Thresholds for transient cavitation produced by pulsed ultrasound in a controlled nuclei environment

Lesion generation through ribs using histotripsy therapy without aberration correction

This study investigates the feasibility of using high-intensity pulsed therapeutic ultrasound, or histotripsy, to non-invasively generate lesions through the ribs. Histotripsy therapy mechanically ablates tissue through the generation of a cavitation bubble cloud, which occurs when the focal pressure exceeds a certain threshold. We hypothesize that histotripsy can generate precise lesions through the ribs without aberration correction if the main lobe retains its shape and exceeds the cavitation initiation threshold and the secondary lobes remain below the threshold. To test this hypothesis, a 750-kHz focused transducer was used to generate lesions in tissue-mimicking phantoms with and without the presence of rib aberrators. In all cases, 8000 pulses with 16 to 18 MPa peak rarefactional pressure at a repetition frequency of 100 Hz were applied without aberration correction. Despite the high secondary lobes introduced by the aberrators, high-speed imaging showed that bubble clouds were generated exclusively at the focus, resulting in well-confined lesions with comparable dimensions. Collateral damage from secondary lobes was negligible, caused by single bubbles that failed to form a cloud. These results support our hypothesis, suggesting that histotripsy has a high tolerance for aberrated fields and can generate confined focal lesions through rib obstacles without aberration correction.

Effect of microbubble contrast agent during high intensity focused ultrasound ablation on rabbit liver in vivo

OBJECTIVE

To evaluate the effect of a microbubble contrast agent (SonoVue) during HIFU ablation of a rabbit liver.

MATERIALS AND METHODS

HIFU ablations (intensity of 400W/cm(2) for 4s, six times, with a 5s interval between exposures) were performed upon 16 in vivo rabbit livers before and after intravenous injection of a microbubble contrast agent (0.8ml). A Wilcoxon signed rank test was used to compare mean ablation volume and time required to tissue ablation on real-time US. Shape of ablation and pattern of coagulative necrosis were analyzed by Fisher's exact test.

RESULTS

The volume of coagulative necrosis was significantly larger in the combination microbubble and HIFU group than in the HIFU alone group (P<0.05). Also, time to reach ablation was shorter in the combination microbubble and HIFU group than in the HIFU alone group (P<0.05). When analyzing the shape of tissue ablation, a pyramidal shape was more prevalently in the HIFU alone group compared to the combination microbubble and HIFU group (P<0.05). Following an analysis of the pattern of coagulative necrosis, non-cavitary necrosis was found in ten and cavitary necrosis in six of the samples in the combination microbubble and HIFU group. Conversely, non-cavitary necrosis occurred in all 16 samples in the HIFU alone group (P<0.05).

CONCLUSION

HIFU of in vivo rabbit livers with a microbubble contrast agent produced larger zones of ablation and more cavitary tissue necrosis than without the use of a microbubble contrast agent. Microbubble contrast agents may be useful in tissue ablation by enhancing the treatment effect of HIFU.

High-intensity focused ultrasound for hepatocellular carcinoma: a single-center experience

OBJECTIVE

This study aims to evaluate the outcome of patients with hepatocellular carcinoma (HCC) treated by high-intensity focused ultrasound (HIFU) in a single tertiary referral center.

BACKGROUND

HIFU is the latest developed local ablation technique for unresectable HCC. The initial experience on its efficacy is promising, but the survival benefit of patients undergoing HIFU for HCC is poorly defined.

METHODS

From October 2006 to December 2008, 49 patients received HIFU for unresectable HCC. Each patient underwent a single session of HIFU with a curative intent. Treatment efficacy and survival outcome were evaluated. Clinicopathologic factors affecting the primary technique effectiveness and overall survival rates were investigated by univariate analysis.

RESULTS

The median size of the treated tumors was 2.2 cm, ranging from 0.9 to 8 cm. The majority of patients had single tumors (n = 41, 83.6%). Thirty-one patients (63.2%) had artificial right pleural effusion during HIFU treatment to reduce damage to the lung and diaphragm. The hospital mortality rate was 2% (n = 1) and the complication rate was 8.1% (n = 4). The primary technique effectiveness rate was 79.5% (39 of 49 patients). It increased from 66.6% in the initial series to 89.2% in the last 28 patients. Tumor size (≥3.0 cm) was the significant risk factor affecting the complete ablation rate. The 1- and 3-year overall survival rates were 87.7% and 62.4%, respectively. Child-Pugh liver function grading was the significant prognostic factor influencing the overall survival rate.

CONCLUSIONS

HIFU is an effective treatment modality for unresectable HCC with a high technique effectiveness rate and favorable survival outcome.

Role of constituents of Optison in Optison-mediated gene transfection enhancement in skeletal muscle in vivo

OBJECTIVES

The mechanism by which Optison (an albumin-shelled, octafluoropropane gas-filled microbubble contrast agent; Amersham Health, Amersham, England) enhances gene transfection in skeletal muscle in vivo with or without ultrasound (US) is unclear. The possible mechanisms were investigated by experimenting with different constituents, both with and without US.

METHODS

Plasmid DNA (10 μg) encoding green fluorescent protein was mixed with Optison or its constituents dissolved in saline (in an equivalent concentration as in Optison) and injected into the tibialis anterior muscle of mice with or without adjunct US (1 MHz, 2 W/cm², 30 seconds, and 20% duty cycle). The efficiencies of green fluorescent protein transgene expression were determined under different experimental conditions: (1) plasmid plus saline as a negative control; (2) plasmid plus Optison as a positive control; (3) plasmid plus heat-treated Optison (without microbubbles); (4) plasmid plus human serum albumin; (5) plasmid plus N-acetyltryptophan; and (6) plasmid plus caprylic acid. Transfection efficiency was assessed by counting the maximum number of green fluorescent protein-positive fibers. Tissue damage was assessed by measuring the damaged area on serial sections.

RESULTS

Heat-treated Optison with or without US and albumin with US showed similarly high levels of transgene expression as Optison in mouse muscle without substantially increased tissue damage. N-Acetyltryptophan and caprylic acid had no effect on the delivery of plasmid green fluorescent protein into mouse muscle but instead showed the potential to increase tissue damage.

CONCLUSIONS

These data suggest that US and albumin separately potentiate transfection in this model. The combination of albumin and perfluoropropane is highly effective, which probably explains why Optison is so effective.

Active focal zone sharpening for high-precision treatment using histotripsy

The goal of this study is to develop a focal zone sharpening strategy that produces more precise lesions for pulsed cavitational ultrasound therapy, or histotripsy. Precise and well-confined lesions were produced by locally suppressing cavitation in the periphery of the treatment focus without affecting cavitation in the center. The local suppression of cavitation was achieved using cavitation nuclei preconditioning pulses to actively control cavitation in the periphery of the focus. A 1-MHz 513-element therapeutic array was used to generate both the therapy and the nuclei preconditioning pulses. For therapy, 10-cycle bursts at 100-Hz pulse repetition frequency with P-/P+ pressure of 21/76 MPa were delivered to the geometric focus of the therapeutic array. For nuclei preconditioning, a different pulse was delivered to an annular region immediately surrounding the focus before each therapy pulse. A parametric study on the effective pressure, pulse duration, and delivery time of the preconditioning pulse was conducted in red blood cell-gel phantoms, where cavitational damage was indicated by the color change resulting from local cell lysis. Results showed that a short-duration (20 μs) preconditioning pulse at a medium pressure (P-/P+ pressure of 7.2/13.6 MPa) delivered shortly before (30 μs) the therapy pulse substantially suppressed the peripheral damage by 77 ± 13% while complete fractionation in the focal center was maintained. High-speed imaging of the bubble cloud showed a substantial decrease in the maximum width of the bubble cloud by 48 ± 24% using focal zone sharpening. Experiments in ex vivo livers confirmed that highly confined lesions were produced in real tissues as well as in the phantoms. This study demonstrated the feasibility of active focal zone sharpening using cavitation nuclei preconditioning, allowing for increased treatment precision compared with the natural focal width of the therapy transducer.

