Remnants of Albunex nucleate acoustic cavitation

Safety of Clinical Ultrasound Neuromodulation

Transcranial ultrasound holds much potential as a safe, non-invasive modality for navigated neuromodulation, with low-intensity focused ultrasound (FUS) and transcranial pulse stimulation (TPS) representing the two main modalities. While neuroscientific and preclinical applications have received much interest, clinical applications are still relatively scarce. For safety considerations, the current literature is largely based on guidelines for ultrasound imaging that uses various physical parameters to describe the ultrasound pulse form and expected bioeffects. However, the safety situation for neuromodulation is inherently different. This article provides an overview of relevant ultrasound parameters with a focus on bioeffects relevant for safe clinical applications. Further, a retrospective analysis of safety data for clinical TPS applications in patients is presented.

In Vivo Temperature Rise Measurements of Rabbit Liver and Femur Bone Surface Exposed to an Acoustic Radiation Force Impulse

Acoustic radiation force impulse (ARFI) imaging and shear wave elastography use a "push pulse." The push pulse, which is referenced as an ARFI in this study, has a longer duration than that of conventional diagnostic pulses (several microseconds). Therefore, there are concerns regarding thermal safety in vivo. However, few in vivo studies have been conducted using living animals. In this study, to suggest a concept for deciding an ARFI output and cooling time while considering thermal safety, the liver (with and without an ultrasound contrast agent) and femur bone surface of living rabbits were exposed to an ARFI, and the maximum temperature rise, temperature rise for 5-min duration, and cooling time were measured via a thermocouple. While testing within the regulation limits of diagnostic ultrasound outputs, a maximum temperature rise on the femur bone surface exceeded the allowable temperature rise (1.5°C) in the British Medical Ultrasound Society (BMUS) statement. However, using the linear relationships between the pulse intensity integral (PII) of a single pulse and the above three temperature parameters, PII may be determined so that the maximum temperature rise is within the allowable temperature rise in the BMUS statement. The cooling time can be estimated from the PII.

The AFSUMB Consensus Statements and Recommendations for the Clinical Practice of Contrast-Enhanced Ultrasound using Sonazoid

The first edition of the guidelines for the use of ultrasound contrast agents was published in 2004, dealing with liver applications. The second edition of the guidelines in 2008 reflected changes in the available contrast agents and updated the guidelines for the liver, as well as implementing some nonliver applications. The third edition of the contrast-enhanced ultrasound (CEUS) guidelines was the joint World Federation for Ultrasound in Medicine and Biology-European Federation of Societies for Ultrasound in Medicine and Biology (WFUMB-EFSUMB) venture in conjunction with other regional US societies such as Asian Federation of Societies for Ultrasound in Medicine and Biology, resulting in a simultaneous duplicate on liver CEUS in the official journals of both WFUMB and EFSUMB in 2013. However, no guidelines were described mainly for Sonazoid due to limited clinical experience only in Japan and Korea. The new proposed consensus statements and recommendations provide general advice on the use of Sonazoid and are intended to create standard protocols for the use and administration of Sonazoid in hepatic and pancreatobiliary applications in Asian patients and to improve patient management.

The AFSUMB Consensus Statements and Recommendations for the Clinical Practice of Contrast-Enhanced Ultrasound using Sonazoid

The first edition of the guidelines for the use of ultrasound contrast agents was published in 2004, dealing with liver applications. The second edition of the guidelines in 2008 reflected changes in the available contrast agents and updated the guidelines for the liver, as well as implementing some nonliver applications. The third edition of the contrast-enhanced ultrasound (CEUS) guidelines was the joint World Federation for Ultrasound in Medicine and Biology-European Federation of Societies for Ultrasound in Medicine and Biology (WFUMB-EFSUMB) venture in conjunction with other regional US societies such as Asian Federation of Societies for Ultrasound in Medicine and Biology, resulting in a simultaneous duplicate on liver CEUS in the official journals of both WFUMB and EFSUMB in 2013. However, no guidelines were described mainly for Sonazoid due to limited clinical experience only in Japan and Korea. The new proposed consensus statements and recommendations provide general advice on the use of Sonazoid and are intended to create standard protocols for the use and administration of Sonazoid in hepatic and pancreatobiliary applications in Asian patients and to improve patient management.

