Cavitation nucleation agents for nonthermal ultrasound therapy

Nanostructured TiO2 cavitation agents for dual-modal sonophotocatalysis with pulsed ultrasound

Current sonochemical methods rely on spatially uncontrolled cavitation for radical species generation to promote chemical reactions. To improve radical generation, sonosensitizers have been demonstrated to be activated by cavitation-based light emission (sonoluminescence). Unfortunately, this process remains relatively inefficient compared to direct photocatalysis, due to the physical separation between cavitation event and sonosensitizing agent. In this study, we have synthesized nanostructured titanium dioxide particles to couple the source for cavitation within a photocatalytic site to create a sonophotocatalyst. In doing so, we demonstrate that site-controlled cavitation from the nanoparticles using pulsed ultrasound at reduced acoustic powers resulted in the sonochemical degradation methylene blue at rates nearly three orders of magnitude faster than other titanium dioxide-based nanoparticles by conventional methods. Sonochemical degradation was directly proportional to the measured cavitation produced by these sonophotocatalysts. Our work suggests that simple nanostructuring of current sonosensitizers to enable on-site cavitation greatly enhances sonochemical reaction rates.

Imparting Functionality to the Hydrogel by Magnetic-Field-Induced Nano-assembly and Macro-response

Hydrogels are composed of 3D hydrophilic networks with an abundance of water; they are analogous to biological soft tissues. Their unique physico-chemical properties endow hydrogels with great potential in many fields, including tissue engineering and flexible sensing. However, inadequate functionality, such as lack of rapid responsiveness, severely limits practical applications in many areas. Therefore, imparting functionality to the hydrogel is a hot research topic. The magnetic field, as an important physical field, provides a new strategy with a variety of advantages. Magnetic-field-induced ordered nano-assembly brought anisotropic properties and novel performance. Furthermore, the magnetic responsiveness of hydrogels with magnetic nanoparticles can lead to the generation of functionality under magnetic fields. Thus, we aim to systematically describe the significant effect of magnetic fields on the functionality of the hydrogel. In this review, magnetic-field-induced assembly of nanomaterials with different dimensions and resulting functional performance are introduced. The functionalities of hydrogels based on magnetic-field-induced macroscopic responses are also summarized. We believe this review will motivate more exploration of the application of magnetic fields to develop functional hydrogel materials.

Delivering Focused Ultrasound to Intervertebral Discs Using Time-Reversal

Chronic low back pain causes more disability worldwide than any other condition and is thought to arise in part through loss of biomechanical function of degenerate intervertebral discs (IVDs). Current treatments can involve replacing part or all of the degenerate IVDs by invasive surgery. Our vision is to develop a minimally invasive approach in which high intensity focused ultrasound (HIFU) is used to mechanically fractionate degenerate tissue in an IVD; a fine needle is then used to first remove the fractionated tissue and then inject a biomaterial able to restore normal physiologic function. The goal of this manuscript is to demonstrate the feasibility of trans-spinal HIFU delivery using simulations of 3-D ultrasound propagation in models derived from patient computed tomography (CT) scans. The CT data were segmented into bone, fat and other soft tissue for three patients. Ultrasound arrays were placed around the waist of each patient model, and time-reversal was used to determine the source signals necessary to create a focus in the center of the disc. The simulations showed that for 0.5 MHz ultrasound, a focus could be created in most of the lumbar IVDs, with the pressure focal gain ranging from 3.2-13.7. In conclusion, it is shown that with patient-specific planning, focusing ultrasound into an IVD is possible in the majority of patients despite the complex acoustic path introduced by the bony structures of the spine.

Biomedical Applications for Gas-Stabilizing Solid Cavitation Agents

For over a decade, advancements in ultrasound-enhanced drug delivery strategies have demonstrated remarkable success in providing targeted drug delivery for a broad range of diseases. In order to achieve enhanced drug delivery, these strategies harness the mechanical effects from bubble oscillations (i.e., cavitation) of a variety of exogenous cavitation agents. Recently, solid cavitation agents have emerged due to their capacity for drug-loading and sustained cavitation duration. Unlike other cavitation agents, solid cavitation agents stabilize gaseous bubbles on hydrophobic surface cavities. Thus, the design of these particles is crucial. In this Review, we provide an overview of the different designs for solid cavitation agents such as nanocups, nanocones, and porous structures, as well as the current status of their development. Considering the numerous advantages of solid cavitation agents, we anticipate further innovations for this new type of cavitation agent across a broad range of biomedical applications.

