RESUMEN
Phase-change droplets are a class of ultrasound contrast agents that can convert into echogenic microbubbles in situ with the application of sufficient acoustic energy. Droplets are smaller and more stable than their microbubble counterparts. However, traditional ultrasound contrast agents are not trackable beyond acoustic feedback measurements, which makes quantifying contrast agent bio-distribution or accumulation ex vivo difficult. Researchers may have to rely on fluorescent or optically absorbent companion diagnostic particles to infer bio-distribution. The purpose of this protocol is to detail steps for creating multi-modal phase-change porphyrin droplets using a condensation method. Porphyrins are fluorescent molecules with distinct absorbance bands that can be conjugated onto lipids and incorporated into droplets to extend droplet versatility, enabling more robust bio-distribution while retaining acoustic properties. Seven formulations with varying porphyrin-lipid and base lipid contents were made to investigate microbubble and droplet size distributions. Characterizations suited to porphyrin-containing structures are also described in the protocol to demonstrate their analytic versatility in-solution. Sizing showed that the post-condensed mean diameters were 1.72 to 2.38 times smaller than precursor populations. Absorbance characterization showed intact assemblies had a Q-band peak of 700 nm while disrupted samples had an absorbance peak at 671 nm. Fluorescence characterization showed intact 30% porphyrin-lipid assemblies were fluorescently quenched (>97%), with fluorescence recovery achieved upon disruption. Acoustic vaporization showed that porphyrin droplets were non-echogenic at lower pressures and could be converted into echogenic microbubbles with sufficient pressures. These characterizations show the potential for porphyrin droplets to eliminate the need for absorbance or fluorescence-based companion diagnostic strategies to quantify ultrasound contrast agent bio-distribution for delivery or therapeutic applications in vivo or ex vivo.
Asunto(s)
Fluorocarburos , Porfirinas , Acústica , Medios de Contraste/química , Fluorocarburos/química , Microburbujas , VolatilizaciónRESUMEN
Ultrasound (US) is known to stimulate endogenous shear-dependent pathways, and can lower microvascular resistance through mediators that are conducted downstream from US exposure. We hypothesized that endovascular US, already in use for thrombolysis in humans, can improve tissue perfusion in the setting of acute limb ischemia through downstream-conducted effects. Models of severe peripheral arterial disease were developed in mice and in rhesus macaques. An endovascular US catheter (2.3 MHz, 0.5-1.1 MPa) was used to expose the limb adductor in mice for 10 min or the femoral artery distal to stenosis in macaques for 15 min. Quantitative contrast-enhanced ultrasound perfusion imaging was performed to assess flow augmentation in the adductor muscle of mice and the calf muscle of macaques. Microvascular blood flow in the ischemic limb relative to the contralateral control limb was reduced to 22 ± 8% in mice and 36 ± 20% in macaques. US produced immediate 2.3- and 3-fold increases (p < 0.05) in the murine and macaque ischemic limbs, respectively. In macaques, perfusion in the ischemic limb was increased to a normal level. We conclude that non-cavitating US produced by endovascular catheters that are used to enhance thrombolysis in humans can reduce vascular resistance and increase limb perfusion in the setting of acute ischemia.
