Correlation of ultrasound-induced hemolysis with cavitation detector output in vitro

2D spatiotemporal passive cavitation imaging and evaluation during ultrasound thrombolysis based on diagnostic ultrasound platform

Acoustic cavitation plays a critical role in various biomedical applications. However, uncontrolled cavitation can lead to undesired damage to healthy tissues. Therefore, real-time monitoring and quantitative evaluation of cavitation dynamics is essential for understanding underlying mechanisms and optimizing ultrasound treatment efficiency and safety. The current research addressed the limitations of traditionally used cavitation detection methods by developing introduced an adaptive time-division multiplexing passive cavitation imaging (PCI) system integrated into a commercial diagnostic ultrasound platform. This new method combined real-time cavitation monitoring with B-mode imaging, allowing for simultaneous visualization of treatment progress and 2D quantitative evaluation of cavitation dosage within targeted area. An improved delay-and-sum (DAS) algorithm, optimized with a minimum variance (MV) beamformer, is utilized to minimize the side lobe effect and improve the axial resolution typically associated with PCI. In additional to visualize and quantitatively assess the cavitation activities generated under varied acoustic pressures and microbubble concentrations, this system was specifically applied to perform 2D cavitation evaluation for ultrasound thrombolysis mediated by different solutions, e.g., saline, nanodiamond (ND) and nitrogen-annealed nanodiamond (N-AND). This research aims to bridge the gap between laboratory-based research systems and real-time spatiotemporal cavitation evaluation demands in practical uses. Results indicate that this improved 2D cavitation monitoring and evaluation system could offer a useful tool for comprehensive evaluating cavitation-mediated effects (e.g., ultrasound thrombolysis), providing valuable insights into in-depth understanding of cavitation mechanisms and optimization of cavitation applications.

Magnetic resonance cavitation imaging for the monitoring of ultrasound therapies

Objective.Focused ultrasound (FUS) is a promising non-invasive therapeutic approach that can be used to generate thermal and non-thermal bioeffects. Several non-thermal FUS therapies rely on FUS-induced oscillations of microbubbles (MBs), a phenomenon referred to as cavitation. Cavitation monitoring in real time is essential to ensure both the efficacy and the safety of FUS therapies. This study aims to introduce a new magnetic resonance (MR) method for cavitation monitoring during FUS therapies.Approach.By finely synchronizing the FUS pulse with an accelerated turbo spin-echo MR sequence, the cavitation effect could be quantitatively estimated on the acquired images at 1-Hz refresh rate. The proposed method was assessed in vitro in a water bath. A series of FUS pulses were generated on a silicone tube filled with MBs at different acoustic pressures (0.07-2.07 MPa) and pulse durations (20-2000μs). MR images and passive cavitation detection (PCD) signals were simultaneously acquired for each FUS pulse.Main results.Inertial cavitation was found to induce a quantitatively interpretable signal loss on the MR image. The transition from stable to inertial cavitation was identified on MR cavitation maps with high repeatability. These results were found to be in good agreement with PCD measurements in terms of pressure thresholds between stable and inertial cavitation. MR cavitation imaging was shown to be sensitive to short and even ultrashort FUS pulses, from 2 ms down to 20μs. The presented theoretical model suggests that the signal loss in MR cavitation imaging relies on susceptibility changes related to the diameter of the oscillating MBs.Significance.The proposed MR cavitation imaging method can both locate and characterize cavitation activity. It has therefore the potential to improve the efficacy and safety of FUS therapies, particularly for localized drug delivery applications.

State of the art on microbubble cavitation monitoring and feedback control for blood-brain-barrier opening using focused ultrasound

Focused ultrasound (FUS) is a non-invasive and highly promising method for targeted and reversible blood-brain barrier permeabilization. Numerous preclinical studies aim to optimize the localized delivery of drugs using this method in rodents and non-human primates. Several clinical trials have been initiated to treat various brain diseases in humans using simultaneous BBB permeabilization and drug injection. This review presents the state of the art ofin vitroandin vivocavitation control algorithms for BBB permeabilization using microbubbles (MB) and FUS. Firstly, we describe the different cavitation states, their physical significance in terms of MB behavior and their translation into the spectral composition of the backscattered signal. Next, we report the different indexes calculated and used during the ultrasonic monitoring of cavitation. Finally, the differentin vitroandin vivocavitation control strategies described in the literature are presented and compared.

Dynamic Behavior of Polymer Microbubbles During Long Ultrasound Tone-Burst Excitation and Its Application for Sonoreperfusion Therapy

OBJECTIVE

Ultrasound (US)-targeted microbubble (MB) cavitation (UTMC)-mediated therapies have been found to restore perfusion and enhance drug/gene delivery. Because of the potentially longer circulation time and relative ease of storage and reconstitution of polymer-shelled MBs compared with lipid MBs, we investigated the dynamic behavior of polymer microbubbles and their therapeutic potential for sonoreperfusion (SRP) therapy.

METHODS

The fate of polymer MBs during a single long tone-burst exposure (1 MHz, 5 ms) at various acoustic pressures and MB concentrations was recorded via high-speed microscopy and passive cavitation detection (PCD). SRP efficacy of the polymer MBs was investigated in an in vitro flow system and compared with that of lipid MBs.

DISCUSSION

Microscopy videos indicated that polymer MBs formed gas-filled clusters that continued to oscillate, fragment and form new gas-filled clusters during the single US burst. PCD confirmed continued acoustic activity throughout the 5-ms US excitation. SRP efficacy with polymer MBs increased with pulse duration and acoustic pressure similarly to that with lipid MBs but no significant differences were found between polymer and lipid MBs.

CONCLUSION

These data suggest that persistent cavitation activity from polymer MBs during long tone-burst US excitation confers excellent reperfusion efficacy.

Proof of Concept: Protein Delivery into Human Erythrocytes Using Stable Cavitation

Human erythrocytes represent candidates of choice as carriers for a wide range of drugs due to their unique biophysical and physiological properties. In this study, we used a sonoporation device generating and monitoring acoustic stable cavitation without any addition of contrast or nucleation agents. The device was evaluated for bovine serum albumin (BSA) delivery into human erythrocytes. After determining the adequate hematocrit percentage compatible with the generation of stable cavitation, we determined the optimal sonoporation conditions allowing BSA delivery while preserving erythrocyte integrity. Our results demonstrate that stable cavitation allows efficient delivery of proteins into human erythrocytes with limited lysis of these cells. In conclusion, our study allowed for the development of a stable and regulated cavitation program and the establishment of sonoporation conditions suitable for intracellular protein delivery while maintaining erythrocyte integrity. Additional investigations are needed to move from the proof of concept to a larger-scale application.

