Enhancement of gas‐filled microbubble R2* by iron oxide nanoparticles for MRI

Gas‐filled microbubbles have the potential to become a unique intravascular MR contrast agent due to their magnetic susceptibility effect, biocompatibility, and localized manipulation via ultrasound cavitation. However, microbubble susceptibility effect is relatively weak when compared with other intravascular MR susceptibility contrast agents. In this study, enhancement of microbubble susceptibility effect by entrapping monocrystalline iron oxide nanoparticles (MIONs) into polymeric microbubbles was investigated at 7 T in vitro. Apparent T₂ enhancement (ΔR₂*) induced by microbubbles was measured to be 79.2 ± 17.5 sec^?1 and 301.2 ± 16.8 sec^?1 for MION‐free and MION‐entrapped polymeric microbubbles at 5% volume fraction, respectively. ΔR₂* and apparent transverse relaxivities (r₂*) for MION‐entrapped polymeric microbubbles and MION‐entrapped solid microspheres (without gas core) were also compared, showing the synergistic effect of the gas core with MIONs. This is the first experimental demonstration of microbubble susceptibility enhancement for MRI application. This study indicates that gas‐filled polymeric microbubble susceptibility effect can be substantially increased by incorporating iron oxide nanoparticles into microbubble shells. With such an approach, microbubbles can potentially be visualized with higher sensitivity and lower concentrations by MRI. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Prolonging the ultrasound signal enhancement from thrombi using targeted microbubbles based on sulfur-hexafluoride-filled gas

RATIONALE AND OBJECTIVES: The objective of this study is to develop and characterize new microbubbles based on lipids and sulfur hexafluoride (SF6) for targeting thrombi as an improved ultrasound contrast agent. MATERIALS AND METHODS: Bioconjugate ligands were inserted into the lipid-coated membranes of SF6 gas microbubbles, and their physicochemical properties were determined. Diagnostic efficacies of SF6-filled microbubbles and the contrast agent SonoVue (Bracco Imaging, Geneve, Switzerland) were compared in dogs. RESULTS: Suspensions of lyophilized powder were reconstituted by injecting saline containing 3.1 x 10(8) SF6 microbubbles/mL with a mean diameter of 4.4 microm. More than 90% of microbubbles had diameters between 1 and 10 microm. After reconstitution, echogenicity and microbubble characteristics were unchanged for 8 hours. Targeted microbubbles increased the echogenicity of thrombi significantly and provided a longer period of optimal signal enhancement compared with nontargeted microbubbles. CONCLUSIONS: Our thrombus-targeting microbubble contrast agent shows high echogenicity and stability and thereby enhances the visualization of intravascular thrombi and prolongs the duration of the diagnostic window.     

手振微泡子宫输卵管声学造影的临床研究

对３２例不孕妇女行手振微泡子宫输卵管声学造影，以探讨手振法制备的微泡造影剂的可靠性及安全性。结果显示手振微泡造影剂造影效果好，副作用小。我们认为手振微泡子宫输卵管声学造影是一种安全可靠的技术，值得推广     

Magnetic targeting and ultrasound mediated drug delivery: Benefits,limitations and combination

This paper reviews the uses of magnetism and ultrasound in therapeutic delivery applications. Emphasis is placed upon magnetic nanoparticles and microbubble ultrasound contrast agents. The underlying physical principles, history, key developments and limitations of these techniques for drug and gene delivery in vitro and in vivo are explored. The combination of ultrasonic and magnetic techniques is also reviewed with particular focus on magnetic microbubbles as delivery agents with the potential to combine the advantages of both methods whilst addressing many of their limitations. Finally, results are presented from a study of a new magnetic microbubble formulation which shows great applicability as a therapeutic delivery vehicle.     

携Sialyl Lewis^X超声微泡靶向探查缺血心肌的炎症分子“印记”

目的探讨应用靶向超声分子成像探查心肌缺血再灌注损伤炎症“印记”的可行性。方法采用“亲和素-生物素”桥接法构建携唾液酸化路易斯（Sibyl Lewis^X）靶向超声微泡（MBsLex）和同型对照微泡（MBc）。10只心肌缺血再灌注小鼠随机先后注入MBsLex和MBc（间隔30min）,分别于注入5min后行心肌对比超声检查,测量心肌缺血区和非缺血区的声强度（Ⅵ）。结果对比超声图像显示MBsLex组缺血区心肌造影见显著增强,Ⅵ值高达23．52±1．08,而在MBc组缺血区心肌造影仅见轻度增强,Ⅵ缸为9．81±0．41,两者之间差异有统计学意义（P〈0．05）。但无论MBsLex组还是MBc组,缺血区心肌Ⅵ值均明显高于非缺血区心肌Ⅵ值（P〈0．05）。两组非缺血区心肌之间Ⅵ值未见明显差异。结论应用携sLex超声微泡行对比超声能够靶向探查心肌缺血再灌注损伤的炎症“印记”。     

