Quantitative acoustic characterization of a new surfactant-based ultrasound contrast agent

Sonobactericide: An Emerging Treatment Strategy for Bacterial Infections

Ultrasound has been developed as both a diagnostic tool and a potent promoter of beneficial bio-effects for the treatment of chronic bacterial infections. Bacterial infections, especially those involving biofilm on implants, indwelling catheters and heart valves, affect millions of people each year, and many deaths occur as a consequence. Exposure of microbubbles or droplets to ultrasound can directly affect bacteria and enhance the efficacy of antibiotics or other therapeutics, which we have termed sonobactericide. This review summarizes investigations that have provided evidence for ultrasound-activated microbubble or droplet treatment of bacteria and biofilm. In particular, we review the types of bacteria and therapeutics used for treatment and the in vitro and pre-clinical experimental setups employed in sonobactericide research. Mechanisms for ultrasound enhancement of sonobactericide, with a special emphasis on acoustic cavitation and radiation force, are reviewed, and the potential for clinical translation is discussed.

Formation of Microbubbles for Targeted Ultrasound Contrast Imaging: Practical Translation Considerations

For preparation of ligand-decorated microbubbles for targeted ultrasound contrast imaging, it is important to maximize the amount of ligand associated with the bubble shell. We describe optimization of the use of a biocompatible cosurfactant in the microbubble formulation media to maximize the incorporation of targeting ligand-lipid conjugate into the microbubble shell, and thus reduce the fraction of ligand not associated with microbubbles, following amalgamation preparation. The influence of the concentration of a helper cosurfactant propylene glycol (PG) on the efficacy of microbubble preparation by amalgamation and on the degree of association of fluorescent PEG-lipid with the microbubble shell was tested. Three sets of targeted bubbles were then prepared: with VCAM-1-targeting peptide VHPKQHRGGSK(FITC)GC-PEG-DSPE, cyclic RGDfK-PEG-DSPE, selective for α_Vβ₃, and control cRADfK-PEG-DSPE, without such affinity. Microbubbles were prepared by 45 s amalgamation, with DSPC and PEG stearate as the main components of the shell, with 15% PG in aqueous saline. In vitro microbubble targeting was assessed with a parallel plate flow chamber with a recombinant receptor coated surface. In vivo targeting was assessed in MC-38 tumor-bearing mice (subcutaneous tumor in hind leg), 10 min after intravenous bolus of microbubble contrast agent (20 million particles per injection). Ultrasound imaging of the tumor and control nontarget muscle tissue in a contralateral leg was performed with a clinical scanner. Amalgamation technique with PG cosurfactant produced microbubbles at concentrations exceeding 2 × 10⁹ particles/mL, and ∼50-60% or more of the added fluorescein-PEG-DSPE or VCAM-1-targeted fluorescent peptide was associated with microbubbles, about 2 times higher than that in the absence of PG. After intravenous injection, peptide-targeted bubbles selectively accumulated in the tumor vasculature, with negligible accumulation in nontumor contralateral leg muscle, or with control nontargeted microbubbles (assessed by contrast ultrasound imaging). For comparison, administration of RGD-decorated microbubbles prepared by traditional sonication, and purified from free peptide-PEG-lipid by repeated centrifugation, resulted in the same accumulation pattern as for translatable amalgamated microbubbles. Following amalgamation in the presence of PG, efficient transfer of ligand-PEG-lipid to microbubble shell was achieved and quantified. Purification of microbubbles from free peptide-PEG-lipid was not necessary, as proven by in vitro and in vivo targeting studies, so PG cosurfactant amalgamation technique generated peptide-targeted microbubbles are amenable for bedside preparation and clinical translation. The pathway to clinical translation is simplified by the fact that most of the materials used in this study either are on the United States Food and Drug Administration GRAS list or can be procured as pharmaceutical grade substances.