Hemocoagulase Combined with Microbubble-Enhanced Ultrasound Cavitation for Augmented Ablation of Microvasculature in Rabbit VX2 Liver Tumors

We investigated a new method for combining microbubble-enhanced ultrasound cavitation (MEUC) with hemocoagulase (HC) atrox. Our goal was to induce embolic effects in the vasculature and combine these with an anti-angiogenic treatment strategy. Fourteen days after being implanted with a single slice of the liver VX2 tumor, rabbits were randomly divided into five groups: (i) a control group injected intra-venously with saline using a micropump; (ii) a group given only an injection of HC; (iii) a group treated only with ultrasound cavitation; (iv) a group treated with MEUC; (v) a group treated with MEUC + HC. Contrast-enhanced ultrasound was performed before treatment and 1 h and 7 d post-treatment to measure tumor size, enhancement and necrosis range. QontraXt software was used to determine the time-intensity curve of tumor blood perfusion and microvascular changes. At 1 h and 7 d after treatment with MEUC + HC, the parameters of the time-intensity curve, which included peak value, regional blood volume, regional blood flow and area under the curve value and which were measured using contrast-enhanced ultrasound, were significantly lower than those of the other treatment groups. The MEUC + HC treatment group exhibited significant growth inhibition relative to the ultrasound cavitation only, HC and MEUC treatment groups. No damage was observed in the surrounding normal tissues. These results support the feasibility of reducing the blood perfusion of rabbit VX2 liver tumors using a new method that combines MEUC and HC.

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.

Measurement of sound pressure and temperature in tissue-mimicking material using an optical fiber Bragg grating sensor

PURPOSE

The experimental investigation of an optical fiber Bragg grating (FBG) sensor for biomedical application is described. The FBG sensor can be used to measure sound pressure and temperature rise simultaneously in biological tissues exposed to ultrasound. The theoretical maximum values that can be measured with the FBG sensor are 73.0 MPa and 30 °C.

METHODS

In this study, measurement of sound pressure up to 5 MPa was performed at an ultrasound frequency of 2 MHz. A maximum temperature change of 6 °C was measured in a tissue-mimicking material.

RESULTS

Values yielded by the FBG sensor agreed with those measured using a thermocouple and a hydrophone.

CONCLUSION

Since this sensor is used to monitor the sound pressure and temperature simultaneously, it can also be used for industrial applications, such as ultrasonic cleaning of semiconductors under controlled temperatures.

JSUM ultrasound elastography practice guidelines: pancreas

Ultrasound elastography is a relatively new diagnostic technique for measuring the elasticity (hardness) of tissue. Eleven years have passed since the debut of elastography. Various elastography devices are currently being marketed by manufacturers under different names. Pancreatic elastography can be used not only with transabdominal ultrasonography but also with endoscopic ultrasonography, but some types of elastography are difficult to perform for the pancreas. These guidelines aim to classify the various types of elastography into two major categories depending on the differences in the physical quantity (strain, shear wave), and to present the evidence for pancreatic elastography and how to use pancreatic elastography in the present day. But the number of reports on ultrasound elastography for the pancreas is still small, and there are no reports on some elastography devices for the pancreas. Therefore, these guidelines do not recommend methods of imaging and analysis by elastography device.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

In vivo gas body efficacy for glomerular capillary hemorrhage induced by diagnostic ultrasound in rats

Glomerular capillary hemorrhage (GCH) in rat kidney provided a model for assessing in vivo gas body efficacy in diagnostic or therapeutic applications of ultrasound. Two diagnostic ultrasound machines were utilized: one monitored the harmonic B-mode contrast enhancement of the left kidney and the other exposed the right kidney for GCH production. Definity contrast agent was infused at 1, 2, 5, or 10 microL/(kg x min) and infusion durations were 30, 60, 120, or 300 s. Exposure of the right kidney was at a peak rarefactional pressure amplitude of 2.3 MPa at 1.5 MHz. The circulating dose was estimated with a simple model of agent dilution and gas body loss. For 300 s infusion at 5 microL/(kg x min), the left kidney image brightness increased to a plateau with an estimated 6.4 +/- 1.3 microL/kg circulating dose with no GCH in histological sections. Exposure of the right kidney with a 1-s image interval reduced the estimated circulating dose to 1.3 +/- 0.3 microL/kg and induced 68.4% GCH. Dose and duration increases gave rapidly diminishing treatment effectiveness per gas body. The effective in vivo agent dose in rats can be reduced greatly due to high gas body destruction in the small animal, complicating predictions for similar conditions of human treatment.