Promoting inertial cavitation by nonlinear frequency mixing in a bifrequency focused ultrasound beam

Enhancing cavitation activity with minimal acoustic intensities could be interesting in a variety of therapeutic applications where mechanical effects of cavitation are needed with minimal heating of surrounding tissues. The present work focuses on the relative efficiency of a signal combining two neighbouring frequencies and a one-frequency signal for initiating ultrasound inertial cavitation. Experiments were carried out in a water tank, using a 550kHz piezoelectric composite spherical transducer focused on targets with 46μm roughness. The acoustic signal scattered, either by the target or by the cavitation bubbles, is filtered using a spectral and cepstral-like method to obtain an inertial cavitation activity measurement. The ultrasound excitations consist of 1.8ms single bursts of single frequency f(0)=550kHz excitation, in the monofrequency case, and of dual frequency f(1)=535kHz and f(2)=565kHz excitation, in the bifrequency case. It is shown that depending on the value of the monofrequency cavitation threshold intensity the bifrequency excitation can increase or reduce the cavitation threshold. The analysis of the thresholds indicates that the mechanisms involved are nonlinear. The progress of the cavitation activity beyond the cavitation threshold is also studied. The slope of the cavitation activity considered as a function of the acoustic intensity is always steeper in the case of the bifrequency excitation. This means that the delimitation of the region where cavitation occurs should be cleaner than with a classical monofrequency excitation.

Ultrasound-assisted thrombolysis for stroke therapy: better thrombus break-up with bubbles

BACKGROUND AND PURPOSE

Ultrasound has been shown to increase recombinant tissue plasminogen activator thrombolysis through stable cavitation, or sustained bubble activity, but this mechanism needs further optimization. Use of low-frequency ultrasound in combination with microbubbles stabilized against dissolution, in the form of ultrasound contrast agents, has resulted in greater lytic efficacy in vitro. Summary of Review-This article reviews the motivation for developing ultrasound-enhanced thrombolysis and the existing evidence for its potential as an intervention for ischemic stroke. Stable cavitation is discussed and current in vitro and ex vivo studies of bubble-mediated recombinant tissue plasminogen activator clot lysis are summarized.

CONCLUSIONS

Ultrasound-driven stable cavitation nucleated by an infusion of an echo contrast agent facilitates recombinant tissue plasminogen activator thrombolysis. Optimization of this gently effervescent phenomenon has the potential to reduce the morbidity and mortality of victims of ischemic stroke.

Doxorubicin as a molecular nanotheranostic agent: effect of doxorubicin encapsulation in micelles or nanoemulsions on the ultrasound-mediated intracellular delivery and nuclear trafficking

Doxorubicin (DOX) is one of the most commonly used chemotherapeutic drugs and is a popular research tool due to the inherent fluorescence of the DOX molecule. After DOX injection, fluorescence imaging of organs or cells can provide information on drug biodistribution. Therapeutic and imaging capabilities combined in a DOX molecule make it an excellent theranostic agent. However, DOX fluorescence depends on a number of factors that should be taken into consideration when interpreting results of DOX fluorescence measurements. Discussing these problems is the main thrust of the current paper. The sensitivity of DOX fluorescence intensity to DOX concentration, local microenvironment, and interaction with model cellular components is illustrated by fluorescence spectra of paired DOX/phospholipid, DOX/histone, DOX/DNA, and triple DOX/histone/DNA and DOX/phospholipid/DNA systems. DOX fluorescence is dramatically quenched upon intercalation into the DNA; DOX fluorescence is also self-quenched at high concentrations of molecularly dissolved DOX; in contrast, DOX fluorescence is increased after binding to the histone or partitioning into the phospholipid phase of PEG-phospholipid micelles or hydrophobic cores of polymeric micelles. While flow cytometry is commonly used for characterization of DOX intracellular uptake, the above aspects of DOX fluorescence may significantly complicate interpretation of flow cytometry results. High cell fluorescence measured by flow cytometry may provide deceptive information on the actual intracellular DOX concentration and may not correlate with the therapeutic efficacy if DOX does not penetrate into the site of action in cell nuclei. These problems are illustrated in the experiments on the intracellular trafficking of DOX encapsulated in poly(ethylene glycol)-co-polycaprolactone (PEG-PCL) micelles or PEG-PCL stabilized perfluorocarbon nanodroplets, with and without the application of ultrasound used as an external trigger. For efficient encapsulation in micelle cores, DOX is usually deprotonated, which removes the positive charge and enhances hydrophobicity of DOX molecule. It was found that the deprotonated DOX accumulated in the cell cytoplasm but did not penetrate into the cell nuclei. The same was true for the DOX encapsulated in micelles or nanodroplets, which may explain their low therapeutic efficacy in the absence of ultrasound. Ultrasound triggers DOX trafficking into the cell nuclei, which is especially pronounced in the presence of nanoemulsions that convert into microbubbles under the ultrasound action. Microbubble cavitation results in the transient permeabilization of both plasma and nuclear membranes, thus allowing DOX penetration into the cell nuclei, which dramatically enhances therapeutic efficacy of DOX-loaded nanodroplet systems.

Current and Future Clinical Applications of High-Intensity Focused Ultrasound (HIFU) for Pancreatic Cancer

High-intensity focused ultrasound (HIFU) is a novel therapeutic modality that permits noninvasive treatment of various benign and malignant solid tumors, including prostatic cancer, uterine fibroids, hepatic tumors, renal tumors, breast cancers, and pancreatic cancers. Several preclinical and clinical studies have investigated the safety and efficacy of HIFU for treating solid tumors, including pancreatic cancer. The results of nonrandomized studies of HIFU therapy in patients with pancreatic cancer have suggested that HIFU treatment can effectively alleviate cancer-related pain without any significant complications. This noninvasive method of delivering ultrasound energy into the body has recently been evolving from a method for purely thermal ablation to harnessing the mechanical effects of HIFU to induce a systemic immune response and to enhance targeted drug delivery. This review provides a brief overview of HIFU, describes current clinical applications of HIFU for pancreatic cancer, and discusses future applications and challenges.

