Sonoporation of monolayer cells by diagnostic ultrasound activation of contrast-agent gas bodies

Sonoporation by single-shot pulsed ultrasound with microbubbles adjacent to cells

In this article, membrane perforation of endothelial cells with attached microbubbles caused by exposure to single-shot short pulsed ultrasound is described, and the mechanisms of membrane damage and repair are discussed. Real-time optical observations of cell-bubble interaction during sonoporation and successive scanning electron microscope observations of the membrane damage with knowledge of bubble locations revealed production of micron-sized membrane perforations at the bubble locations. High-speed observations of the microbubbles visualized production of liquid microjets during nonuniform contraction of bubbles, indicating that the jets are responsible for cell membrane damage. The resealing process of sonoporated cells visualized using fluorescence microscopy suggested that Ca2+-independent and Ca2+-triggered resealing mechanisms were involved in the rapid resealing process. In an experimental condition in which almost all cells have one adjacent bubble, 25.4% of the cells were damaged by exposure to single-shot pulsed ultrasound, and 15.9% (approximately 60% of the damaged cells) were resealed within 5 s. These results demonstrate that single-shot pulsed ultrasound is sufficient to achieve sonoporation when microbubbles are attached to cells.

Microbubbles as ultrasound triggered drug carriers

Originally developed as contrast agents for ultrasound imaging and diagnostics, in the past years, microbubbles have made their way back from the patients' bedside to the researcher's laboratory. Microbubbles are currently believed to have great potential as carriers for drugs, small molecules, nucleic acids, and proteins. This review provides insight into this intriguing new frontier from the perspective of the pharmaceutical scientist. First, basic aspects on the application of ultrasound-targeted microbubble destruction for drug delivery will be presented. Next, we will review the recently applied approaches for manufacturing and drug-loading microbubbles. Important quality issues and characterization techniques for advanced microbubble formulation will be discussed. Finally, we will provide an assessment of the prospects for microbubbles in drug and gene therapy, illustrating the problems and requirements for their future development.

Spatial distribution of ultrasound targeted microbubble destruction increases cardiac transgene expression but not capillary permeability

Ultrasound targeted microbubble destruction (UTMD) has evolved as a promising tool for organ specific gene and drug delivery. Using DNA-loaded microbubbles, cardiac transfection has been shown to be feasible. However, two-dimensional properties of the ultrasound beam limit cardiac transgene expression to the focal zone, thus, reducing its potential therapeutic effect. The aim of this study was to test if spatial distribution of ultrasound targeted microbubble destruction in the heart could lead to augmented transgene expression or increased capillary permeability. Lipid microbubbles containing plasmids with a luciferase transgene were used to target rat hearts. The diagnostic ultrasound probe was fixed in a mid-short axis view with a gel stand-off between the chest and probe. Ultrasound (1.3 MHz) with a mechanical index of 1.6 was intermittently applied to rats during microbubble infusion. Rats were randomized to either stay in that position or move horizontally in a cranio-caudal direction (3 mm sweep) relative to the ultrasound probe during UTMD. After 4 days, organs were harvested and analyzed for reporter gene expression. Another group of rats received Evans Blue, followed by UTMD with unloaded microbubbles. Again, rats were randomized into a static or moving group. Hearts were harvested to evaluate extravasation of Evans Blue. Moving rats in a cranio-caudal direction significantly increased transgene expression by 19-fold in the anterior heart, by sixfold in the posterior heart and by 32-fold in the apex. Interestingly, Evans Blue extravasation was not augmented in the moving group. Spatial distribution of UTMD may increase transgene expression due to sonication of larger areas in the heart. In contrast, capillary permeability does not increase, indicating less capillary damage.

Enhancement of microbubble mediated gene delivery by simultaneous exposure to ultrasonic and magnetic fields

It has been shown in previous studies that gene delivery can be enhanced by a variety of minimally-invasive techniques including: (1) exposure of cells to ultrasound in the presence of DNA and gas microbubbles and (2) exposure of cells to a magnetic field in the presence of DNA conjugated to magnetic nanoparticles. The aim of this work was to investigate whether it was possible to combine the advantages of both these techniques. It was found that transfection of Chinese hamster ovary cells by naked plasmid DNA was enhanced by combined exposure of the cells to ultrasound (10 s at 1 kHz pulse repetition frequency with 40 cycle 1 MHz sinusoidal pulses, 1 MPa peak to peak pressure) and a magnetic field (provided by five square cross-section N52 grade NdFeB magnets 25 x 10 x 10 mm with transversal magnetisation Br = 1.50 T arranged in a Halbach array), in the presence of one of two different microbubble/nanoparticle preparations. The first preparation consisted of phospholipid coated microbubbles mixed with micelles containing magnetic nanoparticles. The second consisted of microbubbles which were themselves magnetically active. These preparations were found to be more effective than either magnetic micelles or phospholipid coated microbubbles alone by a factor of 2.8 (total flux approximately 4 versus 1.4 x 10(6) photon/s) and the results were found to be statistically significant (p < 0.01). Two mechanisms are proposed to explain these observations: firstly, that the magnetic field facilitates close proximity between the cells and the microbubbles and hence increases the likelihood of transfection; second, that there is sensitisation of the cells, as a result of exposure to the magnetic field in the presence of the micelles, which increases their ability to be transfected upon exposure to ultrasound. Further work is in progress to determine which of these mechanisms is the most significant and the potential for other therapeutic applications.

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 and microbubble-targeted delivery of therapeutic compounds: ICIN Report Project 49: Drug and gene delivery through ultrasound and microbubbles

The molecular understanding of diseases has been accelerated in recent years, producing many new potential therapeutic targets. A noninvasive delivery system that can target specific anatomical sites would be a great boost for many therapies, particularly those based on manipulation of gene expression. The use of microbubbles controlled by ultrasound as a method for delivery of drugs or genes to specific tissues is promising. It has been shown by our group and others that ultrasound increases cell membrane permeability and enhances uptake of drugs and genes. One of the important mechanisms is that microbubbles act to focus ultrasound energy by lowering the threshold for ultrasound bioeffects. Therefore, clear understanding of the bioeffects and mechanisms underlying the membrane permeability in the presence of microbubbles and ultrasound is of paramount importance. (Neth Heart J 2009;17:82-6.).

