The potential for thermal damage posed by microbubble ultrasound contrast agents

Advances in nanomedicines for lymphatic imaging and therapy

Lymph nodes play a pivotal role in tumor progression as key components of the lymphatic system. However, the unique physiological structure of lymph nodes has traditionally constrained the drug delivery efficiency. Excitingly, nanomedicines have shown tremendous advantages in lymph node-specific delivery, enabling distinct recognition and diagnosis of lymph nodes, and hence laying the foundation for efficient tumor therapies. In this review, we comprehensively discuss the key factors affecting the specific enrichment of nanomedicines in lymph nodes, and systematically summarize nanomedicines for precise lymph node drug delivery and therapeutic application, including the lymphatic diagnosis and treatment nanodrugs and lymph node specific imaging and identification system. Notably, we delve into the critical challenges and considerations currently facing lymphatic nanomedicines, and futher propose effective strategies to address these issues. This review encapsulates recent findings, clinical applications, and future prospects for designing effective nanocarriers for lymphatic system targeting, with potential implications for improving cancer treatment strategies.

Effects of sub-atmospheric pressure and dissolved oxygen concentration on lesions generated in ex vivo tissues by high intensity focused ultrasound

Background

Acoustic cavitation plays an important role in the medical treatment using high-intensity focused ultrasound (HIFU), but unnecessarily strong cavitation also could deform the morphology and enlarge the size of lesions. It is known that the increase of ambient hydrostatic pressure (P_stat) can control the acoustic cavitation. But the question of how the decrease of P_stat and dissolved oxygen concentration (DOC) influence the strength of cavitation has not been thoroughly answered. In this study, we aimed to investigate the relationship among the P_stat, DOC and the strength of cavitation.

Methods

Ex vivo bovine liver tissues were immersed in degassed water with different DOC of 1.0 mg/L, 1.5 mg/L and 2.0 mg/L. Ultrasound (US) of 1 MHz and the spatial and temporal average intensity (I_sata) of 6500 W/cm² was used to expose two groups of in vitro bovine livers for 2 s; one group was under atmospheric pressure (P_stat = 1 bar) and the other was under sub-atmospheric pressure (P_stat = 0.1 bar). Acoustic cavitation was detected by a passive cavitation detector (PCD) during the exposure process. Echo signals at the focal zone of HIFU were monitored by B-mode ultrasound imaging before and after exposure. The difference between two pressure groups was tested using paired sample t-test. The difference among different DOC groups was evaluated by one-way analysis of variance (ANOVA).

Results

The results demonstrated a significant difference of broadband acoustic emissions from the cavitation bubbles, echo signals on B-mode image, morphology of lesions under various conditions of ambient pressure and DOC. The lesion volume in tissue was increased with the increase of ambient pressure and DOC.

Conclusion

Cavitation could be suppressed through sub-atmospheric pressure and low DOC level in liver tissue, which could provide a method of controlling cavitation in HIFU treatment to avoid unpredictable lesions.

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.

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

PURPOSE

Ultrasound contrast agent microbubbles were combined with magnetic resonance imaging (MRI)-guided focused ultrasound (MRgFUS) as a means to achieve mild hyperthermia at reduced power levels.

METHODS

MRgFUS hyperthermia (42°C for 20 min) was evaluated in rabbit thigh muscle or Vx2 tumors using infusions of microbubbles (Definity, 20 µL/kg) or saline (sham) administered over 5 min. The impact of treatments on drug uptake was assessed with liposomal doxorubicin (Caelyx, 2.5 mg/kg). Applied power levels before and after the injection of microbubbles or saline were compared, and drug uptake was evaluated with fluorometry of tissues harvested 24 hr post-treatment.

RESULTS

MRgFUS hyperthermia in muscle and tumors resulted in accurate temperature control (mean =42.0°C, root mean square error (RMSE) = 0.3°C). The power dropped significantly following the injection of microbubbles in muscle and tumors compared to exposures without microbubbles (-21.9% ± 12.5% vs -5.9% ± 7.8%, p = .009 in muscle; -33.8% ± 9.9% vs -3.0% ± 7.2%, p < .001 in tumors). Cavitation monitoring indicated emission of subharmonic, ultraharmonic, and elevated levels of fourth to sixth harmonic frequencies following microbubble injection. The drug delivery was elevated significantly in muscle with the use of microbubble-assisted relative to conventional heating (0.5 ± 0.5 ng/mg vs 0.20 ± 0.04 ng/mg, p = .05), whereas in tumors similar levels were found (11 ± 3 ng/mg vs 16 ± 4 ng/mg, p = .13).