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Endosonografía/métodos , Extremidades/irrigación sanguínea , Miembro Posterior/irrigación sanguínea , Isquemia/terapia , Enfermedad Arterial Periférica/terapia , Ultrasonografía Intervencional/métodos , Animales , Catéteres , Endosonografía/instrumentación , Macaca mulatta , Masculino , Ratones , Ratones Endogámicos C57BL , Ultrasonografía Intervencional/instrumentaciónRESUMEN
BACKGROUND: Cavitation of microbubble contrast agents with ultrasound produces shear-mediated vasodilation and an increase in tissue perfusion. We investigated the influence of the size of the cavitation volume by comparing flow augmentation produced by two-dimensional (2D) versus three-dimensional (3D) therapeutic ultrasound. We also hypothesized that cavitation could augment flow beyond the ultrasound field through release of vasodilators that are carried downstream. METHODS: In 11 rhesus macaques, cavitation of intravenously administered lipid-shelled microbubbles was performed in the proximal forearm flexor muscles unilaterally for 10 min. Ultrasound cavitation (1.3 MHz, 1.5 MPa peak negative pressure) was performed with 2D or 3D transmission with beam elevations of 5 and 25 mm, respectively, and pulsing intervals (PIs) sufficient to allow complete postdestruction refill (5 and 12 sec for 2D and 3D, respectively). Contrast ultrasound perfusion imaging was performed before and after cavitation, using multiplane assessment within and beyond the cavitation field in 1.5-cm increments. Cavitation in the hindlimb of mice using 2D ultrasound at a PI of 1 or 5 sec was performed to examine microvascular flow changes from cavitation in only arteries versus the microcirculation. RESULTS: In primates, the degree of muscle flow augmentation in the center of the cavitation field was similar for 2D and 3D conditions (five- to sixfold increase for both, P < .01 vs baseline). The spatial extent of flow augmentation was only modestly greater for 3D cavitation because of an increase in perfusion with 2D transmission that was detected outside of the cavitation field. In mice, cavitation in the microvascular compartment (PI 5 sec) produced the greatest degree of flow augmentation, yet cavitation in the arterial compartment (PI 1 sec) still produced a three- to fourfold increase in flow (P < .001 vs control). The mechanism for flow augmentation beyond the cavitation zone was investigated by in vitro studies that demonstrated cavitation-related release of vasodilators, including adenosine triphosphate and nitric oxide, from erythrocytes and endothelial cells. CONCLUSIONS: Compared with 2D transmission, 3D cavitation of microbubbles generates a similar degree of muscle flow augmentation, possibly because of a trade-off between volume of cavitation and PI, and only modestly increases the spatial extent of flow augmentation because of the ability of cavitation to produce conducted effects beyond the ultrasound field.
Asunto(s)
Células Endoteliales , Vasodilatación , Animales , Medios de Contraste , Macaca mulatta , Ratones , Microburbujas , PerfusiónRESUMEN
The ability to monitor cavitation activity during ultrasound and microbubble-mediated procedures is of high clinical value. However, there has been little reported literature comparing the cavitation characteristics of different clinical microbubbles, nor have current clinical scanners been used to perform passive cavitation detection in real time. The goal of this work was to investigate and characterize standard microbubble formulations (Optison, Sonovue, Sonazoid, and a custom microbubble made with similar components as Definity) with a custom passive cavitation detector (two confocal single-element focused transducers) and with a Philips EPIQ scanner with a C5-1 curvilinear probe passively listening. We evaluated three different methods for investigating cavitation thresholds, two from previously reported work and one developed in this work. For all three techniques, it was observed that the inertial cavitation thresholds were between 0.1 and 0.3 MPa for all agents when detected with both systems. Notably, we found that most microbubble formulations in bulk solution behaved generally similarly, with some differences. We show that these characteristics and thresholds are maintained when using a diagnostic ultrasound system for detecting cavitation activity. We believe that a systematic evaluation of the frequency response of the cavitation activity of different microbubbles in order to inform real-time therapy monitoring using a clinical ultrasound device could make an immediate clinical impact.