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

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

In vivo acceleration of ultrasonic tissue heating by microbubble agent

The ultrasonic power absorbed by a microbubble in its continuous wave response is estimated through numerically solving a version of the Rayleigh-Plesset equation. At an ultrasonic frequency of 3 MHz, a resonant microbubble, approximately 1.1 microm in radius, showed an absorption cross section of about 0.005 mm2 in its low power response. This estimation predicts that the tissue ultrasonic absorption will be doubled when such microbubbles are delivered to the tissue at a concentration of about eight bubbles/mm3 in tissue. An exteriorized murine kidney was exposed to focused ultrasound at 3.2 MHz in degassed saline, and the tissue temperature change was measured. With an intravenous bolus administration of a microbubble agent, the ultrasonically induced temperature elevation was multiplied by up to five times. The enhancement in temperature elevation gradually decreased as the microbubble agent was eliminated from the body. The experimental results agreed with the prediction in the order of magnitude. This effect may have a potential use to enhance the throughput as well as the selectivity of focused ultrasound treatment.

Effects of contrast agent infusion rates on thresholds for tissue damage produced by single exposures of high-intensity ultrasound

Stabilized microbubble ultrasound contrast agents (UCA) have potential to aid tissue ablation during ultrasound surgery by enhancing both cavitational and thermal damage mechanisms. Previously, we showed UCA infused at a rate of 1 microL/kg/min prior to ultrasound exposure could reduce the total energy required to produce tissue damage by up to two orders of magnitude. In this paper, we evaluate thresholds for macroscopic tissue damage with UCA infusion rates (IR) of 0.1, 0.3, 1, 3, and 10 microL/kg/min to determine IR potentially effective for ultrasound therapy. Canine kidneys were surgically externalized and insonified with single exposures of focused ultrasound. Incident exposures were 1.44 MHz tone bursts, either 250 ms in duration with intensity between 500 W/cm2 and 3200 W/cm2, or 100 micros to 1 s in duration with intensity equal to 3200 W/cm2. Probabilities of tissue damage occurrence were determined for each set of exposure conditions (intensity, duration, and IR). A threshold intensity and threshold duration, defined as the quantities for which tissue damage occurred with probability equal to 0.5, were estimated for each IR. Results show that, as IR increased from 0.1 to 10 microL/kg/min, the threshold intensity decreased by up to a factor of 3, and threshold duration decreased by up to a factor of 200. Microbubble introduction at IR up to 10 microL/kg/min thus may be effective in aiding ultrasound therapy.

Safety of ultrasound contrast agents

The use of ultrasound contrast agents has increased over recent years. The Contrast Media Safety Committee (CMSC) of the European Society of Urogenital Radiology (ESUR) decided to review the safety of ultrasound contrast agents in humans and to draw up guidelines. A comprehensive literature search and review was carried out. The resulting report was discussed by the CMSC of ESUR and at the 11th European Symposium on Urogenital Radiology in Santiago de Compostela, Spain, in 2004. Ultrasound contrast agents approved for clinical use are well tolerated, and serious adverse reactions are rarely observed. Adverse events are usually minor (e.g. headache, nausea, altered taste, sensation of heat) and self-resolving. These symptoms may not be related to the ultrasound contrast materials as they have also been observed in placebo-control groups. Intolerance to some components may occur. Generalized allergy-like reactions occur rarely. Ultrasound contrast agents are generally safe. The ultrasound scanning time and the acoustic output should be kept to the lowest level consistent with obtaining diagnostic information. Adverse reactions should be treated symptomatically.

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.

Mechanical Bioeffects of Ultrasound

Ultrasound is used widely in medicine as both a diagnostic and therapeutic tool. Through both thermal and nonthermal mechanisms, ultrasound can produce a variety of biological effects in tissues in vitro and in vivo. This chapter provides an overview of the fundamentals of key nonthermal mechanisms for the interaction of ultrasound with biological tissues. Several categories of mechanical bioeffects of ultrasound are then reviewed to provide insight on the range of ultrasound bioeffects in vivo, the relevance of these effects to diagnostic imaging, and the potential application of mechanical bioeffects to the design of new therapeutic applications of ultrasound in medicine.