In vitro characterization of perfluorocarbon droplets for focused ultrasound therapy

Focused ultrasound therapy can be enhanced with microbubbles by thermal and cavitation effects. However, localization of treatment is difficult as bioeffects can occur outside of the target region. Spatial control of bubbles can be achieved by ultrasound-induced conversion of liquid perfluorocarbon droplets to gas bubbles. This study was undertaken to determine the acoustic parameters for bubble production by droplet conversion and how it depends on the acoustic conditions and droplet physical parameters. Lipid-encapsulated droplets containing dodecafluoropentane were manufactured with sizes ranging from 1.9 to 7.2 microm in diameter and diluted to a concentration of 8 x 10(6) droplets mL(-1). The droplets were sonicated in vitro with a focused ultrasound transducer and varying frequency and exposure under flow conditions through an acoustically transparent vessel. The sonications were 10 ms in duration at frequencies of 0.578, 1.736 and 2.855 MHz. The pressure threshold for droplet conversion was measured with an active transducer operating in pulse-echo mode and simultaneous measurements of broadband acoustic emissions were performed with passive acoustic detection. The results show that droplets cannot be converted at low frequency without broadband emissions occurring. However, the pressure threshold for droplet conversion decreased with increasing frequency, exposure and droplet size. The pressure threshold for broadband emissions was independent of the droplet size and was 2.9, 4.4 and 5.3 MPa for 0.578, 1736 and 2.855 MHz, respectively. In summary, we have demonstrated that droplet conversion is feasible for clinically relevant sized droplets and acoustic exposures.

Ultrasound-enhanced delivery of targeted echogenic liposomes in a novel ex vivo mouse aorta model

The goal of this study was to determine whether targeted, Rhodamine-labeled echogenic liposomes (Rh-ELIP) containing nanobubbles could be delivered to the arterial wall, and whether 1-MHz continuous wave ultrasound would enhance this delivery profile. Aortae excised from apolipoprotein-E-deficient (n=8) and wild-type (n=8) mice were mounted in a pulsatile flow system through which Rh-ELIP were delivered in a stream of bovine serum albumin. Half the aortae from each group were treated with 1-MHz continuous wave ultrasound at 0.49 MPa peak-to-peak pressure, and half underwent sham exposure. Ultrasound parameters were chosen to promote stable cavitation and avoid inertial cavitation. A broadband hydrophone was used to monitor cavitation activity. After treatment, aortic sections were prepared for histology and analyzed by an individual blinded to treatment conditions. Delivery of Rh-ELIP to the vascular endothelium was observed, and sub-endothelial penetration of Rh-ELIP was present in five of five ultrasound-treated aortae and was absent in those not exposed to ultrasound. However, the degree of penetration in the ultrasound-exposed aortae was variable. There was no evidence of ultrasound-mediated tissue damage in any specimen. Ultrasound-enhanced delivery within the arterial wall was demonstrated in this novel model, which allows quantitative evaluation of therapeutic delivery.

Sonothrombolysis in the management of acute ischemic stroke

Multiple in vitro and animal models have demonstrated the efficacy of ultrasound to enhance fibrinolysis. Mechanical pressure waves produced by ultrasound energy improve the delivery and penetration of alteplase (recombinant tissue plasminogen activator [tPA]) inside the clot. In human stroke, the CLOTBUST phase II trial showed that the combination of alteplase plus 2 hours of continuous transcranial Doppler (TCD) increased recanalization rates, producing a trend toward better functional outcomes compared with alteplase alone. Other small clinical trials also showed an improvement in clot lysis when transcranial color-coded sonography was combined with alteplase. In contrast, low-frequency ultrasound increased the symptomatic intracranial hemorrhage rate in a clinical trial. Administration of microbubbles (MBs) may further enhance the effect of ultrasound on thrombolysis by lowering the ultrasound-energy threshold needed to induce acoustic cavitation. Initial clinical trials have been encouraging, and a multicenter international study, TUCSON, determined a dose of newly developed MBs that can be safely administered with alteplase and TCD. Even in the absence of alteplase, the ultrasound energy, with or without MBs, could increase intrinsic fibrinolysis. The intra-arterial administration of ultrasound with the EKOS NeuroWave catheter is another ultrasound application for acute stroke that is currently being studied in the IMS III trial. Operator-independent devices, different MB-related techniques, and other ultrasound parameters for improving and spreading sonothrombolysis are being tested.

Sonoporation, drug delivery, and gene therapy

Ultrasound is a very effective modality for drug delivery and gene therapy because energy that is non-invasively transmitted through the skin can be focused deeply into the human body in a specific location and employed to release drugs at that site. Ultrasound cavitation, enhanced by injected microbubbles, perturbs cell membrane structures to cause sonoporation and increases the permeability to bioactive materials. Cavitation events also increase the rate of drug transport in general by augmenting the slow diffusion process with convective transport processes. Drugs and genes can be incorporated into microbubbles, which in turn can target a specific disease site using ligands such as the antibody. Drugs can be released ultrasonically from microbubbles that are sufficiently robust to circulate in the blood and retain their cargo of drugs until they enter an insonated volume of tissue. Local drug delivery ensures sufficient drug concentration at the diseased region while limiting toxicity for healthy tissues. Ultrasound-mediated gene delivery has been applied to heart, blood vessel, lung, kidney, muscle, brain, and tumour with enhanced gene transfection efficiency, which depends on the ultrasonic parameters such as acoustic pressure, pulse length, duty cycle, repetition rate, and exposure duration, as well as microbubble properties such as size, gas species, shell material, interfacial tension, and surface rigidity. Microbubble-augmented sonothrombolysis can be enhanced further by using targeting microbubbles.

Cavitation properties of block copolymer stabilized phase-shift nanoemulsions used as drug carriers

Cavitation properties of block copolymer stabilized perfluoropentane nanoemulsions have been investigated. The nanoemulsions were stabilized by two biodegradable amphiphilic block copolymers differing in the structure of the hydrophobic block, poly(ethylene oxide)-co-poly(L-lactide) (PEG-PLLA) and poly(ethylene oxide)-co-polycaprolactone (PEG-PCL). Cavitation parameters were measured in liquid emulsions and gels as a function of ultrasound pressure for unfocused or focused 1-MHz ultrasound. Acoustic droplet vaporization preceded generation of acoustic cavitation in liquid matrices and gels. Both stable and inertial cavitation was observed for focused ultrasound while only stable cavitation was observed for unfocused ultrasound.

Ultrasound imaging and contrast agents: A safe alternative to MRI?

Microbubble contrast media are used to enhance ultrasound images. Because ultrasound is a real-time investigation, contrast-enhanced ultrasound offers possibilities for perfusion imaging. This review is conducted to evaluate the safety of contrast-enhanced ultrasound and its possible role in medical imaging. The safety of diagnostic ultrasound is still an important field of research. The wanted and unwanted effects of ultrasound and microbubble contrast media as well as the effects of ultrasound on these microbubbles are described. Furthermore, some of the possible applications and indications of contrast-enhanced ultrasound will be discussed. The shared advantages of MRI and ultrasound are the use of non-ionizing radiation and non-nephrotoxic contrast media. From this review it can be concluded that, for certain indications, contrast enhanced ultrasound could be a safe alternative to MRI and a valuable addition to medical imaging.

Cardiovascular therapeutic uses of targeted ultrasound contrast agents

The therapeutic use of ultrasound contrast agents (UCAs) is an emerging methodology with high potential for enhanced directed therapeutic gene, bioactive gas, drug, and stem cell delivery. Ultrasound-targeted microbubble destruction has already demonstrated feasibility for plasmid DNA delivery. Similarly, therapeutic ultrasound for thrombolysis treatment has been taken into the clinical setting, and the addition of UCAs for therapeutic delivery or enhanced effect through cavitation is a natural progression to this investigation. However, as with any new technique, safety needs to be first demonstrated before translation into clinical practice. This review article will focus on the development of UCAs for cardiac and vascular therapeutics as well as the limitations/concerns for the use of therapeutic ultrasound in clinical medicine in order to lay a foundation for investigators planning to enter this exciting field or for those who want to broaden their understanding.