Microbubbles, Nanodroplets and Gas-Stabilizing Solid Particles for Ultrasound-Mediated Extravasation of Unencapsulated Drugs: An Exposure Parameter Optimization Study

Ultrasound-induced cavitation has been proposed as a strategy to tackle the challenge of inadequate extravasation, penetration and distribution of therapeutics into tumours. Here, the ability of microbubbles, droplets and solid gas-trapping particles to facilitate mass transport and extravasation of a model therapeutic agent following ultrasound-induced cavitation is investigated. Significant extravasation and penetration depths on the order of millimetres are achieved with all three agents, including the range of pressures and frequencies achievable with existing clinical ultrasound systems. Deeper but highly directional extravasation was achieved with frequencies of 1.6 and 3.3 MHz compared with 0.5 MHz. Increased extravasation was observed with increasing pulse length and exposure time, while an inverse relationship is observed with pulse repetition frequency. No significant cell death or any haemolytic activity in human blood was observed at clinically relevant concentrations for any of the agents. Overall, solid gas-trapping nanoparticles were found to enable the most extensive extravasation for the lowest input acoustic energy, followed by microbubbles and then droplets. The ability of these agents to produce sustained inertial cavitation activity whilst being small enough to follow the drug out of the circulation and into diseased tissue, combined with a good safety profile and the possibility of real-time monitoring, offers considerable potential for enhanced drug delivery of unmodified drugs in oncological and other biomedical applications.

Vaporization Detection Imaging: A Technique for Imaging Low-Boiling-Point Phase-Change Contrast Agents with a High Depth of Penetration and Contrast-to-Tissue Ratio

Phase-change contrast agents (PCCAs) possess advantages over microbubble contrast agents, such as the ability to extravasate and circulate longer in the vasculature that could enhance the diagnostic capabilities of contrast-enhanced ultrasound. PCCAs typically have a liquid perfluorocarbon (PFC) core that can be vaporized into echogenic microbubbles. Vaporization of submicron agents filled with liquid PFCs at body temperature usually requires therapeutic pressures higher than typically used for diagnostic imaging, but low-boiling-point PCCAs using decafluorobutane or octafluoropropane can be vaporized using pressures in the diagnostic imaging regime. Low-boiling-point PCCAs produce a unique acoustic signature that can be separated from tissue and bubble signals to make images with high contrast-to-tissue ratios. In this work, we explore the effect of pulse length and concentration on the vaporization signal of PCCAs and a new technique to capture and use the signals to make high contrast-to-tissue ratio images in vivo. The results indicate that using a short pulse may be ideal for imaging because it does not interact with created bubbles but still produces strong signals for making images. Furthermore, it was found that capturing PCCA vaporization signals produced higher contrast-to-tissue ratio values and better depth of penetration than imaging the bubbles generated by droplet activation using conventional contrast imaging techniques. The resolution of the vaporization signal images is poor because of the low frequency of the signals, but their high sensitivity may be used for applications such as molecular imaging, where the detection of small numbers of contrast agents is important.

Coalescence of residual histotripsy cavitation nuclei using low-gain regions of the therapy beam during electronic focal steering

Following collapse of a histotripsy cloud, residual microbubbles may persist for seconds, distributed throughout the focus. Their presence can attenuate and scatter subsequent pulses, hindering treatment speed and homogeneity. Previous studies have demonstrated use of separate low-amplitude (~1 MPa) pulses interleaved with histotripsy pulses to drive bubble coalescence (BC), significantly improving treatment speed without sacrificing homogeneity. We propose that by using electronic focal steering (EFS) to direct the therapy focus throughout specially-designed EFS sequences, it is possible to use low-gain regions of the therapy beam to accomplish BC during EFS without any additional acoustic sequence. First, to establish proof of principle for an isolated focus, a 50-foci EFS sequence was constructed with the first position isolated near the geometric focus and remaining positions distributed post-focally. EFS sequences were evaluated in tissue-mimicking phantoms with gas concentrations of 20% and 100% with respect to saturation. Results using an isolated focus demonstrated that at 20% gas concentration, 49 EFS pulses were sufficient to achieve BC in all samples for pulse repetition frequency (PRF)  ⩽  800 Hz and 84.1%  ±  3.0% of samples at 5 kHz PRF. For phantoms prepared with 100% gas concentration, BC was achieved by 49 EFS pulses in 39.2%  ±  4.7% of samples at 50 Hz PRF and 63.4%  ±  15.3% of samples at 5 kHz. To show feasibility of using the EFS-BC method to ablate a large volume quickly, a 1000-foci EFS sequence covering a volume of approximately 27 ml was tested. Results indicate that the BC effect was similarly present. A treatment rate of 27  ±  6 ml min^-1 was achieved, which is signficantly faster than standard histotripsy and ultrasound thermal ablation. This study demonstrates that histotripsy with EFS can achieve BC without employing a separate acoustic sequence which has the potential to accelerate large-volume ablation while minimizing energy deposition.