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Microburbujas , Transductores , Fantasmas de Imagen , UltrasonografíaRESUMEN
Submicron phase-change droplets are an emerging class of ultrasound contrast agent. Compared with microbubbles, their relatively small size and increased stability offer the potential to passively extravasate and accumulate in solid tumors through the enhanced permeability and retention effect. Under exposure to sufficiently powerful ultrasound, these droplets can convert into in situ gas microbubbles and thus be used as an extravascular-specific contrast agent. However, in vivo imaging methods to detect extravasated droplets have yet to be established. Here, we develop an ultrasound imaging pulse sequence within diagnostic safety limits to selectively detect droplet extravasation in tumors. Tumor-bearing mice were injected with submicron perfluorobutane droplets and interrogated with our imaging-vaporization-imaging sequence. By use of a pulse subtraction method, median droplet extravasation signal relative to the total signal within the tumor was estimated to be Etumor=37±5% compared with the kidney Ekidney=-2±8% (p < 0.001). This work contributes toward the advancement of volatile phase-shift droplets as a next-generation ultrasound agent for imaging and therapy.
Asunto(s)
Medios de Contraste , Fluorocarburos , Microburbujas , Neoplasias/diagnóstico por imagen , Volatilización , Animales , Ratones , Ultrasonografía/métodosRESUMEN
Microbubble contrast agents were introduced more than 25 years ago with the objective of enhancing blood echoes and enabling diagnostic ultrasound to image the microcirculation. Cardiology and oncology waited anxiously for the fulfillment of that objective with one clinical application each: myocardial perfusion, tumor perfusion and angiogenesis imaging. What was necessary though at first was the scientific understanding of microbubble behavior in vivo and the development of imaging technology to deliver the original objective. And indeed, for more than 25 years bubble science and imaging technology have evolved methodically to deliver contrast-enhanced ultrasound. Realization of the basic bubbles properties, non-linear response and ultrasound-induced destruction, has led to a plethora of methods; algorithms and techniques for contrast-enhanced ultrasound (CEUS) and imaging modes such as harmonic imaging, harmonic power Doppler, pulse inversion, amplitude modulation, maximum intensity projection and many others were invented, developed and validated. Today, CEUS is used everywhere in the world with clinical indications both in cardiology and in radiology, and it continues to mature and evolve and has become a basic clinical tool that transforms diagnostic ultrasound into a functional imaging modality. In this review article, we present and explain in detail bubble imaging methods and associated artifacts, perfusion quantification approaches, and implementation considerations and regulatory aspects.
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Medios de Contraste , Microburbujas , Ultrasonografía/métodos , HumanosRESUMEN
Breast cancer remains a leading cause of death for women throughout the world. Recent advances in medical imaging technologies and tumor targeting agents signify vast potential for progress toward improved management of this global problem. Phase-change contrast agents (PCCAs) are dynamic imaging agents with practical applications in both the research and clinical settings. PCCAs possess characteristics that allow for cellular uptake where they can be converted from liquid-phase PCCAs to gaseous microbubbles via ultrasound energy. Previously, we reported successful internalization of folate-targeted PCCAs in MDA-MB-231 breast cancer cells followed by ultrasound-mediated activation to produce internalized microbubbles. This study examines the binding, internalization and activation of folate-receptor targeted PCCAs in MDA-MB-231 breast cancer cells as a function of gaseous core compositions, incubation time and ultrasound exposure period. In vitro results indicate that internalization and ultrasound-mediated activation of PCCAs were significantly greater using a 50:50 mixture of decafluorobutane:dodecafluoropentane compared with other core compositions: 50:50 octafluoropropane:decafluorobutane (p < 0.0001), decafluorobutane (p < 0.04) and dodecafluoropentane (p < 0.0001). Furthermore, it was found that PCCAs composed of perfluorocarbons with higher boiling points responded with greater activation efficiency when exposed to 12 s of ultrasound exposure as opposed to 4 s of ultrasound exposure. When evaluating different incubation times, it was found that incubating the PCCAs with breast cancer cells for 60 min did not produce significantly greater internalization and activation compared with incubation for 10 min; this was concluded after comparing the number of microbubbles present per cell before ultrasound versus post-ultrasound, and finding a ratio of intracellular microbubbles post-ultrasound/pre-ultrasound, 3.46 versus 3.14, respectively. The data collected in this study helps illustrate further optimization of folate-receptor targeted PCCAs for breast cancer targeting and imaging.