The utilization of ultrasound and microbubbles for therapy in acute coronary syndromes

Ultrasound has become a useful high resolution imaging modality for examining the cardiac microcirculation. With the use of microbubbles as an ultrasound contrast agent, ultrasound can be utilized to image the microcirculation and detect capillary flow abnormalities in acute ischaemia. A wide range of ultrasound frequencies (including those used for diagnostic transthoracic imaging) have also been utilized therapeutically to augment the effectiveness of fibrinolytic therapy in ST-segment elevation myocardial infarction (STEMI). Ultrasound and microbubbles are now being explored as methods of improving both microcirculatory and epicardial flow in acute STEMI. This article will review the mechanisms by which ultrasound and microbubbles assist in thrombus detection and dissolution. In addition, the pre-clinical studies utilizing transthoracic ultrasound as a therapeutic entity in acute STEMI will be reviewed. Clinical studies, completed and ongoing, will also be presented.

The role of inertial cavitation in acoustic droplet vaporization

The vaporization of a superheated droplet emulsion into gas bubbles using ultrasound--termed acoustic droplet vaporization (ADV)--has potential therapeutic applications in embolotherapy and drug delivery. The optimization of ADV for therapeutic applications can be enhanced by understanding the physical mechanisms underlying ADV, which are currently not clearly elucidated. Acoustic cavitation is one possible mechanism. This paper investigates the relationship between ADV and inertial cavitation (IC) thresholds (measured as peak rarefactional pressures) by studying parameters that are known to influence the IC threshold. These parameters include bulk fluid properties such as gas saturation, temperature, viscosity, and surface tension; droplet parameters such as degree of superheat, surfactant type, and size; and acoustic properties such as pulse repetition frequency and pulse width. In all cases the ADV threshold occurred at a lower rarefactional pressure than the IC threshold, indicating that the phase transition occurs before IC events. The viscosity and temperature of the bulk fluid are shown to influence both thresholds directly and inversely, respectively. An inverse trend is observed between threshold and diameter for droplets in the 1 to 2.5 microm range. Based on a choice of experimental parameters, it is possible to achieve ADV with or without IC.

An in vitro study of the correlation between bubble distribution, acoustic emission, and cell damage by contrast ultrasound

The objective of this study was to investigate the influences of total exposure duration and pulse-to-pulse bubble distribution on contrast-mediated cell damage. Murine macrophage cells were grown as monolayers on thin polyester sheets. Contrast agent microbubbles were attached to these cells by incubation. Focused ultrasound exposures (P(r) = 2 MPa) were implemented at a frequency of 2.25 MHz with 46 cycle pulses and pulse repetition frequencies (PRF) of 1 kHz, 500 Hz, 100 Hz, and 10 Hz in a degassed water bath at 10 or 100 pulses. A 1 MHz receive transducer measured the scattered signal. The frequency spectrum was normalized to a control spectrum from linear scatterers. Photomicrographs were captured before, during, and after exposure at a frame rate of 2000 fps and a pixel resolution of 960 x 720. Results clearly show that cell death is increased, up to 60%, by increasing total exposure duration from 0 ms to 100 ms. There was an increasing difference in cell damage between a 10-pulse exposure and a 100-pulse exposure with increasing PRF. The greatest change in damage occurred at 1000 Hz PRF with a 53% increase between 10-pulse and 100-pulse exposures. For each pulse from 0 to 10, an overlay of the 2 mum bubble count with corresponding emission shows consistent behavior in its pulse-to-pulse changes, indicating a correlation between acoustic emission, bubble distribution, and cell damage.

Ultrasound-enhanced thrombolysis with tPA-loaded echogenic liposomes

BACKGROUND AND PURPOSE

Currently, the only FDA-approved therapy for acute ischemic stroke is the administration of recombinant tissue plasminogen activator (tPA). Echogenic liposomes (ELIP), phospholipid vesicles filled with gas and fluid, can be manufactured to incorporate tPA. Also, transcranial ultrasound-enhanced thrombolysis can increase the recanalization rate in stroke patients. However, there is little data on lytic efficacy of combining ultrasound, echogenic liposomes, and tPA treatment. In this study, we measure the effects of pulsed 120-kHz ultrasound on the lytic efficacy of tPA and tPA-incorporating ELIP (t-ELIP) in an in-vitro human clot model. It is hypothesized that t-ELIP exhibits similar lytic efficacy to that of rt-PA.

METHODS

Blood was drawn from 22 subjects after IRB approval. Clots were made in 20-microL pipettes, and placed in a water tank for microscopic visualization during ultrasound and drug treatment. Clots were exposed to combinations of [tPA]=3.15 microg/ml, [t-ELIP]=3.15 microg/ml, and 120-kHz ultrasound for 30 minutes at 37 degrees C in human plasma. At least 12 clots were used for each treatment. Clot lysis over time was imaged and clot diameter was measured over time, using previously developed imaging analysis algorithms. The fractional clot loss (FCL), which is the decrease in mean clot width at the end of lytic treatment, was used as a measure of lytic efficacy for the various treatment regimens.

RESULTS

The fractional clot loss FCL was 31% (95% CI: 26-37%) and 71% (56-86%) for clots exposed to tPA alone or tPA with 120 kHz ultrasound. Similarly, FCL was 48% (31-64%) and 89% (76-100%) for clots exposed to t-ELIP without or with ultrasound.

CONCLUSIONS

The lytic efficacy of tPA containing echogenic liposomes is comparable to that of tPA alone. The addition of 120 kHz ultrasound significantly enhanced lytic treatment efficacy for both tPA and t-ELIP. Liposomes loaded with tPA may be a useful adjunct in lytic treatment with tPA.

Effects of transcranial ultrasound and intravenous microbubbles on blood brain barrier permeability in a large animal model

We sought to determine whether transtemporal-applied 1-MHz ultrasound-induced microbubble destruction may be a safe method of transiently altering blood brain barrier (BBB) permeability for drug delivery in a large animal model. Endothelial cells are an integral component of the BBB but also prevent passage of potentially therapeutic drugs. Ultrasound-mediated destruction (UMD) of microbubbles has been shown to disrupt this barrier in small animals when ultrasound is delivered through bone windows. However, the effects of temporal bone attenuation and scattering in a large animal may limit the clinical application of such a technique. Twenty-four pigs were studied. One-MHz pulsed-wave ultrasound at 2.0 W/cm(2) (20% duty cycle) across the temporal bone was applied for 30 min after intravenous injections of either albumin-coated perfluorocarbon microbubble (PESDA, 8 pigs), lipid-encapsulated perfluorocarbon microbubbles (LEMB, 8 pigs) or ultrasound alone (8 pigs). BBB leak was quantified at 30 and 120 min after insonation using Evans blue. Serial magnetic resonance imaging (MRI) was performed in nine of the pigs (3 for each group) to quantify Gadolinium leak within the parenchyma. Peak negative pressures decreased ten-fold when ultrasound was transmitted across the pig temporal bone. Despite this, spectrophotometric analysis showed that both IV LEMB and PESDA combined with transtemporal ultrasound resulted in a significant increase in Evans blue extravasation across BBB of the treated side at 30 min after insonation (p < 0.001; compared with ultrasound alone) but not at 120 min. There was significant retention of Gadolinium within the insonified parenchyma at 60 and 90 min after insonation, but not at 120 min. Oxygen saturation and arterial pressures were not changed after any microbubble injection. Intravenous microbubbles, combined with transtemporal ultrasound, can transiently increase BBB permeability in a large animal. This induced opening of BBB is reversible and may be a safe noninvasive method of achieving drug or gene delivery across the BBB.