Ultrasonic imaging of molecular targets

Today nuclear medicine is the only modality that is clinically successful in molecular imaging. However, other modalities compete with its excellent sensitivity in imaging molecular targets. In the last 10 years ultrasound imaging has shown the potential to provide sufficiently high sensitivity for the molecular imaging of vascular targets. These advances are based on the joint development of microbubble contrast media and the methods to image them with high sensitivity. Ultrasound-contrast-enhanced imaging strategies make use of the specific physical properties of microbubbles such as resonance, nonlinear oscillation, and collapse. The size of microbubbles limits their use to the vascular space. Thus, the main applications of ultrasound for molecular imaging are inflammation, thrombi, and angiogenesis, for which successful contrast enhancement has been achieved in animal models. Main molecular targets used to date include selectins, alpha(v)beta(3) or alpha(5)beta(1) integrins, glycoprotein (GP) IIb/IIIa, intracellular adhesion molecule ICAM-1, and vascular endothelial growth factor receptor VEGFR2. Results from animal studies indicate that ultrasound could play a major role in vascular molecular imaging for diagnosis and treatment monitoring. Additional effects of insonified microbubbles (e.g., opening of the blood-brain barrier or increased transfection efficiency in gene therapy) are attributed to the transient opening of cell membranes known as "sonoporation" and demonstrate further potential for integrated diagnosis and therapy.

Therapeutic potential of low-intensity ultrasound (part 2): biomolecular effects, sonotransfection, and sonopermeabilization

Part one of this review focused on the thermal and mechanical effects of low-intensity ultrasound (US). In this second and final part of the review, we will focus on and discuss various aspects of low-intensity US, with emphasis on the biomolecular effects, US-mediated gene transfection (sonotransfection), and US-mediated permeabilization (sonopermeabilization). Sonotransfection of different cell lines in vitro and target tissues in vivo have been reported. Optimization experiments have been done and different mechanisms investigated. It has also been found that several genes can be up-regulated or down-regulated by sonication. As to the potential therapeutic applications, systemic or local sonotransfection might also be a safe and effective gene therapy method in effecting the cure of local and systemic disorders. Gene regulation of target cells may be utilized in modifying cellular response to a treatment, such as increasing the sensitivity of diseased cells while making normal cells resistant to the side effects of a treatment. Advances in sonodynamic therapy and drug sonopermeabilization also offer an ever-increasing array of therapeutic options for low-intensity US.

Therapeutic potential of low-intensity ultrasound (part 1): thermal and sonomechanical effects

In this first part of the review, we will focus on and discuss various aspects of low-intensity ultrasound (US), with emphasis on mild thermal effects, apoptosis induction, and sonomechanical effects. Mild thermal effects of US have been commonly applied to physical therapy. Though US has clear beneficial effects, the advantage of using US over other heating modalities remains unclear. US has also been used in vivo and clinically in the treatment of wounds and fractures, with promising results. On the biomolecular level, studies have shown that US can induce apoptosis and that certain conditions can provide optimal apoptosis induction. As to potential therapeutic applications, in addition to the thermal and other physical effects, apoptosis induction by US may offer direct and rapid treatment of tumors or cancer tissues. Technological advances and rapidly accelerating research in this field are providing an ever-increasing array of therapeutic options for lowintensity US.

Enhanced operation of femtosecond lasers and applications in cell transfection

In this work we present a review and discussion on the enhancement of femtosecond (fs) lasers for use within biophotonics with a particular focus on their use in optical transfection techniques. We describe the broad range of source options now available for the generation of femtosecond pulses before briefly reviewing the application of fs laser in optical transfection studies. We show that major performance enhancements may be obtained by optimising the spatial and temporal performance of the laser source before considering possible future directions in this field. In relation to optical transfection we describe how such laser sources initiate a multiphoton process to permeate the cell membrane in a transient fashion. We look at aspects of this technique including the ability to combine transfection with optical trapping. For future implementation of such transfection we explore the role of new sources and "nondiffracting" light fields.

Cytotoxic effects and in vitro reversal of multidrug resistance by therapeutic ultrasound in human hepatocarcinoma cell line (HepG2)

Multidrug resistance (MDR) is one of the major obstacles to successful chemotherapy of human malignancies. Although many strategies have been explored to overcome MDR, none of them have been proven to be clinically useful until now. The aim of this study was to investigate whether a novel therapeutic ultrasound (US) approach would have useful effects on the reversal of MDR in cancer cells. Wild-type and MDR phenotype (HepG2/ADM) cells of the human hepatocarcinoma cell line HepG2 were exposed to 0.8 MHz US at an intensity of 0.43 W/cm(2) for a 9s exposure (total energy density: 3.87 J/cm(2)). After US exposure, cell number and viability were counted immediately, and flow cytometry was performed to measure retention of rhodamine 123 and adriamycin in HepG2 and HepG2/MDR cells. Both cell lines were then incubated in suspension with adriamycin, vincristine, etoposide, cisplatin and 5-fluorouracil, respectively, and the MTT assay was used to determine cytotoxicity. The results showed that US exposure could significantly increase the uptake of Rh123 and ADM by HepG2/ADM tumor cells. The resistant index for the chemotherapeutic drugs was significantly lower in the US-exposed HepG2/ADM cells than in those not exposed to US. It was therefore concluded that US exposure could enhance the sensitivity of HepG2/ADM tumor cells to these chemotherapeutic agents, and the functional and structural changes induced by previous US exposure in MDR tumor cells may be responsible for it. However, further study is needed to investigate the mechanism behind US-mediated reversal of MDR.

Ultrasound-induced cytolysis of cancer cells is enhanced in the presence of micron-sized alumina particles

Micron-sized alumina particles have been shown to enhance sonochemical free radical formation in aqueous solutions and simultaneously increase the solution temperature and acoustic (white) noise, effects attributable to enhanced inertial cavitation [T. Tuziuti, J. Phys. Chem. A 109 (2005) 4869-4872]. In the current study, the same ultrasound exposure system was applied to in vitro cancer cells as a model system to determine the effect of alumina particles on the long-term survival of cells and on the major pathways of cell death, i.e., either apoptosis or necrosis. Following 6h of incubation after ultrasound treatment, it was found that the cells died mainly through necrosis, irrespective of whether the exposure was conducted in the presence of alumina particles or not. Alumina particles were non-toxic to cells alone, but were found to decrease the long-term survivability of cells that survived the initial exposure. This effect depended on the size and concentration of particles. These results correlated well with the effect of alumina particles on the sonochemical oxidation of KI under the same exposure conditions. Spin-trapping with 5,5-dimethyl-pyroline N-oxide (DMPO) and electron spin resonance spectroscopy indicated that the sonochemical formation of *OH radicals increased in the presence of alumina particles. The current study is consistent with the well known observation that micron-sized particles enhance the acoustic cavitation process.

Ultrasound-mediated microbubble destruction enhances gene transfection in pancreatic cancer cells

INTRODUCTION

The purpose of this study was to determine whether ultrasound exposure combined with microbubble destruction could be used to enhance non-viral gene delivery in human pancreatic carcinoma cells (PANC-1).