Development of an ultrasound sensitive oxygen carrier for oxygen delivery to hypoxic tissue

Radiation therapy is frequently used in the treatment of malignancies, but tumors are often more resistant than the surrounding normal tissue to radiation effects, because the tumor microenvironment is hypoxic. This manuscript details the fabrication and characterization of an ultrasound-sensitive, injectable oxygen microbubble platform (SE61O2) for overcoming tumor hypoxia. SE61O2 was fabricated by first sonicating a mixture of Span 60 and water-soluble vitamin E purged with perfluorocarbon gas. SE61O2 microbubbles were separated from the foam by flotation, then freeze dried under vacuum to remove all perfluorocarbon, and reconstituted with oxygen. Visually, SE61O2 microbubbles were smooth, spherical, with an average diameter of 3.1 μm and were reconstituted to a concentration of 6.5 E7 microbubbles/ml. Oxygen-filled SE61O2 provides 16.9 ± 1.0 dB of enhancement at a dose of 880 μl/l (5.7 E7 microbubbles/l) with a half-life under insonation of approximately 15 min. In in vitro release experiments, 2 ml of SE61O2 (1.3 E8 microbubbles) triggered with ultrasound was found to elevate oxygen partial pressures of 100ml of degassed saline 13.8 mmHg more than untriggered bubbles and 20.6 mmHg more than ultrasound triggered nitrogen-filled bubbles. In preliminary in vivo delivery experiments, triggered SE61O2 resulted in a 30.4 mmHg and 27.4 mmHg increase in oxygen partial pressures in two breast tumor mouse xenografts.

Preclinical acute toxicology study of surfactant-stabilized ultrasound contrast agents in adult rats

Gas-filled microbubbles are used as contrast agents in diagnostic ultrasound imaging. A preclinical, acute toxicity study of 2 surfactant-stabilized ultrasound contrast agents (ST68 and ST44) was conducted. Subjects were 104 Sprague-Dawley rats (experimental doses, 0.1, 0.2, 0.8, and 1.0 mL/kg; control, 1.0 mL/kg saline) that were studied for 14 days after contrast; clinical signs, weight, blood, and urine were evaluated. Histopathology was performed following euthanasia. Of the 40 animals receiving ST44, 4 died prematurely and a dose dependency was demonstrated (P = .011), whereas in the ST68 groups only 1 death occurred (no dose dependency; P = .48). Only the weight of rats injected with ST44 varied significantly (P = .0003). This dependency was also found for 3 of 5 urine parameters and 4 of 36 blood parameters (P < .05). For ST68, only 1 urine parameter showed significance (P < .0001). Giant cell infiltration in the lungs was significantly higher than controls in the ST44 0.1 mL/kg and the ST68 0.8-1.0 mL/kg groups (P < .01). It is concluded that the prudent choice for future nonrodent, toxicology studies and potentially for human clinical trials is ST68 (given the deaths in the ST44 groups).

Plasma sterilization of poly lactic acid ultrasound contrast agents: surface modification and implications for drug delivery

Poly lactic acid (PLA) ultrasound contrast agents (CA) have been developed previously in our laboratory for ultrasound (US) imaging, as well as surface coated with doxorubicin to create a potential targeted platform of chemotherapeutic delivery using focused US. However, we have previously found it impossible to sterilize these agents while at the same time maintaining their acoustic properties, a task that would probably require fabrication within a clean facility. The purpose of this paper is to investigate the feasibility of using plasma to sterilize these CA while maintaining maximum echogenicity, a step that would greatly facilitate in vivo investigations. Effects of plasma exposure time (1, 3 and 6 min) and intensity (low-10 mA, 6.8 W; medium-15 mA, 10.5 W; and high-25 mA, 18 W) on the CAs' acoustic properties, surface morphology, zeta potential, capacity to carry chemotherapeutics and overall sterility are described. Both increases in plasma intensity and exposure time increased CA zeta potential and also significantly increased drug payload. High-intensity plasma exposure for 3 min was found to be an optimal sterilization protocol for maximal (100%) preservation of CA echogenicity. Plasma exposure resulted in sterile samples and maintained original CA enhancement of 20 dB and acoustic half-life over 75 min, while increasing CA zeta potential by 11 mV and doxorubicin loading efficiency by 10%. This study not only shows how a highly temperature- and pressure-sensitive agent can be sterilized using plasma, but also that surface modification can be used to increase surface binding of the drug.

siRNA-mediated gene silencing in the salivary gland using in vivo microbubble-enhanced sonoporation

OBJECTIVES

siRNA-induced gene silencing in the salivary gland using microbubble-enhanced sonoporation was used to develop an in vivo gene knockdown technique.