Quantification of optison bubble size and lifetime during sonication dominant role of secondary cavitation bubbles causing acoustic bioeffects

Acoustic cavitation has been shown to deliver molecules into viable cells, which is of interest for drug and gene delivery applications. To address mechanisms of these acoustic bioeffects, this work measured the lifetime of albumin-stabilized cavitation bubbles (Optison) and correlated it with desirable (intracellular uptake of molecules) and undesirable (loss of cell viability) bioeffects. Optison was exposed to 500 kHz ultrasound (acoustic pressures of 0.6-3.0 MPa and energy exposures of 0.2-200 J/cm2) either with or without the presence of DU145 prostate cancer cells (10(6) cells/ml) bathed in calcein, a cell-impermeant tracer molecule. Bubble lifetime was determined using a Coulter counter and flow cytometer, while bioeffects were evaluated by flow cytometry. The lifetime of Optison cavitation nuclei was found to decrease and bioeffects (molecular uptake and loss of cell viability) were found to increase with increasing acoustic energy exposure. These bioeffects correlated well with the disappearance of bubbles, suggesting that contrast agent destruction either directly or indirectly affected cells, probably involving unstabilized cavitation nuclei created upon the destruction of Optison. Because Optison solutions presonicated to destroy all detectable bubbles also caused significant bioeffects, the indirect mechanism involving secondary cavitation bubbles is more likely.

Microbubble-enhanced cavitation for noninvasive ultrasound surgery

Experiments were conducted to explore the potential of stabilized microbubbles for aiding tissue ablation during ultrasound therapy. Surgically exteriorized canine kidneys were irradiated in situ using single exposures of focused ultrasound. In each experiment, up to eight separate exposures were placed in the left kidney. The right kidney was then similarly exposed, but while an ultrasound contrast agent was continually infused. Kidneys were sectioned and examined for gross observable tissue damage. Tissue damage was produced more frequently, by lower intensity and shorter duration exposures, in kidneys irradiated with the contrast agent present. Using 250-ms exposures, the minimum intensity that produced damage was lower in kidneys with microbubbles than those without (controls) in 10 of 11 (91%) animals. In a separate study using approximately 3200 W/cm2 exposures, the minimum duration that produced damage was shorter after microbubbles were introduced in 11 of 12 (92%) animals. With microbubbles, gross observable tissue damage was produced with exposure intensity > or = approximately 800 W/cm2 and exposure duration > or = 10 micros. The overall intensity and duration tissue damage thresholds were reduced by approximately 2x and approximately 100x, respectively. Results indicate that acoustic cavitation is a primary damage mechanism. Lowering in vivo tissue damage thresholds with stabilized microbubbles acting as cavitation nuclei may make acoustic cavitation a more predictable, and thus practical, mechanism for noninvasive ultrasound surgery.

Contrast ultrasound targeted drug and gene delivery: an update on a new therapeutic modality

The effective delivery of intravascular drugs and genes to regions of pathology is dependent on a number of factors that are often difficult to control. Foremost is the site-specific delivery of the payload to the region of pathology and the subsequent transport of the payload across the endothelial barrier. Ultrasound contrast agent microbubbles, which are typically used for image enhancement, are capable of amplifying both the targeting and transport of drugs and genes to tissue. Microbubble targeting can be achieved by the intrinsic binding properties of the microbubble shells or through the attachment of site-specific ligands. Once microbubbles have been targeted to the region of interest, microvessel walls can be permeabilized by destroying the microbubbles with low-frequency, high-power ultrasound. A second level of targeting specificity can be achieved by carefully controlling the ultrasound field and limiting microbubble destruction to the region of interest. When microbubbles are destroyed, drugs or genes that are housed within them or bound to their shells can be released to the blood stream and then delivered to tissue by convective forces through the permeabilized microvessels. An alternative strategy is to increase payload volume by coinjecting drug- or gene-bearing vehicles, such as liposomes, with the microbubbles. In this manifestation, microbubbles are used for creating sites of microvessel permeabilization that facilitate drug or gene vehicle transport. Recent work in the emerging field of contrast ultrasound-based therapeutics, with particular emphasis on the delivery of drugs and genes to tissue through microvascular networks is reviewed.