METHODS

The study was performed with four experimental groups: Group P, plasmid alone; Group P+M, plasmid and microbubbles; Group P+U, plasmid and ultrasound; Group P+U+M, plasmid with ultrasound and microbubbles. Plasmid DNA encoding enhanced green fluorescent protein (pEGFP) was gently mixed with commercially available ultrasound microbubble contrast agents (SonoVue; Bracco Diagnostics Inc, Milan, Italy) in Group P+M and Group P+U+M. The different combinations of DNA and DNA plus microbubbles were added to cultured PANC-1 cells under different conditions. Transfection efficiency and cell viability were assessed by FACS analysis (Becton Dickinson, San Jose, CA, USA), confocal laser scanning microscopy, and trypan blue staining.

RESULTS

The results demonstrated that microbubbles with ultrasound exposure could significantly enhance the reporter gene expression as compared with other groups (Group P+U+M, 21.4%+/-3.16%; Group P, 2.9%+/-0.45%; Group P+M, 3.1%+/-0.51%; Group P+U, 6.1%+/-1.27%; P<0.01). No statistically significant difference was observed in the PANC-1 cell viability between Group P+U+M and other groups (P>0.05).

CONCLUSION

Our in-vitro findings suggest that ultrasound-mediated microbubble destruction has the potential to promote efficient gene transfer into PANC-1 cells without significant cell death. This non-invasive gene transfer method may be a useful tool for safe clinical gene therapy of pancreatic cancer in the future.

Phospholipids-based microbubbles sonoporation pore size and reseal of cell membrane cultured in vitro

OBJECTIVE

To investigate phospholipids-based microbubbles induced sonoporation and cell membrane reseal in vitro under various conditions.

METHODS

A breast cancer cell line SK-BR-3 was used to investigate ultrasonic sonoporation under various conditions. Atomic force microscopy (AFM) scanning techniques were employed to observe the change of membrane pores.

RESULTS

Normal SK-BR-3 cells membrane pores were evenly distributed and less than 1 microm. After ultrasound exposure, membrane pores were enlarged at different degree depending on ultrasound exposure durations, filling gas species and microbubble suspension concentration. With microbubble suspension concentration being increased to 5% or ultrasound exposure reached 30 s, membrane pores in fluorocarbon (C(3)F(8) or SF(6))-filled microbubble groups exceeded 1 microm, which were significantly larger than that of air-filled microbubble group. Membrane pores were about 2-3 microm under ultrasound 60 s with 5% fluorocarbon-filled microbubble suspension. After 24 h of incubation, most of the enlarged membrane pores could reseal to normal size, which corresponded to cell viability.

CONCLUSIONS

Membrane pores can be obviously enlarged by ultrasonic sonoporation of fluorocarbon-filled microbubbles, whose reseal time depended on ultrasound exposure duration and microbubble suspension concentration.

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.

Deformation and rupture of lipid vesicles in the strong shear flow generated by ultrasound-driven microbubbles

Considering the elastic response of the membrane of a lipid vesicle (artificial cell) in an arbitrary three-dimensional shear flow, we derive analytical predictions of vesicle shape and membrane tension for vesicles close to a spherical shape. Large amplitude deviations from sphericity are described using boundary integral numerical simulations. Two possible modes of vesicle rupture are found and compared favourably with experiments: (i) for large enough shear rates the tension locally exceeds a rupture threshold and a pore opens at the waist of the vesicle and (ii) for large elongations the local tension becomes negative, leading to buckling and tip formation near a pole of the vesicle. We experimentally check these predictions in the case of strong acoustic streaming flow generated near ultrasound-driven microbubbles, such as those used in medical applications.

Ultrasound-based nonviral gene delivery induces bone formation in vivo

Nonviral gene delivery is a promising, safe, therapeutic tool in regenerative medicine. This study is the first to achieve nonviral, ultrasound-based, osteogenic gene delivery that leads to bone tissue formation, in vivo. We hypothesized that direct in vivo sonoporation of naked DNA encoding for the osteogenic gene, recombinant human bone morphogenetic protein-9 (rhBMP-9) would induce bone formation. A luciferase plasmid (Luc), encoding rhBMP-9 or empty pcDNA3 vector mixed with microbubbles, was injected into the thigh muscles of mice. After injection, noninvasive sonoporation was applied. Luc activity was monitored noninvasively, and quantitatively using bioluminescence imaging in vivo, and found for 14 days with a peak expression on day 7. To examine osteogenesis in vivo, rhBMP-9 plasmid was sonoporated into the thigh muscles of transgenic mice that express the Luc gene under the control of a human osteocalcin promoter. Following rhBMP-9 sonoporation, osteocalcin-dependent Luc expression lasted for 24 days and peaked on day 10. Bone tissue was formed in the site of rhBMP-9 delivery, as was shown by micro-computerized tomography and histology. The sonoporation method was also compared with previously developed electrotransfer-based gene delivery and was found significantly inferior in its efficiency of gene delivery. We conclude that ultrasound-mediated osteogenic gene delivery could serve as a therapeutic solution in conditions requiring bone tissue regeneration after further development that will increase the transfection efficiency.

Ultrasound targeted microbubble destruction increases capillary permeability in hepatomas

Ultrasound-targeted microbubble destruction (UTMD) has evolved as a promising tool for organ-specific gene and drug delivery. Taking advantage of high local concentrations of therapeutic substances and transiently increased capillary permeability, UTMD could be used for the treatment of ultrasound accessible tumors. The aim of this study was to evaluate if UTMD can locally increase capillary permeability in a hepatoma model of the rat. Furthermore, we evaluated whether UTMD can transfect DNA into such tumors. Subcutaneous Morris hepatomas were induced in both hind limbs of ACI rats by cell injection. A total of 18 rats were divided into three groups. Only one tumor per rat was treated by ultrasound. The first group received injection of Evans blue, followed by UTMD. The second group received a phosphate-buffered saline solution infusion and ultrasound to the target tumor after Evans blue injection. The third group received UTMD first, followed by Evans blue injection. Tumors and control organs were harvested, and Evans blue extravasation was quantified. Another 12 rats received DNA-loaded microbubbles by UTMD to one tumor, encoding for luciferase. Evans blue injection followed by UTMD showed about fivefold higher Evans blue amount in the target tumors compared with the control tumors. In contrast, no significant difference in Evans blue content was detected between target and control tumors when ultrasound was applied without microbubbles or when UTMD was performed before Evans blue injection. Plasmid transfection was not successful. In conclusion, ultrasound targeted microbubble destruction is able to transiently increase capillary permeability in hepatomas. Using naked DNA, this technique does not seem to be feasible for noninvasive transfection of hepatomas.