Lytic Release of Cellular ATP: Physiological Relevance and Therapeutic Applications

The lytic release of ATP due to cell and tissue injury constitutes an important source of extracellular nucleotides and may have physiological and pathophysiological roles by triggering purinergic signalling pathways. In the lungs, extracellular ATP can have protective effects by stimulating surfactant and mucus secretion. However, excessive extracellular ATP levels, such as observed in ventilator-induced lung injury, act as a danger-associated signal that activates NLRP3 inflammasome contributing to lung damage. Here, we discuss examples of lytic release that we have identified in our studies using real-time luciferin-luciferase luminescence imaging of extracellular ATP. In alveolar A549 cells, hypotonic shock-induced ATP release shows rapid lytic and slow-rising non-lytic components. Lytic release originates from the lysis of single fragile cells that could be seen as distinct spikes of ATP-dependent luminescence, but under physiological conditions, its contribution is minimal <1% of total release. By contrast, ATP release from red blood cells results primarily from hemolysis, a physiological mechanism contributing to the regulation of local blood flow in response to tissue hypoxia, mechanical stimulation and temperature changes. Lytic release of cellular ATP may have therapeutic applications, as exemplified by the use of ultrasound and microbubble-stimulated release for enhancing cancer immunotherapy in vivo.

The relation of Bleomycin Delivery Efficiency to Microbubble Sonodestruction and Cavitation Spectral Characteristics

The concurrent assessment of principal sonoporation factors has been accomplished in a single systemic study. Microbubble sonodestruction dynamics and cavitation spectral characteristics, ultrasound scattering and attenuation, were examined in relation to the intracellular delivery of anticancer drug, bleomycin. Experiments were conducted on Chinese hamster ovary cells coadministered with Sonovue microbubbles. Detailed analysis of the scattering and attenuation temporal functions culminated in quantification of metrics, inertial cavitation dose and attenuation rate, suitable for cavitation control. The exponents, representing microbubble sonodestruction kinetics were exploited to derive dosimetric, microbubble sonodestruction rate. High intracorrelation between empirically-attained metrics defines the relations which indicate deep physical interdependencies within inherent phenomena. Subsequently each quantified metric was validated to be well-applicable to prognosticate the efficacy of bleomycin delivery and cell viability, as indicated by strong overall correlation (R² > 0.85). Presented results draw valuable insights in sonoporation dosimetry and contribute towards the development of universal sonoporation dosimetry model. Both bleomycin delivery and cell viability reach their respective plateau levels by the time, required to attain total microbubble sonodestruction, which accord with scattering and attenuation decrease to background levels. This suggests a well-defined criterion, feasible through signal-registration, universally employable to set optimal duration of exposure for efficient sonoporation outcome.

Spatiotemporal Assessment of the Cellular Safety of Cavitation-Based Therapies by Passive Acoustic Mapping

Many useful therapeutic bio-effects can be generated using ultrasound-induced cavitation. However, cavitation is also capable of causing unwanted cellular and vascular damage, which should be monitored to ensure treatment safety. In this work, the unique opportunity provided by passive acoustic mapping (PAM) to quantify cavitation dose across an entire volume of interest during therapy is utilised to provide setup-independent measures of spatially localised cavitation dose. This spatiotemporally quantifiable cavitation dose is then related to the level of cellular damage generated. The cavitation-mediated destruction of equine red blood cells mixed with one of two types of cavitation nuclei at a variety of concentrations is investigated. The blood is placed within a 0.5-MHz ultrasound field and exposed to a range of peak rarefactional pressures up to 2 MPa, with 50 to 50,000 cycle pulses maintaining a 5% duty cycle. Two co-planar linear arrays at 90° to each other are used to generate 400-µm-resolution frequency domain robust capon beamforming PAM maps, which are then used to generate estimates of cavitation dose. A relationship between this cavitation dose and the levels of haemolysis generated was found which was comparable regardless of the applied acoustic pressure, pulse length, cavitation agent type or concentration used. PAM was then used to monitor cellular damage in multiple locations within a tissue phantom simultaneously, with the damage-cavitation dose relationship being similar for the two experimental models tested. These results lay the groundwork for this method to be applied to other measures of safety, allowing for improved ultrasound monitoring of cavitation-based therapies.

Cell-cycle-dependences of membrane permeability and viability observed for HeLa cells undergoing multi-bubble-cell interactions

Microbubble-mediated sonoporation is a promising strategy for intracellular gene/drug delivery, but the biophysical mechanisms involved in the interactions between microbubbles and cells are not well understood. Here, HeLa cells were synchronized in individual cycle phases, then the cell-cycle-dependences of the membrane permeability and viability of HeLa cells undergoing multi-bubble sonoporation were evaluated using focused ultrasound exposure apparatus coupled passive cavitation detection system. The results indicated that: (1) the microbubble cavitation activity should be independent on cell cycle phases; (2) G₁-phase cells with the largest Young's modulus were the most robust against microbubble-mediated sonoporation; (3) G₂/M-phase cells exhibited the greatest accumulated FITC uptake with the lowest viability, which should be mainly attributed to the chemical effect of synchronization drugs; and (4) more important, S-phase cells with the lowest stiffness seemed to be the most susceptible to the mechanical effect generated by microbubble cavitation activity, which resulted in the greatest enhancement in sonoporation-facilitated membrane permeabilization without further scarifying their viability. The current findings may benefit ongoing efforts aiming to pursue rational utilization of microbubble-mediated sonoporation in cell-cycle-targeted gene/drug delivery for cancer therapy.

Bubble dynamics in boiling histotripsy

Boiling histotripsy is a non-invasive, cavitation-based ultrasonic technique which uses a number of millisecond pulses to mechanically fractionate tissue. Though a number of studies have demonstrated the efficacy of boiling histotripsy for fractionation of solid tumours, treatment monitoring by cavitation measurement is not well studied because of the limited understanding of the dynamics of bubbles induced by boiling histotripsy. The main objectives of this work are to (a) extract qualitative and quantitative features of bubbles excited by shockwaves and (b) distinguish between the different types of cavitation activity for either a thermally or a mechanically induced lesion in the liver. A numerical bubble model based on the Gilmore equation accounting for heat and mass transfer (gas and water vapour) was developed to investigate the dynamics of a single bubble in tissue exposed to different High Intensity Focused Ultrasound fields as a function of temperature variation in the fluid. Furthermore, ex vivo liver experiments were performed with a passive cavitation detection system to obtain acoustic emissions. The numerical simulations showed that the asymmetry in a shockwave and water vapour transport are the key parameters which lead the bubble to undergo rectified growth at a boiling temperature of 100°C. The onset of rectified radial bubble motion manifested itself as (a) an increase in the radiated pressure and (b) the sudden appearance of higher order multiple harmonics in the corresponding spectrogram. Examining the frequency spectra produced by the thermal ablation and the boiling histotripsy exposures, it was observed that higher order multiple harmonics as well as higher levels of broadband emissions occurred during the boiling histotripsy insonation. These unique features in the emitted acoustic signals were consistent with the experimental measurements. These features can, therefore, be used to monitor (a) the different types of acoustic cavitation activity for either a thermal ablation or a mechanical fractionation process and (b) the onset of the formation of a boiling bubble at the focus in the course of HIFU exposure.