A comparison of the hemolytic potential of Optison and Albunex in whole human blood in vitro: acoustic pressure, ultrasound frequency, donor and passive cavitation detection considerations

This project tested the hypothesis that a "second-generation" ultrasound (US) contrast agent (Optison), offering extended echogenicity over that of its "first-generation" predecessor (Albunex), would have the greater potential for sonolysis of human erythrocytes in vitro. Whole human blood, obtained from apparently healthy donors, was anticoagulated and subsequently exposed in vitro to US in the presence of one of each or neither of the two US contrast agents. The US exposures were for 30 s and involved frequency (1.0, 2.2 and 3.4 MHz) and amplitude (approximately 2.8 to 0.38 MPa P(-)) regimens; pulse duration (200 micros) and interpulse interval (20 ms) were held constant. The data supported the hypothesis, with an overall ratio of approximately 2.5 for relative extent of background-corrected US-induced hemolysis of the Optison/Albunex regimens. Passive cavitation detection analyses corroborated the results obtained with hemolysis.

Thresholds for inertial cavitation in albunex suspensions under pulsed ultrasound conditions

Stabilized microbubbles used as echo-contrast agents can be destroyed by ultrasonic irradiation. We have identified two pressure thresholds at which these microbubbles undergo inertial cavitation (here, defined as the collapse of gas bubbles followed by emission of an acoustic broadband noise). The first threshold (P1) corresponds to the pressure at which all the microbubbles in a cavitation field lose their property as an effective scatterer because of fragmentation or deflation. The second threshold (P2) is associated with the acoustic reactivation of the remnants of the contrast agents and is related to the onset of more violent inertial cavitation. P1 and P2 were measured as a function of the concentration of Albunex (Molecular Biosystems Inc., San Diego, CA) contrast agent, the number of transmitting acoustic cycles, and the pulse repetition frequency (PRF). The ultrasound frequency used was 1.1 MHz, and the peak negative acoustic pressures ranged from 0 to 8 MPa. Our results, measured in Isoton II (Coulter Diagnostics, Miami, FL) and whole blood solutions, showed that P1 increased with increasing Albunex concentration and decreased with increasing PRF, whereas P2 decreased with increasing Albunex concentration and was independent of the PRF. Both P1 and P2 decreased with increasing number of acoustic cycles N for N < 10 and were independent of the number of cycles for N > 10. Ultrasound images of Albunex acquired by a commercial scanner showed echo enhancement not only at pressure levels below P1 but also at levels above P2. The threshold P2 was achieved at ultrasound energies above the diagnostic level. Inertial cavitation produced at P2 was associated with a higher level of hemolysis compared with P1. The results of this investigation have potential significance for both diagnostic and therapeutic ultrasound applications.

The search for cavitation in vivo

Until the mid 1970s, it was generally assumed that, with the short pulses of ultrasound (US) used in medical diagnosis, there was little need for concern about the possibility of inertial cavitation in vivo. This assumption came into question when experimental evidence indicated that killing of fruit fly larvae by diagnostically relevant US was associated with the presence of gas in the respiratory apparatus of the organisms. Independent theoretical contributions by Flynn and Apfel in the early 1980s made it clear that complacency in regard to cavitation was not warranted. Later, the mammalian lung, as with larva, was shown to be particularly vulnerable when it contained air. Yet, overall evidence suggests that lung hemorrhage is not consistent with the classical picture of inertial cavitation. Most recently, however, hemolysis and hemorrhage associated with the use of contrast agents have provided nearly incontrovertible evidence of the occurrence of cavitation in vivo.