Therapeutic ultrasound facilitates antiangiogenic gene delivery and inhibits prostate tumor growth

Gene therapy clinical trials are limited due to several hurdles concerning the type of vector used, particularly, the viral vectors, and transfection efficacy when non-viral vectors are used. Therapeutic ultrasound is a promising non-viral technology that can be used in the clinical setting. Here, for the first time, we show the efficacy of therapeutic ultrasound to deliver genes encoding for hemopexin-like domain fragment (PEX), an inhibitor of angiogenesis, to prostate tumors in vivo. Moreover, the addition of an ultrasound contrast agent (Optison) to the transfection process was evaluated. Prostate cancer cells and endothelial cells (EC) were transfected in vitro with cDNA-PEX using therapeutic ultrasound alone (TUS + pPEX) or with Optison (TUS + pPEX + Optison). The biological activity of the expressed PEX was assessed using proliferation, migration, and apoptosis assays done on EC and prostate cancer cells. TUS + pPEX + Optison led to the inhibition of EC and prostate cancer cell proliferation (<65%), migration (<50%), and an increase in apoptosis. In vivo, C57/black mice were inoculated s.c. with prostate cancer cells. The tumors were treated with TUS + pPEX and TUS + pPEX + Optison either once or repeatedly. Tumor growth was evaluated, after which histology and immunohistochemistry analyses were done. A single treatment of TUS + pPEX led to a 35% inhibition in tumor growth. Using TUS + PEX + Optison led to an inhibition of 50%. Repeated treatments of TUS + pPEX + Optison were found to significantly (P < 0.001) inhibit prostate tumor growth by 80%, along with the angiogenic indices, with no toxicity to the surrounding tissues. These results depict the efficacy of therapeutic ultrasound as a non-viral technology to efficiently deliver genes to tumors in general, and to deliver angiogenic inhibitors to prostate cancer in particular.

Impact of microbubbles on shock wave-mediated DNA uptake in cells in vitro

Gas-filled microbubbles have been successfully used as gene delivery reagents in combination with diagnostic ultrasound. Although shock wave exposure has been shown to transfect cells with naked DNA in vitro, it has not been tested whether the addition of microbubbles would augment DNA uptake under those conditions. Therefore, the aim of this study was to test the impact of microbubbles on transgene expression in vitro under shock wave exposure conditions. HEK 293 cells were treated with 60 or 120 pulses of shock waves at varying energy levels. Cells were mixed with either 100 microg/mL luciferase expressing plasmid DNA or with microbubbles that were produced with the same amount of this DNA. Cell death was evaluated after 1 h and transgene expression, after 24 h. In the presence of microbubbles, transgene expression was significantly higher (as much as 29-fold) relative to that obtained without microbubbles. Cells exposed to 120 pulses demonstrated higher transgene expression (as high as 2.7-fold) compared with cells exposed to 60 pulses. The use of microbubbles resulted in greater cell death, varying from 26% (low energy) to 78% (high energy). In conclusion, DNA-loaded microbubbles can significantly increase shock wave mediated gene transfer. However, this effect is associated with increased levels of cell destruction.

A novel phase assignment protocol and driving system for a high-density focused ultrasound array

Currently, most phased-array systems intended for therapy are one-dimensional (1-D) and use between 5 and 200 elements, with a few two-dimensional (2-D) systems using several hundred elements. The move toward lambda/2 interelement spacing, which provides complete 3-D beam steering, would require a large number of closely spaced elements (0.15 mm to 3 mm). A solution to the resulting problem of cost and cable assembly size, which this study examines, is to quantize the phases available at the array input. By connecting elements with similar phases to a single wire, a significant reduction in the number of incoming lines can be achieved while maintaining focusing and beam steering capability. This study has explored the feasibility of such an approach using computer simulations and experiments with a test circuit driving a 100-element linear array. Simulation results demonstrated that adequate focusing can be obtained with only four phase signals without large increases in the grating lobes or the dimensions of the focus. Experiments showed that the method can be implemented in practice, and adequate focusing can be achieved with four phase signals with a reduction of 20% in the peak pressure amplitude squared when compared with the infinite-phase resolution case. Results indicate that the use of this technique would make it possible to drive more than 10,000 elements with 33 input lines. The implementation of this method could have a large impact on ultrasound therapy and diagnostic devices.

Contrast imaging with chirped excitation

Coded excitation has been successfully used in imaging to increase the signal-to-noise ratio (SNR) and penetration depth. With a contrast agent, wideband signals have been hypothesized to increase the contrast-to-tissue ratio (CTR). However, nonlinear properties of contrast agents make decoding difficult when applying coded excitation to contrast imaging. We propose two chirped excitation methods to image contrast agents, with a mechanical index (MI) ranging from 0.05 to 0.34. In the single chirp method, one chirp is transmitted, followed by a clutter filter to reject tissue echoes, then a matched filter is used to recover range resolution. In the chirp sequence method, an increasing and decreasing chirp sequence is transmitted followed by subtraction of the compressed echoes to reject tissue echoes (assuming tissue is a linear scatterer at low MI). Ten independent acoustic experiments were performed to evaluate the CTR for chirp and tone burst insonation, with the same spatial peak temporal averaged intensity (I(SPTA)). A significant increase in CTR, ranging from 4 dB to 8 dB, is observed for chirped excitation as compared with tone burst insonation, at an I(SPTA) of 0.1 and 0.3 mW/cm2 (P < or = 5e-3). To achieve the same CTR of 15 dB, the spatial peak pulse averaged intensity (I(SPPA)) can be decreased by 6 dB for chirp insonation as compared with tone burst insonation (P < 1e-5). Additionally, an increase of more than 10 dB in tissue rejection ratio (TRR) is observed for a chirp sequence insonation compared to tone burst phase inversion for this set of parameters (P < or = 1e-9). Deconvolution of the linear microbubble response from the received echoes is proposed as a method to recover spatial resolution. The difference in the axial resolution resulting from chirp and three-cycle tone burst insonation is approximately 220 microm. The difference in the mainlobe width between experimental and predicted compressed echoes is less than 20%. The side-lobe amplitude is 9 dB to 16 dB below the mainlobe with a transmitted I(SPTA) from 0.1 to 6.6 mW/cm2.

Phagocytosis of ultrasound contrast agent microbubbles by Kupffer cells

Delayed parenchymal phase images of the liver more than 5 min after IV injection of ultrasound contrast agents are thought to be related to the phagocytosis of contrast agent microbubbles by macrophages. In this study, we examined whether liver-specific macrophages, Kupffer cells, phagocytosed the microbubbles and whether their elimination affected the delayed parenchymal images of the liver. Phase-contrast microscope observations showed that Kupffer cells phagocytosed various contrast agents in vitro. Among the contrast agents used, 99% of Sonazoid and Optison, and 47% of Levovist were phagocytosed, whereas only 7.3% of SonoVue and 0% of Imavist were phagocytosed. Elimination of Kupffer cells in vivo by gadolinium chloride (GdCl(3)) resulted in decreased intensity of the delayed parenchymal images with Sonazoid and Levovist, while SonoVue showed no changes compared with control. Our findings suggested that Kupffer cells phagocytosed contrast agents and they were responsible for the delayed images of contrast ultrasound in the liver.

