Peak Negative Acoustic Pressure Research Articles

Experimental study on high-frequency subharmonic scattering characteristics of ultrasound contrast agent microbubbles under low ambient pressure

Correlation between nonlinear subharmonic scattering of ultrasound contrast agent microbubbles and ambient pressure is expected to be used for local brain tissue pressure monitoring. Although high-frequency ultrasound has achieved high-resolution imaging of intracranial microvessels, the research on high-frequency subharmonic scattering characteristics of microbubbles is insufficient at present, which restricts the research progress of estimating local brain tissue pressure based on high-frequency subharmonic scattering of microbubbles. Therefore, under the excitation of 10 MHz high-frequency ultrasound, the effects of different acoustic pressures and ambient pressures on the high-frequency subharmonic scattering characteristics of three different ultrasound contrast agents including SonoVue, Sonazoid and Huashengxian were investigated in this in vitro study. Results showed that the subharmonic scattering amplitudes of the three microbubbles increased with the increase of ambient pressure at the peak negative acoustic pressures of 696, 766 and 817 kPa, and there was a favorable linear correlation between subharmonic amplitude and ambient pressure. Under the above three acoustic pressures, the highest correlation coefficient of SonoVue was 0.948 ( P = 0.03), the highest sensitivity of pressure measurement was 0.248 dB/mm Hg and the minimum root mean square error (RMSE) was 2.64 mm Hg. Sonazoid's highest correlation coefficient was 0.982 ( P < 0.01), the highest sensitivity of pressure measurement was 0.052 dB/mm Hg and the minimum RMSE was 1.51 mm Hg. The highest correlation coefficient of Huashengxian was 0.969 ( P = 0.02), the highest sensitivity of pressure measurement was 0.098 dB/mm Hg and the minimum RMSE was 2.00 mm Hg. The above in vitro experimental results indicate that by selecting ultrasound contrast agent microbubbles and optimizing acoustic pressure, the correlation between high-frequency subharmonic scattering of microbubbles and ambient pressure can be improved, the sensitivity of pressure measurement can be upgraded, and the measurement error can be reduced to meet the clinical demand for local brain tissue pressure measurement, which provided an important experimental basis for subsequent research in vivo.

Three-dimensional super resolution ultrasound imaging with a multi-frequency hemispherical phased array.

High resolution imaging of the microvasculature plays an important role in both diagnostic and therapeutic applications in the brain. However, ultrasound pulse-echo sonography imaging the brain vasculatures has been limited to narrow acoustic windows and low frequencies due to the distortion of the skull bone, which sacrifices axial resolution since it is pulse length dependent. To overcome the detect limit, a large aperture 256-module sparse hemispherical transmit/receive array was used to visualize the acoustic emissions of ultrasound-vaporized lipid-coated decafluorobutane nanodroplets flowing through tube phantoms and within rabbit cerebral vasculature in vivo via passive acoustic mapping and super resolution techniques. Nanodroplets were vaporized with 55kHz burst-mode ultrasound (burst length=145μs, burst repetition frequency=9-45Hz, peak negative acoustic pressure=0.10-0.22MPa), which propagates through overlying tissues well without suffering from severe distortions. The resulting emissions were received at a higher frequency (612 or 1224kHz subarray) to improve the resulting spatial resolution during passive beamforming. Normal resolution three-dimensional images were formed using a delay, sum, and integrate beamforming algorithm, and super-resolved images were extracted via Gaussian fitting of the estimated point-spread-function to the normal resolution data. With super resolution techniques, the mean lateral (axial) full-width-at-half-maximum image intensity was 16±3 (32±6)μm, and 7±1 (15±2)μm corresponding to ∼1/67 of the normal resolution at 612 and 1224kHz, respectively. The mean positional uncertainties were ∼1/350 (lateral) and ∼1/180 (axial) of the receive wavelength in water. In addition, a temporal correlation between nanodroplet vaporization and the transmit waveform shape was observed, which may provide the opportunity to enhance the signal-to-noise ratio in future studies. Here, we demonstrate the feasibility of vaporizing nanodroplets via low frequency ultrasound and simultaneously performing spatial mapping via passive beamforming at higher frequencies to improve the resulting spatial resolution of super resolution imaging techniques. This method may enable complete four-dimensional vascular mapping in organs where a hemispherical array could be positioned to surround the target, such as the brain, breast, or testicles.

