Optison Microbubbles Research Articles

Focus Ultrasound‐Induced Blood‐Brain Barrier opening enhances anti‐pGlu3 Aβ mAb delivery and amyloid‐beta plaque clearance

AbstractBackgroundAlzheimer’s Disease (AD) is a neurodegenerative disorder characterized by the accumulation of amyloid plaques and hyperphosphorylated tau in the brain. Currently, therapeutic agents targeting amyloid appear promising for AD, however, delivery to the CNS is limited due to the blood‐brain‐barrier (BBB). Focused ultrasound (FUS) is a method to induce a temporary opening of the BBB to enhance delivery of therapeutic agents to CNS. Our lab has shown that an mAb (07/2a), targeting N‐terminally truncated and modified toxic Aβ species, reduced pGlu3 Aβ and general Aβ cerebral plaques in aged APP/PS1dE9 mice. Here, we asked whether FUS‐induced BBB opening enhances the delivery of 07/2a mAb and whether it has any additive effect on cognition or plaque clearance.MethodsFirst, 24 mo‐old APP/PS1dE9 mice were i.v. infused with a single dose of 300 µg 07/2a with or without hippocampal FUS sonication (n=5/group) with i.v. infused Optison microbubbles and euthanized 4 or 72 h later; brain levels of antibodies were measured by ELISA. Next, 16 mo‐old APP/PS1dE9 mice were treated weekly for 3 weeks with PBS (n=9), 500 µg 07/2a alone (n=9), FUS alone (n=7) or 07/2a + FUS combination (n=6). Behavioral testing in the Water T Maze (WTM) was performed 1‐2 weeks later followed by euthanasia.ResultsFUS treatment increased 07/2a mAb levels in brain by 5.9‐fold 4 hr (41.7 pg/mg for mAb alone, vs. 244 pg/mg mAb+FUS; t=3.48, p<0.005) and 5.5‐fold (mean=31.5 pg/mg for mAb alone, vs. 173 pg/mg for mAb+FUS; t=2.9, p<0.05) 72 hr post‐treatment. Immunohistochemistry confirmed a significant increase in IgG2a mAb staining in the mAb+FUS treated mice at 4 hr (p=0.007) and a strong trend in the mAb+FUS treated mice at 72 hr after treatment compared to mAb alone. Three weekly treatments of 07/2a improved cognition however, when combined with FUS, this improvement occurred faster and in a higher percentage of mice and led to reduced hippocampal plaque load compared to PBS control mice.ConclusionsTransient opening of the BBB by FUS increased 07/2a delivery to the brain, improved cognition, and enhanced Ab clearance in aged APP/PS1dE9 mice, suggesting that FUS increased efficacy of the 07/2a mAb.

Effect of optison microbubbles- mediated B-mode ultrasound- guided focused low- level confocal dual pulse electrohydraulic shock wave- assisted streptokinase thrombolysis on embolic carotid artery

Background and Aims: In this study, we developed an experimental focused low- level confocal dual pulse electrohydraulic shock wave thrombolytic therapy system, and investigated its effectiveness on common carotid artery recanalization, accompanied by intravenous streptokinase and Optison microbubbles administration, wherein diagnostic ultrasound imaging system is combined with the therapy system, with a goal of increased safety.

Inertial cavitation of lyophilized and rehydrated nanoparticles of poly(L-lactic acid) at 835\u2009kHz and 1.8\u2009MPa ultrasound