Performance characterisation of a passive cavitation detector optimised for subharmonic periodic shock waves from acoustic cavitation in MHz and sub-MHz ultrasound

We describe the design, construction and characterisation of a broadband passive cavitation detector, with the specific aim of detecting low frequency components of periodic shock waves, with high sensitivity. A finite element model is used to guide selection of matching and backing layers for the shock wave passive cavitation detector (swPCD), and the performance is evaluated against a commercially available device. Validation of the model, and characterisation of the swPCD is achieved through experimental detection of laser-plasma bubble collapse shock waves. The final swPCD design is 20 dB more sensitive to the subharmonic component, from acoustic cavitation driven at 220 kHz, than the comparable commercial device. This work may be significant for monitoring cavitation in medical applications, where sensitive detection is critical, and higher frequencies are more readily absorbed by tissue.

Effect of pulse duration and pulse repetition frequency of cavitation histotripsy on erosion at the surface of soft material

Cavitation histotripsy with the short pulse duration (PD) but high pulse repetition frequency (PRF) disintegrates the tissue at a fluid interface. However, longer PD and lower PRF are used in the other focused ultrasound applications, where the acoustic radiation force, streaming, and cavitation are different, and their effects on erosion are unknown. In this study, the erosion at the surface of phantom/ex vivo tissue and the characteristics of induced bubble cloud captured by high-speed photography, passive cavitation detection, and light transmission during histotripsy exposure at varied PDs and PRFs but the same duty cycle were compared. The peak negative pressure of 6.6 MPa at the PD of 20 ms and PRF of 1 Hz began to erode the phantom, which becomes more significant with the increase of peak negative pressure, PD, and interval time between bursts. The increase of the PRF from 1 Hz to 1000 Hz, while the decrease of the PD from 20 ms to 20 μs (duty cycle of 2%) at the same energy was delivered to the gel phantom immersed in the degassed water led to the decrease of erosion volume but a slight increase of the erosion area and smoother surface. Low PRF and long PD produce the significant tissue deformation, acoustic wave refocusing, confinement of bubbles in a conical region, and more bubble dissolution after the collapse for the high acoustic scattering and light transmission signals. In comparison, high PRF and low PD produce a wide distribution of bubbles with only little wave refocusing at the beginning of cavitation histotripsy and high inertial cavitation. Acoustic emission dose has a good correlation with the erosion volume. The erosion on the porcine kidney at the varied PRFs and PDs with the same energy output showed similar trends as those in the phantom but at a slow rate. In summary, the PRF and PD are important parameters for the cavitation histotripsy-induced erosion at the interface of fluid and soft material, and they should be optimized for the best outcome.

Mechanical and Biological Effects of Ultrasound: A Review of Present Knowledge

Ultrasound is widely used for medical diagnosis and increasingly for therapeutic purposes. An understanding of the bio-effects of sonography is important for clinicians and scientists working in the field because permanent damage to biological tissues can occur at high levels of exposure. Here the underlying principles of thermal mechanisms and the physical interactions of ultrasound with biological tissues are reviewed. Adverse health effects derived from cellular studies, animal studies and clinical reports are reviewed to provide insight into the in vitro and in vivo bio-effects of ultrasound.

Investigation of Microbubble Cavitation-Induced Calcein Release from Cells In Vitro

In the present study, microbubble (MB) cavitation signal analysis was performed together with calcein release evaluation in both pressure and exposure duration domains of the acoustic field. A passive cavitation detection system was used to simultaneously measure MB scattering and attenuation signals for subsequent extraction efficiency relative to MB cavitation activity. The results indicate that the decrease in the efficiency of extraction of calcein molecules from Chinese hamster ovary cells, as well as cell viability, is associated with MB cavitation activity and can be accurately predicted using inertial cavitation doses up to 0.18 V × s (R² > 0.9, p < 0.0001). No decrease in additional calcein release or cell viability was observed after complete MB sonodestruction was achieved. This indicates that the optimal exposure duration within which maximal sono-extraction efficiency is obtained coincides with the time necessary to achieve complete MB destruction. These results illustrate the importance of MB inertial cavitation in the sono-extraction process. To our knowledge, this study is the first to (i) investigate small molecule extraction from cells via sonoporation and (ii) relate the extraction process to the quantitative characteristics of MB cavitation acoustic spectra.

Inertial cavitation initiated by polytetrafluoroethylene nanoparticles under pulsed ultrasound stimulation

Nanoscale gas bubbles residing on a macroscale hydrophobic surface have a surprising long lifetime (on the order of days) and can serve as cavitation nuclei for initiating inertial cavitation (IC). Whether interfacial nanobubbles (NBs) reside on the infinite surface of a hydrophobic nanoparticle (NP) and could serve as cavitation nuclei is unknown, but this would be very meaningful for the development of sonosensitive NPs. To address this problem, we investigated the IC activity of polytetrafluoroethylene (PTFE) NPs, which are regarded as benchmark superhydrophobic NPs due to their low surface energy caused by the presence of fluorocarbon. Both a passive cavitation detection system and terephthalic dosimetry was applied to quantify the intensity of IC. The IC intensities of the suspension with PTFE NPs were 10.30 and 48.41 times stronger than those of deionized water for peak negative pressures of 2 and 5MPa, respectively. However, the IC activities were nearly completely inhibited when the suspension was degassed or ethanol was used to suspend PTFE NPs, and they were recovered when suspended in saturated water, which may indicates the presence of interfacial NBs on PTFE NPs surfaces. Importantly, these PTFE NPs could sustainably initiate IC for excitation by a sequence of at least 6000 pulses, whereas lipid microbubbles were completely depleted after the application of no more than 50 pulses under the same conditions. The terephthalic dosimetry has shown that much higher hydroxyl yields were achieved when PTFE NPs were present as cavitation nuclei when using ultrasound parameters that otherwise did not produce significant amounts of free radicals. These results show that superhydrophobic NPs may be an outstanding candidate for use in IC-related applications.