Theranostics in the vasculature: bioeffects of ultrasound and microbubbles to induce vascular shutdown.

Ultrasound-triggered microbubbles destruction leading to vascular shutdown have resulted in preclinical studies in tumor growth delay or inhibition, lesion formation, radio-sensitization and modulation of the immune micro-environment. Antivascular ultrasound aims to be developed as a focal, targeted, non-invasive, mechanical and non-thermal treatment, alone or in combination with other treatments, and this review positions these treatments among the wider therapeutic ultrasound domain. Antivascular effects have been reported for a wide range of ultrasound exposure conditions, and evidence points to a prominent role of cavitation as the main mechanism. At relatively low peak negative acoustic pressure, predominantly non-inertial cavitation is most likely induced, while higher peak negative pressures lead to inertial cavitation and bubbles collapse. Resulting bioeffects start with inflammation and/or loose opening of the endothelial lining of the vessel. The latter causes vascular access of tissue factor, leading to platelet aggregation, and consequent clotting. Alternatively, endothelium damage exposes subendothelial collagen layer, leading to rapid adhesion and aggregation of platelets and clotting. In a pilot clinical trial, a prevalence of tumor response was observed in patients receiving ultrasound-triggered microbubble destruction along with transarterial radioembolization. Two ongoing clinical trials are assessing the effectiveness of ultrasound-stimulated microbubble treatment to enhance radiation effects in cancer patients. Clinical translation of antivascular ultrasound/microbubble approach may thus be forthcoming.

Physical field simulation of the ultrasonic radiation method: An investigation of the vessel, probe position and power

In this paper, the effects of ultrasonic probe position, vessel shape, and ultrasonic input power on the sound pressure distribution in the reactor were investigated by solving the Helmholtz equation using COMSOL Multiphysis@ software. Three different types of glass containers were used in the study, which are beaker, Erlenmeyer flask, and round bottom flask. The maximum value of sound pressure in the three containers will gradually increase when the distance between the probe and the bottom of the container decreases. When the distance decreases, the area of the high acoustic pressure region in the round bottom flask does not change significantly, while the area of the high acoustic pressure region in the beaker and Erlenmeyer flask increases sharply, which means that the use of the round bottom flask can reduce the influence of the dead zone on the preparation of nanomaterials. In addition, the change in power increases the value of the peak negative acoustic pressure in the vessel, enhancing the response efficiency of ultrasonic cavitation.

Improving the Therapeutic Effect of Ultrasound Combined With Microbubbles on Muscular Tumor Xenografts With Appropriate Acoustic Pressure.

Ultrasound combined with microbubbles (USMB) is a promising antitumor therapy because of its capability to selectively disrupt tumor perfusion. However, the antitumor effects of repeated USMB treatments have yet to be clarified. In this study, we established a VX2 muscular tumor xenograft model in rabbits, and performed USMB treatments at five different peak negative acoustic pressure levels (1.0, 2.0, 3.0, 4.0, or 5.0 MPa) to determine the appropriate acoustic pressure. To investigate whether repeated USMB treatments could improve the antitumor effects, a group of tumor-bearing rabbits was subjected to one USMB treatment per day for three consecutive days for comparison with the single-treatment group. Contrast-enhanced ultrasonic imaging and histological analyses showed that at an acoustic pressure of 4.0 MPa, USMB treatment contributed to substantial cessation of tumor perfusion, resulting in severe damage to the tumor cells and microvessels without causing significant effects on the normal tissue. Further, the percentages of damaged area and apoptotic cells in the tumor were significantly higher, and the tumor growth inhibition effect was more obvious in the multiple-treatment group than in the single USMB treatment group. These findings indicate that with an appropriate acoustic pressure, the USMB treatment can selectively destroy tumor vessels in muscular tumor xenograft models. Moreover, the repeated treatments strategy can significantly improve the antitumor effect. Therefore, our results provide a foundation for the clinical application of USMB to treat solid tumors using a novel therapeutic strategy.