Recent applications of ultrasound: diagnosis and treatment of hepatocellular carcinoma

Ultrasound (US) has the advantages of real-time observation, simple technique, and a noninvasive procedure compared to other imaging modalities. The recent development of digital technologies has enabled the observation of sonograms with improved signal-to-noise ratio, penetration, and spatial and contrast resolutions. Furthermore, microbubble contrast agents have increased the diagnostic ability of US examination, and the use of three-dimensional sonograms is now not unusual. These advances have furthered the usefulness of US for liver tumors in clinical practice. This article reviews the recent applications of US in the diagnosis and treatment of hepatocellular carcinoma.

Efficient transfection of tumors facilitated by long-term therapeutic ultrasound in combination with contrast agent: from in vitro to in vivo setting

Therapeutic ultrasound (TUS) is a promising non-viral clinical approach for the delivery of genes. This study demonstrates the efficient delivery and localization of DNA in subcutaneous tumors facilitated by TUS application and examines the contribution of ultrasound contrast-agent (USCA) addition on transfection. The study addresses the importance of in vivo optimization when using long-term TUS and USCA based on data achieved in vitro. In vitro results showed that transfection of TrampC2 prostate cancer (Pca) cells using genes encoding for luciferase and green fluorescent protein was enhanced when DNA and Optison were added together and TUS was applied for 20 or 30 min. In vivo results showed that the highest transfection was achieved when Optison and DNA were co-injected intratumorally, and TUS was applied for 20 min. Using Optison significantly increased protein distribution in the tumor. However, in vivo expression level was decreased by two and four fold at 7 and 14 days, respectively, post-TUS. The study establishes the potential of intratumoral delivery of DNA-Optison, followed by TUS as an effective, non-toxic, gene delivery method that could provide a safe, clinical alternative to current viral gene delivery approaches where short-term gene expression is needed.

Microdevice-based delivery of gene products using sonoporation

This paper presents a proof-of-concept miniature device for delivery of antisense oligonucleotides (ASO). A piezoelectric, lead zirconate titanate (PZT) plate (0.5 cm(2) x 0.75 mm) is used to transfect cells using cavitation-induced sonoporation. Both human umbilical vein endothelial cells (HUVEC) and human prostate cancer cells (PC3) are investigated in vitro. Preliminary results show that after sonication, the transfection rate for HUVEC increases by 96% compared to controls (p < 0.01). For PC3, the transfection rate increases by 31% compared to controls (p < 0.02). This research can potentially be applied in realizing a microelectromechanical system (MEMS)-based device for gene therapy in cancer treatment.

Ultrasound-biophysics mechanisms

Ultrasonic biophysics is the study of mechanisms responsible for how ultrasound and biological materials interact. Ultrasound-induced bioeffect or risk studies focus on issues related to the effects of ultrasound on biological materials. On the other hand, when biological materials affect the ultrasonic wave, this can be viewed as the basis for diagnostic ultrasound. Thus, an understanding of the interaction of ultrasound with tissue provides the scientific basis for image production and risk assessment. Relative to the bioeffect or risk studies, that is, the biophysical mechanisms by which ultrasound affects biological materials, ultrasound-induced bioeffects are generally separated into thermal and non-thermal mechanisms. Ultrasonic dosimetry is concerned with the quantitative determination of ultrasonic energy interaction with biological materials. Whenever ultrasonic energy is propagated into an attenuating material such as tissue, the amplitude of the wave decreases with distance. This attenuation is due to either absorption or scattering. Absorption is a mechanism that represents that portion of ultrasonic wave that is converted into heat, and scattering can be thought of as that portion of the wave, which changes direction. Because the medium can absorb energy to produce heat, a temperature rise may occur as long as the rate of heat production is greater than the rate of heat removal. Current interest with thermally mediated ultrasound-induced bioeffects has focused on the thermal isoeffect concept. The non-thermal mechanism that has received the most attention is acoustically generated cavitation wherein ultrasonic energy by cavitation bubbles is concentrated. Acoustic cavitation, in a broad sense, refers to ultrasonically induced bubble activity occurring in a biological material that contains pre-existing gaseous inclusions. Cavitation-related mechanisms include radiation force, microstreaming, shock waves, free radicals, microjets and strain. It is more challenging to deduce the causes of mechanical effects in tissues that do not contain gas bodies. These ultrasonic biophysics mechanisms will be discussed in the context of diagnostic ultrasound exposure risk concerns.

Microbubble-enhanced ultrasound exposure improves gene transfer in vascular endothelial cells

AIM: To explore the effects of ultrasound exposure combined with microbubble contrast agent (SonoVue) on the permeability of the cellular membrane and on the expression of plasmid DNA encoding enhanced green fluorescent protein (pEGFP) transfer into human umbilical vein endothelial cells (HUVECs).

METHODS: HUVECs with fluorescein isothiocyanate-dextran (FD500) and HUVECs with pEGFP were exposed to continuous wave (1.9 MHz, 80.0 mW/cm²) for 5 min, with or without a SonoVue. The percentage of FD500 taken by the HUVECs and the transient expression rate of pEGFP in the HUVECs were examined by fluorescence microscopy and flow cytometry, respectively.

RESULTS: The percentage of FD500-positive HUVECs in the group of ultrasound exposure combined with SonoVue was significantly higher than that of the group of ultrasound exposure alone (24.0% ± 5.5% vs 66.6% ± 4.1%, P < 0.001). Compared with the group of ultrasound exposure alone, the transfection expression rate of pEGFP in HUVECs was markedly increased with the addition of SonoVue (16.1% ± 1.9% vs 1.5% ± 0.2%, P < 0.001). No statistical significant difference was observed in the HUVECs survival rates between the ultrasound group with and without the addition of SonoVue (94.1% ± 2.3% vs 91.1% ± 4.1%).

CONCLUSION: The cell membrane permeability of HUVECs and the transfection efficiency of pEGFP into HUVECs exposed to ultrasound are significantly increased after addition of an ultrasound contrast agent without obvious damage to the survival of HUVECs. This non-invasive gene transfer method may be a useful tool for clinical gene therapy of hepatic tumors.