Ultrasound Effect on Cancerous versus Non-Cancerous Cells

Previous studies have found that cancer cells whose metastatic potential is low are more vulnerable to mechanical stress-induced trauma to their cytoskeleton compared with benign cells. Because ultrasound induces mechanical stresses on cells and tissues, it is postulated that there may be a way to apply ultrasound to tumors to reduce their ability to metastasize. The difference between low-malignant-potential cancer cells and benign cells could be a result of their different responses to the mechanical stress insonation induced. This hypothesis was tested in vitro and in vivo. Low-malignant-potential cells were found to be more sensitive to insonation, resulting in a significantly higher mortality rate compared with that of benign cells, 89% versus 21%, respectively. This effect can be controlled by varying ultrasound parameters: intensity, duration, and duty cycle. Thus, the results presented in this study suggest the application of ultrasound to discriminate between benign and malignant cells.

Dynamic Behavior of Microbubbles during Long Ultrasound Tone-Burst Excitation: Mechanistic Insights into Ultrasound-Microbubble Mediated Therapeutics Using High-Speed Imaging and Cavitation Detection

Ultrasound (US)-microbubble (MB)-mediated therapies have been found to restore perfusion and enhance drug/gene delivery. On the presumption that MBs do not persist during long US exposure under high acoustic pressures, most schemes use short US pulses when a high US pressure is employed. However, we recently observed an enhanced thrombolytic effect using long US pulses at high acoustic pressures. Therefore, we explored the fate of MBs during long tone-burst exposures (5 ms) at various acoustic pressures and MB concentrations via direct high-speed optical observation and passive cavitation detection. MBs first underwent stable or inertial cavitation depending on the acoustic pressure and then formed gas-filled clusters that continued to oscillate, break up and form new clusters. Cavitation detection confirmed continued, albeit diminishing, acoustic activity throughout the 5-ms US excitation. These data suggest that persisting cavitation activity during long tone bursts may confer additional therapeutic effects.

Analysis of Metrics for Molecular Sonotransfer in Vitro

Ultrasound induced microbubble (MB) cavitation is used to significantly enhance cell membrane permeabilization, thereby allowing delivery of various therapeutic agents into cells. In order to monitor and quantitatively control the extent of cavitation the uniform dosimetry model is needed. In present study we have simultaneously performed quantitative evaluation of three main sonoporation factors: (1) MB concentration, (2) MB cavitation extent, and (3) doxorubicin (DOX) sonotransfer into Chinese hamster ovary cells. MB concentration measurement results and passively recorded MB cavitation signals were used for MB sonodestruction rate and spectral root-mean-square (RMS) calculations, respectively. Subsequently, time to maximum value of RMS and inertial cavitation dose (ICD) quantifications were performed for every acoustic pressure value. This comprehensive research has led not only to explanation of relation of ICD and MB sonodestruction rate but also to the development of a new sonoporation metric: the inverse of time to maximum value of RMS (1/time to maximum value of RMS). ICD and MB sonodestruction rate intercorrelation and correlation with DOX sonotransfer suggest inertial cavitation to be the key mechanism for cell sonoporation. All these metrics were successfully used for doxorubicin sonotransfer prediction (R(2) > 0.9, p < 0.01) and therefore shows feasibility to be applied for future dosimetric applications for ultrasound-mediated drug and gene delivery.

Acoustic Cavitation-Mediated Delivery of Small Interfering Ribonucleic Acids with Phase-Shift Nano-Emulsions

Localized, targeted delivery of small interfering ribonucleic acid (siRNA) has been the foremost hurdle in the use of siRNA for the treatment of various diseases. Major advances have been achieved in the synthesis of siRNA, which have led to greater target messenger RNA (mRNA) silencing and stability under physiologic conditions. Although numerous delivery strategies have shown promise, there are still limited options for targeted delivery and release of siRNA administered systemically. In this in vitro study, phase-shift nano-emulsions (PSNE) were explored as cavitation nuclei to facilitate free siRNA delivery to cancer cells via sonoporation. A cell suspension containing varying amounts of PSNE and siRNA was exposed to 5-MHz pulsed ultrasound at fixed settings (6.2-MPa peak negative pressure, 5-cycle pulses, 250-Hz pulse repetition frequency (PRF) and total exposure duration of 100 s). Inertial cavitation emissions were detected throughout the exposure using a passive cavitation detector. Successful siRNA delivery was achieved (i.e., >50% cell uptake) with high (>80%) viability. The percentage of cells with siRNA uptake was correlated with the amount of inertial cavitation activity generated from vaporized PSNE. The siRNA remained functional after delivery, significantly reducing expression of green fluorescent protein in a stably transfected cell line. These results indicate that vaporized PSNE can facilitate siRNA entry into the cytosol of a majority of sonicated cells and may provide a non-endosomal route for siRNA delivery.

On the relationship between microbubble fragmentation, deflation and broadband superharmonic signal production

Acoustic angiography imaging of microbubble contrast agents uses the superharmonic energy produced from excited microbubbles and enables high-contrast, high-resolution imaging. However, the exact mechanism by which broadband harmonic energy is produced is not fully understood. To elucidate the role of microbubble shell fragmentation in superharmonic signal production, simultaneous optical and acoustic measurements were performed on individual microbubbles at transmit frequencies from 1.75 to 3.75 MHz and pressures near the shell fragmentation threshold for microbubbles of varying diameter. High-amplitude, broadband superharmonic signals were produced with shell fragmentation, whereas weaker signals (approximately 25% of peak amplitude) were observed in the presence of shrinking bubbles. Furthermore, when populations of stationary microbubbles were imaged with a dual-frequency ultrasound imaging system, a sharper decline in image intensity with respect to frame number was observed for 1-μm bubbles than for 4-μm bubbles. Finally, in a study of two rodents, increasing frame rate from 4 to 7 Hz resulted in decreases in mean steady-state image intensity of 27% at 1000 kPa and 29% at 1300 kPa. Although the existence of superharmonic signals when bubbles shrink has the potential to prolong the imaging efficacy of microbubbles, parameters such as frame rate and peak pressure must be balanced with expected re-perfusion rate to maintain adequate contrast during in vivo imaging.