METHODS

siRNA targeting rat glyceraldehyde-3-phosphate dehydrogenas (GAPDH) was mixed with echo-enhanced microbubbles and reverse-injected into rat parotid glands using transdermal ultrasound. To compare direct and transdermal ultrasound efficiencies, an incision was made on the lateral neck to expose the parotid glands for direct application. The efficiency of gene suppression was determined using quantitative reverse transcription-polymerase chain reaction 24-72 h after siRNA delivery. Cytotoxicity was assessed using histological analysis.

RESULTS

Expression of rat GAPDH in the parotid glands was silenced 48 h after siRNA was delivered by ultrasound (frequency: 1 MHz; intensity: 2 W cm(-2); exposure time: 2 min). High-intensity ultrasound induced tissue damage and apoptotic change. Echo-enhanced microbubbles significantly improved siRNA-induced gene silencing by 10-50%. Compared with transdermal application, direct-exposure ultrasound was only slightly effective, and no significant difference in gene expression was observed.

CONCLUSION

The results indicate that microbubble-enhanced sonoporation can yield in vivo siRNA gene silencing in the rat parotid gland. This technique could be applied to provide gene knockdown organs for functional genomic analyses and to develop siRNA-based gene therapy.

Single microbubble response using pulse sequences: initial results

The study of acoustic scattering by single microbubbles has the potential to offer improved signal processing techniques. A microacoustic system that employs a hydrodynamically-focused flow was used to detect radiofrequency (RF) backscatter from single microbubbles. RF data were collected using a commercial scanner. Results are presented for two agents, namely Definity (Lantheus Medical Imaging, N. Billerica, MA, USA) and biSphere (Point Biomedical Corp, San Carlos, CA, USA). The agents were insonified with amplitude-modulated pulses, and it was observed in both agents that a subpopulation of microbubbles did not produce a measurable echo from the first-half amplitude pulse, but did produce a response from the full amplitude pulse and from a subsequent half amplitude pulse. The number of microbubbles in this subpopulation was seen to increase with increasing transmit amplitude. These results do not bear out the simple theory of microbubble-pulse sequence interaction and invite a reassessment of signal processing approaches.

Response of contrast agents to ultrasound

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

Acoustic characterization of a new trisacryl contrast agent. Part I: In vitro study

The objective of the study was to acoustically characterize trisacryl polymeric microparticles (TMP), which are derived from biocompatible embolic agents. With significant acoustic properties, these polymeric particles could be potentially used as targeted ultrasound contrast agents, directed towards a specific site, with ligands conjugation on the polymeric network surface. In the in vitro study, a pulser/receiver (PRF of 1 kHz), associated to different transducers (5, 10 and 15 MHz), was used to measure the acoustic properties of the TMP inserted in a Couette flow device. Acoustic characterization according to TMP concentration (0.12-15.63 mg/ml), frequency (4.5-17 MHz, defined by each transducer bandwidth), ultrasound pressure (137-378 kPa) and exposure time (0-30 min) was conducted. Particle attenuation was also evaluated according to TMP concentration and emission frequency. Backscattering increased non linearly with concentration and maximum enhancement was of 16.4 dB+/-0.89 dB above 7.8 mg/ml. This parameter was found non-linear with increasing applied pressure and no harmonic oscillation could be noticed. Attenuation reached approximately 1.4 dB/cm at 15 MHz and for the 15.6 mg/ml suspension. The TMP have revealed in vitro ultrasound properties comparable to those observed with known contrast agents, studied in similar in vitro systems. However, such set-ups combined with a rather aqueous suspending medium, have some limitations and further investigations need now to be conducted to approach in vivo conditions in terms of flow and blood environment.