Nanoparticles of poly-L-lactic acid dispersed in water and of approximately 120 nm diameter were prepared by a nanoprecipitation method followed by lyophilization together with trehalose. After rehydration, the nanodispersion was exposed to ultrasound at 835 kHz frequency and 1.8 MPa peak negative sound pressure. Substantial levels of broadband noise were surprisingly detected which are attributed to the occurance of inertial cavitation of bubbles present in the dispersion. Inertial cavitation encompasses the formation and growth of gas cavities in the rarefaction pressure cycle which collapse in the compression cycle because of the inwardly-acting inertia of the contracting gas-liquid interface. The intensity of this inertial cavitation over 600 s was similar to that produced by Optison microbubbles used as contrast agents for diagnostic ultrasound. Non-lyophilized nanodispersions produced negligible broadband noise showing that lyophilization and rehydration are requirements for broadband activity of the nanoparticles. Photon correlation spectroscopy indicates that the nanoparticles are not highly aggregated in the nanodispersion and this is supported by scanning (SEM) and transmission (TEM) electron micrographs. TEM visualized non-spherical nanoparticles with a degree of irregular, non-smooth surfaces. Although the presence of small aggregates with inter-particulate gas pockets cannot be ruled out, the inertial cavitation activity can be explained by incomplete wetting of the nanoparticle surface during rehydration of the lyophilizate. Nano-scale gas pockets may be trapped in the surface roughness of the nanoparticles and may be released and coalesce to the size required to nucleate inertial cavitation on insonation at 835 kHz/1.8 MPa.

MRI-guided transurethral insonation of silica-shell phase-shift emulsions in the prostate with an advanced navigation platform.

In this study, the efficacy of transurethral prostate ablation in the presence of silica-shell ultrasound-triggered phase-shift emulsions (sUPEs) doped with MR contrast was evaluated. The influence of sUPEs on MR imaging assessment of the ablation zone was also investigated. sUPEs were doped with a magnetic resonance (MR) contrast agent, Gd2 O3 , to assess ultrasound transition. Injections of saline (sham), saline and sUPEs alone, and saline and sUPEs with Optison microbubbles were performed under guidance of a prototype interventional MRI navigation platform in a healthy canine prostate. Treatment arms were evaluated for differences in lesion size, T1 contrast, and temperature. In addition, non-perfused areas (NPAs) on dynamic contrast-enhanced (DCE) MRI, 55°C isotherms, and areas of 240 cumulative equivalent minutes at 43°C (CEM43 ) dose or greater computed from MR thermometry were measured and correlated with ablated areas indicated by histology. For treatment arms including sUPEs, the computed correlation coefficients between the histological ablation zone and the NPA, 55°C isotherm, and 240 CEM43 area ranged from 0.96-0.99, 0.98-0.99, and 0.91-0.99, respectively. In the absence of sUPEs, the computed correlation coefficients between the histological ablation zone and the NPA, 55°C isotherm, and 240 CEM43 area were 0.69, 0.54, and 0.50, respectively. Across all treatment arms, the areas of thermal tissue damage and NPAs were not significantly different (P=0.47). Areas denoted by 55°C isotherms and 240 CEM43 dose boundaries were significantly larger than the areas of thermal damage, again for all treatment arms (P=0.009 and 0.003, respectively). No significant differences in lesion size, T1 contrast, or temperature were observed between any of the treatment arms (P>0.0167). Lesions exhibiting thermal fixation on histological analysis were present in six of nine insonations involving sUPE injections and one of five insonations involving saline sham injections. Significantly larger areas (P=0.002), higher temperatures (P=0.004), and more frequent ring patterns of restricted diffusion on exvivo diffusion-weighted imaging (P=0.005) were apparent in lesions with thermal fixation. T1 contrast suggesting sUPE transition was not evident in sUPE treatment arms. The use of MR imaging metrics to predict prostate ablation was not diminished by the presence of sUPEs. Lesions generated in the presence of sUPEs exhibited more frequent thermal fixation, though there were no significant changes in the ablation areas when comparing arms with and without sUPEs. Thermal fixation corresponded to some qualitative imaging features.