Renal thermal ablative therapy

Energy targeting is greatly enhanced through imaging modalities, which greatly assist needle placement or energy delivery to the optimal location for maximal effectiveness. When vital structures obscure access to the renal lesion, laparoscopic mobilization of these structures with direct visualization of the tumor can increase the likelihood of ablation success and minimize complication risk. Ablative therapies are attractive because of their minimal impact on patient quality of life in addition to their morbidity and cost. Although they show promise of efficacy, they must be evaluated with long-term follow-up before they are considered the standard of oncologic care. Renal masses can be treated with a laparoscopic or percutaneous approach depending on tumor location, size, and the available technology and experience of the center.

Effect of modulated ultrasound parameters on ultrasound-induced thrombolysis

The potential of ultrasound to enhance enzyme-mediated thrombolysis by application of constant operating parameters (COP) has been widely demonstrated. In this study, the effect of ultrasound with modulated operating parameters (MOP) on enzyme-mediated thrombolysis was investigated. The MOP protocol was applied to an in vitro model of thrombolysis. The results were compared to a COP with the equivalent soft tissue thermal index (TIS) over the duration of ultrasound exposure of 30 min (p < 0.14). To explore potential differences in the mechanism responsible for ultrasound-induced thrombolysis, a perfusion model was used to measure changes in average fibrin pore size of clot before, after and during exposure to MOP and COP protocols and cavitational activity was monitored in real time for both protocols using a passive cavitation detection system. The relative lysis enhancement by each COP and MOP protocol compared to alteplase alone yielded values of 33.69 +/- 12.09% and 63.89 +/- 15.02% in a thrombolysis model, respectively (p < 0.007). Both COP and MOP protocols caused an equivalent significant increase in average clot pore size of 2.09 x 10(-2) +/- 0.01 microm and 1.99 x 10(-2) +/- 0.004 microm, respectively (p < 0.74). No signatures of inertial or stable cavitation were observed for either acoustic protocol. In conclusion, due to mechanisms other than cavitation, application of ultrasound with modulated operating parameters has the potential to significantly enhance the relative lysis enhancement compared to application of ultrasound with constant operating parameters.

Do bubble characteristics affect recanalization in stroke patients treated with microbubble-enhanced sonothrombolysis?

Administration of microbubbles (MB) may augment the effect of ultrasound-enhanced systemic thrombolysis in acute stroke. Bubble structural characteristics may influence the effect of MB on sonothrombolysis. We aimed to compare the effects of galactose-based air-filled MB (Levovist) and sulphur hexafluoride-filled MB (Sonovue) on recanalization and clinical outcome. One hundred thirty-eight i.v. recombinant tissue plasminogen activator-(tPA-) treated patients with middle cerebral artery (MCA) occlusion were studied. Presence and location of arterial occlusion and recanalization (RE) were assessed using the thrombolysis in brain ischemia (TIBI) flow grading system. Patients underwent 2 h of continuous transcranial Doppler (TCD) monitoring and received three bolus of MB after 2, 20 and 40 min of tPA bolus. Ninety-one patients received Levovist (LV) and 47 received Sonovue (SV). NIHSS scores were obtained at baseline and after 24 h. Modified Rankin Scale (mRS) score was used to assess outcome at 3 mo. Median admission NIHSS was 17. On TCD, 96 (69.6%) patients had a proximal and 42 (30.4%) a distal MCA occlusion. Age, baseline NIHSS, clot location, stroke subtypes and time to treatment were similar between LV and SV groups. Recanalization rates after 1 h (32.2%/35.6%), 2 h (50.0%/46.7%) and 6 h (63.8%/54.5%) were similar in LV/SV groups (p > 0.3). Clinical improvement (NIHSS decrease >or= 4 points) at 24 h was similar in both groups (54.9%/51.1%, p = 0.400), as well as symptomatic intracranial haemorrhage rate (3.3%/2.1%, p = 0.580) and in-hospital mortality (8.1%/9.3%, p = 0.531). Similarly, the type of MB administered did not affect long-term outcome after sonothrombolysis. Forty-four percent of patients in the LV group and 48.5% in the SV group achieved functional independence (mRS <or= 2) at 3 mo (p = 0.440). MB administration during sonothrombolysis is associated with a high RE rate. However, RE rates, clinical course and long-term outcome are comparable when administering galactose-based air-filled MB (Levovist) or sulphur hexafluoride-filled MB (Sonovue).

Toward a reference ultrasonic cavitation vessel: Part 2--investigating the spatial variation and acoustic pressure threshold of inertial cavitation in a 25 kHz ultrasound field

As part of an ongoing project to establish a reference facility for acoustic cavitation at the National Physical Laboratory (NPL), carefully controlled studies on a 25 kHz, 1.8 kW cylindrical vessel are described. Using a patented high-frequency acoustic emission detection method and a sonar hydrophone, results are presented of the spatial variation of inertial acoustic cavitation with increasing peak-negative pressure. Results show that at low operating levels, inertial acoustic cavitation is restricted to, and is strongly localized on, the vessel axis. At intermediate power settings, inertial acoustic cavitation also occurs close to the vessel walls, and at higher settings, a complex spatial variation is seen that is not apparent in measurements of the 25 kHz driving field alone. At selected vessel locations, a systematic investigation of the inertial cavitation threshold is described. This was carried out by making simultaneous measurements of the peak-negative pressures leading to inertial cavitation and the resultant MHz-frequency emissions, and indicates an inertial cavitation threshold of 101 kPa +/- 14% (estimated expanded uncertainty). However, an intermediate threshold at 84 kPa +/- 14% (estimated expanded uncertainty) is also seen. The results are discussed alongside theoretical predictions and recent experimental findings.

Ultrasound enhanced thrombolysis in acute arterial ischemia

In vitro and animal studies have shown that thrombolysis with intravenous tissue plasminogen activator (tPA) can be enhanced with ultrasound. Ultrasound delivers mechanical pressure waves to the clot, thus exposing more thrombus surface to circulating drug. Moreover, intravenous gaseous microspheres with ultrasound have been shown to be a potential alternative to fibrinolytic agents to recanalize discrete peripheral thrombotic arterial occlusions or acute arteriovenous graft thromboses. Small phase I-II randomized and non-randomized clinical trials have shown promising results concerning the potential applications of ultrasound-enhanced thrombolysis in the setting of acute cerebral ischemia. CLOTBUST was an international four-center phase II trial, which demonstrated that, in patients with acute ischemic stroke, transcranial Doppler (TCD) monitoring augments tPA-induced arterial recanalization (sustained complete recanalization rates: 38% vs. 13%) with a non-significant trend toward an increased rate of clinical recovery from stroke, as compared with placebo. The rates of symptomatic intracerebral hemorrhage (sICH) were similar in the active and placebo group (4.8% vs. 4.8%). Smaller single-center clinical trials using transcranial color-coded sonography (TCCD) reported recanalization rates ranging from 27% to 64% and sICH rates of 0-18%. A separate clinical trial evaluating the safety and efficacy of therapeutic low-frequency ultrasound was discontinued because of a concerning sICH rate of 36% in the active group. To further enhance the ability of tPA to break up thrombi, current ongoing clinical trials include phase II studies of a single beam 2 MHz TCD with perflutren-lipid microspheres. Moreover, potential enhancement of intra-arterial tPA delivery is being clinically tested with 1.7-2.1 MHz pulsed wave ultrasound (EKOS catheter) in ongoing phase II-III clinical trials. Intravenous platelet-targeted microbubbles with low-frequency ultrasound are currently investigated as a rapid noninvasive technique to identify thrombosed intracranial and peripheral vessels. Multi-national dose escalation studies of microspheres and the development of an operator independent ultrasound device are underway.