Comparative sensitivity of human and bovine erythrocytes to sonolysis by 1-MHz ultrasound

This project tested the hypothesis that human erythrocytes, being larger than bovine erythrocytes, would be the more sensitive to sonolysis induced by inertial cavitation. The rationale behind this hypothesis was an earlier demonstration that, among sized populations of erythrocytes, an inverse relation existed between erythrocyte volume and mechanically-induced shear forces in the surrounding medium; viz, the larger the cell, the less shear force required to rupture the cell's membrane. At low erythrocyte densities (i.e., approximately 5% hematocrit) the hypothesis was supported; at high cell densities (i.e., approximately 35% hematocrit) it was not supported. The data are consistent with an ultrasound (US)-induced symmetric implosion of affected gas nuclei as causing the effect at low cell densities; under such conditions there is ample spacing among cells for US-induced symmetric growth and collapse of gas nuclei and the concomitant production of radially-expanding shock waves (which lyse the cells); at high cell densities there is not sufficient spacing among cells for US-induced symmetric growth and collapse of bubbles and an alternative mechanism, possibly asymmetric bubble collapse, becomes operational.

Diagnostic ultrasound activation of contrast agent gas bodies induces capillary rupture in mice

Interaction of diagnostic ultrasound with gas bodies produces a useful contrast effect in medical images, but the same interaction also represents a mechanism for bioeffects. Anesthetized hairless mice were scanned by using a 2.5-MHz transducer (610-ns pulses with 3.6-kHz repetition frequency and 61-Hz frame rate) after injection of Optison and Evans blue dye. Petechial hemorrhages (PHs) in intestine and abdominal muscle were counted 15 min after exposure to characterize capillary rupture, and Evans blue extravasation was evaluated in samples of muscle tissue. For 5 ml small middle dotkg(-1) contrast agent and exposure to 10 alternating 10-s on and off periods, PH counts in muscle were approximately proportional to the square of peak negative pressure amplitude and were statistically significant above 0.64 MPa. PH counts in intestine and Evans blue extravasation into muscle tissue were significant above 1. 0 MPa. The PH effect in muscle was proportional to contrast dose and was statistically significant for the lowest dose of 0.05 ml small middle dotkg(-1). The effects decreased nearly to sham levels if the exposure was delayed 5 min. The PH effect in abdominal muscle was significant and statistically indistinguishable for uninterrupted 100-s exposure, 10-s exposure, 100 scans repeated at 1 Hz, and even for a single scan. The results confirms a previous report of PH induction by diagnostic ultrasound with contrast agent in mammalian skeletal muscle [Skyba, D. M., Price, R. J., Linka, A. Z., Skalak, T. C. & Kaul, S. (1998) Circulation 98, 290-293].

Destruction of contrast microbubbles and the association with inertial cavitation

The destruction of insonified Sonazoid microbubbles and its association with inertial cavitation in vitro utilizing an active acoustic detector was investigated. The experimental observation indicated that contrast microbubbles could be damaged at moderate acoustic pressures of 0.6-1.6 MPa (0.4-1.0 in mechanical index, MI). A damaged bubble could be dissolved into the medium on the order of 1 ms, implying that the destruction at moderate pressures is a relatively slow (relative to inertial bubble collapse), nonviolent dissolution process following the disruption of encapsulating surface materials. Inertial cavitation events in the presence of contrast microbubbles were observed using multiple highly intense ultrasound (US) pulses (>1.6 MPa). This observation suggested that intense US might disintegrate contrast microbubbles, and fragments of disintegrated microbubbles could be activated by an upcoming highly intense imaging pulse. The above results imply that inertial cavitation is unlikely to take place in the presence of Sonazoid contrast microbubbles when exposed to diagnostic US with an MI <1.

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

Human (A431 epidermoid carcinoma) cells were grown as monolayers on 5 microm thick Mylar sheets, which formed the upper window for a 1-mm thick, 23-mm diameter disc-shaped exposure chamber. A 3.5-MHz curved linear-array transducer was aimed upward at the chamber, 7 cm away, in a 37 degrees C water bath. The chamber contained phosphate-buffered saline (PBS) with 10 mg/mL fluorescent dextran and 1% Optison ultrasound (US) contrast agent. Significant fluorescent cell counts, indicative of membrane damage (i.e., sonoporation), up to about 10% of cells within a 1-mm diameter field of view, were noted for spectral Doppler and two-dimensional (2-D) scan mode with or without a tissue-mimicking phantom. The effect was only weakly dependent on pulse-repetition frequency or exposure duration, but was strongly dependent on contrast agent concentration below 2%. Thus, diagnostic US activation of contrast-agent gas bodies can produce cell membrane damage.