Optimizing Acoustic Activation of Phase Change Contrast Agents With the Activation Pressure Matching Method: A Review

Submicrometer phase-change contrast agents (PCCAs) consist of a liquid perfluorocarbon (PFC) core that can be vaporized by ultrasound (acoustic droplet vaporization) to generate contrast with excellent spatial and temporal control. When these agents, commonly referred to as nanodroplets, are formulated with cores of low boiling-point PFCs such as decafluorobutane and octafluoropropane, they can be activated with low-mechanical-index (MI) imaging pulses for diagnostic applications. Since the utilization of minimum MI is often desirable to avoid unnecessary biological effects, enabling consistent activation of these agents in an acoustic field is a challenge because the energy that must be delivered to achieve the vaporization threshold increases with depth due to attenuation. A novel vaporization approach called activation pressure matching (APM) has been developed to deliver the same pressure throughout a field of view in order to produce uniform nanodroplet vaporization and to limit the amount of energy that is delivered. In this paper, we discuss the application of this method with a Verasonics V1 Research Ultrasound System to modulate the output pressure from an ATL L11-5 transducer. Vaporization-pulse spacing optimization can be used in addition to matching the activation pressure through depth, and we demonstrate the feasibility of this approach both in vivo and in vitro. The use of optimized vaporization parameters increases the amount of time a single bolus of nanodroplets can generate useful contrast and provides consistent image enhancement in vivo. Therefore, APM is a useful technique for maximizing the efficacy of PCCA while minimizing delivered acoustic energy.

Blinking Phase-Change Nanocapsules Enable Background-Free Ultrasound Imaging

Microbubbles are widely used as contrast agents to improve the diagnostic capability of conventional, highly speckled, low-contrast ultrasound imaging. However, while microbubbles can be used for molecular imaging, these agents are limited to the vascular space due to their large size (> 1 μm). Smaller microbubbles are desired but their ultrasound visualization is limited due to lower echogenicity or higher resonant frequencies. Here we present nanometer scale, phase changing, blinking nanocapsules (BLInCs), which can be repeatedly optically triggered to provide transient contrast and enable background-free ultrasound imaging. In response to irradiation by near-infrared laser pulses, the BLInCs undergo cycles of rapid vaporization followed by recondensation into their native liquid state at body temperature. High frame rate ultrasound imaging measures the dynamic echogenicity changes associated with these controllable, periodic phase transitions. Using a newly developed image processing algorithm, the blinking particles are distinguished from tissue, providing a background-free image of the BLInCs while the underlying B-mode ultrasound image is used as an anatomical reference of the tissue. We demonstrate the function of BLInCs and the associated imaging technique in a tissue-mimicking phantom and in vivo for the identification of the sentinel lymph node. Our studies indicate that BLInCs may become a powerful tool to identify biological targets using a conventional ultrasound imaging system.

Contrast-enhanced ultrasound imaging and in vivo circulatory kinetics with low-boiling-point nanoscale phase-change perfluorocarbon agents

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.

Spatial and temporal observation of phase-shift nano-emulsions assisted cavitation and ablation during focused ultrasound exposure

BACKGROUND

Phase-shift nano-emulsions (PSNEs) with a small initial diameter in nanoscale have the potential to leak out of the blood vessels and to accumulate at the target point of tissue. At desired location, PSNEs can undergo acoustic droplet vaporization (ADV) process, change into gas bubbles and enhance focused ultrasound efficiency. The threshold of droplet vaporization and influence of acoustic parameters have always been research hotspots in order to spatially control the potential of bioeffects and optimize experimental conditions. However, when the pressure is much higher than PSNEs' vaporization threshold, there were little reports on their cavitation and thermal effects.

OBJECT

In this study, PSNEs induced cavitation and ablation effects during pulsed high-intensity focused ultrasound (HIFU) exposure were investigated, including the spatial and temporal information and the influence of acoustic parameters.

METHODS

Two kinds of tissue-mimicking phantoms with uniform PSNEs were prepared because of their optical transparency. The Sonoluminescence (SL) method was employed to visualize the cavitation activities. And the ablation process was observed as the heat deposition could produce white lesion.

RESULTS

Precisely controlled HIFU cavitation and ablation can be realized at a relatively low input power. But when the input power was high, PSNEs can accelerate cavitation and ablation in pre-focal region. The cavitation happened layer by layer advancing the transducer. While the lesion appeared to be separated into two parts, one in pre-focal region stemmed from one point and grew quickly, the other in focal region grew much more slowly. The influence of duty cycle has also been examined. Longer pulse off time would cause heat transfer to the surrounding media, and generate smaller lesion. On the other hand, this would give outer layer bubbles enough time to dissolve, and inner bubbles can undergo violent collapse and emit bright light.

