experiments. DLLM⁺ combined with UTMD significantly suppressed proliferation and promoted the apoptosis of HGCCs with more cells arrested in the G₂/M phase. In addition, DLLM⁺ combined with UTMD suppressed the proliferation and induced apoptosis by arresting cells in the G₂/M phase, which led to a great inhibition of GC progression. Thus, our results indicated that the combination of DLLM⁺ and UTMD might represent a novel and promising approach to chemotherapy for GC.

Biomedical nanobubbles and opportunities for microfluidics

The use of bulk nanobubbles in biomedicine is increasing in recent years, which is attributable to the array of therapeutic and diagnostic tools promised by developing bulk nanobubble technologies. From cancer drug delivery and ultrasound contrast enhancement to malaria detection and the diagnosis of acute donor tissue rejection, the potential applications of bulk nanobubbles are broad and diverse. Developing these technologies to the point of clinical use may significantly impact the quality of patient care. This review compiles and summarizes a representative collection of the current applications, fabrication techniques, and characterization methods of bulk nanobubbles in biomedicine. Current state-of-the-art generation methods are not designed to create nanobubbles of high concentration and low polydispersity, both characteristics of which are important for several bulk nanobubble applications. To date, microfluidics has not been widely considered as a tool for generating nanobubbles, even though the small-scale precision and real-time control offered by microfluidics may overcome the challenges mentioned above. We suggest possible uses of microfluidics for improving the quality of bulk nanobubble populations and propose ways of leveraging existing microfluidic technologies, such as organ-on-a-chip platforms, to expand the experimental toolbox of researchers working to develop biomedical nanobubbles.

The use of bulk nanobubbles in biomedicine is increasing in recent years. This translates into new opportunities for microfluidics, which may enable the generation of higher quality nanobubbles that lead to advances in diagnostics and therapeutics.

Super-Resolution Contrast-Enhanced Ultrasound Methodology for the Identification of In Vivo Vascular Dynamics in 2D

OBJECTIVES

The aim of this study was to provide an ultrasound-based super-resolution methodology that can be implemented using clinical 2-dimensional ultrasound equipment and standard contrast-enhanced ultrasound modes. In addition, the aim is to achieve this for true-to-life patient imaging conditions, including realistic examination times of a few minutes and adequate image penetration depths that can be used to scan entire organs without sacrificing current super-resolution ultrasound imaging performance.

METHODS

Standard contrast-enhanced ultrasound was used along with bolus or infusion injections of SonoVue (Bracco, Geneva, Switzerland) microbubble (MB) suspensions. An image analysis methodology, translated from light microscopy algorithms, was developed for use with ultrasound contrast imaging video data. New features that are tailored for ultrasound contrast image data were developed for MB detection and segmentation, so that the algorithm can deal with single and overlapping MBs. The method was tested initially on synthetic data, then with a simple microvessel phantom, and then with in vivo ultrasound contrast video loops from sheep ovaries. Tracks detailing the vascular structure and corresponding velocity map of the sheep ovary were reconstructed. Images acquired from light microscopy, optical projection tomography, and optical coherence tomography were compared with the vasculature network that was revealed in the ultrasound contrast data. The final method was applied to clinical prostate data as a proof of principle.

RESULTS

Features of the ovary identified in optical modalities mentioned previously were also identified in the ultrasound super-resolution density maps. Follicular areas, follicle wall, vessel diameter, and tissue dimensions were very similar. An approximately 8.5-fold resolution gain was demonstrated in vessel width, as vessels of width down to 60 μm were detected and verified (λ = 514 μm). Best agreement was found between ultrasound measurements and optical coherence tomography with 10% difference in the measured vessel widths, whereas ex vivo microscopy measurements were significantly lower by 43% on average. The results were mostly achieved using video loops of under 2-minute duration that included respiratory motion. A feasibility study on a human prostate showed good agreement between density and velocity ultrasound maps with the histological evaluation of the location of a tumor.