Accelerated Clearance of Ultrasound Contrast Agents Containing Polyethylene Glycol is Associated with the Generation of Anti-Polyethylene Glycol Antibodies

Emerging evidence suggests that the immune system can recognize polyethylene glycol (PEG), leading to the accelerated blood clearance (ABC) of PEGylated particles. Our aim here was to study the generation of anti-PEG immunity and changes in PEGylated microbubble pharmacokinetics during repeated contrast-enhanced ultrasound imaging in rats. We administered homemade PEGylated microbubbles multiple times over a 28-d period and observed dramatically accelerated clearance (4.2 × reduction in half-life), which was associated with robust anti-PEG IgM and anti-PEG IgG antibody production. Dosing animals with free PEG as a competition agent before homemade PEGylated microbubble administration significantly prolonged microbubble circulation, suggesting that ABC was largely driven by circulating anti-PEG antibodies. Experiments with U.S. Food and Drug Administration-approved Definity microbubbles similarly resulted in ABC and the generation of anti-PEG antibodies. Experiments repeated with non-PEGylated Optison microbubbles revealed a slight shift in clearance, indicating that immunologic factors beyond anti-PEG immunity may play a role in ABC, especially of non-PEGylated agents.

The Effects of Oxygen on Ultrasound-Induced Blood–Brain Barrier Disruption in Mice

Numerous researchers are investigating the use of microbubble-enhanced ultrasound to disrupt the blood–brain barrier (BBB) and deliver drugs to the brain. This study investigated the impact of using oxygen as a carrier gas for anesthesia on microbubble activity and BBB disruption. Targets in mice were sonicated in combination with administration of Optison microbubbles (100 μL/kg) under isoflurane anesthesia with either oxygen or medical air. A 690-kHz focused ultrasound transducer applied 10-ms bursts at peak pressure amplitudes of 0.46–0.54 MPa (n = 2) or 0.34–0.36 MPa (n = 5). After sonication of two locations in one hemisphere, the carrier gas for the anesthesia was changed and the sonications were repeated in the contralateral hemisphere. The BBB disruption, measured via contrast-enhanced magnetic resonance imaging, was significantly greater (p < 0.001) with medical air than with oxygen. Harmonic emissions were also greater with air (p < 0.001), while the decay rate of the harmonic emissions was 1.5 times faster with oxygen. A good correlation (R2, 0.46) was observed between the harmonic emissions strength and magnetic resonance imaging signal enhancement. At 0.46–0.54 MPa, both the occurrence and strength of wideband emissions were greater with medical air. However, at lower peak pressure amplitudes of 0.34–0.36 MPa, the strength and probability for wideband emissions were higher with oxygen. Little or no effects were observed in histology at 0.34–0.36 MPa. These findings show that use of oxygen as a carrier gas can result in a substantial diminution of BBB disruption. These results should be taken into account when comparing studies from different researchers and in translating this method to humans.

Enhancing effect of ultrasound–mediated microbubble destruction on gene delivery into rat kidney via different administration routes

ObjectiveTo investigate the efficiency of β-galactosidase gene transfer into rat kidney with ultrasound-mediated microbubble destruction via different injection routes. MethodsA total of 25 Wistar rats were randomly divided into 5 groups. Four groups received a mixture of optison microbubbles (0.2 mL) and lacz plasmids (25 μg) injection via renal artery, tail vein, anterior tibial muscle and renal parenchyma, respectively. The control group received a mixture of PBS (xx mL) and lacz plasmids (25 μg) via renal artery. Three days after the gene transfer, ultrasound with fixed frequency and power (1 MHz, xxW) was delivered to the kidneys for 3 min. The efficiency of the gene transfer and expression was evaluated on the basis of β-galactosidase expression. The side effects of this method were evaluated by immunohistological method. Resultsβ-galactosidase expression could be observed only in tubules but not in glomeruli and interstitial area. The efficiency of renal artery group was higher than that of tail vein, anterior tibial muscle and renal parenchyma group (P<0.05). Immunohistochemical analysis revealed co-expression of β-galactosidase with a roximal tubule marker, megalin, which suggested that ultrasound enhanced gene transfer into the proximal tubular epithelial cells. No β-galactosidase expression was observed in the extrarenal organs. There were no evident pathological and biochemical changes after gene transfer. ConclusionsUltrasound-mediated microbubble destruction can transfer gene into kidney via renal artery, tail vein, anterior tibial muscle and renal parenchyma. Compared with renal artery, administrating microbubbles via tail vein and anterior tibial muscle are more convenient and less vulnerarious.