Ultrasound-Targeted Microbubble Cavitation with Sodium Nitrite Synergistically Enhances Nitric Oxide Production and Microvascular Perfusion

Microvascular obstruction is a common repercussion of percutaneous coronary intervention for distal microembolization, ischemia–reperfusion injury and inflammation, which increases post-myocardial infarction heart failure and mortality. Ultrasound-targeted microbubble cavitation (UTMC) may resolve microvascular obstruction while activating endothelial nitric oxide synthase (eNOS) and increasing endothelium-derived nitric oxide (NO) bioavailability. Nitrite, a cardioprotective agent, offers an additional source of NO and potential synergy with UTMC. UTMC and nitrite co-therapy increased microvascular perfusion and NO concentration in a rat hindlimb model. Using N-nitro-L-arginine methyl ester for eNOS blockade, we found a three-way interaction effect between nitrite, UTMC and eNOS on microvascular perfusion and NO production. Modulating ultrasound peak negative acoustic pressure (0.33–1.5 MPa) significantly affected outcomes, while microbubble dosage (2 × 108 bubbles/mL, 1.5 mL/h to 1 × 109 bubbles/mL, 3 mL/h) did not. Nitrite co-therapy also protected against oxidative stress. Comparison of nitrite to sodium nitroprusside with UTMC revealed synergistic effects were specific to nitrite. Synergy between UTMC and nitrite holds therapeutic potential for cardiovascular disease.

MR image-guided focused ultrasound immune modulation for glioma therapy

Abstract Glioblastoma (GB) is the most common and malignant brain tumor. Despite standard treatment with surgery, radiation and chemotherapy, its diffuse nature and proclivity for recurrence renders it largely intractable. Immunotherapy (ITx) approaches (e.g. anti-PD1) may hold promise for treating GB; however, the blood-brain (BBB) and blood-tumor (BTB) barriers hinder delivery of systemically administered ITx drugs. A potential approach to enhancing ITx delivery is MRI-guided focused ultrasound (FUS), a non-invasive technique that, when combined with concomitant systemic injection of microbubbles (MB), can transiently disrupt the BBB/BTB and mechanically perturb the tumor microenvironment. Here, we investigate whether localized BBB/BTB disruption with FUS+MB enhances anti-tumor immune responses and inhibits tumor growth, as a prelude to eventual combination with immune checkpoint blockade. One week after FUS+MB (peak negative acoustic pressure=0.6 MPa) treatment of a murine glioma model stably transfected with luciferase (GL261-luc2), CD86 mean fluorescence intensity on dendritic cells (DC) increased ~3-fold in deep cervical lymph nodes, intratumoral CD4+ T cells doubled, and intratumoral CD8+ T cells increased by ~17% (by flow cytometry). Serial bioluminescence imaging of tumors revealed significant reduction in total photon flux as early as 6 days following FUS+MB (p=0.0106), indicating tumor growth inhibition. We conclude that FUS+MB can promote DC maturity and potentially mediate adaptive immunity against glioma, independent of drug delivery. Ongoing studies entail combining FUS+MB with anti-PD-1 delivery to evaluate whether an allied treatment approach can promote an even more robust anti-glioma response.

Acoustic nonlinearity and the generation of large tensile pressures to explain atomization in drop-chain acoustic fountains

An ultrasound beam propagating upward in a liquid creates an acoustic fountain at a gas interface in the form of a drop chain. High-speed photography shows that one or several drops in such a fountain explode in less than a millisecond, resulting in liquid atomization [Simon et al. J. Fluid Mech., 2015, 766, pp. 129-146]. To explain this phenomenon, a nonlinear theory involving an isolated spherical drop is developed. The model considers an initial excitation in the form of a spherical standing acoustic wave at the lowest resonance frequency, i.e., when the drop diameter coincides with a wavelength. If higher harmonics are generated inside the drop due to acoustic nonlinearity, these harmonics will also have the form of standing spherical waves. At higher frequencies, more of the energy of each harmonic is localized near the drop center. Calculations demonstrate that harmonic generation can lead to large increases in both peak positive and peak negative acoustic pressure at the drop center. Such large tensile pressures may exceed the intrinsic cavitation threshold, leading to the nucleation of a bubble at the center and explosion of the drop as the bubble grows rapidly. [Work supported by RFBR 17-02-00261 and NIH R01EB007643.]