Acoustic characterization of a new trisacryl contrast agent. Part II: Flow phantom study and in vivo quantification

The biocompatible trisacryl particles (TMP) are made of a cross-linked acrylic copolymer. Their inherent acoustic properties, studied for a contrast agent application, have been previously demonstrated in a in vitro Couette device. To measure their acoustic behaviour under circulating blood conditions, the TMP backscatter enhancement was further evaluated on a home-made flow phantom at different TMP doses (0.12-15.6 mg/ml) suspended in aqueous and blood media, and in nude mice (aorta and B16 grafted melanoma). Integrated backscatter (IB) was measured by spectral analysis of the Doppler signals recorded from an ultrasound system (Aplio) combined with a 12-MHz probe. Doppler phantom experiments revealed a maximal IB of 17+/-0.88 dB and 7.5+/-0.7 dB in aqueous and blood media, respectively. IB measured on mice aorta, in pulsed Doppler mode, confirmed a constant maximal value of 7.29+/-1.72 dB over the first minutes after injection of a 7.8 mg/ml TMP suspension. Following the injection, a 60% enhancement of intratumoral vascularization detection was observed in power Doppler mode. A preliminary histological study revealed inert presence of some TMP in lungs 8 and 16 days after injection. Doppler phantom experiments on whole blood allowed to anticipate the in vivo acoustic behaviour. Both protocols demonstrated TMP effectiveness in significantly increasing Doppler signal intensity and intratumoral vascularization detection. However, it was also shown that blood conditions seemed to shadow the TMP contrast effect, as compared to in vitro observations. These results encourage further investigations on the specific TMP targeting and on their bio-distribution in the different tissues.

Contrast-enhanced ultrasound measurement of microvascular perfusion relevant to nutrient and hormone delivery in skeletal muscle: a model study in vitro

Contrast-enhanced ultrasound (CEU) has been used to measure muscle microvascular perfusion in vivo in response to exercise and insulin. In the present study we address whether CEU measurement of capillary volume is influenced by bulk flow and if measured capillary filling rate allows discrimination of different flow pattern changes within muscle. Three in vitro models were used: (i) bulk flow rate was varied within a single length of capillary tubing; (ii) at constant bulk flow, capillary volume was increased 3-fold by joining lengths of capillary in series, and compared to a single length; and (iii) at constant bulk flow, capillary volume was increased by sharing flow between a number of lengths of identical capillaries in parallel. The contrast medium for CEU was gas-filled albumin microbubbles. Pulsing interval (time) versus acoustic-intensity curves were constructed and from these, capillary volume and capillary filling rate were calculated. CEU estimates of capillary volume were not affected by changes in bulk flow. Furthermore, as CEU estimates of capillary volume increased, measures of capillary filling rate decreased, regardless of whether capillaries were connected in series or parallel. Therefore, CEU can detect a change in filling rate of the microvascular volume under measurement, but it can not be used to discriminate between different flow patterns within muscle that might account for capillary recruitment in vivo.

The application of microbubbles for targeted drug delivery

Interest in microbubbles as vehicles for drug delivery has grown in recent years, due in part to characteristics that make them well suited for this role and in part to the need the for localized delivery of drugs in a number of applications. Microbubbles are inherently small, allowing transvascular passage, they can be functionalized for targeted adhesion, and can be acoustically driven, which facilitates ultrasound detection, production of bioeffects and controlled release of the cargo. This article provides an overview of related microbubble biofluid mechanics and reviews recent developments in the application of microbubbles for targeted drug delivery. Additionally, related advances in non-bubble microparticles for drug delivery are briefly described in the context of targeted adhesion.

Comparison of in vitro and in vivo acoustic response of a novel 50:50 PLGA contrast agent