Physical parameters affecting ultrasound/microbubble-mediated gene delivery efficiency in vitro

Ultrasound (US)/microbubble-mediated gene delivery is a technology with many potential advantages suited to clinical application. Previous studies have demonstrated transfection but many are unsatisfactory in respect to the exposure apparatus, lack of definition of the US field or the limitations on parameters that can be explored using clinical diagnostic US machines. We investigated individual exposure parameters using a system minimising experimental artefacts and allowing control of many parameters of the US field. Using a 1-MHz transducer we systematically varied US parameters, the duration of exposure and the microbubble and DNA concentrations to optimise gene delivery. Delivery was achieved, using lipid microbubbles (SonoVue) and clinically acceptable US exposures, to adherent cells at efficiencies of approximately 4%. The acoustic pressure amplitude (0.25 MPa peak-negative), pulse repetition frequency (1-kHz) and duration of exposure (10 s) were important in optimising gene delivery with minimal impact on cell viability. These findings support the hypothesis that varying the physical parameters of US-mediated gene delivery has an affect on both efficiency and cell viability. These data are the first in terms of their thorough exploration of the US parameter space and will be the basis for more-informed approaches to developing clinical applications of this technology.

Spatial and acoustic pressure dependence of microbubble-mediated gene delivery targeted using focused ultrasound

BACKGROUND

Ultrasound/microbubble-mediated gene delivery has the potential to be targeted to tissue deep in the body by directing the ultrasound beam following vector administration. Application of this technology would be minimally invasive and benefit from the widespread clinical experience of using ultrasound and microbubble contrast agents. In this study we evaluate the targeting ability and spatial distribution of gene delivery using focused ultrasound.

METHODS

Using a custom-built exposure tank, Chinese hamster ovary cells in the presence of SonoVue microbubbles and plasmid encoding beta-galactosidase were exposed to ultrasound in the focal plane of a 1 MHz transducer. Gene delivery and cell viability were subsequently assessed. Characterisation of the acoustic field and high-resolution spatial analysis of transfection were used to examine the relationship between gene delivery efficiency and acoustic pressure.

RESULTS

In contrast to that seen in the homogeneous field close to the transducer face, gene delivery in the focal plane was concentrated on the ultrasound beam axis. Above a minimum peak-to-peak value of 0.1 MPa, transfection efficiency increased as acoustic pressure increased towards the focus, reaching a maximum above 1 MPa. Delivery was microbubble-dependent and cell viability was maintained.

CONCLUSIONS

Gene delivery can be targeted using focused ultrasound and microbubbles. Since delivery is dependent on acoustic pressure, the degree of targeting can be determined by appropriate transducer design to modify the ultrasound field. In contrast to other physical gene delivery approaches, the non-invasive targeting ability of ultrasound makes this technology an attractive option for clinical gene therapy.

A new imaging strategy using wideband transient response of ultrasound contrast agents

High-resolution clinical systems operating near 15 MHz are becoming more available; however, they lack sensitive harmonic imaging modes for ultrasound contrast agent (UCA) detection, primarily due to limited bandwidth. When an UCA is driven to nonlinear oscillation, a very wideband acoustic transient response is produced that extends beyond 15 MHz. We propose a novel strategy using two separate transducers at widely separated frequencies and arranged confocally to simultaneously excite and receive acoustic transients from UCAs. Experiments were performed to demonstrate that this new mode shows similar resolution, higher echo amplitudes, and greatly reduced attenuation compared to transmission at a higher frequency, and superior resolution compared to transmission and reception at a lower frequency. The proposed method is shown to resolve two 200 microm tubes with centers separated by 400 microm. Strong acoustic transients were detected for rarefaction-first 1-cycle pulses with peak-negative pressures above 300 kPa. The results of this work may lead to uses in flow and/or targeted imaging in applications requiring very high sensitivity to contrast agents.

Dose-dependent disintegration of human endothelial monolayers by contrast echocardiography

Biological effects on endothelium induced by contrast ultrasound (US) may be relevant for transferring drugs into the tissue. An in vitro tissue-mimicking phantom was developed to simulate clinical precordial echocardiography of three modalities (two-dimensional (2DE), pulsed wave (PW), and Power Doppler echocardiography) with gradual increases of acoustic output (mechanical index (MI) 0.0-1.6 and thermal index soft tissue (TIS) 0.0-1.3, respectively; transmit-frequency 1.8 MHz in second harmonic mode (SHI) by 2DE, 1.8 MHz for PW-Doppler, and 3.2 MHz for Power Doppler) as well as contrast agent (CA) concentrations (0.002-4 mg/mL Levovist). Disintegration of the endothelial monolayer was quantitatively analyzed by counting intercellular gaps in light microscopy. No gaps were observed in CA application without sonication. Only few gaps appeared at sonication without CA application in 2DE at MI=1.6 and in PW- and Power Doppler at TIS > or =0.4 and MI > or =0.4. The number of gaps increased significantly with the gradual increase of US output and to a comparably lesser but also significant extent with CA concentrations. Diagnostic contrast echocardiography may induce endothelial disintegrations dependent on US output as well as on CA concentrations. This aspect might be helpful in further in vivo series on local drug delivery.

Retroviral transduction of adherent cells in resonant acoustic fields

Ultrasound-induced cavitation has been extensively used to enhance the efficiency of nonviral-based gene delivery. Although such unique mechanical force could possibly augment the efficacy of retrovirus-mediated gene transfer, we harnessed an alternative approach, a resonant acoustic field, to facilitate the retroviral transduction rate. NIH 3T3 fibroblast cells suspended in a culture well and mixed with ecotropic retroviruses were co-treated with megahertz resonant acoustic fields (RAF). Suspended NIH 3T3 cells under RAF treatment agglomerated at acoustic nodal planes by primary radiation force within a short exposure time. These first arrived and agglomerated cells formed bands as nucleating sites for nanometer-sized ecotropic retroviruses circulated between nodal planes to attach on and thereby increased cell-virus encounters. According to the neomycin-resistant colony assay, 2-fold increment of retroviral transduction rate was obtained by exposing cells and retroviruses in the RAF for 6 min in the presence of 8 microg/mL Polybrene.

The effects of Levovist and DD-723 in activating platelets and damaging hepatic cells of rats

OBJECTIVE

The purpose of this study was to compare platelet activation and hepatic cell damage produced by 2 ultrasonographic contrast agents with flow cytometric and ultrastructural analysis.