Inactivation of Planktonic Escherichia coli by Focused 2-MHz Ultrasound

This study was motivated by the desire to develop a non-invasive means to treat abscesses, and represents the first steps toward that goal. Non-thermal, high-intensity focused ultrasound (HIFU) was used to inactivate Escherichia coli (∼1 × 109 cells/mL) in suspension. Cells were treated in 96-well culture plate wells using 1.95-MHz ultrasound and incident focal acoustic pressures as high as 16 MPa peak positive and 9.9 MPa peak negative (free field measurements). The surviving fraction was assessed by coliform culture and the alamarBlue assay. No biologically significant heating was associated with ultrasound exposure. Bacterial inactivation kinetics were well described by a half-life model, with a half-time of 1.2 min. At the highest exposure levels, a 2log inactivation was typically achieved within 10 min. The free field-equivalent peak negative acoustic pressure threshold for inactivation was ∼7 MPa. At the highest acoustic pressures used, inactivation efficacy was insensitive to reciprocal changes in pulse length and pulse repetition frequency at constant duty factor. Although treated volumes were very small, proof of principle was provided by these experiments.

Enzyme-Degradable Hybrid Polymer/Silica Microbubbles as Ultrasound Contrast Agents.

The fabrication of an enzyme-degradable polymer/silica hybrid microbubble is reported that produces an ultrasound contrast image. The polymer, a triethoxysilane end-capped polycaprolactone (SiPCL), is used to incorporate enzyme-degradable components into a silica microbubble synthesis, and to impart increased elasticity for enhanced acoustic responsiveness. Formulations of 75, 85, and 95 wt % SiPCL in the polymer feed produced quite similar ratios of SiPCL and silica in the final bubble but different surface properties. The data suggest that different regions of the microbubbles were SiPCL-rich: the inner layer next to the polystyrene template core and the outer surface layer, thereby creating a sandwiched silica-rich layer of the bubble shell. Overall, the thickness of the microbubble shell was dependent on the starting TEOS concentration and the reaction time. Despite the layered structure, the microbubble could be efficiently degraded by lipase enzyme, but was stable without enzyme. The ultrasound contrast showed a general trend of increase in image intensity with SiPCL feed ratio, although the 95 wt % SiPCL bubbles did not produce a contrast image, probably due to bubble collapse. At higher normalized peak negative acoustic pressure (mechanical index, MI), a nonlinear frequency response also emerges, characterized by the third harmonic at around 3f0, and increases with MI. The threshold MI transition from linear to nonlinear response increased with decrease in SiPCL.

Targeted microbubble mediated sonoporation of endothelial cells in vivo.

Ultrasound contrast agents as drug-delivery systems are an emerging field. Recently, we reported that targeted microbubbles are able to sonoporate endothelial cells in vitro. In this study, we investigated whether targeted microbubbles can also induce sonoporation of endothelial cells in vivo, thereby making it possible to combine molecular imaging and drug delivery. Live chicken embryos were chosen as the in vivo model. αvß3-targeted microbubbles attached to the vessel wall of the chicken embryo were insonified at 1 MHz at 150 kPa (1 × 10,000 cycles) and at 200 kPa (1 × 1000 cycles) peak negative acoustic pressure. Sonoporation was studied by intravital microscopy using the model drug propidium iodide (PI). Endothelial cell PI uptake was observed in 48% of microbubble-vessel-wall complexes at 150 kPa (n = 140) and in 33% at 200 kPa (n = 140). Efficiency of PI uptake depended on the local targeted microbubble concentration and increased up to 80% for clusters of 10 to 16 targeted microbubbles. Ultrasound or targeted microbubbles alone did not induce PI uptake. This intravital microscopy study reveals that sonoporation can be visualized and induced in vivo using targeted microbubbles.