Asunto(s)
Neoplasias de la Mama/diagnóstico por imagen , Aumento de la Imagen/métodos , Microburbujas , Ultrasonografía/métodos , Células Cultivadas , Medios de Contraste , Femenino , HumanosRESUMEN
Three-dimensional contrast-enhanced ultrasound (CEUS) imaging presents a clear advantage over its 2-D counterpart in detecting and characterizing suspicious lesions as it properly surveys the inherent heterogeneity of tumors. However, 3-D CEUS is also slow compared to 2-D CEUS and tends to undersample the microbubble wash-in. This makes it difficult to resolve the feeding vessels, an important oncogenic marker, from the background perfusion cloud. Contrast-enhanced Doppler is helpful in isolating this conduit flow, but requires too many pulses in conventional line-by-line beamforming design. Recent breakthroughs in plane-wave imaging have greatly accelerated the volumetric imaging frame rate, but volumetric Doppler angiography still remains challenging when considering real-time limitations on the Doppler ensemble length. In this work, we demonstrate the feasibility of volumetric CEUS angiography subjected to real-time imaging constraints. Namely, we show how principal curvature detection can significantly improve 3-D rendering of relatively noisy ultrasound angiograms without degrading the spatial resolution while subjected to a reasonable Doppler ensemble size. Singular value decomposition is also shown to be capable of identifying the quasi-stationary capillary perfusion.
Asunto(s)
Angiografía/métodos , Imagenología Tridimensional/métodos , Ultrasonografía Doppler/métodos , Algoritmos , Animales , Medios de Contraste/química , Miembro Posterior/irrigación sanguínea , Miembro Posterior/diagnóstico por imagen , Microburbujas , Neoplasias Experimentales/irrigación sanguínea , Neoplasias Experimentales/diagnóstico por imagen , Conejos , Procesamiento de Señales Asistido por ComputadorRESUMEN
In this paper, we assess the importance of microbubble shell composition for contrast-enhanced imaging sequences commonly used on clinical scanners. While the gas core dynamics are primarily responsible for the nonlinear harmonic response of microbubbles at diagnostic pressures, it is now understood that the shell rheology plays a dominant role in the nonlinear response of microbubbles subjected to low acoustic pressures. Of particular interest here, acoustic pressures of tens of kilopascal can cause a reversible phase transition of the phospholipid coatings from a stiff elastic organized state to a less stiff disorganized buckled state. Such a transition from elastic to buckled shell induces a steep variation of the shell elasticity, which alters the microbubble acoustic scattering properties. We demonstrate in this paper that this mechanism plays a dominant role in contrast pulse sequences that modulate the amplitude of the insonifying pulse pressure. The contrast-to-tissue ratio (CTR) for amplitude modulation (AM), pulse inversion (PI), and amplitude modulation pulse inversion (AMPI) is measured in vitro for Definity, Sonazoid, both lipid-encapsulted microbubbles, and the albumin-coated Optison. It is found that pulse sequences using AM significantly enhanced the nonlinear response of all studied microbubbles compared to PI (up to 15 dB more) when low insonation pressures under 200 kPa were used. Further investigation reveals that the origin of the hyperechoicity is a small phase lag occurring between the echoes from the full-and half-amplitude driving pulses, and that the effect could be attributed to the shell softening dynamics of lipid and albumin coatings. We assess that this additional phase in microbubble ultrasound scattering can have a dominant role in the CTR achieved in contrast sequences using AM. We also show that the pressure dependent phase lag is a specific marker for microbubbles with no equivalent in tissue, which can be used to segment microbubbles from the tissue harmonics and significantly increase the CTR.