Phase transitions of perfluorocarbon nanoemulsion induced with ultrasound: a mathematical model

While ultrasound has been used in many medical and industrial applications, only recently has research been done on phase transformations induced by ultrasound. This paper presents a numerical model and the predicted results of the phase transformation of a spherical nanosized droplet of perfluorocarbon in water. Such a model has applications in acoustic droplet vaporization, the generation of gas bubbles for medical imaging, therapeutic delivery and other biomedical applications. The formation of a gas phase and the subsequent bubble dynamics were studied as a function of acoustic parameters, such as frequency and amplitude, and of the physical aspects of the perfluorocarbon nanodroplets, such as chemical species, temperature, droplet size and interfacial energy. The model involves simultaneous applications of mass, energy and momentum balances to describe bubble formation and collapse, and was developed and solved numerically. It was found that, all other parameters being constant, the maximum bubble size and collapse velocity increases with increasing ultrasound amplitude, droplet size, vapor pressure and temperature. The bubble size and collapse velocity decreased with increasing surface tension and frequency. These results correlate with experimental observations of acoustic droplet vaporization.

Improving the performance of phase-change perfluorocarbon droplets for medical ultrasonography: current progress, challenges, and prospects

Over the past two decades, perfluorocarbon (PFC) droplets have been investigated for biomedical applications across a wide range of imaging modalities. More recently, interest has increased in "phase-change" PFC droplets (or "phase-change" contrast agents), which can convert from liquid to gas with an external energy input. In the field of ultrasound, phase-change droplets present an attractive alternative to traditional microbubble agents for many diagnostic and therapeutic applications. Despite the progress, phase-change PFC droplets remain far from clinical implementation due to a number of challenges. In this review, we survey our recent work to enhance the performance of phase-change agents for ultrasound through a variety of techniques in order to provide increased efficacy in therapeutic applications of ultrasound and enable previously unexplored applications in diagnostic and molecular imaging.

Phase change events of volatile liquid perfluorocarbon contrast agents produce unique acoustic signatures

Phase-change contrast agents (PCCAs) provide a dynamic platform to approach problems in medical ultrasound (US). Upon US-mediated activation, the liquid core vaporizes and expands to produce a gas bubble ideal for US imaging and therapy. In this study, we demonstrate through high-speed video microscopy and US interrogation that PCCAs composed of highly volatile perfluorocarbons (PFCs) exhibit unique acoustic behavior that can be detected and differentiated from standard microbubble contrast agents. Experimental results show that when activated with short pulses PCCAs will over-expand and undergo unforced radial oscillation while settling to a final bubble diameter. The size-dependent oscillation phenomenon generates a unique acoustic signal that can be passively detected in both time and frequency domain using confocal piston transducers with an 'activate high' (8 MHz, 2 cycles), 'listen low' (1 MHz) scheme. Results show that the magnitude of the acoustic 'signature' increases as PFC boiling point decreases. By using a band-limited spectral processing technique, the droplet signals can be isolated from controls and used to build experimental relationships between concentration and vaporization pressure. The techniques shown here may be useful for physical studies as well as development of droplet-specific imaging techniques.

Phase-transition thresholds and vaporization phenomena for ultrasound phase-change nanoemulsions assessed via high-speed optical microscopy

Ultrasonically activated phase-change contrast agents (PCCAs) based on perfluorocarbon droplets have been proposed for a variety of therapeutic and diagnostic clinical applications. When generated at the nanoscale, droplets may be small enough to exit the vascular space and then be induced to vaporize with high spatial and temporal specificity by externally-applied ultrasound. The use of acoustical techniques for optimizing ultrasound parameters for given applications can be a significant challenge for nanoscale PCCAs due to the contributions of larger outlier droplets. Similarly, optical techniques can be a challenge due to the sub-micron size of nanodroplet agents and resolution limits of optical microscopy. In this study, an optical method for determining activation thresholds of nanoscale emulsions based on the in vitro distribution of bubbles resulting from vaporization of PCCAs after single, short (<10 cycles) ultrasound pulses is evaluated. Through ultra-high-speed microscopy it is shown that the bubbles produced early in the pulse from vaporized droplets are strongly affected by subsequent cycles of the vaporization pulse, and these effects increase with pulse length. Results show that decafluorobutane nanoemulsions with peak diameters on the order of 200 nm can be optimally vaporized with short pulses using pressures amenable to clinical diagnostic ultrasound machines.