CONCLUSIONS

The finding that microbubbles reduce the applied power requirements for hyperthermia has considerable clinical implications. The elevated levels of drug found in muscle but not tumor tissue suggest a complex interplay between the heating effects of microbubbles with those of enhanced permeabilization and possible vascular damage.

Observation of flow variation in capillaries of artificial blood vessel by producing microbubble aggregations

Microbubbles form their aggregations between the neighboring microbubbles by the effect of secondary Bjerknes force under ultrasound exposure. However, because of the difficulty to reproduce a capillary-mimicking artificial blood vessel, the behavior of aggregations in a capillary has not been predicted. Thus we prepared artificial blood vessels including a capillary model, which was made of poly(vinyl alcohol) (PVA) by grayscale lithography method, with minimum diameter of the path of 0.5 mm. By using this model we investigated the possibility of artificial embolization, where the microbubble aggregations might block entire vessels not to penetrate flow in downstream. Confirming that the sizes of flown aggregation were greater than the section area of the minimum path in the capillary model, we investigated the probability of path block in it. As the results we confirmed the probability increased in proportion to sound pressure and inversely to flow velocity. We are going to investigate with more kinds of parameters to enhance the possibility of artificial embolization.

Ultrasound mediated delivery of drugs and genes to solid tumors

It has long been shown that therapeutic ultrasound can be used effectively to ablate solid tumors, and a variety of cancers are presently being treated in the clinic using these types of ultrasound exposures. There is, however, an ever-increasing body of preclinical literature that demonstrates how ultrasound energy can also be used non-destructively for increasing the efficacy of drugs and genes for improving cancer treatment. In this review, a summary of the most important ultrasound mechanisms will be given with a detailed description of how each one can be employed for a variety of applications. This includes the manner by which acoustic energy deposition can be used to create changes in tissue permeability for enhancing the delivery of conventional agents, as well as for deploying and activating drugs and genes via specially tailored vehicles and formulations.

Bioeffects considerations for diagnostic ultrasound contrast agents

Diagnostic ultrasound contrast agents have been developed for enhancing the echogenicity of blood and for delineating other structures of the body. Approved agents are suspensions of gas bodies (stabilized microbubbles), which have been designed for persistence in the circulation and strong echo return for imaging. The interaction of ultrasound pulses with these gas bodies is a form of acoustic cavitation, and they also may act as inertial cavitation nuclei. This interaction produces mechanical perturbation and a potential for bioeffects on nearby cells or tissues. In vitro, sonoporation and cell death occur at mechanical index (MI) values less than the inertial cavitation threshold. In vivo, bioeffects reported for MI values greater than 0.4 include microvascular leakage, petechiae, cardiomyocyte death, inflammatory cell infiltration, and premature ventricular contractions and are accompanied by gas body destruction within the capillary bed. Bioeffects for MIs of 1.9 or less have been reported in skeletal muscle, fat, myocardium, kidney, liver, and intestine. Therapeutic applications that rely on these bioeffects include targeted drug delivery to the interstitium and DNA transfer into cells for gene therapy. Bioeffects of contrast-aided diagnostic ultrasound happen on a microscopic scale, and their importance in the clinical setting remains uncertain.

The antivascular action of physiotherapy ultrasound on a murine tumor: role of a microbubble contrast agent

This study investigated whether a microbubble-containing ultrasound contrast agent had a role in the antivascular action of physiotherapy ultrasound on tumor neovasculature. Ultrasound images (B-mode and contrast-enhanced power Doppler [0.02 mL Definity]) were made of 22 murine melanomas (K1735(22)). The tumor was insonated (I(SATA) = 1.7 W cm(-2), 1 MHz, continuous output) for 3 min and the power Doppler observations of the pre- and postinsonation tumor vascularities were analyzed. Significant reductions (p = 0.005 for analyses of color-weighted fractional area) in vascularity occurred when a contrast-enhanced power Doppler study occurred before insonation. Vascularity was unchanged in tumors without a pretherapy Doppler study. Histologic studies revealed tissue structural changes that correlated with the ultrasound findings. The underlying etiology of the interaction between the physiotherapy ultrasound beam, the microbubble-containing contrast agent and the tumor neovasculature is unknown. It was concluded that contrast agents play an important role in the antivascular effects induced by physiotherapy ultrasound.