Sonothrombolysis: the contribution of stable and inertial cavitation to clot lysis

Microbubble-mediated sonothrombolysis (STL) is a remarkable approach to vascular occlusion therapy. However, STL remains a complex process with multiple interactions between clot, ultrasound (US), microbubbles (MB) and thrombolytic drug. The aim of this study was to evaluate the ability of combining US and MB to degrade fibrin and, more specifically, to assess the roles of both stable (SC) and inertial (IC) cavitation. Human blood clots containing radiolabeled fibrin were exposed to different combinations of recombinant tissue plasminogen activator (rtPA), US (1 MHz) and phospholipid MB. Three acoustic pressures were tested: 200, 350 and 1,300 kPa (peak-negative pressure). Clot lysis was assessed by diameter loss and release of radioactive fibrin degradation products. The combination rtPA + US + MB clearly revealed that IC (1,300 kPa) was able to enhance fibrin degradation significantly (66.3 ± 1.8%) compared with rtPA alone (51.7 ± 2.0%, p < 0.001). However, SC failed to enhance fibrin degradation at an acoustic pressure of 200 kPa. At 350 kPa, a synergistic effect between rtPA and US + MB was observed with an absolute increase of 6% compared to rtPA alone (p < 0.001). Conversely, without rtPA, the combination of US + MB was unable to degrade the fibrin network (0.3 ± 0.1%, p > 0.05 vs. control), but induced a distinct loss of red blood cells throughout the entire thickness of the clot, implying that MB were able to penetrate and cavitate inside the clot.

Real-time feedback of histotripsy thrombolysis using bubble-induced color Doppler

Histotripsy thrombolysis is a non-invasive, drug-free, image-guided therapy that fractionates blood clots using well-controlled acoustic cavitation alone. Real-time quantitative feedback is highly desired during histotripsy thrombolysis treatment to monitor the progress of clot fractionation. Bubble-induced color Doppler (BCD) monitors the motion after cavitation generated by each histotripsy pulse, which has been found in gel and ex vivo liver tissue to be correlated with histotripsy fractionation. We investigated the potential of BCD to quantitatively monitor histotripsy thrombolysis in real time. To visualize clot fractionation, transparent three-layered fibrin clots were developed. Results indicated that a coherent motion follows the cavitation generated by each histotripsy pulse with a push and rebound pattern. The temporal profile of this motion expands and saturates as treatment progresses. A strong correlation exists between the degree of histotripsy clot fractionation and two metrics extracted from BCD: time of peak rebound velocity (tPRV) and focal mean velocity at a fixed delay (Vf,delay). The saturation of clot fractionation (i.e., treatment completion) matches well the saturations detected using tPRV and Vf,delay. The mean Pearson correlation coefficients between the progression of clot fractionation and the two BCD metrics were 93.1% and 92.6%, respectively. To validate BCD feedback in in vitro clots, debris volumes from histotripsy thrombolysis were obtained at different therapy doses and compared with Vf,delay. There is also good agreement between the increasing and saturation trends of debris volume and Vf,delay. Finally, a real-time BCD feedback algorithm to predict complete clot fractionation during histotripsy thrombolysis was developed and tested. This work illustrates the potential of BCD to monitor histotripsy thrombolysis treatment in real time.

Ultrasonically triggered drug delivery: breaking the barrier

The adverse side-effects of chemotherapy can be minimized by delivering the therapeutics in time and space to only the desired target site. Ultrasound offers one fairly non-invasive method of accomplishing such precise delivery because its energy can disrupt nanosized containers that are designed to sequester the drug until the ultrasonic event. Such containers include micelles, liposomes and solid nanoparticles. Conventional micelles and liposomes are less acoustically sensitive to ultrasound because the strongest forces associated with ultrasound are generated by gas-liquid interfaces, which both of these conventional constructs lack. Acoustically activated carriers often incorporate a gas phase, either actively as preformed bubbles, or passively such as taking advantage of dissolved gasses that form bubbles upon insonation. Newer concepts include using liquids that form gas when insonated. This review focuses on the ultrasonically activated delivery of therapeutics from micelles, liposomes and solid particles. In vitro and in vivo results are summarized and discussed. Novel structural concepts from micelles and liposomes are presented. Mechanisms of ultrasonically activated release are discussed. The future of ultrasound in drug delivery is envisioned.

Relations between acoustic cavitation and skin resistance during intermediate- and high-frequency sonophoresis

Enhanced skin permeability is known to be achieved during sonophoresis due to ultrasound-induced cavitation. However, the mechanistic role of cavitation during sonophoresis has been extensively investigated only for low-frequency (LFS, <100 kHz) applications. Here, mechanisms of permeability-enhancing stable and inertial cavitation were investigated by passively monitoring subharmonic and broadband emissions arising from cavitation isolated within or external to porcine skin in vitro during intermediate- (IFS, 100-700 kHz) and high-frequency sonophoresis (HFS, >1 MHz). The electrical resistance of skin, a surrogate measure of the permeability of skin to a variety of compounds, was measured to quantify the reduction and subsequent recovery of the skin barrier during and after exposure to pulsed (1 second pulse, 20% duty cycle) 0.41 and 2.0 MHz ultrasound over a range of acoustic powers (0-21.7 W) for 30 min. During IFS, significant skin resistance reductions and acoustic emissions from cavitation were measured exclusively when cavitation was isolated outside of the skin. Time-dependent skin resistance reductions measured during IFS correlated significantly with subharmonic and broadband emission levels. During HFS, significant skin resistance reductions were accompanied by significant acoustic emissions from cavitation measured during trials that isolated cavitation activity either outside of skin or within skin. Time-dependent skin resistance reductions measured during HFS correlated significantly greater with subharmonic than with broadband emission levels. The reduction of the skin barrier due to sonophoresis was reversible in all trials; however, effects incurred during IFS recovered more slowly and persisted over a longer period of time than HFS. These results quantitatively demonstrate the significance of cavitation during sonophoresis and suggest that the mechanisms and post-treatment longevity of permeability enhancement due to IFS and HFS treatments are different.

Flow rate and duty cycle effects in lysis of Chlamydomonas reinhardtii using high-energy pulsed focused ultrasound

To consider microalgae lipid biofuel as a viable energy source, it is a necessity to maximize algal cell lysis, lipid harvest, and thus biofuel production versus the energy used to lyse the cells. Previous techniques have been to use energy consumptive ultrasound waves in the 10-40 kHz range in a stationary exposure environment. This study evaluated the potential of using 1.1 MHz ultrasound pulses in a new flow through type chamber on Chlamydomonas reinhardtii as a model organism for cell breakage. The ultrasound was generated using a spherically focused transducer with a focal length of 6.34 cm and an active diameter of 6.36 cm driven by 20 cycle sine-wave tone bursts at varied pulse repetition frequencies. First, variations in flow rate were examined at a constant duty cycle of 3.6%. After assessing flow rates, the duty cycle was varied to further explore the dependence on the tone burst parameters. Cell lysis was assessed by quantifying protein and chlorophyll release into the supernatant as well as by lipid extractability. Appropriate flow rates with higher duty cycles led to statistically significant increases in cell lysis relative to controls and other exposure conditions.