P85, Optison microbubbles and ultrasound cooperate in mediating plasmid DNA transfection in mouse skeletal muscles in vivo

Pluronic block copolymers, a kind of non-ionic surfactant, also known as poloxamers, and ultrasound-targeted microbubble destruction have been respectively investigated as vectors for gene delivery in vitro and in vivo. However, they are limited for clinical application due to the relatively low transfer efficiency of each individual vector. In the present study, we explored if the combination of P85, a pluronic block copolymer, Optison, a microbubble contrast agent and ultrasound enhances the transfection of plasmid DNA in vivo using mouse skeletal muscle models. Plasmid encoding green fluorescent protein (GFP) was respectively conjugated with 0.05%P85, 10%Optison, or 0.05%P85 plus 10%Optison, and injected into mouse tibialis anterior (TA) muscles with or without ultrasound irradiation (1 MHz, 1 W/cm 2, 2 min and 20% duty cycle). Mice were sacrificed 1 week after injection. The TA muscles were collected and cryo-sectioned into a series of 7 μm slices. To assess the efficiency of plasmid DNA transfection, tissue sections were counterstained with DAPI and scored by counting the number of GFP-positive fibers. Meanwhile the area of damaged muscles was measured based on the tissues stained with hematoxylin and eosin. Both P85 and Optison significantly enhanced the delivery of plasmid DNA in mouse TA skeletal muscles ( P < 0.01 and P < 0.05 respectively, compared to saline control). In combination with Ultrasound irradiation, P85 ( P < 0.01, compared to P85 alone) but not Optison ( P > 0.05, compared to Optison alone) exerted a more pronounced effect on the transfection efficiency. Furthermore P85-induced gene delivery was higher than that by Optison regardless of the presence of ultrasound ( P < 0.01). The highest transfection efficiency was observed when P85, Optison and ultrasound irradiation were administrated together ( P < 0.01, compared to any other treatment in this study). The area of damaged muscles was enlarged by ultrasound irradiation in the presence of Optison microbubbles ( P < 0.01, compared to those groups without ultrasound irradiation). These results suggest that P85, microbubbles and ultrasound irradiation synergistically enhance plasmid DNA delivery in mouse skeletal muscles in vivo.

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

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

Ultrasound-Induced Calcium Oscillations and Waves in Chinese Hamster Ovary Cells in the Presence of Microbubbles

This study investigated the effects of ultrasound on the intracellular [Ca 2+] of Chinese hamster ovary cells in the presence of albumin-encapsulated Optison microbubbles. Cells were exposed to 1 MHz ultrasound (tone burst of 0.2 s duration, 0.45 MPa peak pressure) while immersed in solution of 0.9 mM Ca 2+. Calcium imaging of the cells was performed using digital video fluorescence microscopy and Ca 2+-indicator dye fura-2AM. Experimental evidence indicated that ultrasound caused a direct microbubble-cell interaction resulting in the breaking and eventual dissolution of the microbubble and concomitant permeabilization of the cells to Ca 2+. These cells exhibited a large influx of Ca 2+ over 3–4 s and did not return to their equilibrium levels. Subsequently, some cells exhibited one or more Ca 2+ oscillations with the onset of oscillations delayed by 10–80 s after the ultrasound pulse. A variety of oscillations were observed including decaying oscillations returning to the baseline value over 35–100 s, oscillations superimposed on a more gradual recovery over 150–200 s, and oscillations continued with increased amplitude caused by a second ultrasound tone burst. The delays in onset appeared to result from calcium waves that propagated across the cells after the application of the ultrasound pulse.