CONCLUSIONS

The feasibility of a 2-dimensional contrast-enhanced ultrasound-based super-resolution method was demonstrated using in vitro, synthetic and in vivo animal data. The method reduces the examination times to a few minutes using state-of-the-art ultrasound equipment and can provide super-resolution maps for an entire prostate with similar resolution to that achieved in other studies.

Resolving Ultrasound Contrast Microbubbles Using Minimum Variance Beamforming

Minimum Variance (MV) beamforming is known to improve the lateral resolution of ultrasound images and enhance the separation of isolated point scatterers. This paper aims to evaluate the adaptive beamformer's performance with flowing microbubbles (MBs) which are relevant to super-resolution ultrasound imaging. Simulations using point scatterer data from single emissions were complemented by an experimental investigation performed using a capillary tube phantom and the Synthetic Aperture Real-time Ultrasound System (SARUS). The MV performance was assessed by the minimum distance that allows the display of two scatterers positioned side-by-side, the lateral Full-Width-at-Half-Maximum (FWHM), and the Peak-Sidelobe-Level (PSL). In the tube, scatterer responses separated by down to [Formula: see text] (or 1.05λ ) were distinguished by the MV method, while the standard Delay-And-Sum (DAS) beamformers were unable to achieve such separation. Up to ninefold FWHM decrease was also measured in favor of the MV beamformer for individual echoes from MBs. The lateral distance between two scatterers impacted on their FWHM value, and additional differences in the scatterers' axial or out-of-plane position also impacted on their size and appearance. The simulation and experimental results were in agreement in terms of lateral resolution. The point scatterer study showed that the proposed MV imaging scheme provided clear resolution benefits compared to DAS. Current super-resolution methods mainly depend on DAS beamformers. Instead, the use of the MV method may provide a larger number of detected, and potentially better localized, MB scatterers.

Targeted microbubbles: a novel application for the treatment of kidney stones

Kidney stone disease is endemic. Extracorporeal shockwave lithotripsy was the first major technological breakthrough where focused shockwaves were used to fragment stones in the kidney or ureter. The shockwaves induced the formation of cavitation bubbles, whose collapse released energy at the stone, and the energy fragmented the kidney stones into pieces small enough to be passed spontaneously. Can the concept of microbubbles be used without the bulky machine? The logical progression was to manufacture these powerful microbubbles ex vivo and inject these bubbles directly into the collecting system. An external source can be used to induce cavitation once the microbubbles are at their target; the key is targeting these microbubbles to specifically bind to kidney stones. Two important observations have been established: (i) bisphosphonates attach to hydroxyapatite crystals with high affinity; and (ii) there is substantial hydroxyapatite in most kidney stones. The microbubbles can be equipped with bisphosphonate tags to specifically target kidney stones. These bubbles will preferentially bind to the stone and not surrounding tissue, reducing collateral damage. Ultrasound or another suitable form of energy is then applied causing the microbubbles to induce cavitation and fragment the stones. This can be used as an adjunct to ureteroscopy or percutaneous lithotripsy to aid in fragmentation. Randall's plaques, which also contain hydroxyapatite crystals, can also be targeted to pre-emptively destroy these stone precursors. Additionally, targeted microbubbles can aid in kidney stone diagnostics by virtue of being used as an adjunct to traditional imaging methods, especially useful in high-risk patient populations. This novel application of targeted microbubble technology not only represents the next frontier in minimally invasive stone surgery, but a platform technology for other areas of medicine.

Characterization of individual ultrasound microbubble dynamics with a light-scattering system

Ultrasound microbubbles are contrast agents used for diagnostic ultrasound imaging and as carriers for noninvasive payload delivery. Understanding the acoustic properties of individual microbubble formulations is important for optimizing the ultrasound imaging parameters for improved image contrast and efficient payload delivery. We report here a practical and simple optical tool for direct real-time characterization of ultrasound contrast microbubble dynamics based on light scattering. Fourier transforms of raw linear and nonlinear acoustic oscillations, and microbubble cavitations are directly recorded. Further, the power of this tool is demonstrated by comparing clinically relevant microbubble cycle-to-cycle dynamics and their corresponding Fourier transforms.