Lysis of Chlamydomonas reinhardtii by high-intensity focused ultrasound as a function of exposure time

Efficient lysis of microalgae for lipid extraction is an important concern when processing biofuels. Historically, ultrasound frequencies in the range of 10-40 kHz have been utilized for this task. However, greater efficiencies might be achievable if higher frequencies could be used. In our study, we evaluated the potential of using 1.1 MHz ultrasound to lyse microalgae for biofuel production while using Chlamydomonas reinhardtii as a model organism. The ultrasound was generated using a spherically focused transducer with a focal length of 6.34 cm and an active diameter of 6.36 cm driven by 20 cycle sine-wave tone bursts at a pulse repetition frequency of 2 kHz (3.6% duty cycle). The time-average acoustic power output was 26.2 W while the spatial-peak-pulse-average intensity (ISPPA) for each tone burst was 41 kW/cm(2). The peak compressional and rarefactional pressures at the focus were 102 and 17 MPa, respectively. The exposure time was varied for the different cases in the experiments from 5s to 9 min and cell lysis was assessed by quantifying the percentage of protein and chlorophyll release into the supernate as well as the lipid extractability. Free radical generation and lipid oxidation for the different ultrasound exposures were also determined. We found that there was a statistically significant increase in lipid extractability for all of the exposures compared to the control. The longer exposures also completely fragmented the cells releasing almost all of the protein and chlorophyll into the supernate. The cavitation activity did not significantly increase lipid oxidation while there was a minor trend of increased free radical production with increased ultrasound exposure.

Variations of bubble cavitation and temperature elevation during lesion formation by high-intensity focused ultrasound

High-intensity focused ultrasound (HIFU) is emerging as an effective therapeutic modality in both thermal ablations for solid tumor/cancer and soft-tissue fragmentation. Mechanical and thermal effects, which play an important role in the HIFU treatment simultaneously, are dependent on the operating parameters and may vary with the progress of therapy. Mechanical erosion in the shape of a "squid," a "dumbbell" lesion with both mechanical and thermal lesions, or a "tadpole" lesion with mechanical erosion at the center and thermal necrosis on the boundary in the transparent gel phantom could be produced correspondingly with the pulse duration of 5-30 ms, which is much longer than histotripsy burst but shorter than the time for tissue boiling, and pulse repetition frequency (PRF) of 0.2-5 Hz. Meanwhile, variations of bubble cavitation (both inertial and stable cavitation) and temperature elevation in the focal region (i.e., z = -2.5, 0, and 2.5 mm) were measured by passive cavitation detection (PCD) and thermocouples during the therapeutic procedure, respectively. Stable cavitation increased with the pulse duration, PRF, and the number of pulses delivered. However, inertial cavitation was found to increase initially and then decrease with long pulse duration and high PRF. Temperature in the pre-focal region is always higher than those at the focal and post-focal position in all tests. Great variations of PCD signals and temperature elevation are due to the generation and persistence of large bubble, which is resistant to collapse and occurs with the increase of pulse duration and PRF. Similar lesion pattern and variations were also observed in ex vivo porcine kidneys. Hyperechoes in the B-mode ultrasound image were comparable to the shape and size of lesions in the dissected tissue. Thermal lesion volume increased with the increase of pulse duration and PRF, but mechanical erosion reached its maximum volume with the pulse duration of 20 ms and PRF of 1 Hz. Altogether, bubble cavitation and thermal field vary with the progress of HIFU treatment with different sonication parameters, which provide insights into the interaction of ultrasound burst with the induced bubbles for both soft tissue fractionation and enhancement in thermal accumulation. Appropriate synergy and monitoring of mechanical and thermal effects would broaden the HIFU application and enhance its efficiency as well as safety.

Microbubble cavitation imaging

Ultrasound cavitation of microbubble contrast agents has a potential for therapeutic applications such as sonothrombolysis (STL) in acute ischemic stroke. For safety, efficacy, and reproducibility of treatment, it is critical to evaluate the cavitation state (moderate oscillations, stable cavitation, and inertial cavitation) and activity level in and around a treatment area. Acoustic passive cavitation detectors (PCDs) have been used to this end but do not provide spatial information. This paper presents a prototype of a 2-D cavitation imager capable of producing images of the dominant cavitation state and activity level in a region of interest. Similar to PCDs, the cavitation imaging described here is based on the spectral analysis of the acoustic signal radiated by the cavitating microbubbles: ultraharmonics of the excitation frequency indicate stable cavitation, whereas elevated noise bands indicate inertial cavitation; the absence of both indicates moderate oscillations. The prototype system is a modified commercially available ultrasound scanner with a sector imaging probe. The lateral resolution of the system is 1.5 mm at a focal depth of 3 cm, and the axial resolution is 3 cm for a therapy pulse length of 20 μs. The maximum frame rate of the prototype is 2 Hz. The system has been used for assessing and mapping the relative importance of the different cavitation states of a microbubble contrast agent. In vitro (tissue-mimicking flow phantom) and in vivo (heart, liver, and brain of two swine) results for cavitation states and their changes as a function of acoustic amplitude are presented.

Application of cavitation promoting surfaces in management of acute ischemic stroke

High frequency, low intensity ultrasound has the potential to accelerate the clearance of thrombotic occlusion in the absence of cavitation. At high frequency ultrasound, high acoustic pressures, >5.2MPa, are required to generate cavitation in thrombus. The focus of this study was to reduce the cavitation threshold by applying materials with appropriate nucleation sites at the transducer-thrombus boundary to further augment sonothrombolysis. Heterogeneous and homogenous nucleation sites were generated on the outer surface of a polyimide tube (PI) using microfringed (MPI) and laser induced (LPI) microcavities. The cavitation threshold of these materials was determined using a passive cavitation detection system. Furthermore, the biological impact of both materials was investigated in vitro. The results revealed that both MPI and LPI have the potential to induce cavitation at acoustic pressure levels as low as 2.3MPa. In the presence of cavitation, thrombolysis rate could be enhanced by up to two times without any evidence of hemolysis that is generally associated with cavitation activities in blood. A prototype ultrasonic catheter operating at 1.7MHz frequency and acoustic pressure of 2.3MPa with either of MPI or LPI could be considered as a viable option for treatment of acute ischemic stroke.