Use of ultrasound to enhance nonviral lung gene transfer in vivo

We have assessed if high-frequency ultrasound (US) can enhance nonviral gene transfer to the mouse lung. Cationic lipid GL67/pDNA, polyethylenimine (PEI)/pDNA and naked plasmid DNA (pDNA) were delivered via intranasal instillation, mixed with Optison microbubbles, and the animals were then exposed to 1 MHz US. Addition of Optison alone significantly reduced the transfection efficiency of all three gene transfer agents. US exposure did not increase GL67/pDNA or PEI/pDNA gene transfer compared to Optison-treated animals. However, it increased naked pDNA transfection efficiency by approximately 15-fold compared to Optison-treated animals, suggesting that despite ultrasound being attenuated by air in the lung, sufficient energy penetrates the tissue to increase gene transfer. US-induced lung haemorrhage, assessed histologically, increased with prolonged US exposure. The left lung was more affected than the right and this was mirrored by a lesser increase in naked pDNA gene transfer, in the left lung. The positive effect of US was dependent on Optison, as in its absence US did not increase naked pDNA transfection efficiency. We have thus established proof of principle that US can increase nonviral gene transfer, in the air-filled murine lung.

Effect of 810 kHz cw ultrasound on bacterial biofilms

Biofilms of florescent E. coli bacteria were grown in protein‐rich media on coverslips that served as the floor of an ultrasound exposure chamber. Two opposite walls of the chamber were strips of lead zirconate titanate (PZT‐4) driven at their resonance frequency of 810 kHz in continuous wave mode, thereby setting up acoustic standing waves within the 12 mm by 16 mm by 0.5 mm chamber. A convolution confocal florescence microscope was used to visualize the biofilm before, during, and after 10‐minute exposures at various acoustic pressure amplitudes, with and without Optison microbubbles present. Quantifiable changes to the biofilm structure suggest that stable cavitation is a mechanism of interaction that may provide a method of enhancing antibiotic action in medical applications. [Work supported by HHMI grant ♯52005202.]

Difference frequency and its harmonic emitted by microbubbles under dual frequency excitation

Vibro-acoustography is an elasticity imaging method that uses two ultrasound beams of slightly different frequency to excite an object and detects the resulting acoustic emission (AE) at the difference frequency. This method is especially sensitive to bubbles due to their nonlinearity. This study explores the harmonic acoustic emission (HAE) at twice the difference frequency emitted from bubbles. A perturbation method based on the dynamic bubble equation is used to derive the AE and HAE from a single bubble excited by dual frequency waves. Simulation shows that HAE is generated only by microbubbles whose resonant frequencies match the incident ultrasound frequencies. In contrast, AE is more sensitive to resonance at the difference frequency, which is relevant to sub-millimeter bubbles. This finding was confirmed by experiments where HAE was produced from Optison microbubbles, but not from larger air bubbles which are off resonance at the incident ultrasound frequency. In conclusion, harmonic acoustic emission is present for microbubbles. It is very sensitive to the size of the bubble and may be used for selective detection of microbubbles.

The effects of albumin-coated microbubbles in DNA delivery mediated by therapeutic ultrasound

The application of therapeutic ultrasound (TUS) in combination with contrast agents (USCA) to mediate gene delivery relies on the understanding of the bioeffects involved. The objective of this study was to evaluate the various bioeffects generated by albumin-coated microbubbles: Optison, an USCA, when applied with TUS operated for 10–30 min, on cells and on DNA transfection. This study reveals that Optison microbubbles were still acoustically active after long-term TUS application of 30 min. Optison enhances TUS-gene transfection by increasing the number of plasmids in the cells and also by distributing the plasmids to more cells, without significant decrease in cell viability. Optison also interacts with the DNA to further enhance transfection in a mechanism not necessarily involving cavitation. However, Optison affects mainly the cell cytoplasmatic membrane, without interfering with DNA intracellular trafficking. Using high-resolution scanning electron microscopy (HRSEM), the bioeffects on cell membrane induced by TUS–Optison were observed, demonstrating that Optison lead to a rougher surface, characterized by depressions that are reversible within 24-h post TUS. These effects are different from those observed when only TUS was applied. The findings from this study suggest that albumin-coated microbubbles enhances transfection when using TUS for 10–30 min, and that microbubbles play a major role in elevating cell transfection level and efficiency.