Delivery by shock waves of active principle embedded in gelatin-based capsules

PURPOSE

Delivering a drug close to the targeted cells improves its benefit versus risk ratio. A possible method for local drug delivery is to encapsulate the drug into solid microscopic carriers and to release it by ultrasound. The objective of this work was to use shock waves for delivering a molecule loaded in polymeric microcapsules.

MATERIAL AND METHODS

Ethyl benzoate (EBZ) was encapsulated in spherical gelatin shells by complex coacervation. A piezocomposite shock wave generator (120 mm in diameter, focused at 97 mm, pulse length 1.4 micros) was used for sonicating the capsules and delivering the molecule. Shock parameters (acoustic pressure, number of shocks and shock repetition frequency) were varied in order to measure their influence on EBZ release. A cavitation-inhibitor liquid (Ablasonic) was then used to evaluate the role of cavitation in the capsule disruption.

RESULTS

The measurements showed that the mean quantity of released EBZ was proportional to the acoustic pressure of the shock wave (r2 > 0.99), and increased with the number of applied shocks. Up to 88% of encapsulated EBZ could be released within 4 min only (240 shocks, 1 Hz). However, the quantity of released EBZ dropped at high shock rates (above 2Hz). Ultrasound imaging sequences showed that cavitation clouds might form, at high shock rates, along the acoustic axis making the exposure inefficient. Measurements done in Ablasonic showed that cavitation plays a major role in microcapsules disruption.

CONCLUSIONS

In this study, we designed polymeric capsules that can be disrupted by shock waves. This type of microcapsule is theoretically a suitable vehicle for carrying hydrophobic drugs. Following these positive results, encapsulation of drugs is considered for further medical applications.

Micelles and nanoparticles for ultrasonic drug and gene delivery

Drug delivery research employing micelles and nanoparticles has expanded in recent years. Of particular interest is the use of these nanovehicles that deliver high concentrations of cytotoxic drugs to diseased tissues selectively, thus reducing the agent's side effects on the rest of the body. Ultrasound, traditionally used in diagnostic medicine, is finding a place in drug delivery in connection with these nanoparticles. In addition to their non-invasive nature and the fact that they can be focused on targeted tissues, acoustic waves have been credited with releasing pharmacological agents from nanocarriers, as well as rendering cell membranes more permeable. In this article, we summarize new technologies that combine the use of nanoparticles with acoustic power both in drug and gene delivery. Ultrasonic drug delivery from micelles usually employs polyether block copolymers and has been found effective in vivo for treating tumors. Ultrasound releases drug from micelles, most probably via shear stress and shock waves from the collapse of cavitation bubbles. Liquid emulsions and solid nanoparticles are used with ultrasound to deliver genes in vitro and in vivo. The small packaging allows nanoparticles to extravasate into tumor tissues. Ultrasonic drug and gene delivery from nanocarriers has tremendous potential because of the wide variety of drugs and genes that could be delivered to targeted tissues by fairly non-invasive means.

Response of contrast agents to ultrasound

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

Ultrasound with microbubbles enhances gene expression of plasmid DNA in the liver via intraportal delivery

Current ultrasound (US)-mediated gene delivery methods are inefficient due, in part, to a lack of US optimization. We systematically explored the use of microbubbles (MBs), US parameters and plasmid delivery routes to improve gene transfer into the mouse liver. Co-presentation of plasmid DNA (pDNA), 10% Optison MBs and pulsed 1-MHz US at a peak negative pressure of 4.3 MPa significantly increased luciferase gene expression with pDNA delivered by intrahepatic injection to the left liver lobe. Intraportal injection delivered pDNA and MBs to the whole liver; with insonation, all lobes expressed the transgene, thus increasing total gene expression. Gene expression was also dependent on acoustic pressure over the range of 0-4.3 MPa, with a peak effect at 3 MPa. An average of 85-fold enhancement in gene delivery was achieved. No enhancement was observed below 0.25 MPa. Increasing pulse length while decreasing pulse repetition frequency and exposure time to maintain a constant total energy during exposure did not further improve transfection efficiency, nor did extend the US exposure pre- or postinjection of pDNA. The results indicate that coupled with MBs, US can more efficiently and dose-dependently enhance gene expression from pDNA delivered via portal vein injection by an acoustic mechanism of inertial cavitation.

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.

High-intensity focused ultrasound: current potential and oncologic applications

OBJECTIVE

The objective of this article is to introduce the reader to the principles and applications of high-intensity focused ultrasound (HIFU).

CONCLUSION

Although a great deal about HIFU physics is understood, its clinical applications are currently limited, and multiple trials are underway worldwide to determine its efficacy.

Cavitation threshold of microbubbles in gel tunnels by focused ultrasound

The investigation of inertial cavitation in micro-tunnels has significant implications for the development of therapeutic applications of ultrasound such as ultrasound-mediated drug and gene delivery. The threshold for inertial cavitation was investigated using a passive cavitation detector with a center frequency of 1 MHz. Micro-tunnels of various diameters (90 to 800 microm) embedded in gel were fabricated and injected with a solution of Optison(trade mark) contrast agent of concentrations 1.2% and 0.2% diluted in water. An ultrasound pulse of duration 500 ms and center frequency 1.736 MHz was used to insonate the microbubbles. The acoustic pressure was increased at 1-s intervals until broadband noise emission was detected. The pressure threshold at which broadband noise emission was observed was found to be dependent on the diameter of the micro-tunnels, with an average increase of 1.2 to 1.5 between the smallest and the largest tunnels, depending on the microbubble concentration. The evaluation of inertial cavitation in gel tunnels rather than tubes provides a novel opportunity to investigate microbubble collapse in a situation that simulates in vivo blood vessels better than tubes with solid walls do.

High speed imaging of bubble clouds generated in pulsed ultrasound cavitational therapy--histotripsy

Our recent studies have demonstrated that mechanical fractionation of tissue structure with sharply demarcated boundaries can be achieved using short (< 20 micros), high intensity ultrasound pulses delivered at low duty cycles. We have called this technique histotripsy. Histotripsy has potential clinical applications where noninvasive tissue fractionation and/or tissue removal are desired. The primary mechanism of histotripsy is thought to be acoustic cavitation, which is supported by a temporally changing acoustic backscatter observed during the histotripsy process. In this paper, a fast-gated digital camera was used to image the hypothesized cavitating bubble cloud generated by histotripsy pulses. The bubble cloud was produced at a tissue-water interface and inside an optically transparent gelatin phantom which mimics bulk tissue. The imaging shows the following: (1) Initiation of a temporally changing acoustic backscatter was due to the formation of a bubble cloud; (2) The pressure threshold to generate a bubble cloud was lower at a tissue-fluid interface than inside bulk tissue; and (3) at higher pulse pressure, the bubble cloud lasted longer and grew larger. The results add further support to the hypothesis that the histotripsy process is due to a cavitating bubble cloud and may provide insight into the sharp boundaries of histotripsy lesions.

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.