The influence of ultrasound frequency and gas-body composition on the contrast agent-mediated enhancement of vascular bioeffects in mouse intestine

The induction by ultrasound (US) of petechiae and hemorrhages in mouse intestine was examined with injection of gas body-based contrast agents. Production of petechiae in the intestinal wall was enhanced by contrast agents for both continuous and pulsed (10 micros pulses repeated at 1 kHz) exposure relative to a gas body-free blank. For pulsed exposure with 10 mL/kg of Albunex, apparent thresholds for peak negative pressure amplitude were 0.42 MPa at 0.4 MHz, 0.85 MPa at 1.09 MHz and 2.3 MPa at 2.4 MHz. Results at these frequencies were the same for 10-11 cycle pulses with fixed duty cycle (0.01). Thresholds for hemorrhage into the intestinal lumen were not appreciably enhanced by added Albunex, and appear to be compatible with previously reported lithotripsy data when duty factor differences are considered. The agents PESDA, Optison and Levovist had lower thresholds (for example, 1.8 MPa for Levovist) than Albunex at 2.3 MHz, and yielded more petechiae. The thresholds for petechiae induction by US with contrast agents encroach upon the exposure range relevant to diagnostic US practice.

Erosion of artificial endothelia in vitro by pulsed ultrasound: acoustic pressure, frequency, membrane orientation and microbubble contrast agent dependence

The erosion of cells from fibroblast monolayers simulating the vascular endothelium by 20 micros pulses of ultrasound at 500 Hz PRF was studied in relation to the peak negative acoustic pressure (P-; 0.0-2.5 MPa), ultrasound (US) frequency (1.0, 2.1 or 3.5 MHz), orientation of the monolayer (i.e., simulating the sites of ultrasound entry/exit from a blood vessel) and the presence or absence of a microbubble contrast agent (3 Vol% Albunex). The a priori hypotheses were that erosion of the monolayers would: 1. arise due to insonation treatment, 2. arise as a consequence of cavitation activity and, thus, increase with increasing P- at constant frequency, and decrease with increasing frequency at constant P-, 3. be significantly increased by the presence of a microbubble contrast agent, and 4. have a weak dependence on monolayer orientation. The data support these hypotheses. Under the most severe exposure conditions used, most of the affected cells appeared to have been lysed; however, a substantial number of viable cells were dislodged from the monolayer surface.

Sonolysis of Albunex-supplemented, 40% hematocrit human erythrocytes by pulsed 1-MHz ultrasound: pulse number, pulse duration and exposure vessel rotation dependence

The hypotheses tested were that sonolysis of erythrocytes in the presence of a gas-based ultrasound contrast agent in vitro will be related quantitatively to the duration and number of ultrasound pulses applied using a constant pulse repetition period and, at least qualitatively, to the total exposure duration (i.e., the product of pulse number x pulse duration). An objective was to determine the influence of sample rotation during insonation on the amount of hemolysis produced under these conditions. Human erythrocytes, suspended to 40% hematocrit in autologous plasma containing 3.6% (V:V) Albunex, were exposed/sham-exposed to 1-100 pulses of 1-MHz ultrasound (6.2 MPa peak positive, 3.6 MPa peak negative acoustic pressures; I(SPTP) approximately 800 W/cm2) using a 1-s pulse repetition period. Pulse durations ranged from 20-20,000 micros; samples were either stationary or rotated (200 rpm) during insonation. Hemolysis was independent of vessel rotation treatment at all tested pulse durations and pulse numbers. Levels of hemolysis statistically greater than in sham-exposed samples were obtained with > or = 50 pulses of 20 micros duration, and > or = 1 pulse of 200, 2000 or 20,000 micros duration. Hemolysis increased with increasing pulse number and pulse duration. Approximately equivalent levels of hemolysis were produced by different pulse number x pulse duration combinations, yielding the same total exposure duration.