Dependence of the reversibility of focused- ultrasound-induced blood-brain barrier opening on pressure and pulse length in vivo

The most challenging aspect of intravenously-administered drugs currently developed to treat central nervous system (CNS) diseases is their impermeability through the blood-brain barrier (BBB), a specialized vasculature system protecting the brain microenvironment. Focused ultrasound (FUS) in conjunction with systemically administered microbubbles has been shown to open the BBB locally, noninvasively, and reversibly. The objective of this study was to investigate the effect of FUS (center frequency: 1.5 MHz) pulse length (PL), ranging here from 67 μs to 6.7 ms, on the physiology of the FUS-induced BBB opening. Dynamic contrast-enhanced (DCE) and T1-weighted magnetic resonance imaging (MRI) were used to quantify the permeability changes using transfer rate (Ktrans) mapping, the volume of BBB opening (VBBB) and the reversibility timeline of the FUS-induced BBB opening, with the systemic administration of microbubbles at different acoustic pressures, ranging from 0.30 to 0.60 MPa. Permeability and volume of opening were both found to increase with the acoustic pressure and pulse length. At 67-μs PL, the opening pressure threshold was 0.45 MPa, with BBB opening characteristics similar to those induced with 0.60 MPa at the same PL, as well as with 0.67-ms PL/0.30 MPa. On average, these cases had Ktrans = 0.0049 ± 0.0014 min-1 and VBBB = 3.7 ± 4.3 mm(3), and closing occurred within 8 h. The 6.7-ms PL/0.30 MPa induced similar opening with 0.67-ms PL/0.45 MPa, and a closing timeline of 24 to 48 h. On average, Ktrans was 0.0091 ± 0.0029 min-1 and VBBB was 14.13 ± 7.7 mm(3) in these cases. Also, there were no significant differences between the 6.7-ms PL/0.45 MPa, 0.67-ms PL/0.60 MPa and 6.7-ms PL/0.60 MPa cases, yielding on average a Ktrans of 0.0100 ± 0.0023 min-1 and VBBB equal to 20.1 ± 5.7 mm(3). Closing occurred within 48 to 72 h in these cases. Stacked histograms of the Ktrans provided further insight to the nonuniform spatial distribution of permeability changes and revealed a correlation with the closing timeline. These results also suggest a beneficial complementary relationship between the elongation of the PL and the decrease of the peak negative acoustic pressures, and vice versa. Linear regression between Ktrans and VBBB showed a good correlation fit. Also, the time required for closing linearly increased with VBBB. The volume rate of decrease was measured to be 11.4 ± 4.0 mm3 per day, suggesting that the closing timeline could be predicted from the initial volume of opening. Finally, no histological damage was detected in any of the cases 7 d post-FUS, indicating the safety of the methodology and parameters used.

Effect of Acoustic Conditions on Microbubble-Mediated Microvascular Sonothrombolysis

Ultrasound (US) mediated microbubble (MB) destruction facilitates thrombolysis of the epicardial coronary artery in acute myocardial infarction (AMI) but its effect on microvascular thromboemboli remains largely unexplored. We sought to define the acoustic requirements for effective microvascular sonothrombolysis. To model microembolization, microthrombi were injected and entrapped in a 40 μm pore mesh, increasing upstream pressure, which was measured as an index of thrombus burden. MBs (2.0 × 106 MBs/mL) were then infused while pulsed US (1 MHz) was delivered to induce MB destruction immediately adjacent to the thrombus. Upstream pressure decreased progressively during US delivery, indicating a reduction in thrombus burden. More rapid and complete lysis occurred with increasing peak negative acoustic pressure (1.5 MPa > 0.6 MPa) and increasing pulse length (5000 cycles > 100 cycles). Additionally, similar lytic efficacy was achieved at 1.5 MPa without tPA as was at 1.0 MPa with tPA. This model uniquely provides a means to systematically evaluate multiple acoustic and microbubble parameters for the optimization of microvascular sonothrombolysis. This treatment approach for thrombotic microvascular obstruction may obviate the need for adjunctive rt-PA and could have important clinical cost and safety benefits.