The assessment of microvascular flow and tissue perfusion using ultrasound imaging

Imaging microvascular flow is of diagnostic value for a wide range of diseases including cancer, inflammation, and cardiovascular disease. The introduction of microbubbles as ultrasound contrast agents offers significant signal enhancement to the otherwise weakly scattered signal from blood in the circulation. Microbubbles provide maximum impedance mismatch, but are not linear scatterers. Their complex response to ultrasound has generated research on both their behaviour and their scattered-signal processing. Nearly 20 years ago signal processing started with simple spectral filtering of harmonics showing contrast-enhanced images. More recent pulse encoding techniques have achieved good cancellation of tissue echoes. The good quality contrast-only images enabled ultrasound contrast-imaging applications to be established in microvascular measurements in the liver and the myocardium. The field promises to advance the quantification of microvascular flow kinetics.

Nanomechanics of biocompatible hollow thin-shell polymer microspheres

The nanomechanical properties of biocompatible thin-shell hollow polymer microspheres with approximately constant ratio of shell thickness to microsphere diameter were measured by nanocompression tests in aqueous conditions. These microspheres encapsulate an inert gas and are used as ultrasound contrast agents by releasing free microbubbles in the presence of an ultrasound field as a result of free gas leakage from the shell. The tests were performed using an atomic force microscope (AFM) employing the force-distance curve technique. An optical microscope, on which the AFM was mounted, was used to guide the positioning of tipless cantilevers on top of individual microspheres. We performed a systematic study using several cantilevers with spring constants varying from 0.08 to 2.3 N/m on a population of microspheres with diameters from about 2 to 6 microm. The use of several cantilevers with various spring constants allowed a systematic study of the mechanical properties of the microsphere thin shell at different regimes of force and deformation. Using thin-shell mechanics theory for small deformations, the Young's modulus of the thin wall material was estimated and was shown to exhibit a strong size effect: it increased as the shell became thinner. The Young's modulus of thicker microsphere shells converged to the expected value for the macroscopic bulk material. For high applied forces, the force-deformation profiles showed a reversible and/or irreversible nonlinear behavior including "steps" and "jumps" which were attributed to mechanical instabilities such as buckling events.

Hydrogel based tissue mimicking phantom for in-vitro ultrasound contrast agents studies

Ultrasound medical imaging (UMI) is the most widely used image analysis technique, and often requires advanced in-vitro set up to perform morphological and functional investigations. These studies are based on contrast properties both related to tissue structure and injectable contrast agents (CA). In this work, we present a three-dimensional structure composed of two different hydrogels reassembly the microvascular network of a human tissue. This phantom was particularly suitable for the echocontrastographic measurements in human microvascular system. This phantom has been characterized to present the acoustic properties of an animal liver, that is, acoustic impedance (Z) and attenuation coefficient (AC), in UMI signal analysis in particular; the two different hydrogels have been selected to simulate the target organ and the acoustic properties of the vascular system. The two hydrogels were prepared starting from cellulose derivatives to simulating the target organ parenchyma and using a PEG-diacrylate to reproduce the vascular system. Moreover, harmonic analysis was performed on the hydrogel mimicking the liver parenchyma hydrogel to evaluate the ultrasound (US) distortion during echographic measurement. The phantom was employed in the characterization of an experimental US CA. Perfect agreement was found when comparing the hydrogel acoustical properties materials with the corresponding living reference tissues (i.e., vascular and parenchimal tissue).

Single microbubble response using pulse sequences: initial results

The study of acoustic scattering by single microbubbles has the potential to offer improved signal processing techniques. A microacoustic system that employs a hydrodynamically-focused flow was used to detect radiofrequency (RF) backscatter from single microbubbles. RF data were collected using a commercial scanner. Results are presented for two agents, namely Definity (Lantheus Medical Imaging, N. Billerica, MA, USA) and biSphere (Point Biomedical Corp, San Carlos, CA, USA). The agents were insonified with amplitude-modulated pulses, and it was observed in both agents that a subpopulation of microbubbles did not produce a measurable echo from the first-half amplitude pulse, but did produce a response from the full amplitude pulse and from a subsequent half amplitude pulse. The number of microbubbles in this subpopulation was seen to increase with increasing transmit amplitude. These results do not bear out the simple theory of microbubble-pulse sequence interaction and invite a reassessment of signal processing approaches.