On the Usefulness of the Mechanical Index Displayed on Clinical Ultrasound Scanners for Predicting Contrast Microbubble Destruction

The purpose of this study was to evaluate the mechanical index (MI) displayed on clinical ultrasound scanners as a predictor of exposure conditions related to the destruction of sonographic microbubble contrast agents. Sonazoid (GE Healthcare, Oslo, Norway) and Optison (GE Healthcare, Princeton, NJ) microbubbles were injected into a tissue-mimicking flow phantom. Gray scale imaging was performed with 4 different scanners and 3 different transducers (3.5 MHz curved linear, 2.5 MHz convex, and 7.5 MHz linear array), and the MI displayed by the scanner was varied from 0.2 to 1.5 by changing the system output power. All other scanning parameters were kept constant. Downstream changes in echogenicity were monitored with a PowerVision 7000 scanner (Toshiba America Medical Systems, Tustin, CA) as an indirect measure of bubble destruction. Video intensity changes within the flow tube were determined as a function of MI for the different scanner/transducer combinations, and the best linear fit was determined. At a displayed MI of 0.7, different scanner/transducer combinations exhibited a range in video intensity from +16% to -3% of baseline for Sonazoid and from +8% to -71% for Optison. At an MI of 0.3, reductions in video intensity of up to 32% were produced. These results indicate a wide range in bubble destruction at identical MI values. Likewise, regression analysis found no linear fits for all scanner/transducer combinations (r2 < 0.046). The MI displayed on clinical ultrasound scanners does not predict the degree of microbubble destruction and should not be used by itself to define exposure conditions for destruction of microbubble contrast agents.

Microbubble dynamics in contrast agent shell rupture

A passive cavitation detector (PCD) was used to study the features of insonified contrast agent microbubble rupture. When Optison microbubbles ruptured, the PCD signal received from single microbubbles showed the presence of post-excitation echoes identified as inertial cavitation and rebounds. Using these signals, the collapse threshold was evaluated as a function of insonation frequency, pulse duration, and pressure amplitude. For each excitation signal, 128 received time traces were recorded (200 MHz sampling, 12-bit), and frequency-domain analysis of the received signals was performed to characterize microbubble collapse. The peak rarefactional pressure threshold for detection of these emissions increased with frequency (e.g., 0.53, 0.87, and 0.99 MPa for 0.9, 2.8, and 4.6 MHz, respectively; 5-cycle pulse duration) and decreased with pulse duration. The modified Herring equation was used to model the bubble response at experimentally determined threshold conditions. These predicted microbubble dynamics were used to determine microbubble rupture indices based on: radial strain, peak velocity, and peak acceleration. This work provides a clearer understanding of the role of contrast agent dynamics in bubble destruction. [Work supported by the cooperative project for biomedical engineering between the University of Illinois at Urbana-Champaign, USA and the Centre National de la Recherche Scientifique, France.]

Observation on the integrity of the blood-brain barrier after microbubble destruction by diagnostic transcranial color-coded sonography.

To investigate alteration of the blood-brain barrier from ultrasonic contrast agent destruction by diagnostic transcranial color-coded sonography using gadolinium-enhanced magnetic resonance imaging. Healthy male volunteers received 10 mL (400 mg/dL) of Levovist (SH U 508A; Schering AG, Berlin, Germany; n = 6) or 3 mL of Optison (FS069; Mallinckrodt Inc, St Louis, MO; n = 4) followed by 0.3 mmol/kg magnetic resonance imaging contrast agent (Magnevist; Schering) intravenously. Then transcranial color-coded sonography was performed with a conventional color duplex sonographic system, which insonated the brain in a slightly angulated axial plane with temporal average intensity of less than 700 mW/cm2 or acoustic pressure amplitude of less than 2.69 MPa, attenuated by the temporal bone. Before, immediately after, and 2 hours after insonation, T1-weighted axial magnetic resonance imaging was performed. All magnetic resonance images were individually assessed, and T1 signal intensities were measured in 2 regions of interest in both hemispheres at the 3 time points. No focal contrast enhancement or damage to the brain and no significant difference between T1 signal intensities in the right and left brain regions could be detected during early or late phases when either ultrasonic contrast agent was used. This bioeffects study gives further evidence of the safety of ultrasonic destruction of Levovist and Optison microbubbles by diagnostic transcranial color-coded sonography. However, more subtle local effects may have been missed by gadolinium-enhanced magnetic resonance imaging. Studies on diagnostic contrast-enhanced transcranial color-coded sonography as well as microbubble-based drug delivery strategies should consider ultrasonic contrast agent microbubble characteristics and concentration as well as ultrasound transmission power levels.