METHODS

Suspension samples were made by mixing Levovist (SH U508A; Schering AG, Berlin, Germany) or DD-723 (Nycomed; Amersham Health, Princeton, NJ) with whole blood. The final concentrations of Levovist in citrated whole blood were 0, 15, and 75 mg/mL, and those of DD-723 were 0, 5, and 50 microL/mL. After exposure to ultrasound in vitro, flow cytometric analysis was performed to determine the concentration of the CD62P activation-specific antigen. To compare the hepatic cell damage associated with these 2 agents, we divided 15 rats into 5 groups as follows: group 1, sham operation; group 2, Levovist injection only; group 3, DD-723 injection only; group 4, Levovist injection (contrast agent) and ultrasound exposure; and group 5, DD-723 injection and ultrasound exposure. The ultrasonographic contrast agents Levovist and DD-723 were administered through the femoral vein and sonicated continuously for the first minute; this was followed by sweeping for 5 minutes 10 seconds after the contrast agent was injected. The rats were perfused via the heart with a fixative solution immediately after the sweeping, and then the liver was excised; the specimens were studied with electron and light microscopy.

RESULTS

The percentage of CD62P-expressing platelets increased in both contrast agent-ultrasound exposure groups, and the percentage of CD62P-expressing platelets was greater in the Levovist group. We observed vacuolation and round deposits in the hepatocytes in both contrast agent-ultrasound exposure groups. Microbubbles were observed in the rat Kupffer cells, and a few hepatocytes were seen unexpectedly in the DD-723 group but were found in neither the Kupffer cells nor the hepatocytes in the Levovist group.

CONCLUSIONS

Both contrast agents, Levovist and DD-723, produced platelet activation and structural change in the rat hepatic cells, but only the microbubbles of DD-723 were taken up by the Kupffer cells and a few hepatocytes.

Study of sonoporation dynamics affected by ultrasound duty cycle

Sonoporation is the ultrasound-induced membrane porosity and has been investigated as a means for intracellular drug delivery and nonviral gene transfection. The dynamic characteristics of sonoporation, such as formation, duration and resealing of the pores in the cell membrane, determine the process of intracellular uptake of molecules or agents of interest that are otherwise obstructed by the cell membrane barrier. Sonoporation dynamics is also important for postultrasound cell survival. In this study, we investigated the effects of ultrasound duty cycle on sonoporation dynamics using Xenopus oocyte as a model system. Transducer with a center frequency of 0.96 MHz was used to generate pulsed ultrasound of desired duty cycle (5%, 10% and 15%) at a pulse repetition frequency of 1 Hz and an acoustic pressure of 0.4 MPa in our experiments. Employing voltage clamp techniques, we measured the transmembrane current as the direct result of decreased membrane resistance due to pore formation induced by ultrasound application. We characterized the sonoporation dynamics from these time-resolved recordings of transmembrane current to indicate cell membrane status, including pore formation, extension and resealing. We observed that the transmembrane current amplitude increased with increasing duty cycle, while the recovering process of membrane pores and cell survival rate decreased at higher duty cycles.

Vascular effects induced by combined 1-MHz ultrasound and microbubble contrast agent treatments in vivo

Previous in vivo studies have demonstrated that microvessel hemorrhages and alterations of endothelial permeability can be produced in tissues containing microbubble-based ultrasound contrast agents when those tissues are exposed to MHz-frequency pulsed ultrasound of sufficient pressure amplitudes. The general hypothesis guiding this research was that acoustic (viz., inertial) cavitation, rather than thermal insult, is the dominant mechanism by which such effects arise. We report the results of testing five specific hypotheses in an in vivo rabbit auricular blood vessel model: (1) acoustic cavitation nucleated by microbubble contrast agent can damage the endothelia of veins at relatively low spatial-peak temporal-average intensities, (2) such damage will be proportional to the peak negative pressure amplitude of the insonifying pulses, (3) damage will be confined largely to the intimal surface, with sparing of perivascular tissues, (4) greater damage will occur to the endothelial cells on the side of the vessel distal to the source transducer than on the proximal side and (5) ultrasound/contrast agent-induced endothelial damage can be inherently thrombogenic, or can aid sclerotherapeutic thrombogenesis through the application of otherwise subtherapeutic doses of thrombogenic drugs. Auricular vessels were exposed to 1-MHz focused ultrasound of variable peak pressure amplitude using low duty factor, fixed pulse parameters, with or without infusion of a shelled microbubble contrast agent. Extravasation of Evans blue dye and erythrocytes was assessed at the macroscopic level. Endothelial damage was assessed via scanning electron microscopy (SEM) image analysis. The hypotheses were supported by the data. We discuss potential therapeutic applications of vessel occlusion, e.g., occlusion of at-risk gastric varices.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Use of ultrasound contrast agents for gene or drug delivery in cardiovascular medicine

The clinical utility of ultrasound contrast agents has been established in diagnostic echocardiography. Recently, the use of such agents has been promoted for transport and delivery of various bioactive substances, thus providing a technique for non-invasive gene therapy and organ-specific drug delivery. In this review, we give a critical update of published studies using ultrasound contrast agents for therapeutic use. We discuss the potential applications and limitations of this technique and suggest future applications in cardiovascular medicine.

Theoretical gas body pulsation in relation to empirical gas-body destabilization and to cell membrane damage thresholds

Contrast agent gas bodies attached to phagocytic monolayer cells pulsate in response to ultrasound exposure and damage the cells above thresholds, which increase in proportion to frequency. This study considered the physical basis for the thresholds and their frequency dependence. Theory for the pulsation was evaluated using empirical pulse waveforms acquired at thresholds for 1.0, 2.25, 3.5, 5.0, 7.5, and 10 MHz. For optimum-sized gas bodies, the amplitudes calculated at the thresholds were about 11% of the initial radii. At the cell membrane damage thresholds, theoretical negative shell stresses were approximately constant with frequency at about 50 MPa. This stress appears to be sufficient to induce failure of the shell, and gas body destabilization was confirmed by increases in transmission of ultrasound pulses through the monolayer and by microscopically-observed shrinkage of the gas bodies. A model of acoustic microstreaming was used to calculate the shear stress during the pulses. The maximum shear stress increased from about 1500 to 4500 Pa from 1 to 10 MHz, sufficient for the cell membrane damage. This theoretical analysis shows that both the gas body destabilization and the cell membrane damage could be expected at similar peak rarefactional pressure amplitudes, with thresholds having the observed proportionality to frequency.

Biological effects of low intensity ultrasound: the mechanism involved, and its implications on therapy and on biosafety of ultrasound

The biological effects of low intensity ultrasound (US) in vitro; the mechanisms involved; and the factors that can enhance or inhibit these effects are reviewed. The lowest possible US intensities required to induce cell killing or to produce free radicals were determined. Following sonication in the region of these intensities, the effects of US in combination with either hyperthermia, hypotonia, echo-contrast agents (ECA), CO2, incubation time, high cell density or various agents were examined. The results showed that hyperthermia, hypotonia and microbubbles are good enhancers of the bioeffects, while CO2, incubation time and high cell density are good inhibitors. Cellular membrane damage is pivotal in the events leading to cell death, with the cellular damage-and-repair mechanism as an important determinant of the fate of the damaged cells. The optimal level of apoptosis (with minimal lysis) and optimal gene transfection efficiency were attained using a pulsed low intensity US. In summary, the findings suggest that low intensity US is potentially useful in therapy, while on the other hand, they also call for further investigation of such clinical scenarios as high-grade fever, edema or use of ECA which may lead to the lowering of the threshold for bioeffects with diagnostic US.