Effects of acoustic parameters on bubble cloud dynamics in ultrasound tissue erosion (histotripsy)

High intensity pulsed ultrasound can produce significant mechanical tissue fractionation with sharp boundaries ("histotripsy"). At a tissue-fluid interface, histotripsy produces clearly demarcated tissue erosion and the erosion efficiency depends on pulse parameters. Acoustic cavitation is believed to be the primary mechanism for the histotripsy process. To investigate the physical basis of the dependence of tissue erosion on pulse parameters, an optical method was used to monitor the effects of pulse parameters on the cavitating bubble cloud generated by histotripsy pulses at a tissue-water interface. The pulse parameters studied include pulse duration, peak rarefactional pressure, and pulse repetition frequency (PRF). Results show that the duration of growth and collapse (collapse cycle) of the bubble cloud increased with increasing pulse duration, peak rarefactional pressure, and PRF when the next pulse arrived after the collapse of the previous bubble cloud. When the PRF was too high such that the next pulse arrived before the collapse of the previous bubble cloud, only a portion of histotripsy pulses could effectively create and collapse the bubble cloud. The collapse cycle of the bubble cloud also increased with increasing gas concentration. These results may explain previous in vitro results on effects of pulse parameters on tissue erosion.

Effect of surfactants on inertial cavitation activity in a pulsed acoustic field

It has previously been reported that the addition of low concentrations of ionic surfactants enhances the steady-state sonoluminescence (SL) intensity relative to water (Ashokkumar; et al. J. Phys. Chem. B 1997, 101, 10845). In the current study, both sonoluminescence and passive cavitation detection (PCD) were used to examine the acoustic cavitation field generated at different acoustic pulse lengths in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS). A decrease in the SL intensity was observed in the presence of low concentrations of SDS and short acoustic pulse lengths. Under these conditions, the inhibition of bubble coalescence by SDS leads to a population of smaller bubbles, which dissolve during the pulse "off time". As the concentration of surfactant was increased at this pulse length, an increase in the acoustic cavitation activity was observed. This increase is partly attributed to enhanced growth rate of the bubbles by rectified diffusion. Conversely, at long pulse lengths acoustic cavitation activity was enhanced at low SDS concentrations as a larger number of the smaller bubbles could survive the pulse "off time". The effect of reduced acoustic shielding and an increase in the "active" bubble population due to electrostatic repulsion between bubbles are also significant in this case. Finally, as the surfactant concentration was increased further, the effect of electrostatic induced impedance shielding or reclustering dominates, resulting in a decrease in the SL intensity.

Acoustic droplet vaporization threshold: effects of pulse duration and contrast agent

The use of superheated liquid perfluorocarbon droplets encased in albumin shells has been proposed as a minimally invasive alternative to current treatment of cancer by means of occlusion therapy. In response to an applied acoustic field, these droplets, which are small enough to pass through capillaries, vaporize into large gas bubbles that subsequently lodge in the vasculature. This technique, known as acoustic droplet vaporization (ADV) has been shown to successfully reduce blood flow in vivo, but for in situ conditions where attenuation is present, lower acoustic frequency and ADV threshold may be desirable. Thus, two methods to lower the ADV threshold at a lower 1.44 MHz were explored. The first part of this study investigated the role of pulse duration on ADV. The second part investigated the role of inertial cavitation (IC) external to a droplet by lowering the IC threshold in the host liquid with the presence of Definity contrast agent (CA). The threshold was found to be 5.5-5.9 MPa for short microsecond pulses and decreased for millisecond pulses (3.8-4.6 MPa). When CAs were present and long millisecond pulses were used, the ADV threshold decreased to values as low as 0.41 MPa.

Detection of acoustic cavitation in the heart with microbubble contrast agents in vivo: a mechanism for ultrasound-induced arrhythmias

Ultrasound fields can produce premature cardiac contractions under appropriate exposure conditions. The pressure threshold for ultrasound-induced premature contractions is significantly lowered when microbubble contrast agents are present in the vasculature. The objective of this study was to measure directly ultrasound-induced cavitation in the murine heart in vivo and correlate the occurrence of cavitation with the production of premature cardiac contractions. A passive cavitation detection technique was used to quantify cavitation activity in the heart. Experiments were performed with anesthetized, adult mice given intravenous injections of either a contrast agent (Optison) or saline. Murine hearts were exposed to ultrasound pulses (200 kHz, 1 ms, 0.1-0.25 MPa). Premature beats were produced in mice injected with Optison and the likelihood of producing a premature beat increased with increasing pressure amplitude. Similarly, cavitation was detected in mice injected with Optison and the amplitude of the passive cavitation detector signal increased with increasing exposure amplitude. Furthermore, there was a direct correlation between the extent of cavitation and the likelihood of ultrasound producing a premature beat. Neither premature beats nor cavitation activity were observed in animals injected with saline and exposed to ultrasound. These results are consistent with acoustic cavitation as a mechanism for this bioeffect.

Cavitation pressure in water

We investigate the limiting mechanical tension (negative pressure) that liquid water can sustain before cavitation occurs. The temperature dependence of this quantity is of special interest for water, where it can be used as a probe of a postulated anomaly of its equation of state. After a brief review of previous experiments on cavitation, we describe our method which consists in focusing a high amplitude sound wave in the bulk liquid, away from any walls. We obtain highly reproducible results, allowing us to study in detail the statistics of cavitation, and to give an accurate definition of the cavitation threshold. Two independent pressure calibrations are performed. The cavitation pressure is found to increase monotonically from -26 MPa at 0 degrees C to -17 MPa at 80 degrees C. While these values lie among the most negative pressures reported in water, they are still far away from the cavitation pressure expected theoretically and reached in the experiment by Angell and his group [Zheng, Science 254, 829 (1991)] (around -120 MPa at 40 degrees C). Possible reasons for this discrepancy are considered.

Correlation of cavitation with ultrasound enhancement of thrombolysis

Pulsed ultrasound, when used as an adjuvant to recombinant tissue plasminogen activator (rt-PA), has been shown to enhance thrombolysis in the laboratory as well as in clinical trials for the treatment of ischemic stroke. The exact mechanism of this enhancement has not yet been elucidated. In this work, stable and inertial cavitation (SC and IC) are investigated as possible mechanisms for this enhancement. A passive cavitation detection scheme was utilized to measure cavitation thresholds at 120 kHz (80% duty cycle, 1667 Hz pulse repetition frequency) for four host fluid and sample combinations: plasma, plasma with rt-PA, plasma with clot and plasma with clot and rt-PA. Following cavitation threshold determination, clots were exposed to pulsed ultrasound for 30 min in vitro using three separate ultrasound treatment regimes: (1) no cavitation (0.15 MPa), (2) SC alone (0.24 MPa) or (3) SC + IC combined (0.36 MPa) in the presence of rt-PA. Percent clot mass loss after each treatment was used to determine thrombolysis efficacy. The highest percent mass loss was observed in the stable cavitation regime (26%), followed by the combined stable and inertial cavitation regime (20.7%). Interestingly, the percent mass loss in clots exposed to ultrasound without cavitation (13.7%) was not statistically significantly different from rt-PA alone (13%) [p > 0.05]. Significant enhancement of thrombolysis correlates with presence of cavitation and stable cavitation appears to play a more important role in the enhancement of thrombolysis. (E-mail: ).