Progress in Sonothrombolysis for the Treatment of Stroke

In 1974, Sobbe et al1 applied 26.5-kHz ultrasound (US) to recanalize thrombosed iliofemoral arteries in dogs with minimal complications. These pioneer efforts were followed by studies showing that catheter-based or transcutaneous US can enhance the effect of fibrinolytic agents in recanalizing thrombosed arteries,2–8 thus paving the way for first clinical studies evaluating the adjunct effect of US in treating patients with ischemic stroke. ### Mechanisms #### US Thrombolysis Despite numerous studies documenting a thrombolytic effect of US, the mechanisms remain poorly understood. Inertial cavitation (ie, the formation and violent collapse of gas-filled bubbles in a fluid exposed to US) gave rise to transient microjets that disintegrate thrombus mechanically.9 Stable cavitation (ie, sustainable nonlinear periodic contraction or expansion of a gas body or bubble) may be more effective than inertial cavitation in clot lysis.10 US also facilitates penetration of fibrinolytic drugs into the thrombus and binding to fibrin.11 This is because US promotes the motion of fluids around the clot surface through a process called microstreaming. Moreover, pressure waves may increase the permeation of tissue-type plasminogen activator (tPA) into the interior of the fibrin network.12 Heating is uniformly present in tissue exposed to US but has been deemed too mild to explain thrombolytic effects. #### Microbubble-Enhanced Thrombolysis With tPA Significant amplification of lysis occurs with the addition of microbubbles to the combination of thrombolytic drug and US.13–15 Microbubbles, composed of lipid, albumin, or galactose shells and ranging in size from 0.5 to 5 μm, lower the threshold for thrombolysis by providing a pre-existing bubble that easily can be made to cavitate by US. Stable cavitation can produce microstreaming in the area and dramatically enlarge the bubble momentarily. This will cause localized mechanical stress on the adjacent clot. The surface of the clot will erode, and even penetration and numerous microscopic …

Sonoporation of endothelial cells by vibrating targeted microbubbles

Molecular imaging using ultrasound makes use of targeted microbubbles. In this study we investigated whether these microbubbles could also be used to induce sonoporation in endothelial cells. Lipid-coated microbubbles were targeted to CD31 and insonified at 1 MHz at low peak negative acoustic pressures at six sequences of 10 cycle sine-wave bursts. Vibration of the targeted microbubbles was recorded with the Brandaris-128 high-speed camera (~ 13 million frames per second). In total, 31 cells were studied that all had one microbubble (1.2–4.2 micron in diameter) attached per cell. After insonification at 80 kPa, 30% of the cells (n = 6) had taken up propidium iodide, while this was 20% (n = 1) at 120 kPa and 83% (n = 5) at 200 kPa. Irrespective of the peak negative acoustic pressure, uptake of propidium iodide was observed when the relative vibration amplitude of targeted microbubbles was greater than 0.5. No relationship was found between the position of the microbubble on the cell and induction of sonoporation. This study shows that targeted microbubbles can also be used to induce sonoporation, thus making it possible to combine molecular imaging and drug delivery.

Drug uptake by endothelial cells through targeted microbubble sonoporation.

Molecular ultrasound imaging uses targeted contrast agents consisting of microbubbles. Drug uptake using microbubbles can be induced by sonoporation, a method that induces transient cell membrane pores by oscillating or jetting microbubbles so that therapeutics can enter the cell. This study focuses on inducing sonoporation with CD31‐targeted microbubbles in endothelial cells at low acoustic pressures. Biotinylated lipid coated microbubbles with a C4F10 gas core were made by sonication including CD31 antibody conjugation. Microbubble‐cell behavior upon insonification at 1 MHz (6 × 10 cycle sine‐wave bursts) at 80, 120, and 200 kPa peak negative acoustic pressure was studied with the Brandaris 128 high‐speed camera at a frame rate of 12 Mfs. The cell‐impermeable propidium iodide was used as indicator for increased cell membrane permeability due to sonoporation and was detected using fluorescence imaging. A total of 31 cells were studied, wherein all had one microbubble attached per cell. After insonifying the targeted microbubbles at 80, 120, and 200 kPa, only 6 burst of each 10 cycles, 12 cells PI uptake. This study reveals that targeted microbubbles can induce sonoporation. This feature may now be used in molecular imaging using ultrasound, thereby combining imaging and drug delivery.