Gas-body-based contrast agent enhances vascular bioeffects of 1.09 MHz ultrasound on mouse intestine

Anesthetized hairless mice were exposed to continuous or pulsed 1.09-MHz ultrasound with or without prior injection of a gas-body-based ultrasound contrast agent. Albunex at a dose of 10 mL/kg increased the production of intestinal hyperemia, petechia and hemorrhages by continuous ultrasound. For pulsed ultrasound, with 10 micros pulses and 0.01 duty cycle, petechiae were produced for exposures as low as 1 MPa spatial peak pressure amplitude with added gas bodies. The enhancement of petechiae production was robust for pulsed exposure; for example, at 2.8 MPa, an average of 227 petechiae was obtained with added gas bodies, which was 30 times more than without the agent. The production of petechia was roughly proportional to the dosage of Albunex for pulsed exposure. Results did not appear to be strongly dependent on pulsing parameters, but long bursts (0.1 s) were somewhat more effective than pulses (10 micros). The observed vascular bioeffects appeared to involve both thermal and nonthermal mechanisms for continuous exposure, but to result primarily from gas-body activation for pulsed exposure.

Frequency relationships for ultrasonic activation of free microbubbles, encapsulated microbubbles, and gas-filled micropores

The ultrasonic activation of free microbubbles, encapsulated microbubbles, and gas-filled micropores was explored using available linear theory. Encapsulated microbubbles, used in contrast agents for diagnostic ultrasound, have relatively high resonance frequencies and damping. At 2 MHz the resonance radii are 1.75 microns for free microbubbles, 4.0 microns for encapsulated microbubbles, and 1.84 microns for gas-filled micropores. Higher-pressure amplitudes are needed to elicit equivalent subharmonic, fundamental, or second-harmonic responses from the encapsulated microbubbles, and this behavior increases for higher frequencies. If an encapsulated microbubble becomes destabilized during exposure,the resulting liberated microbubble would be about twice the linear resonance size, which would be likely to produce subharmonic signals. Scattered signals used for medical imaging purposes may be indicative of bioeffects potential: The second harmonic signal is proportional to local shear stress for a microbubble on a boundary, and a strong subharmonic signal may imply destabilization and nucleation of free-microbubble cavitation activity. The potential for bioeffects from contrast agent gas bodies decreases rapidly with increasing frequency. This information should be valuable for understanding of the etiology of bioeffects related to contrast agents and for developing exposure indices and risk management strategies for their use in diagnostic ultrasound.

Shock waves in vascular diseases: an in-vitro study

Three human aortic specimens were used for this in-vitro study on the effects of shock waves on the arterial wall. Specimen one was from a normal (for age) healthy aorta. The full abdominal length was used (including mesenteric and renal arteries and the aortoiliac bifurcation), divided into six pieces (3 cm). The pieces were placed and fixed into degassed water. Shock waves (SW) were focused onto the aortic wall by means of a B-mode ultrasound imager. An SW generator (Minilith SL1, Storz Medical AG, Kreuzlingen, Switzerland) was used for setting of energy flux density between 0.03 and 0.5 mJ/mm2. The six aortic pieces (excluding piece 1, placed in water and left untreated as control) were treated with SW at increasing energy levels. A second aortic specimen of a man with arteriosclerotic plaques was also used and the experiment repeated at energy levels 1, 5, and 8. Another specimen of normal thoracic aorta was exposed at energy levels 1 and 8 only. Energy levels delivered onto the aortic walls were selected from theoretically destructive levels to minimal levels known not to alter vascular tissues. High-resolution ultrasounds of the aortic segments were performed with a 10 MHz high-resolution, broad-band (ATL 3000, USA) probe in water before and after SW application to detect structural changes in the wall after SW. Histology was performed with a standard hematoxylin-eosin staining.

RESULTS

The aortic pieces did not show macroscopic damages at visual examination, and at the ultrasound examination no visible changes were observed even at higher levels of SW energy. Also no effects were seen by histology. In conclusion, no damaging effects were observed, visually, by ultrasound, or by histology. At these energy levels SW appear to be safe and do not produce any damage to the aortic wall. Therefore, SW could be considered a safe, nondamaging procedure for potential treatment (ie, thrombolysis) in which vessel walls could be involved. Theoretically it is possible that functional changes could be observed in vivo including cell permeability modifications and other alterations (including changes in the potential of the cells in SW fields to modify themselves and to divide). At the energy levels described in this study SW could, theoretically be, safely used for vascular applications (ie, treating venous and arterial thrombi or in arterial plaques modification) without altering major, structural, arterial wall characteristics. Lesions or alterations that have a different density from the normal wall (thrombi or plaques) could be differently sensitive to the same dosage of SW. These differences in acoustic impedance characteristics could be used for potential treatments with SW without damaging the arterial wall.