A comparison between in vitro and in vivo experiments conducted to investigate the acoustic properties of a novel, 1.2 microm diameter poly(lactic-co-glycolic acid) (50:50) (PLGA) ultrasound contrast agent, the development of which was described previously by us, is presented. A pulse-echo setup was used to determine enhancement in vitro. Additional in vitro studies further characterized the hollow microcapsules, including resonance frequency from attenuation measurements (from 2.25 to 15 MHz) and temperature effects (25 degrees C vs. 37 degrees C). In vivo, four rabbits received intravenous injections of the agent (dose range: 0.005-0.13 ml/kg). Quantitative in vivo dose-responses were calculated off-line using spectral power analysis of audio Doppler signals acquired from a custom-made 10 MHz cuff transducer placed around the surgically exposed distal aorta. This frequency was chosen since the very shallow scanning depths encountered in rabbits, in particular for the cuff transducer placed directly around the vessel, necessitates the use of high frequency imaging devices with sufficient spatial resolution to enable meaningful measurements. For qualitative assessments, two rabbits were imaged pre- and post-contrast administration (dose: 0.1 ml/kg) in power Doppler mode. Significant acoustic enhancements (up to 24 dB) were reported both in vitro and in vivo. Moreover, the rabbits did not show any adverse side effects from multiple injections (>20) of the agent. Measured in vitro resonance frequency between 3.09 and 3.49 MHz was lower than predicted for a similar sized free bubble, potentially due to capsule wall structure. Minimal loss of signal (approximately 4 dB) was observed at 25 degrees C over 20 min of insonation at 5 MHz but at 37 degrees C the signal dropped close to base line within the first 5 min. This temperature sensitivity could be due to loss of capsule integrity (and hence loss of gas). Potential causes include increased hydrolysis or polymer softening and increased water uptake by the shell at temperatures closer to the glass transition temperature (T(g)).

Surfactant-stabilized contrast agent on the nanoscale for diagnostic ultrasound imaging

Ultrasound contrast agents (CA) are generally micron-sized stabilized gas bubbles, injected IV. However, to penetrate beyond the vasculature and accumulate in targets such as tumors, CA must be an order of magnitude smaller. We describe a method of achieving nanometer-sized, surfactant-stabilized CA by differential centrifugation. High g force was shown to destroy bubble integrity. Optimal conditions (300 rpm for 3 min) produced an agent with a mean diameter of 450 nm, which gave 25.5 dB enhancement in vitro at a dose of 10 microL/mL, with a 13 min half-life. In vivo, the CA produced excellent power Doppler and grey-scale pulse inversion harmonic images at low acoustic power when administered. In vivo dose-response curves obtained in three rabbits showed enhancement between 20 and 25 dB for dosages above 0.025 mL/kg. These results encourage further investigation of the possible diagnostic and therapeutic benefits of using nanoparticles as CA, including passive targeting and accumulation in tumors.

Effect of shell type on the in vivo backscatter from polymer-encapsulated microbubbles

This study compared in vivo enhancement from four different polymer-encapsulated ultrasound (US) contrast agents. The agents were produced with a rigid shell composed of the biodegradable block copolymer poly[D,L-lactide-co-glycolide] (PLGA) with the lactic and glycolic acid ratios 50:50, 75:25, 85:15 and 100:0 (i.e., increasingly hydrophobic shell compositions). Approximately the same bubble diameter (1.2 microm) and concentration (0.4 g/mL) were obtained for each agent. In four rabbits, audio Doppler signals were acquired from a 10 MHz cuff transducer placed around a surgically exposed vessel (contrast dose: 0.0125 to 0.15 mL/kg). In vivo dose responses were calculated off-line (in dB). Nine rabbit kidneys were imaged during contrast administration (0.1 mL/kg) in power Doppler and grey-scale pulse inversion harmonic (PIHI) modes using an HDI 5000 scanner (Philips Medical Systems, Bothell, WA). Time-intensity curves were produced and the time-to-peak, peak intensity, slope, area under the curve (AUC) and total duration of enhancement for each agent were compared. All agents produced marked Doppler enhancement with increasing duration from the 50:50 agent (48 +/- 10 s) to the 75:25 agent (166 +/- 46 s), the 85:15 agent (403 +/- 83 s) and with the 100:0 agent (603 +/- 93 s) lasting longest (p < 0.02). No other parameters changed significantly, except the AUC of the 85:15 agent, which was greater than that of the 50:50 agent (190.75 vs. 61.58; p = 0.02). The in vivo dose-response curves were similar for all agents, with mean enhancement up to 20.6 +/- 1.11 dB (p = 0.17). In conclusion, contrast duration increases by an order of magnitude as the lactic acid component in the polymer-encapsulated bubbles increases and the shell, thus, becomes increasingly hydrophobic.