Response of contrast agents to ultrasound

Microbubbles are used as ultrasonic contrast agents that enhance the ultrasound signals of the vascular bed. The recent development of site-targeted microbubbles opened up the possibility for molecular imaging as well as localised drug and gene delivery. Initially the microbubbles' physical properties and their response to the ultrasound beam were not fully understood. However, the introduction of fast acquisition microscopy has allowed the observation of the microbubble behaviour in the presence of ultrasound. In addition, acoustical techniques can determine the scatter of single microbubbles. Sonoporation experiments promise high-specificity drug and gene delivery, but the responsible physical mechanisms, particularly for in vivo applications, are not fully understood. An improvement of microbubble technology may address variability related problems in both imaging and drug/gene delivery.

SURF imaging: in vivo demonstration of an ultrasound contrast agent detection technique

A dual-band method for ultrasound contrast agent detection is demonstrated in vivo in an animal experiment using pigs. The method is named Second -order UltRasound Field Imaging, abbreviated SURF Imaging. It relies on simultaneously transmitting two ultrasound pulses with a large separation in frequency. Here, a low-frequency pulse of 0.9 MHz is combined with a high-frequency pulse of 7.5 MHz. The low-frequency pulse is used to manipulate the properties of the contrast agent, and the high frequency pulse is used for high-resolution contrast detection and imaging. An annular array capable of transmitting the low- and high-frequency pulses simultaneously was constructed and fitted to a mechanically scanned probe used in a GE Vingmed System 5 ultrasound scanner. The scanner was modified and adapted for the dual-band transmit technique. In-house software was written for post-processing of recorded IQ-data. Contrast-processed B-mode images of pig kidneys after bolus injections of 1 mL of Sonovuer are presented. The images display contrast detection with contrast-to-tissue ratios ranging from 15-40 dB. The results demonstrate the potential of SURF Imaging as an ultrasound contrast detection technique for clinically high ultrasound frequencies. This may allow ultrasound contrast imaging to be available for a wide range of applications.

Destruction thresholds of echogenic liposomes with clinical diagnostic ultrasound

Echogenic liposomes (ELIP) are submicron-sized phospholipid vesicles that contain both gas and fluid. With antibody conjugation and drug incorporation, these liposomes can be used as novel targeted diagnostic and therapeutic ultrasound contrast agents. The utility of liposomes for contrast depends upon their stability in an acoustic field, whereas the use of liposomes for drug delivery requires the liberation of encapsulated gas and drug payload at the desired treatment site. The objective of this study was twofold: (1) to characterize the stability of liposome echogenicity after reconstitution and (2) to quantitate the acoustic destruction thresholds of liposomes as a function of peak rarefactional pressure (P(r)), pulse duration (PD) and pulse repetition frequency (PRF). The liposomes were insonified in an anechoic sample chamber using a Philips HDI 5000 diagnostic ultrasound scanner with a L12-5 linear array. Liposome stability was evaluated with 6.9-MHz fundamental and 4.5-MHz harmonic B-mode pulses at various P(r) at a fixed PRF. Liposome destruction thresholds were determined using 6.0-MHz Doppler pulses, by varying the PD with a fixed PRF of 1.25 kHz and by varying the PRF with a fixed PD of 3.33 micros. Videos or freeze-captured images were acquired during each insonation experiment and analyzed for echogenicity in a fixed region of interest as a function of time. An initial increase in echogenicity was observed for fundamental and harmonic B-mode imaging pulses. The threshold for acoustically driven diffusion of gas out of the liposomes using 6.0-MHz Doppler pulses was weakly dependent upon PRF and PD. The rapid fragmentation thresholds, however, were highly dependent upon PRF and PD. The quantification of acoustic destruction thresholds of ELIP is an important first step in their development as diagnostic and drug delivery agents.