Oscillating bubbles: a versatile tool for lab on a chip applications

With the fast development of acoustic and multiphase microfluidics in recent years, oscillating bubbles have drawn more-and-more attention due to their great potential in various Lab on a Chip (LOC) applications. Many innovative bubble-based devices have been explored in the past decade. In this article, we first briefly summarize current understanding of the physics of oscillating bubbles, and then critically summarize recent advancements, including some of our original work, on the applications of oscillating bubbles in microfluidic devices. We intend to highlight the advantages of using oscillating bubbles along with the challenges that accompany them. We believe that these emerging studies on microfluidic oscillating bubbles will be revolutionary to the development of next-generation LOC technologies.

Validation of an acoustic cavitation dose with hydroxyl radical production generated by inertial cavitation in pulsed mode: application to in vitro drug release from liposomes

The purpose of this study was to define and validate an inertial cavitation dose (CD) based on the detection of broadband noise, designed to monitor ultrasound-mediated drug release from liposomes. The validation consists of using the terephthalate dosimeter to quantify by fluorescence measurements the extent of hydroxyl radical (()OH) production during inertial cavitation. Sonication of samples was performed using tone bursts (pulse repetition frequency (PRF): 10 Hz(-1) kHz, duty cycle (dc): 5-25%, Isppa: 4100-12,200 W/cm(2)) generated by a 1 MHz focused transducer. Three sets of ultrasound parameters with different PRF and dc were selected to be more precisely compared. Results demonstrated an excellent correlation between *OH radical production and CD for each set of parameters, but significant differences in hydroxyl radical levels were observed among the sets of parameters. The results were compared with other studies, and the same tendency of variation with pulse duration was demonstrated. Results also showed that the CD was not distorted by peak intensity variations and was a much more reliable indicator than sonication time. Consequently, one validated parameter was selected to monitor drug release from two liposome formulations, and compare their ultrasound sensitivity.

A study of bubble activity generated in ex vivo tissue by high intensity focused ultrasound

Cancer treatment by extracorporeal high-intensity focused ultrasound (HIFU) is constrained by the time required to ablate clinically relevant tumour volumes. Although cavitation may be used to optimize HIFU treatments, its role during lesion formation is ambiguous. Clear differentiation is required between acoustic cavitation (noninertial and inertial) effects and bubble formation arising from two thermally-driven effects (the vapourization of liquid into vapour, and the exsolution of formerly dissolved permanent gas out of the liquid and into gas spaces). This study uses clinically relevant HIFU exposures in degassed water and ex vivo bovine liver to test a suite of cavitation detection techniques that exploit passive and active acoustics, audible emissions and the electrical drive power fluctuations. Exposure regimes for different cavitation activities (none, acoustic cavitation and, for ex vivo tissue only, acoustic cavitation plus thermally-driven gas space formation) were identified both in degassed water and in ex vivo liver using the detectable characteristic acoustic emissions. The detection system proved effective in both degassed water and tissue, but requires optimization for future clinical application.

An ex vivo study of the correlation between acoustic emission and microvascular damage

The objective of this study was to conduct an ex vivo examination of correlation between acoustic emission and tissue damage. Intravital microscopy was employed in conjunction with ultrasound exposure in cremaster muscle of male Wistar rats. Definity microbubbles were administered intravenously through the tail vein (80microL.kg(-1).min(-1)infusion rate) with the aid of a syringe pump. For the pulse repetition frequency (PRF) study, exposures were performed at four locations of the cremaster at a PRF of 1000, 500, 100 and 10Hz (one location per PRF per rat). The 100-pulse exposures were implemented at a peak rarefactional pressure (P(r)) of 2MPa, frequency of 2.25MHz with 46 cycle pulses. For the pressure amplitude threshold study, 100-pulse exposures (46 cycle pulses) were conducted at various peak rarefactional pressures from 0.5MPa to 2MPa at a frequency of 2.25MHz and PRF of 100Hz. Photomicrographs were captured before and 2-min postexposure. On a pulse-to-pulse basis, the 10Hz acoustic emission was considerably higher and more sustained than those at other PRFs (1000, 500, and 100Hz) (p<0.05). Damage, measured as area of extravasation of red blood cells (RBCs), was also significantly higher at 10Hz PRF than at 1000, 500 and 100Hz (p<0.01). The correlation of acoustic emission to tissue damage showed a trend of increasing damage with increasing cumulative function of the relative integrated power spectrum (CRIPS; R(2)=0.75). No visible damage was present at P(r)< or =0.85MPa. Damage, however, was observed at P(r)> or =1.0MPa and it increased with increasing acoustic pressure.

The correlation between bubble-enhanced HIFU heating and cavitation power

It has been established that while the inherent presence of bubbles increases heat generation due to scattering and absorption, inertial cavitation is responsible for elevated heating during high-intensity focused ultrasound (HIFU) application. The contribution of bubble-induced heating can be an important factor to consider, as it can be several times greater than the expected heat deposition from absorption of energy from the primary ultrasound field. The temperature and cavitation signal near the focus were measured for 5.5-s continuous-wave 1.1-MHz HIFU sonications in tissue mimicking phantoms. The measured temperature was corrected for heating predicted from the primary ultrasound absorption to isolate the temperature rise from the bubble activity. The temperature rise induced from cavitation correlates well with a measurement of the instantaneous "cavitation power" as indicated by the mean square voltage output of a 15-MHz passive cavitation detector. The results suggest that careful processing of the cavitation signals can serve as a proxy for measuring the heating contribution from inertial cavitation.

Ultrasound bioeffects and safety

The main mechanisms by which ultrasound can induce biological effects as it passes through the body are thermal and mechanical in nature. The mechanical effects are primarily related to the presence of gas, whether drawn out of solution by the negative going ultrasound pressure wave (acoustic cavitation), a naturally occurring gas body (such as lung alveoli), or deliberately introduced into the blood stream to increase imaging contrast (microbubble contrast agents). Observed biological effects are discussed in the context of these mechanisms and their relevance to ultrasound safety is discussed.

Applications of ultrasonic skin permeation in transdermal drug delivery

Transdermal ultrasound-mediated drug delivery has been studied as a method for needle-less, non-invasive drug administration. Potential obstacles include the stratum corneum, which is not sufficiently passively permeable to allow effective transfer of many medications into the bloodstream without active methods. A general review of the transdermal ultrasound drug delivery literature has shown that this technology offers promising potential for non-invasive drug administration. Included in this review are the reported acoustic parameters used for achieving delivery, along with the known intensities and exposure times. Ultrasound mechanisms are discussed as well as spatial field characteristics. Accurate and precise quantification of the acoustic field used in drug delivery experiments is essential to ensure safety versus efficacy and to avoid potentially harmful bioeffects.

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.