Radiofrequency spectral analysis of attenuated ultrasound signals in experiments with echo contrast microbubbles

Conventional gray-scale myocardial contrast echocardiography cannot distinguish perfused but attenuated from nonperfused myocardium because both may appear similar at low image intensity. We hypothesized that with radiofrequency spectral analysis of attenuated ultrasound signals, the harmonic-to-fundamental frequency ratio of the peak power spectrum (HFRP) could determine the presence of contrast microbubbles. We measured frequency responses of Optison microbubbles at defined degrees of ultrasound signal attenuation with different formulations of silicone (55D, 80A, and 3M); gray-scale intensities of Optison plus water compared with degassed water were analyzed at different attenuation settings (−25, −32, and −44 dB, respectively). HFRP values of Optison plus water were significantly higher than reference values of degassed water at each attenuation setting (55D, −14 ± 2 dB versus −30 ± 2 dB, P <.001; 80A, −19 ± 2 dB versus −30 ± 3 dB, P <.01; 3M, −22 ± 2 dB versus −30 ± 3 dB, P <.05), even though conventional videodensitometric analysis could not distinguish them. HFRP analysis objectively detects microbubbles in clinically relevant conditions of attenuation. (J Am Soc Echocardiogr 2001;14:789-97.)

Ultrasonic characterization of the nonlinear properties of contrast microbubbles

The nonlinear properties of microbubble contrast agents have been used to create contrast-specific imaging modalities such as harmonic imaging and subharmonic imaging. Thus, a better understanding of the nonlinear performance of contrast microbubbles may enhance the diagnostic capabilities of medical ultrasound (US) imaging. The first and second harmonic, the 1/2 order subharmonic and the 3/2 order ultraharmonic components in spectra of scattered signals from Optison microbubbles insonified at 2 and 4 MHz have been investigated using an in vitro laboratory pulse-echo system. The development of these signal components over time is quite different for 2-MHz insonification compared to 4-MHz insonification. Scattered subharmonic and ultraharmonic signals are much more time-dependent than first and second harmonic echoes. The dependence of the first and second harmonic, subharmonic and ultraharmonic components on acoustic pressure for 2-MHz insonification is similar to that for 4-MHz insonification. The first and second harmonic components increase linearly with acoustic pressure (in double logarithmic scales) and the subharmonic and ultraharmonic amplitudes undergo rapid growths in the intermediate acoustic pressure range and much slower increases at both lower and higher acoustic pressures.

Real-time photoacoustic microscopy : Liu, Z-M. Dissertation Abstracts International, Vol. 48, No. 4, p. 1080 (Oct. 1987)

The nonlinear properties of microbubble contrast agents have been used to create contrast-specific imaging modalities such as harmonic imaging and subharmonic imaging. Thus, a better understanding of the nonlinear performance of contrast microbubbles may enhance the diagnostic capabilities of medical ultrasound (US) imaging. The first and second harmonic, the 1/2 order subharmonic and the 3/2 order ultraharmonic components in spectra of scattered signals from Optison microbubbles insonified at 2 and 4 MHz have been investigated using an in vitro laboratory pulse-echo system. The development of these signal components over time is quite different for 2-MHz insonification compared to 4-MHz insonification. Scattered subharmonic and ultraharmonic signals are much more time-dependent than first and second harmonic echoes. The dependence of the first and second harmonic, subharmonic and ultraharmonic components on acoustic pressure for 2-MHz insonification is similar to that for 4-MHz insonification. The first and second harmonic components increase linearly with acoustic pressure (in double logarithmic scales) and the subharmonic and ultraharmonic amplitudes undergo rapid growths in the intermediate acoustic pressure range and much slower increases at both lower and higher acoustic pressures.