Interactions of inertial cavitation bubbles with stratum corneum lipid bilayers during low-frequency sonophoresis

Interactions of acoustic cavitation bubbles with biological tissues play an important role in biomedical applications of ultrasound. Acoustic cavitation plays a particularly important role in enhancing transdermal transport of macromolecules, thereby offering a noninvasive mode of drug delivery (sonophoresis). Ultrasound-enhanced transdermal transport is mediated by inertial cavitation, where collapses of cavitation bubbles microscopically disrupt the lipid bilayers of the stratum corneum. In this study, we describe a theoretical analysis of the interactions of cavitation bubbles with the stratum corneum lipid bilayers. Three modes of bubble-stratum corneum interactions including shock wave emission, microjet penetration into the stratum corneum, and impact of microjet on the stratum corneum are considered. By relating the mechanical effects of these events on the stratum corneum structure, the relationship between the number of cavitation events and collapse pressures with experimentally measured increase in skin permeability was established. Theoretical predictions were compared to experimentally measured parameters of cavitation events.

Membrane damage thresholds for 1- to 10-MHz pulsed ultrasound exposure of phagocytic cells loaded with contrast agent gas bodies in vitro

Monolayers of mouse macrophage-like cells provide a model system for the study of bioeffects of pulsed ultrasound (US) activation of contrast agent gas bodies. In this study, the dependence of membrane damage on ultrasonic frequency was examined for gas bodies attached to the cells. The monolayers cultured on the inside of one window of an exposure chamber were incubated with 2% Optison (Amersham Health Inc., Princeton, NJ) and then rinsed to remove unattached gas bodies. The chamber was filled with culture medium plus 20% trypan blue stain solution and was mounted at the 3.8-cm focus of an US transducer in a 37 degrees C water bath. Transducers were used with center frequencies of 1.0, 2.25, 3.5, 5.0, 7.5 and 10.0 MHz. The 1-min pulsed exposures utilized two-cycle excitation with 1% duty cycle. After exposure, cells in the focal zone were scored for trypan blue dye exclusion, with stained nuclei indicative of cell membrane damage. Exposure-response functions were approximated by performing a series of exposures with peak rarefactional pressure amplitudes differing by a factor of radical 2 (i.e., 3 dB apart). Linear regressions were performed on selected data to determine a threshold pressure amplitude at each frequency. Thresholds ranged from 0.066 MPa at 1.0 MHz to 0.62 MPa at 10 MHz and were approximately proportional to the frequency. These thresholds are less than the pressure amplitudes needed for nucleation of inertial cavitation and have a different frequency dependence than the general Mechanical Index.

A bubble-driven microfluidic transport element for bioengineering

Microfluidics typically uses channels to transport small objects by actuation forces such as an applied pressure difference or thermocapillarity. We propose that acoustic streaming is an alternative means of directional transport at small scales. Microbubbles on a substrate establish well controlled fluid motion on very small scales; combinations ("doublets") of bubbles and microparticles break the symmetry of the motion and constitute flow transport elements. We demonstrate the principle of doublet streaming and describe the ensuing transport. Devices based on doublet flow elements work without microchannels and are thus potentially cheap and highly parallelizable.

Membrane damage thresholds for pulsed or continuous ultrasound in phagocytic cells loaded with contrast agent gas bodies

Cell membrane damage induced by pulsed or continuous ultrasound (US) activation of attached contrast agent gas bodies was examined in an in vitro model system. Monolayers of mouse macrophage-like cells were cultured on the inside of one window of an exposure chamber. The monolayers were incubated with Optison (Amersham Health Inc., Princeton, NJ) or Definity (Bristol-Myers Squib Medical Imaging, North Billerica, MA) and then rinsed to remove unattached gas bodies. A 3.5-MHz focused transducer was aimed at the chamber 3.7 cm away in a 37 degrees C water bath. The cells were scored for Trypan blue dye exclusion, with stained nuclei indicative of cell membrane damage. Exposure-response functions were approximated with exposure levels spaced 3-dB apart. Thresholds were located between the lowest exposure with statistically significant counts of blue-stained cells relative to sham exposures, and the next lower level. Thresholds with Optison included 0.05 MPa for 60-s continuous exposure duration, and 0.21 MPa for 0.6-micros pulses with 60-micros repetition period for 60-s pulsed exposure duration. Results were similar for Definity. Thresholds changed slowly with changes in timing parameters; for example, the threshold for a 0.6-micros continuous exposure (i.e., one pulse) was 0.84 MPa. Compared to 60-s exposure, this represents a factor of 16.8 increase in threshold for a factor of 10(8) decrease in exposure duration. The thresholds are less than the pressure amplitudes needed for nucleation of inertial cavitation, which suggests classification of the phenomenon as a form of gas body activation.

Endothelial cell injury and platelet aggregation induced by contrast ultrasonography in the rat hepatic sinusoid

OBJECTIVE

To determine whether contrast ultrasonography can affect the sinusoidal cells and platelets of the liver by using ultrastructural analysis in vivo.

METHODS

Fifteen Wistar rats were placed into the following 5 groups of 3 rats each: 3 control groups comprising a sham operation group, a contrast agent injection-alone group, and an ultrasound exposure-alone group; and 2 contrast agent injection with ultrasound exposure groups, split according to excision time. After a dose of an echo contrast agent (100 mg/kg of body weight) was administered through the femoral vein, the rats that received injections were subjected to ultrasound for the first minute, no ultrasound for the next 4 minutes, and then ultrasound sweep scanning for 10 seconds. The rats were perfused via the heart with cold physiologic saline containing 2% paraformaldehyde and 2.5% glutaraldehyde solution buffered with 0.1-mol/L phosphate. The livers of the rats in 4 of the groups were excised immediately. The livers of the rats in 1 of the 2 contrast agent with ultrasound exposure groups were excised by the same procedure 5 hours after they received the injections. All specimens were studied with light and electron microscopy.

RESULTS

Platelet aggregation and injury to endothelial cells were more severe in the contrast agent injection and ultrasound exposure groups than in the other groups.

CONCLUSIONS

Contrast ultrasonography can cause platelet aggregation and endothelial cell damage in the rat hepatic sinusoid.