Evolution of acoustically vaporized microdroplets in gas embolotherapy

Acoustic vaporization dynamics of a superheated dodecafluoropentane (DDFP) microdroplet inside a microtube and the resulting bubble evolution is investigated in the present work. This work is motivated by a developmental gas embolotherapy technique that is intended to treat cancers by infarcting tumors using gas bubbles. A combined theoretical and computational approach is utilized and compared with the experiments to understand the evolution process and to estimate the resulting stress distribution associated with vaporization event. The transient bubble growth is first studied by ultra-high speed imaging and then theoretical and computational modeling is used to predict the entire bubble evolution process. The evolution process consists of three regimes: an initial linear rapid spherical growth followed by a linear compressed oval shaped growth and finally a slow asymptotic nonlinear spherical bubble growth. Although the droplets are small compared to the tube diameter, the bubble evolution is influenced by the tube wall. The final bubble radius is found to scale linearly with the initial droplet radius and is approximately five times the initial droplet radius. A short pressure pulse with amplitude almost twice as that of ambient conditions is observed. The width of this pressure pulse increases with increasing droplet size whereas the amplitude is weakly dependent. Although the rise in shear stress along the tube wall is found to be under peak physiological limits, the shear stress amplitude is found to be more prominently influenced by the initial droplet size. The role of viscous dissipation along the tube wall and ambient bulk fluid pressure is found to be significant in bubble evolution dynamics.

Phase transitions of nanoemulsions using ultrasound: experimental observations

The ultrasound-induced transformation of perfluorocarbon liquids to gases is of interest in the area of drug and gene delivery. In this study, three independent parameters (temperature, size, and perfluorocarbon species) were selected to investigate the effects of 476-kHz and 20-kHz ultrasound on nanoemulsion phase transition. Two levels of each factor (low and high) were considered at each frequency. The acoustic intensities at gas bubble formation and at the onset of inertial cavitation were recorded and subsequently correlated with the acoustic parameters. Experimental data showed that low frequencies are more effective in forming and collapsing a bubble. Additionally, as the size of the emulsion droplet increased, the intensity required for bubble formation decreased. As expected, perfluorohexane emulsions require greater intensity to form cavitating bubbles than perfluoropentane emulsions.

Phase-shift, stimuli-responsive drug carriers for targeted delivery

The intersection of particles and directed energy is a rich source of novel and useful technology that is only recently being realized for medicine. One of the most promising applications is directed drug delivery. This review focuses on phase-shift nanoparticles (that is, particles of submicron size) as well as micron-scale particles whose action depends on an external-energy triggered, first-order phase shift from a liquid to gas state of either the particle itself or of the surrounding medium. These particles have tremendous potential for actively disrupting their environment for altering transport properties and unloading drugs. This review covers in detail ultrasound and laser-activated phase-shift nano- and micro-particles and their use in drug delivery. Phase-shift based drug-delivery mechanisms and competing technologies are discussed.

Phase-change contrast agents for imaging and therapy

Phase-change contrast agents (PCCAs) for ultrasound-based applications have resulted in novel ways of approaching diagnostic and therapeutic techniques beyond what is possible with microbubble contrast agents and liquid emulsions. When subjected to sufficient pressures delivered by an ultrasound transducer, stabilized droplets undergo a phase-transition to the gaseous state and a volumetric expansion occurs. This phenomenon, termed acoustic droplet vaporization, has been proposed as a means to address a number of in vivo applications at the microscale and nanoscale. In this review, the history of PCCAs, physical mechanisms involved, and proposed applications are discussed with a summary of studies demonstrated in vivo. Factors that influence the design of PCCAs are discussed, as well as the need for future studies to characterize potential bioeffects for administration in humans and optimization of ultrasound parameters.

Phase-shift, stimuli-responsive perfluorocarbon nanodroplets for drug delivery to cancer

This review focuses on phase-shift perfluorocarbon nanoemulsions whose action depends on an ultrasound-triggered phase shift from a liquid to gas state. For drug-loaded perfluorocarbon nanoemulsions, microbubbles are formed under the action of tumor-directed ultrasound and drug is released locally into tumor volume in this process. This review covers in detail mechanisms involved in the droplet-to-bubble transition as well as mechanisms of ultrasound-mediated drug delivery.