Development of a novel method for synthesis of a polymeric ultrasound contrast agent

Medical ultrasound is a highly valuable diagnostic tool, especially when compared with other imaging modalities. It is a noninvasive, real-time, portable, extremely safe method compared with X-ray and inexpensive relative to magnetic resonance imaging. However, ultrasound is limited in its ability to distinguish between diseased and normal tissue. This limitation has led to the development of contrast agents. We have produced novel poly (lactic-co-glycolic) acid air-filled microcapsules that work well as ultrasound contrast agents, giving up to 24 and 25 dB enhancement when insonated in the medical imaging range at 5 and 7.5 MHz, respectively. The capsules were fabricated by modifying a double emulsion method to encapsulate camphor in the oil phase and ammonium carbonate in the aqueous phase, and later sublime the encapsulated material, leaving voids capable of being filled with a gas in their place. The role of the surfactant, poly vinyl alcohol, solution temperature, was studied and found to play an important role in the morphology of the capsules, altering their acoustic response.

A review of physical phenomena associated with ultrasonic contrast agents and illustrative clinical applications

Successful clinical applications of contrast agents involve an understanding of the physical interaction of ultrasound (US) with contrast agents. This paper reviews the physical phenomena involved in these interactions and discusses the relevant theoretical background for modeling US-contrast agent interactions. Measurement techniques using US to obtain information regarding contrast agents are summarized. Illustrative clinical applications are given in the second part of the paper. Recent developments in nonlinear imaging techniques and transient techniques are reviewed. New methods, such as depletion perfusion measurement, and high-frequency applications are included.

Generation of ultraharmonics in surfactant based ultrasound contrast agents: use and advantages

A unique distinction between surfactant stabilized ultrasound contrast agent ST68 and water (or tissue), is the enhanced ability of the agent to generate non-linear frequencies such as sub-harmonics (f0/2), higher harmonics (2fo, 3fo, 4fo,...), and ultraharmonics (3f0/2, Sf0/2, 7f0/2,...), when insonated with fundamental frequency f0. Currently, second harmonics (2f0) have been predominantly researched, to exploit the diagnostic benefits of the contrast-specific non-linear imaging. However, we found that at normal imaging pressures (100 kPa-1 MPa), ST68 agent-generated second harmonic enhancements dropped to approximately 8 dB at 100 kPa and approximately 2 dB at 1 MPa. Moreover, at these pressures water (or tissue) produced strong second harmonics due to non-linear propagation. Ultraharmonics and sub-harmonics on the other hand, were generated only by the agent, and were not produced due to the non-linear propagation of ultrasound in either water or tissue. Additionally, ultraharmonic (3f0/2) enhancements of approximately 23 dB at 100 kPa, approximately 35 dB at 0.5 MPa and approximately 41dB at 1.1 MPa for ST68-PFC, offer much greater signal to noise ratio than higher harmonics.

Study on the lifetime and attenuation properties of microbubbles coated with carboxylic acid salts

Four kinds of surfactants, sodium laurate, sodium myristate, sodium palmitate and sodium oleate were used to study the effects of surfactant coatings on the lifetime and attenuation of microbubbles. The changes in the size distribution of microbubbles prepared with these surfactants in saline were measured with a Coulter Multisizer (Coulter Electronics Ltd., Luton, UK). Frequency characteristics of ultrasonic attenuation of the microbubble suspensions were measured between 400 kHz and 6 MHz. From the changes in attenuation in the microbubble suspensions over time, it was found that the lifetime of microbubbles in a suspension also depends on the frequency of the irradiating ultrasound. The effect of surfactants on the frequency characteristics of attenuation was also studied, and characteristics of the surfactant coating, including shell elasticity and shell friction parameters were calculated from the measurement results. Microbubbles produced with sodium palmitate had the longest lifetime and the smallest average size. The shell had very little effect on the ultrasonic properties of microbubbles produced with sodium palmitate, suggesting that the sodium palmitate microbubbles behaved ultrasonically as free microbubbles.