Asunto(s)
Medios de Contraste/química , Aumento de la Imagen/métodos , Microburbujas , Ultrasonografía/métodos , Algoritmos , Animales , Riñón/diagnóstico por imagen , Fantasmas de Imagen , ConejosRESUMEN
Phase-shift droplets can be converted by sound from low-echogenicity, liquid-core agents into highly echogenic microbubbles. Many proposed applications in imaging and therapy take advantage of the high spatiotemporal control over this dynamic transition. Although some studies have reported increased circulation time of the droplets compared with microbubbles, few have directly explored the impact of encapsulation on droplet performance. With the goal of developing nanoscale droplets with increased circulatory persistence, we first evaluate the half-life of several candidate phospholipid encapsulations in vitro at clinical frequencies. To evaluate in vivo circulatory persistence, we develop a technique to periodically measure droplet vaporization from high-frequency B-mode scans of a mouse kidney. Results show that longer acyl chain phospholipids can dramatically reduce droplet degradation, increasing median half-life in vitro to 25.6 min-a 50-fold increase over droplets formed from phospholipids commonly used for clinical microbubbles. In vivo, the best-performing droplet formulations showed a median half-life of 18.4 min, more than a 35-fold increase in circulatory half-life compared with microbubbles with the same encapsulation in vivo. These findings also point to possible refinements that may improve nanoscale phase-shift droplet performance beyond those measured here.
Asunto(s)
Fluorocarburos , Riñón/anatomía & histología , Ultrasonografía/métodos , Animales , Procesamiento de Imagen Asistido por Computador/métodos , Técnicas In Vitro , Riñón/diagnóstico por imagen , Ratones , Ratones Endogámicos C3H , Modelos Animales , Fosfolípidos , VolatilizaciónRESUMEN
While plane-wave imaging can improve the performance of power Doppler by enabling much longer ensembles than systems using focused beams, the long-ensemble averaging of the zero-lag autocorrelation R(0) estimates does not directly decrease the mean noise level, but only decreases its variance. Spatial variation of the noise due to the time-gain compensation and the received beamforming aperture ultimately limits sensitivity. In this paper, we demonstrate that the performance of power Doppler imaging can be improved by leveraging the higher lags of the autocorrelation [e.g., R(1), R(2), ] instead of the signal power (R(0)). As noise is completely uncorrelated from pulse-to-pulse while the flow signal remains correlated significantly longer, weak signals just above the noise floor can be made visible through the reduction of the noise floor. Finally, as coherence decreases proportionally with respect to velocity, we demonstrate how signal coherence can be targeted to separate flows of different velocities. For instance, we show how long-time-range coherence of microbubble contrast-enhanced flow specifically isolates slow capillary perfusion (as opposed to conduit flow).
Asunto(s)
Ultrasonografía Doppler , Velocidad del Flujo Sanguíneo , Microburbujas , Fantasmas de ImagenRESUMEN
Phase-shift perfluorocarbon droplets have been investigated for over 20 years as pre-clinical ultrasound contrast agents with distinctive advantages in imaging and therapy. A number of formulation strategies exist, each with inherent advantages and limitations. In this note, we demonstrate a unique opportunity: that phase-shift droplets can be generated directly from commercially available microbubbles. This may facilitate pre-clinical and translational development by reducing the in-house synthesis expertise and resources required to generate high concentration droplet emulsions. Proof-of-principle in vitro and in vivo is given using droplets created from Definity and MicroMarker. The results demonstrate the role of perfluorocarbon choice in the trade-off between thermal stability and vaporization threshold, and suggest that commercial microbubbles with decafluorobutane cores may be ideal for this approach.