Contrast‐enhanced in vivo magnetic resonance microscopy of the mouse brain enabled by noninvasive opening of the blood‐brain barrier with ultrasound

The use of contrast agents for neuroimaging is limited by the blood-brain barrier (BBB), which restricts entry into the brain. To administer imaging agents to the brain of rats, intracarotid infusions of hypertonic mannitol have been used to open the BBB. However, this technically challenging approach is invasive, opens only a limited region of the BBB, and is difficult to extend to mice. In this work, the BBB was opened in mice, using unfocused ultrasound combined with an injection of microbubbles. This technique has several notable features: it (a) can be performed transcranially in mice; (b) takes only 3 min and uses only commercially available components; (c) opens the BBB throughout the brain; (d) causes no observed histologic damage or changes in behavior (with peak-negative acoustic pressures of 0.36 MPa); and (e) allows recovery of the BBB within 4 h. Using this technique, Gadopentetate Dimeglumine (Gd-DTPA) was administered to the mouse brain parenchyma, thereby shortening T(1) and enabling the acquisition of high-resolution (52 × 52 × 100 micrometers(3)) images in 51 min in vivo. By enabling the administration of both existing anatomic contrast agents and the newer molecular/sensing contrast agents, this technique may be useful for the study of mouse models of neurologic function and pathology with MRI.

Ultrasound-triggered release of materials entrapped in microbubble–liposome constructs: A tool for targeted drug delivery

We investigated the preparation of ultrasound-triggered drug delivery system, based on a pendant complex of microbubble coated with liposomes. Biotinylated decafluorobutane microbubbles were coated with biotinylated liposomes via a streptavidin linker. Liposomes were prepared incorporating calcein and thrombin. Based on initial concentration of calcein, over 1um3 payload volume per each microbubble–liposome particle was achieved, when 100nm liposomes were used.Insonation of microbubble–liposome pendants in vitro resulted in the complete destruction of microbubbles and triggered release of a significant fraction of the entrapped material. Treatment with 1MHz ultrasound (5 pulses, 100ms, 7MPa peak negative acoustic pressure) resulted in the release of ~30% of entrapped calcein, as estimated by the fluorescence quenching assay.Thrombin release from liposomes complexed with microbubbles (11% of entrapped material) due to ultrasound treatment was estimated by a chromogenic substrate study. Prior to insonation, substrate hydrolysis was at background level. Ultrasound-triggered release of thrombin from the pendant complexes caused an acceleration of blood clotting.

Dual-high-frequency ultrasound excitation on microbubble destruction volume

Objective and motivation The goal of this work was to test experimentally that exposing air bubbles or ultrasound contrast agents in water to amplitude modulated wave allows control of inertial cavitation affected volume and hence could limit the undesirable bioeffects. Methods Focused transducer operating at the center frequency of 10 MHz and having about 65% fractional bandwidth was excited by 3 μs 8.5 and 11.5 MHz tone-bursts to produce 3 MHz envelope signal. The 3 MHz frequency was selected because it corresponds to the resonance frequency of the microbubbles used in the experiment. Another 5 MHz transducer was used as a receiver to produce B-mode image. Peak negative acoustic pressure was adjusted in the range from 0.5 to 3.5 MPa. The spectrum amplitudes obtained from the imaging of SonoVue TM contrast agent when using the envelope and a separate 3 MHz transducer were compared to determine their cross-section at the – 6 dB level. Results The conventional 3 MHz tone-burst excitation resulted in the region of interest (ROI) cross-section of 2.47 mm while amplitude modulated, dual-frequency excitation with difference frequency of 3 MHz produced cross-section equal to 1.2 mm. Conclusion These results corroborate our hypothesis that, in addition to the considerably higher penetration depth of dual-frequency excitation due to the lower attenuation at 3 MHz than that at 8.5 and 11.5 MHz, the sample volume of dual-frequency excitation is also smaller than that of linear 3-MHz method for more spatially confined destruction of microbubbles.

Safety and bio-effects of ultrasound contrast agents

The use of gas-filled microbubbles as ultrasound contrast agents raises potential safety concerns for diagnostic ultrasound imaging. A number of biological effects have been seen in experimental systems, including the induction of physiological response to cardiac exposures (premature ventricular contractions) and damage at a microvascular level (microvascular rupture and petechial haemorrhage). The literature indicates that a mechanical index (MI) of 0.4 represents the threshold above which microvascular bio-effects are seen in in vivo studies. Above this value, the extent of biological effects appears to increase rapidly with both increasing in situ peak negative acoustic pressure amplitude and with contrast agent concentration. While there is no proven evidence of harm resulting from clinical use of these agents, caution is recommended when contrast-enhanced imaging is undertaken.