Influence of environmental conditions on a new surfactant-based contrast agent: ST68

Environmental influences on the new surfactant-stabilized bubbles, ST68, were investigated. We have developed a new surfactant-based contrast agent ST68, which is prepared by insonating buffered mixtures of Span 60 and Tween 80 in the presence of either air, PFC, or SF(6) gas. The effect of dilution, shear, and sonication on size distribution of ST68 showed that PFC-containing bubbles (ST68-PFC) were most stable. ST68-PFC bubbles lasted more than 15 min with approximately 30 dB backscatter enhancement in degassed phosphate-buffered saline, (pH 7.4), and air bubbles lasted approximately 3 s, suggesting the effects of diffusion. Additionally, it was found that the ionic strength of the suspending medium (for example, PBS), did not have any effect on ST68 bubbles containing SF(6) or PFC, but had a dramatic impact on bubbles containing air.

Fundamental studies on contrast images from different-sized microbubbles: analytical and experimental studies

Microbubbles are very useful as ultrasound (US) contrast agents because of their excellent scattering properties. Because microbubbles of different sizes can be used for this purpose, the contrast images produced by different-sized microbubbles are studied in this paper. The contrast images from microbubbles of average sizes 35.5 microm and 2.1 microm were investigated experimentally. Although a low concentration of microbubbles produces contrast-enhanced images without artefacts, an excess of microbubbles results in distorted images. From experimental observation, the distortion of an image caused by microbubbles of average size 35.5 microm was mainly due to multiple scattering, and that by 2.1-microm microbubbles was due to the acoustic shadowing effect. With the use of the tissue-mimicking phantoms of known acoustical properties, the brightness of the contrast images from the microbubble suspension was calculated. The calculated and experimental results of the contrast images produced from microbubbles of average size 35.5 microm were closer to each other when there was no image distortion. When image distortion caused by multiple-scattering occurred, the experimental pixel brightness was higher. For smaller microbubbles of average size 2.1 microm, calculated results of free microbubbles showed a weaker contrast effect than the experimental results. By taking the effect of the coatings of microbubbles into consideration, the calculated brightness of contrast images became much closer to the experimental one.

Effect of filling gases on the backscatter from contrast microbubbles: theory and in vivo measurements

Two surfactant-based contrast agents, ST44 and ST68, were produced according to US Patent # 5,352,436 and filled with either air, C4F10 (perfluorobutane) or SF6 (sulfur hexaflouride). Ten rabbits received i.v. injections of each agent/gas combination with 5 repetitions of each dose (range: 0.005-0.13 mL/kg). A custom-made 10-MHz cuff transducer was placed around the surgically exposed distal aorta and audio Doppler signals were acquired in vivo. Quantitative in vivo dose responses were calculated off-line using spectral power analysis and compared to a theoretical model of microbubble dissolution and enhancement. For qualitative comparisons, 10 rabbits were imaged pre- and postcontrast administration (dose: 0.1 mL/kg) in gray-scale and colour. All agent/gas combinations produced marked Doppler enhancement with air bubbles enhancing least of all (p < 0.0001) and ST68-SF6 best of all (maximum: 27.6 +/- 2.04 dB; p < 0.012). There were no significant differences between other agent/gas combinations (0.30 < p < 0.70). Theoretical enhancement was within 1 order of magnitude of the experimental observations (i.e., deviations of up to 10 dB). The duration of contrast enhancement was 1-2 min for air-filled bubbles, 3-5 min for SF6-filled bubbles and more than 7 min for C4F10-filled bubbles. In conclusion, ST68-SF6 microbubbles produced most in vivo enhancement of the agent/gas combinations studied. Theory matched the measurements within an order of magnitude.

Subharmonic imaging with microbubble contrast agents: initial results

The subharmonic emission from insonified contrast microbubbles was used to create a new imaging modality called Subharmonic Imaging. The subharmonic response of contrast microbubbles to ultrasound pulses was first investigated for determining adequate acoustic transmit parameters. Subharmonic A-lines and gray scale images were then obtained using a laboratory pulse-echo system in vitro and a modified ultrasound scanner in vivo. Excellent suppression of all backscattered signals other than from contrast microbubbles was achieved for subharmonic A-lines in vitro while further optimization is required for in vivo gray scale subharmonic images.