Asunto(s)
Medios de Contraste/química , Fluorocarburos/química , Aumento de la Imagen/métodos , Microburbujas , Ultrasonografía/métodos , Animales , Hígado/diagnóstico por imagen , Ratones , Ratones Endogámicos C3H , Modelos Animales , Fantasmas de ImagenRESUMEN
Continued advances in the field of ultrasound and ultrasound contrast agents have created new approaches to imaging and medical intervention. Phase-shift perfluorocarbon droplets, which can be vaporized by ultrasound energy to transition from the liquid to the vapor state, are one of the most highly researched alternatives to clinical ultrasound contrast agents (i.e., microbubbles). In this paper, part of a special issue on methods in biomedical ultrasonics, we survey current techniques to prepare ultrasound-activated nanoscale phase-shift perfluorocarbon droplets, including sonication, extrusion, homogenization, microfluidics, and microbubble condensation. We provide example protocols and discuss advantages and limitations of each approach. Finally, we discuss best practice in characterization of this class of contrast agents with respect to size distribution and ultrasound activation.
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Medios de Contraste/química , Fluorocarburos/química , Microburbujas , Ultrasonografía/métodos , Tamaño de la PartículaRESUMEN
While long Doppler ensembles are, in principle, beneficial for velocity estimates, short acoustic pulses must be used in microbubble contrast-enhanced (CE) Doppler to mitigate microbubble destruction. This introduces inherent tradeoffs in velocity estimates with autocorrelators, which are studied here. A model of the autocorrelation function adapted to the microbubble Doppler signal accounting for transit time, the echo frequency uncertainty, and contrast-agent destruction is derived and validated in vitro. It is further demonstrated that a local measurement of the center frequency of the microbubble echo is essential in order to avoid significant bias in velocity estimates arising from the linear and nonlinear frequency-dependent scattering of microbubbles and compensate for the inherent speckle nature of the received echo frequency. For these reasons, broadband Doppler estimators (2-D autocorrelator and Radon projection) are better suited than simpler narrow-band estimators (1-D autocorrelator and 1-D Fourier transform) for CE flow assessment. A case study of perfusion in a VX-2 carcinoma using CE plane-wave Doppler is also shown. We demonstrate that even when considering all uncertainties associated with microbubble-related decorrelation (destruction, pulse bandwidth, transit time, and flow gradient) and the need for real-time imaging, a coefficient of variation of 4% on the axial velocity is achievable with plane-wave imaging.
Asunto(s)
Microburbujas , Procesamiento de Señales Asistido por Computador , Ultrasonografía Doppler/métodos , Animales , Velocidad del Flujo Sanguíneo , Procesamiento de Imagen Asistido por Computador , Conejos , Reproducibilidad de los ResultadosRESUMEN
Breast cancer is a diverse and complex disease that remains one of the leading causes of death among women. Novel, outside-of-the-box imaging and treatment methods are needed to supplement currently available technologies. In this study, we present evidence for the intracellular delivery and ultrasound-stimulated activation of folate receptor (FR)-targeted phase-change contrast agents (PCCAs) in MDA-MB-231 and MCF-7 breast cancer cells in vitro. PCCAs are lipid-coated, perfluorocarbon-filled particles formulated as nanoscale liquid droplets capable of vaporization into gaseous microbubbles for imaging or therapy. Cells were incubated with 1:1 decafluorobutane (DFB)/octafluoropropane (OFP) PCCAs for 1h, imaged via confocal microscopy, exposed to ultrasound (9MHz, MI=1.0 or 1.5), and imaged again after insonation. FR-targeted PCCAs were observed intracellularly in both cell lines, but uptake was significantly greater (p<0.001) in MDA-MB-231 cells (93.0% internalization at MI=1.0, 79.5% at MI=1.5) than MCF-7 cells (42.4% internalization at MI=1.0, 35.7% at MI=1.5). Folate incorporation increased the frequency of intracellular PCCA detection 45-fold for MDA-MB-231 cells and 7-fold for MCF-7 cells, relative to untargeted PCCAs. Intracellularly activated PCCAs ranged from 500nm to 6µm (IQR=800nm-1.5µm) with a mean diameter of 1.15±0.59 (SD) microns. The work presented herein demonstrates the feasibility of PCCA intracellular delivery and activation using breast cancer cells, illuminating a new platform toward intracellular imaging or therapeutic delivery with ultrasound.