Design of ultrasonically-activatable nanoparticles using low boiling point perfluorocarbons

Recently, an interest has developed in designing biomaterials for medical ultrasonics that can provide the acoustic activity of microbubbles, but with improved stability in vivo and a smaller size distribution for extravascular interrogation. One proposed alternative is the phase-change contrast agent. Phase-change contrast agents (PCCAs) consist of perfluorocarbons (PFCs) that are initially in liquid form, but can then be vaporized with acoustic energy. Crucial parameters for PCCAs include their sensitivity to acoustic energy, their size distribution, and their stability, and this manuscript provides insight into the custom design of PCCAs for balancing these parameters. Specifically, the relationship between size, thermal stability and sensitivity to ultrasound as a function of PFC boiling point and ambient temperature is illustrated. Emulsion stability and sensitivity can be 'tuned' by mixing PFCs in the gaseous state prior to condensation. Novel observations illustrate that stable droplets can be generated from PFCs with extremely low boiling points, such as octafluoropropane (b.p. -36.7 °C), which can be vaporized with acoustic parameters lower than previously observed. Results demonstrate the potential for low boiling point PFCs as a useful new class of compounds for activatable agents, which can be tailored to the desired application.

Decafluorobutane as a phase-change contrast agent for low-energy extravascular ultrasonic imaging

Currently available microbubbles used for ultrasound imaging and therapeutics are limited to intravascular space due to their size distribution in the micron range. Phase-change contrast agents (PCCAs) have been proposed as a means to overcome this limitation, since droplets formed in the hundred nanometer size range might be able to extravasate through leaky microvasculature, after which they could be activated to form larger highly echogenic microbubbles. Existing PCCAs in the sub-micron size range require substantial acoustic energy to be vaporized, increasing the likelihood of unwanted bioeffects. Thus, there exists a need for PCCAs with reduced acoustic activation energies for use in imaging studies. In this article, it is shown that decafluorobutane, which is normally a gas at room temperature, can be incorporated into metastable liquid sub-micron droplets with appropriate encapsulation methods. The resulting droplets are activatable with substantially less energy than other favored PCCA compounds. Decafluorobutane nanodroplets may present a new means to safely extend ultrasound imaging beyond the vascular space.

Optimal dosage of ultrasound contrast agent for ultrasound surgery: thermal effect of linear plane wave

The optimal dosage of ultrasound contrast agent model for ultrasound surgery was explored. A specific ultrasound contrast agent Albunex was chosen for simulation. The model was developed based on a dilute bubbly liquid model proposed by Ye and Ding [Z. Ye, L. Ding, Acoustic dispersion and attenuation relations in bubbly mixture, J. Acoust. Soc. Am. 98 (3) (1995) 1629-1636]. The numerical simulation suggests that 2 MHz is more efficient than 1 MHz to thermally treat cancer in deep tissue with the optimal dosage of 3 ml. On the other hand, the simulation also suggests 3 MHz center frequency with the optimal dosage of 1.6 ml is adequate for prostate cancer treatment with transrectal equipment. The simulation is expected to valid up to 2 MPa incident pressure due to the limitation of the linearized UCA model. Even though it is developed from a single ultrasound contrast agent, this model is expected to be useful for any ultrasound contrast agent as long as the necessary parameters are provided.

Power density requirement of a 4 MHz micro-ultrasonic transducer for sonodynamic therapy

In this paper, we propose the use of micro-ultrasonic transducers (MUTs) for a therapeutic application in combination with a cancer drug. In particular, sonodynamic enhancement of doxorubicin cytotoxicity was investigated in vitro using human prostate cancer cells (PC3). Cells in suspensions were found to be two to three times more prone to the cytotoxic effect of ultrasound than adherent cells. With 60 s of tone-burst ultrasound (4 MHz, 50 ms repetition period, and 25% duty cycle) at 40 Watt/cm(2) (spatial average-temporal average), cytotoxicity of doxorubicin treatment of adherent cells increased from 27 to 91%. The threshold ultrasonic power density required for any cytotoxicity enhancement to be observable was found to be 15 Watt/cm(2) for PC3 cells with doxorubicin and tone burst ultrasound at 4 MHz. This is a level achievable by MUTs. The long term vision is to design implantable MUTs for sonodynamic therapy with the goal of improving treatment efficacy.

Role of acoustic cavitation in the delivery and monitoring of cancer treatment by high-intensity focused ultrasound (HIFU)

Acoustic cavitation has been shown to play a key role in a wide array of novel therapeutic ultrasound applications. This paper presents a brief discussion of the physics of thermally relevant acoustic cavitation in the context of high-intensity focussed ultrasound (HIFU). Models for how different types of cavitation activity can serve to accelerate tissue heating are presented, and results suggest that the bulk of the enhanced heating effect can be attributed to the absorption of broadband acoustic emissions generated by inertial cavitation. Such emissions can be readily monitored using a passive cavitation detection (PCD) scheme and could provide a means for real-time treatment monitoring. It is also shown that the appearance of hyperechoic regions (or bright-ups) on B-mode ultrasound images constitutes neither a necessary nor a sufficient condition for inertial cavitation activity to have occurred during HIFU exposure. Once instigated at relatively large HIFU excitation amplitudes, bubble activity tends to grow unstable and to migrate toward the source transducer, causing potentially undesirable pre-focal damage. Potential means of controlling inertial cavitation activity using pulsed excitation so as to confine it to the focal region are presented, with the intention of harnessing cavitation-enhanced heating for optimal HIFU treatment delivery. The role of temperature elevation in mitigating bubble-enhanced heating effects is also discussed, along with other bubble-field effects such as multiple scattering and shielding.