Potential role of pulsed-high intensity focused ultrasound in gene therapy

As the understanding of human cancer biology increases, new potential strategies for gene therapy are being proposed and evaluated. However, safe and efficient gene transfer continues to be the major hurdle for its implementation in the clinic. Preclinical studies have shown how pulsed-high intensity focused ultrasound (HIFU) exposures can be combined with different modes of administration (local, intravascular and systemic) to improve local delivery of genes and other therapeutic agents. Using image guidance, exposures are given, where short pulses of energy create predominantly mechanical/structural effects in the tissues as opposed to thermal ones. The result is an increase in both extravasation and interstitial diffusion of macromolecules, which occur non-destructively and reversibly. Ultrasound contrast agents can also be added, which enhance acoustic cavitation activity and consequently sonoporation. By being able to locally increase the uptake and expression of DNA, pulsed-HIFU holds much promise to further the use and applications of gene therapy for treating cancer and other pathological conditions.

Spatial control of gas bubbles and their effects on acoustic fields

Because microbubbles can enhance therapy, such as by cavitation or by thermal means, treatment could be confined with localization of microbubbles. This spatial control can be achieved by the vaporization of liquid-filled droplets present throughout the medium in a process known as acoustic droplet vaporization (ADV). Bubbles in the form of an orthogonal plane or "wall" can thus be created and can scatter ultrasound to enhance the proximal acoustic field while shielding distal tissues. To investigate the possible effects of a preexistent bubble wall, tissue-mimicking polyacrylamide gels embedded with perfluorocarbon droplets were insonified under various conditions. The preliminary results presented in this paper show that a bubble wall can successfully cause proximal ADV at approximately half the transmitted pressures that are required without the use of a bubble wall, while also serving as a viable shield against ADV and potential damage in distal areas. The results seen here in a gel medium are promising and suggest further development in vivo is needed.

Targeted Therapeutic Applications of Acoustically Active Microspheres in the Microcirculation

The targeted delivery of intravascular drugs and genes across the endothelial barrier with only minimal side effects remains a significant obstacle in establishing effective therapies for many pathological conditions. Recent investigations have shown that contrast agent microbubbles, which are typically used for image enhancement in diagnostic ultrasound, may also be promising tools in emergent, ultrasound-based therapies. Explorations of the bioeffects generated by ultrasound-microbubble interactions indicate that these phenomena may be exploited for clinical utility such as in the targeted revascularization of flow-deficient tissues. Moreover, development of this treatment modality may also include using ultrasound-microbubble interactions to deliver therapeutic material to tissues, and reporter genes and therapeutic agents have been successfully transferred from the microcirculation to tissue in various animal models of normal and pathological function. This article reviews the recent studies aimed at using interactions between ultrasound and contrast agent microbubbles in the microcirculation for therapeutic purposes. Furthermore, the authors present investigations involving microspheres that are of a different design compared to current microbubble contrast agents, yet are acoustically active and demonstrate potential as tools for targeted delivery. Future directions necessary to address current challenges and advance these techniques to clinical practicality are also discussed.

Enhanced heat deposition using ultrasound contrast agent--modeling and experimental observations

Ultrasound contrast agents (UCA), created originally for visualization and diagnostic purposes, recently have been suggested as efficient enhancers of ultrasonic power deposition in tissue. The ultrasonic energy absorption by the contrast agents, considered as problematic in diagnostic imaging, might have beneficial impact in therapeutic applications such as targeted hyperthermia-based or ablation treatments. Introduction of gas microbubbles into the tissue to be treated can improve the effectiveness of current treatments by limiting the temperature rise to the treated site and minimizing the damage to the surrounding healthy tissues. To this end, proper assessment of the governing parameters of energy absorption by ultrasonically induced stabilized bubbles is important for both diagnostic and therapeutic ultrasound applications. The current study was designed to predict theoretically and measure experimentally the dissipation and heating effects of encapsulated UCA in a well-controlled and calibrated environment. The ultrasonic effects of the microbubble concentration, transmitted intensity, and frequency on power dissipation and stability of the UCA have been studied. The maximal temperature elevation obtained during 300 s experiments was 21 degrees C, in a 10 ml volume target containing UCA, insonifled by unfocused 3.2 MHz continuous wave (CW) at spatial average intensity of 1.1 W/cm2 (182 kPa). The results also suggest that higher frequencies are more efficiently absorbed by commonly used UCA. In particular, for spatial average intensity of 1.1 W/cm2 and concentration of 5 x 10(6) microspheres/cm3, no significant reduction of UCA absorption was noticed during the first 150 s for insonation at 3.2 MHz and the first 100 s for insonation at 1 MHz. In addition, when lower average intensity of 0.5 W/cm2 (160 kPa) at 3.2 MHz was used, the UCA absorptivity sustained for almost 200 s. Thus, when properly activated, UCA may be suitable for localized hyperthermic therapies.