Asunto(s)
Neoplasias de la Mama/metabolismo , Medios de Contraste/administración & dosificación , Sistemas de Liberación de Medicamentos , Receptores de Folato Anclados a GPI/metabolismo , Neoplasias de la Mama/diagnóstico por imagen , Línea Celular Tumoral , Femenino , Fluorocarburos/química , Humanos , Células MCF-7 , Microscopía Confocal , Tamaño de la Partícula , Distribución Tisular , Ultrasonido/métodosRESUMEN
Phase-shift perfluorocarbon droplets are designed to convert from the liquid to the gas state by the external application of acoustic or optical energy. Although droplet vaporization has been investigated extensively at ultrasonic frequencies between 1 and 10 MHz, few studies have characterized performance at the higher frequencies commonly used in small animal imaging. In this study, we use standard B-mode imaging sequences on a pre-clinical ultrasound platform to both image and activate sub-micron decafluorobutane droplet populations in vitro and in vivo at center frequencies in the range of 20-40 MHz. Results show that droplets remain stable against vaporization at low imaging pressures but are vaporized at peak negative pressures near 3.5 MPa at the three frequencies tested. This study also found that a small number of size outliers present in the distribution can greatly influence droplet performance. Removal of these outliers results in a more accurate assessment of the vaporization threshold and produces free-flowing microbubbles upon vaporization in the mouse kidney.
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Medios de Contraste/química , Gases/síntesis química , Ondas de Choque de Alta Energía , Riñón/diagnóstico por imagen , Nanopartículas/química , Ultrasonografía/métodos , Animales , Línea Celular Tumoral , Medios de Contraste/efectos de la radiación , Gases/efectos de la radiación , Riñón/química , Ratones , Ratones Endogámicos C3H , Nanopartículas/efectos de la radiación , Nanopartículas/ultraestructura , Neoplasias Experimentales/química , Neoplasias Experimentales/diagnóstico por imagen , Transición de Fase/efectos de la radiación , Soluciones , Sonicación/métodosRESUMEN
Liquid-filled perfluorocarbon droplets emit a unique acoustic signature when vaporized into gas-filled microbubbles using ultrasound. Here, we conducted a pilot study in a tissue-mimicking flow phantom to explore the spatial aspects of droplet vaporization and investigate the effects of applied pressure and droplet concentration on image contrast and axial and lateral resolution. Control microbubble contrast agents were used for comparison. A confocal dual-frequency transducer was used to transmit at 8 MHz and passively receive at 1 MHz. Droplet signals were of significantly higher energy than microbubble signals. This resulted in improved signal separation and high contrast-to-tissue ratios (CTR). Specifically, with a peak negative pressure (PNP) of 450 kPa applied at the focus, the CTR of B-mode images was 18.3 dB for droplets and -0.4 for microbubbles. The lateral resolution was dictated by the size of the droplet activation area, with lower pressures resulting in smaller activation areas and improved lateral resolution (0.67 mm at 450 kPa). The axial resolution in droplet images was dictated by the size of the initial droplet and was independent of the properties of the transmit pulse (3.86 mm at 450 kPa). In post-processing, time-domain averaging (TDA) improved droplet and microbubble signal separation at high pressures (640 kPa and 700 kPa). Taken together, these results indicate that it is possible to generate high-sensitivity, high-contrast images of vaporization events. In the future, this has the potential to be applied in combination with droplet-mediated therapy to track treatment outcomes or as a standalone diagnostic system to monitor the physical properties of the surrounding environment.