Quantitative relations of acoustic inertial cavitation with sonoporation and cell viability

Ultrasound-induced acoustic cavitation assists gene delivery, possibly by increasing the permeability of the cell membranes. How the cavitation dose is related to the sonoporation rate and the cell viability is still unknown and so this in vitro study quantitatively investigated the effects of cavitation induced by 1-MHz pulsed ultrasound waves and the contrast agent Levovist (containing microbubbles when reconstituted by adding saline and shaken) on the delivery of short DNA-FITC molecules into HeLa cells. The concentrations of cells and DNA-FITC were 2 x 10(5) cells/mL and 40 microg/mL, respectively. The cavitation was quantified as the inertial cavitation dose (ICD), corresponding to the spectral broadband signal enhancement during microbubble destruction. The relations of ICD with sonoporation and cell viability were examined for various acoustic pressures (0.48-1.32 MPa), Levovist concentrations (1.12 x 10(5)-1.12 x 10(7) bubbles/mL) and pulse durations (1-10 cycles). The linear regressions of the sonoporation rate versus ICD and the cell viability versus ICD were y = 28.67x + 10.71 (R(2) = 0.95) and z = -62.83x + 91.18 (R(2) = 0.84), respectively, where x is ICD, y is the sonoporation rate and z is the cell viability. These results show that the sonoporation rate and the cell viability are highly correlated with the ICD, indicating that sonoporation results may be potentially predicted using ICD.

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.

Lithotripter shockwave-induced enhancement of mouse melanoma lung metastasis: dependence on cavitation nucleation

PURPOSE

To confirm a previous report of metastasis enhancement by lithotripter shockwaves (LSW) and to test the hypothesis that this effect is attributable to cavitation.

MATERIALS AND METHODS

The metastatic B16-D5 melanoma cell line was implanted on the hind legs of female C57/b16 mice 12 days before tumor treatment. The tumors were treated with 500 LSW in a waterbath arrangement. The effect of augmented cavitation nucleation was tested by intratumor injection of air bubbles or ultrasound contrast agent gas bodies (UCAGB). The primary tumor was surgically removed on day 1 after treatment. The six groups of mice were sham, LSW, sham + air bubbles, LSW + air bubbles, sham + UCAGB, and LSW + UCAGB. Data were collected for the 113 mice that survived at least 25 days. Lung evaluations were performed blind after 2 weeks of bleaching in Fekete's solution.

RESULTS

The outcomes of the three sham groups were very similar and indicated that the simple injection of material into the tumor did not increase metastasis. In comparison with the pooled shams, both the LSW + air bubbles and LSW + UCAGB groups had statistically significant increases in metastasis counts. Only the LSW + UCAGB group had a significant increase in incidence of metastasis relative to the pooled shams. The LSW + UCAGB also had significantly reduced survival.

CONCLUSION

Shockwaves can enhance metastasis from tumors, and this effect is attributable to cavitation.

Tumor growth reduction and DNA transfer by cavitation-enhanced high-intensity focused ultrasound in vivo

The potential application of high-intensity focused ultrasound (US), HIFU, was investigated for nonthermal gene transfer and tumor ablation. Renal carcinoma (RENCA) tumors were implanted on the hind leg of BALB/c mice and injected with a marker plasmid. Optison US contrast agent was also injected into the tumor (IT) or into the venous (IV) circulation. HIFU at 1.55 MHz was applied to the tumors with guidance from diagnostic US images. One test of transfection was also performed with lithotripter shock waves. In one set of exposures, tumor volume was followed for 4 days and a beta-galactosidase marker plasmid was used for localization of transfected cells. A second set of exposures employed a luciferase marker plasmid for assessing overall transfection after 2 days. Use of 100-ms bursts at 8-MPa peak rarefactional pressure amplitude stopped tumor growth during the 4-day period, compared to a 2.8-fold growth in shams and yielded luciferase expression 34-fold greater than in shams. Longer bursts or higher pressure amplitudes led to decreases in tumor growth, but did not yield increases in transfection. The HIFU results were similar to those of shock waves for cavitation enhanced by IT Optison. These results should aid in optimizing the application of HIFU for nonthermal tumor treatment.