Investigating the significance of multiple scattering in ultrasound contrast agent particle populations

The majority of the existing models describing the behavior of microbubble ultrasound contrast agents consider single, isolated microbubbles suspended in infinite media. The behavior of a microbubble population is predicted by summing the results for single microbubbles and ignoring multiple scattering effects. The aim of this investigation is to determine the significance of multiple scattering in microbubble populations and establish whether an alternative approach is required. In the first part of the work, linear models are derived to identify approximately the conditions under which multiple scattering may be expected. A nonlinear model for sound propagation in a microbubble suspension then is developed and used to examine multiple scattering at higher insonation pressures. Broadband attenuation measurements are described for two different types of microbubble suspension (albumin encapsulated octofluropropane and copolymer encapsulated isobutane) to ascertain whether or not multiple scattering may be observed experimentally. The results from the simulation work indicate that multiple scattering effects would be discernible at moderate concentrations (10(6) microbubbles/ml) such as may be present in vivo. The effect upon attenuation in the suspension would be pronounced, however, only if the population contained a sufficient proportion of relatively large (> 4 microm radius) microbubbles excited at their resonance frequency. This also is found to be the case experimentally. These findings may have important implications for the characterization of ultrasound contrast agents and their use in quantitative diagnostic techniques.

Ultrasonographic contrast media: has the time come in obstetrics and gynecology?

OBJECTIVE

The aim of this work was to review the technical aspects and clinical applications of contrast media (microbubbles and nanomolecular agents) in obstetric and gynecologic ultrasonographic imaging.

METHODS

With the use of a computerized database (MEDLINE) and several Web-based search engines (Google Scholar and Copernic), relevant articles on ultrasonographic contrast media were reviewed. References cited in these articles and not obtained via the search engines were also reviewed.

RESULTS

Ultrasonographic contrast media constitute a new and expanding technology. They are frequently used, for example, in adult cardiology. Extensive research in laboratory setups, animals, and human subjects has shown their safety and huge potential as an adjunctive tool in clinical practice. They increase signals returning from insonated tissues and are particularly effective as intravascular agents, enhancing color and Doppler signals, for instance. Preliminary results in tumor imaging are encouraging. The ultrasonographic contrast media permit pharmacokinetic perfusion studies, which may be of enormous clinical importance in the study of early cancer development. Targeted imaging and therapies are becoming a reality. Microbubbles have already brought a new dimension to diagnostic ultrasonographic imaging. Many authors have described the clinical value of these agents in liver, prostate, and breast imaging, among others. Newer types of media, the nanomolecules, are now emerging as the latest in imaging enhancers as well as therapeutic agent carriers.

CONCLUSIONS

Although showing potential in imaging of the uterus and fallopian tubes as well as some obstetric applications, the contrast media, in particular the nanomolecules, seem to be most promising in ovarian cancer.

Ultrasonic drug delivery--a general review

Ultrasound has an ever-increasing role in the delivery of therapeutic agents, including genetic material, protein and chemotherapeutic agents. Cavitating gas bodies, such as microbubbles, are the mediators through which the energy of relatively non-interactive pressure waves is concentrated to produce forces that permeabilise cell membranes and disrupt the vesicles that carry drugs. Thus, the presence of microbubbles enormously enhances ultrasonic delivery of genetic material, proteins and smaller chemical agents. Numerous reports show that the most efficient delivery of genetic material occurs in the presence of cavitating microbubbles. Attaching the DNA directly to the microbubbles, or to gas-containing liposomes, enhances gene uptake even further. Ultrasonic-enhanced gene delivery has been studied in various tissues, including cardiac, vascular, skeletal muscle, tumour and even fetal tissue. Ultrasonic-assisted delivery of proteins has found most application in transdermal transport of insulin. Cavitation events reversibly disrupt the structure of the stratus corneum to allow transport of these large molecules. Other hormones and small proteins could also be delivered transdermally. Small chemotherapeutic molecules are delivered in research settings from micelles and liposomes exposed to ultrasound. Cavitation appears to play two roles: it disrupts the structure of the carrier vesicle and releases the drug; and makes cell membranes and capillaries more permeable to drugs. There remains a need to better understand the physics of cavitation of microbubbles and the impact that such cavitation has on cells and drug-carrying vesicles.

Pediatric applications of abdominal vascular Doppler: Part II

Ultrasound is a remarkably powerful and versatile modality for pediatric imaging, without requiring exposure to radiation or sedatives. By providing information on blood flow, Doppler sonography can reveal details about normal physiology and disease processes not discernable from gray-scale anatomic images alone. In part I, the basics of hemodynamics and effects on the Doppler waveform were discussed, along with clinical applications in hepatic disease. In part II, the application of Doppler in renal disease and in conditions affecting the deep abdominal vessels are discussed. The role of ultrasound contrast agents in pediatric Doppler imaging is briefly reviewed.