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Fluorocarburos/química , Microscopía Acústica/métodos , Ultrasonografía/métodos , Microburbujas , Fantasmas de Imagen , TransductoresRESUMEN
Quantitative imaging is a crucial component of the assessment of therapies that target the vasculature of angiogenic or inflamed tissue. Dynamic contrast-enhanced ultrasound (DCE-US) using microbubble contrast offers the advantages of being sensitive to perfusion, non-invasive, cost effective and well suited to repeated use at the bedside. Uniquely, it employs an agent that is truly intravascular. This papers reviews the principles and methodology of DCE-US, especially as applied to anti-angiogenic cancer therapies. Reproducibility is an important attribute of such a monitoring method: results are discussed. More recent technical advances in parametric and 3D DCE-US imaging are also summarised and illustrated.
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Medios de Contraste , Aumento de la Imagen , Inflamación/diagnóstico por imagen , Enfermedades Inflamatorias del Intestino/diagnóstico por imagen , Neoplasias/diagnóstico por imagen , Humanos , Imagenología Tridimensional , Inflamación/terapia , Enfermedades Inflamatorias del Intestino/terapia , Microburbujas , Neoplasias/terapia , Reproducibilidad de los Resultados , Ultrasonografía Doppler en ColorRESUMEN
BACKGROUND: During high-intensity focused ultrasound (HIFU) surgical procedures, there is a need to rapidly ablate pathological tissue while minimizing damage to healthy tissue. Current techniques are limited by relatively long procedure times and risks of off-target heating of healthy tissue. One possible solution is the use of microbubbles, which can improve the efficiency of thermal energy delivery during HIFU procedures. However, microbubbles also suffer from limitations such as low spatial selectivity and short circulation time in vivo. In this study, the use of a dual-perfluorocarbon nanodroplet that can enhance thermal ablation, yet retains high spatial selectivity and circulation half-life, was evaluated in vivo and compared to traditional microbubble agents during HIFU ablations of rat liver. METHODS: High-intensity focused ultrasound (1.1 MHz, 4.1 MPa, 15-s continuous wave) was applied to rat liver in vivo, and heating was monitored during sonication by magnetic resonance thermometry. Thermometry data were analyzed to quantify temperature rise and ablated area, both at the target and prefocally, for HIFU applied 5, 15, or 95 min after intravenous injection of either nanodroplet or microbubble agents. Sham control experiments (no injected agents) were also performed. RESULTS: At all three time points, nanodroplets significantly enhanced thermal delivery to the target, achieving temperatures 130 % higher and ablated areas 30 times larger than no-agent control sonications. Nanodroplets did not significantly enhance off-target surface heating. Microbubbles also resulted in significantly greater thermal delivery, but heating was concentrated at the proximal surface of the animal, causing skin burns. Furthermore, microbubbles resulted in lower thermal delivery to the desired target than even the control case, with the notable exception of the 95-min time point. CONCLUSIONS: Results indicate that the nanodroplet formulation studied here can substantially increase thermal delivery at the acoustic focus while avoiding prefocal heating. In contrast, microbubbles resulted in greater prefocal heating and less heating at the target. Furthermore, nanodroplets are sufficiently stable to enhance HIFU ablation in vivo for at least 1.5 h after injection. The use of a dual-perfluorocarbon nanodroplet formulation as described herein could substantially reduce HIFU procedure times without increasing the risk of skin burns.
RESUMEN
Many studies have explored phase-change contrast agents (PCCAs) that can be vaporized by an ultrasonic pulse to form microbubbles for ultrasound imaging and therapy. However, few investigations have been published on the utility and characteristics of PCCAs as contrast agents in vivo. In this study, we examine the properties of low-boiling-point nanoscale PCCAs evaluated in vivo and compare data with those for conventional microbubbles with respect to contrast generation and circulation properties. To do this, we develop a custom pulse sequence to vaporize and image PCCAs using the Verasonics research platform and a clinical array transducer. Results indicate that droplets can produce contrast enhancement similar to that of microbubbles (7.29 to 18.24 dB over baseline, depending on formulation) and can be designed to circulate for as much as 3.3 times longer than microbubbles. This study also reports for the first time the ability to capture contrast washout kinetics of the target organ as a measure of vascular perfusion.