A new strategy to enhance cavitational tissue erosion using a high-intensity, Initiating sequence

Our previous studies have shown that pulsed ultrasound can physically remove soft tissue through cavitation. A new strategy to enhance the cavitation-induced erosion is proposed wherein tissue erosion is initiated by a short, high-intensity sequence of pulses and sustained by lower intensity pulses. We investigated effects of the initiating sequence on erosion and cavitation sustained by lower intensity pulses. Multiple three-cycle pulses at a pulse repetition frequency of 20 kHz delivered by a 788-kHz focused transducer were used for tissue erosion. Fixing the initiating sequence at I(SPPA) of 9000 W/cm2, 16 combinations of different numbers of pulses within the initiating sequence and different sustaining pulse intensities were tested. Results showed: the initiating sequence increases the probability of erosion occurrence and the erosion rate with only slight overall increases in propagated energy; the initiating sequence containing more pulses does not increase the sustained cavitation period; and if extinguished and reinitiated, the sustained cavitation period becomes shorter after each initiation, although the waiting time between adjacent cavitation periods is random. The high-intensity, initiating sequence enhances cavitational tissue erosion and enables erosion at intensities significantly lower than what is required to initiate erosion.

The relationship of acoustic emission and pulse-repetition frequency in the detection of gas body stability and cell death

The effect of pulse-repetition frequency (PRF) and number of exposures on membrane damage and subsequent death of contrast agent-attached phagocytic cells was examined. Phagocytic cells of a mouse macrophage cell line were grown as monolayers on thin Mylar sheets. Optison microbubbles were attached to these cells by incubation. Focused ultrasound exposures (Pr = 2 MPa) were implemented at a frequency of 2.25 MHz with 46 cycle pulses and clinically relevant PRFs of 1 kHz, 100 Hz, 10 Hz, 1 Hz and 0.1 Hz in a degassed water bath. A 1-MHz receive transducer measured the scattered signal. The frequency spectrum was normalized to a control spectrum from linear scatterers. Photomicrographs of the cell monolayer were made before and after exposure, and a dye exclusion test (Trypan blue) was used to find the percentage of blue-stained cells indicating cell death, which was then related to acoustic emission. For 10 acoustic pulses and a high prerinse gas body concentration, there was less cell death and correspondingly lower change in the acoustic emissions at a PRF of 1 kHz than with PRFs of 100 Hz, 10 Hz, 1 Hz and 0.1 Hz (p < 0.001). The reduced effect at high PRF may be indicative of some evolution of the shelled microbubble that requires significant total exposure duration (> 10 ms, but < 100 ms).

Low-intensity-ultrasound-accelerated nerve regeneration using cell-seeded poly(D,L-lactic acid-co-glycolic acid) conduits: an in vivo and in vitro study

This study investigated the effects of low intensity ultrasound on seeded Schwann cells within poly(DL-lactic acid-co-glycolic acid) (PLGA) conduits by in vitro and in vivo trials for peripheral nerve regeneration. The possible differences in the ultrasonic effects when using biodegradable and non-biodegradable materials as the conduits were also studied, using silicone rubber tubes as comparisons. In the in vitro study, seeded Schwann cells were cultured in serum deprivation culture medium that simulated the environment of mechanical trauma on injury nerve site. After 12, 24, and 48 h, only the PLGA conduit groups exposed to 0.05 W/cm(2), 3 min/treatment of ultrasound exhibited decreased LDH release and increased MTT values compared to the sham groups. Based on the results of the in vitro experiment in LDH and MTT testing, the silicone conduits with seeded Schwann cells group was ignored in the in vivo study. The PLGA nerve conduits seeded with Schwann cells (9 x 10(3) cells) were implanted to 15-mm right sciatic nerve defects in rats. Each conduit received 12 ultrasonic treatment sessions over 2 weeks after 1 day of rest. Ultrasound was applied as follows: frequency, 1MHz; intensity, 0.3 W/cm(2) (SATP); treatment, 5 min/day. Implanted graft specimens were harvested for histological analysis at 8 weeks following surgery. PLGA groups (with and without Schwann cells) treated with pulsed ultrasonic stimulation were found to have significantly greater number and area of regenerated axons at the mid-conduit of implanted grafts, as compared to the sham groups. Ultrasonic stimulation on silicone groups was found to induce a mass of fibrous tissues that covered the nerve conduits and retarded axon regeneration.

Hyperoxia potentiated sonothrombolysis as a method of acute ischemic stroke therapy

The main goal in the treatment of acute ischemic stroke is prompt arterial recanalization. Thrombolysis with recombinant tissue plasminogen activator (rtPA) is efficient in humans, but shows significant problems including slow and incomplete recanalization and frequent bleeding complications. Limited therapeutic window (the first three hours after onset) is the major limitation resulting in reach too few patients. Therefore, adjunctive therapies extending the reperfusion time window, increasing efficacy and reducing side effects of rtPA are needed. Ultrasound augmentation of rtPA-mediated thrombolysis is suggested to overcome some of these problems, but low-frequency ultrasound (less than 1 MHz) is not safe and high frequency ultrasound (2 MHz) is not much effective. We suggest that normobaric hyperoxia (NBO) may increase the efficacy of ultrasound and rtPA combination in addition to its own efficacy in acute ischemic stroke. Briefly, NBO increases arterial partial oxygen pressure (pO(2)) significantly up to 6-fold. Increase of pO(2) results in an increase of dissolved oxygen in the blood according to Henry's law. Enhanced dissolved oxygen increases gas nuclei formation around and inside of the clot, and decreases the Blake threshold. Under ultrasound field, these small gas nuclei form nano bubbles which fuel inertial cavitation as substrates, and therefore increase the clot fragmentation and lysis. This hypothesis has not been tested so far. The combination of rtPA, therapeutic ultrasound and NBO may be more efficacious than rtPA alone or its combination with ultrasound as acute stroke treatment modality, because each has different and probably additive mechanism of action.

Investigation of intensity thresholds for ultrasound tissue erosion

Our previous studies have shown that short intense pulses delivered at certain pulse repetition frequencies (PRF) can achieve localized, clean erosion in soft tissue. In this paper, the intensity thresholds for ultrasound induced erosion and the effects of pulse intensity on erosion characterized by axial erosion rate, perforation area and volume erosion rate were investigated on in vitro porcine atrial wall tissue. Ultrasound pulses with a 3-cycle pulse duration and a 20-kHz PRF were delivered by a 788-kHz single element focused transducer. I(SPPA) values of 1000 to 9000 W/cm2 were tested. Results show the following: (1) the estimated intensity threshold for generating erosion was at I(SPPA) of 3220 W/cm2; (2) the axial erosion rate increased with higher intensity at I(SPPA) < or = 5000 W/cm2, while decreased with higher intensity at I(SPPA) > or = 5000 W/cm2; and (3) the perforation area and the volume erosion rate increased with higher intensity.

An investigation of the physical forces leading to thrombosis disruption by cavitation

Ultrasound therapy has proven to be an efficient and safe modality for the treatment of acute arterial occlusions, and the use of therapeutic ultrasound for the treatment of thrombosis and vascular diseases holds great promise in overcoming the limitations of other available therapies. Still, there exists little published work that covers the different phenomena that take place in a thorough and comprehensive way. In this paper, we endeavor to address the subject by reviewing work on the physical properties of ultrasound propagation in the blood arteries as it relates to the cavitation of microbubbles, and we compare the impact of the different forces at work for clot disruption. Our conclusion is that the most important effect of ultrasound in the treatment of thrombotic disorders is the liquid-jet impact forces that result from strong bubble collapses in the vicinity of solid boundaries.