Lithotripter shock waves with cavitation nucleation agents produce tumor growth reduction and gene transfer in vivo

Cavitation nucleation agents (CNA) can greatly enhance DNA transfer and cell killing for therapeutically useful applications of nonthermal bioeffects of ultrasound (US). Renal carcinoma (RENCA) tumor cells were implanted and grown to about 400 microL tumor volumes on the hind legs of syngeneic Balb/c mice. Before treatment, mice were anesthetized, the tumor region was shaved and depilated, and a DNA plasmid coding for marker proteins was injected into the tumor. Two sets of tests were completed: the first set involved measurement of tumor growth for 4 days and use of a beta-galactosidase marker plasmid for localization of transfection, and the second set involved 2 days of growth and use of a luciferase marker plasmid for assessing overall protein expression. Either saline, Optison US contrast agent, a vaporizing perfluoropentane droplet suspension (SDS) or air bubble was also injected intratumorally at 10% of tumor volume as a CNA. In some tests, droplets or contrast agent were injected IV. Shock waves (SW) were generated from a spark-gap lithotripter at 7.4 MPa peak negative pressure amplitude. For sham exposure, tumor volume increased by a factor of 3.6 in 4 days. With 500-SW treatment, all the CNA reduced 4-day tumor growth about the same amount (to factors of 1.2 to 1.9). Marker gene expression was generally localized to the region around the needle injection path. All the agents, except saline, produced statistically significant increases of 11.8- to 14.6-fold in luciferase expression after 2 days, relative to sham exposure. IV injection of Optison or droplet nucleation agents before SW treatment reduced tumor growth to factors of 1.0 and 0.7, but did not increase transfection. These results demonstrate the efficacy of CNA in vivo and should lead to improved strategies for simultaneous SW tumor ablation and cancer gene therapy.

Combined shock-wave and immunogene therapy of mouse melanoma and renal carcinoma tumors

The effects of ultrasonic shock waves (SW), recombinant interleukin-12 (rIL-12) protein and DNA plasmids coding for interleukin-12 (pIL-12) were investigated on progression of mouse B16 melanoma and RENCA renal carcinoma tumors. Tumor cells were implanted and grown on the hind legs of syngeneic mice. Before treatment, mice were anesthetized and the tumor region was shaved and depilated. Air bubbles at 10% of tumor volume and an equal volume of phosphate buffered saline (PBS), either with rIL-12 or pIL-12 were injected into the tumor. SW treatment consisted of 500 SWs (7.4-MPa peak negative pressure) from a spark-gap lithotripter. Tumor volume was measured every other day and tumor growth was statistically modeled. SW treatment augmented by air injection induced a tumor growth delay for a few days immediately after exposure. Intratumor rIL-12 injection enhanced the SW effect on tumor progression, to the extent that a statistically significant increase in survival was realized in both tumor models. pIL-12 injection alone, which is known to produce some gene transfer, provided no detectable tumor-growth reduction. The combination of SW and pIL-12 injection provided a statistically significant reduction in tumor growth relative to SW alone for both tumor models. IL-12 expression due to SW-induced gene transfer was confirmed in ELISA assays. This research demonstrates a potentiality for further development of ultrasound (US)-enhanced cancer gene therapy.

Photodisruptive laser nucleation of ultrasonic cavitation for biomedical applications

Pulses of high intensity laser light, when focused into transparent materials, may produce localized electron-ion plasmas through optical breakdown. By simultaneously incorporating the resulting volume of vaporized material within the focal volume of a high intensity ultrasound source, the photodisruption (1.05 microm wavelength) void served as a nucleation site for ultrasonic cavitation. Dilute suspensions of canine erythrocytes in phosphate buffered saline were exposed in a flow-through exposure chamber and the percentage of lysed cells was used as a measure of the biologically effective cavitation activity produced in the chamber. Brief (about 30 micros) acoustic emissions were detected from the photodisruption alone (indicating laser nucleation of bubbles), but the cell lysis produced was undetectable against the background. However, combined exposure greatly increased both the duration of the acoustic emissions (up to 1.5 ms) and the amount of cell lysis above an ultrasonic pressure amplitude threshold of about 4.3 MPa at 2.5 MHz. The amount of cell lysis (sometimes approaching 100%) increased with increasing ultrasonic intensity, laser pulse energy and laser PRF. Addition of 5% serum albumin enhanced the effect, apparently by stabilizing bubbles and nuclei. Photodisruptive laser nucleation of ultrasonic cavitation can provide controlled and synergistic enhancement of bioeffects.