[Ultrasound contrast agents. Pharmaceutical drug safety and bioeffects]

In this overview safety aspects of ultrasound contrast agents (USCA) are described and discussed. In general USCA are very safe drugs. However, allergic adverse reactions can rarely occur, particularly due to the colloidal structure of USCA. In addition, the use of USCA could reduce the threshold for acoustically induced bioeffects and has the potential to increase these effects. In in vitro studies and animal trials USCA caused petechial hemorrhages, vascular damage, and the formation of free radicals. Even DNA damage with single strand breaks could be demonstrated. In human studies and clinical practice none of these bioeffects could be observed. In contrast-enhanced echocardiography a higher rate of premature ventricular contractions has been reported when imaging was triggered at the end systole. Compared with other contrast agents contrast-enhanced ultrasound showed no nephrotoxic effects and could prove to be an alternative diagnostic method for patients with renal failure.

The use of ultrasound in transfection and transgene expression

The interaction of ultrasound with tissue leads to radiation pressure, heat generation, and cavitation. These phenomena have been utilised for local gene delivery, transfection and control of expression. Specially designed nanocarriers or adapted ultrasound contrast agents can further enhance local delivery by: (1) increased permeability of cell membranes; (2) local release of genes. Biological carriers may also be used for local gene delivery. Stem cells and immune cells appear especially promising because of their homing capabilities to lesion sites. Imaging methods can be employed for pharmacodistribution and pharmacokinetics. MRI contrast agents can serve as non-invasive reporters on gene distribution when co-delivered with the gene. They can be used to label nanocarriers and cellular transport systems in gene therapy strategies such as those based on stem cells. Finally, ultrasound heating together with the use of a temperature sensitive promoter allows a local, physical, spatio-temporal control of transgene expression, in particular when combined with MRI temperature mapping for monitoring and even controlling ultrasound heating.

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.

Quantitative relations of acoustic inertial cavitation with sonoporation and cell viability

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

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.

Intravascular inertial cavitation activity detection and quantification in vivo with Optison

Inertial cavitation (IC) is an important mechanism by which ultrasound (US)-induced bioeffects can be produced. It has been reported that US-induced in vitro mechanical bioeffects with the presence of ultrasound contrast agents (UCAs) are highly correlated with quantified IC "dose" (ICD: cumulated root-mean-squared broadband noise amplitude in the frequency domain). The ICD has also been used to quantify IC activity in ex vivo perfused rabbit ear vessels. The in vivo experiments reported here using a rabbit ear vessel model were designed to: (1) detect and quantify IC activity in vivo within the constrained environment of rabbit auricular veins with the presence of Optison and (2) measure the temporal evolution of microbubble IC activity and the ICD generated during insonation treatment, as a function of acoustic parameters. Preselected regions-of-interest (ROI) in the rabbit ear vein were exposed to pulsed focused US (1.17 MHz, 1 Hz PRF). Experimental acoustic variables included peak rarefaction pressure amplitude ([PRPA]: 1.1, 3.0, 6.5 or 9.0 MPa) and pulse length (20, 100, 500 or 1000 cycles). ICD was quantified based on passive cavitation detection (PCD) measurements. The results show that: (1) after Optison injection, the time to onset of measurable microbubble IC activity was relatively consistent, approximately 20 s; (2) after reaching its peak value, the IC activity decayed exponentially and the half-life decay coefficient (t(1/2)) increased with increasing PRPA and pulse length; and (3) the normalized ICD generated by pulsed US exposure increased significantly with increasing PRPA and pulse length.

Correlation between inertial cavitation dose and endothelial cell damage in vivo

Previous in vivo studies have demonstrated that vascular endothelial damage can result when vessels containing gas-based microbubble ultrasound contrast agent (UCA) are exposed to MHz-frequency pulsed ultrasound (US) of sufficient pressure amplitudes, presumably as a result of inertial cavitation (IC). The hypothesis guiding this research was that IC is the primary mechanism by which the vascular endothelium (VE) is damaged when a vessel is exposed to pulsed 1-MHz frequency US in the presence of circulating UCA. The expectation was that a correlation should exist between the magnitude and duration of IC activity and the degree of VE damage. Rabbit auricular vessels were exposed in vivo to 1.17-MHz focused US of variable peak rarefaction pressure amplitude (1, 3, 6.5 or 9 MPa), using low duty factors (0.04% or 0.4%), pulse lengths of 500 or 5000 cycles, with varying treatment durations and with or without infusion of a shelled microbubble contrast agent. A broadband passive cavitation detection system was used to measure IC activity in vivo within the targeted segment of the blood vessel. The magnitude of the detected IC activity was quantified using a previously reported measure of IC dose. Endothelial damage was assessed via scanning electron microscopy image analysis. The results supported the hypothesis and demonstrate that the magnitude of the measured IC dose correlates with the degree of VE damage when UCA is present. These results have implications for therapeutic US-induced vascular occlusion.

Overview of experimental studies of biological effects of medical ultrasound caused by gas body activation and inertial cavitation

Ultrasound exposure can induce bioeffects in mammalian tissue by the nonthermal mechanism of gas body activation. Pre-existing bodies of gas may be activated even at low-pressure amplitudes. At higher-pressure amplitudes, violent cavitation activity with inertial collapse of microbubbles can be generated from latent nucleation sites or from the destabilization of gas bodies. Mechanical perturbation at the activation sites leads to biological effects on nearby cells and structures. Shockwave lithotripsy was the first medical ultrasound application for which significant cavitational bioeffects were demonstrated in mammalian tissues, including hemorrhage and injury in the kidney. Lithotripter shockwaves can also cause hemorrhage in lung and intestine by activation of pre-existing gas bodies in these tissues. Modern diagnostic ultrasound equipment develops pressure amplitudes sufficient for inertial cavitation, but the living body normally lacks suitable cavitation nuclei. Ultrasound contrast agents (UCAs) are suspensions of microscopic gas bodies created to enhance the echogenicity of blood. Ultrasound contrast agent gas bodies also provide nuclei for inertial cavitation. Bioeffects from contrast-aided diagnostic ultrasound depend on pressure amplitude, UCA dose, dosage delivery method and image timing parameters. Microvascular leakage, capillary rupture, cardiomyocyte killing, inflammatory cell infiltration, and premature ventricular contractions have been reported for myocardial contrast echocardiography with clinical ultrasound machines and clinically relevant agent doses in laboratory animals. Similar bioeffects have been reported in intestine, skeletal muscle, fat, lymph nodes and kidney. These microscale bioeffects could be induced unknowingly in diagnostic examinations; however, the medical significance of bioeffects of diagnostic ultrasound with contrast agents is not yet fully understood in relation to the clinical setting.

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: ).