Analysis of in vitro Transfection by Sonoporation Using Cationic and Neutral Microbubbles

The objective of the study was to examine the role of acoustic power intensity and microbubble and plasmid concentrations on transfection efficiency in HEK-293 cells using a sonoporator with a 1-MHz transducer. A green fluorescent protein (GFP) reporter plasmid was delivered in as much as 80% of treated cells, and expression of the GFP protein was observed in as much as 75% of cells, using a power intensity of 2 W/cm² with a 25% duty cycle. In addition, the relative transfection abilities of a lipid noncationic and cationic microbubble platform were investigated. As a positive control, cells were transfected using Lipofectamine reagent. Cell survival and transfection efficiency were inversely proportional to acoustic power and microbubble concentration. Our results further demonstrated that high-efficiency transfection could be achieved, but at the expense of cell loss. Moreover, direct conjugation of plasmid to the microbubble did not appear to significantly enhance transfection efficiency under the examined conditions, although this strategy may be important for targeted transfection in vivo. (E-mail: mbl2a@virginia.edu)     

Nonlinear Emission from Individual Bound Microbubbles at High Frequencies

Targeted microbubbles detected with high-frequency ultrasound can establish the molecular expression of blood vessels with submillimeter resolution. To improve microbubble-specific imaging at high frequencies, the subharmonic and second harmonic signal from individual microbubbles were measured as a function of size and pressure. Single phospholipid-shell microbubbles (1.1 to 5.0 μm in diameter) bound to gelatin, co-aligned with an optical microscope and transducer, were insonated with 30 MHz Gaussian-enveloped pulses at pressures from 20 kPa to 1 MPa with –6 dB one-way bandwidths of 11%, 20% and 45%. A subharmonic signal (15 MHz) was detected above a pressure threshold of 110 kPa—independent of bandwidth. The signal peaked for microbubbles 1.60 μm in diameter subject to 20% and 11% bandwidth pulses, and 1.80 μm for 45% bandwidth pulses, for pressures up to 400 kPa, agreeing with the notion that microbubbles insonated at twice their resonant frequency preferentially emit a subharmonic component. For pressures between 400 kPa and 1 MPa, a broader range of microbubbles emitted a subharmonic signal, and microbubbles below 1.70 μm in diameter were disrupted. The second harmonic signal measured, within the limited experimental conditions, was consistent with nonlinear propagation. Further, the results shed light on the effect of the shell on the phase of the subharmonic signal with respect to the fundamental signal. (E-mail: michael.sprague@sri.utoronto.ca)     

Targeting an Ultrasound Contrast Agent to Folate Receptors on Ovarian Cancer Cells

Objective. The purpose of the study was to synthesize and characterize folate‐targeted microbubbles (MB_F) as an ultrasound contrast agent and to evaluate their affinity to the folate receptor (FR) in vitro. Methods. Folate‐targeted microbubbles were prepared by incorporating 1,2‐distearoyl‐sn‐glycero‐3‐phosphoethanolamine‐N‐[amino(polyethylene glycol)‐2000]‐folate into the lipid membrane of microbubbles. The diameter and concentration of the MB_F were determined by a cell counter and sizer. The MB_F, control microbubbles (MB_C), and MB_F with a free folic acid block‐ade were tested for binding specificity to human ovarian carcinoma SKOV3 cells, which overexpress the FR, by microscopy and confocal imaging, respectively. Results. The basic physical characteristics of MB_F were similar to those of MB_C. In the cell binding test, the adherence efficiency of MB_F to the SKOV3 cells (mean ± SD, 16 ± 5 microbubbles per cell; P < .01) was significantly higher than that of MB_C (0.7 ± 0.4 microbubbles per cell) or MB_F with the free folic acid blockade (0.7 ± 0.6 microbubbles per cell). Conclusions. Folate‐targeted microbubbles showed high affinity to SKOV3 cells with FR overexpression. They are potentially useful for ultrasonic molecular imaging and treatment of FR‐positive tumors and warrant further investigation.     

静注手振氟碳微泡左心声学造影的实验研究

目的探讨静注手振氟碳微泡造影剂左室腔及心肌显影的能力.材料和方法对8只兔子分别静注3种2个剂量手振氟碳微泡造影剂,观察注射后左室腔及心肌显影情况.结果所有注射可见左室腔显影,部分手振糖蛋白氟碳微泡可见心肌显影,所有手振高渗糖氟碳微泡及盐水蛋白氟碳微泡无心肌显影.结论静注手振氟碳微泡能使左室腔显影,部分糖蛋白氟碳微泡可见心肌显影,具有较大的潜在应用价值.     

Delivery of Drugs with Ultrasound

In this article we discuss the potential role of microbubbles, traditionally used as ultrasound contrast agents, for site-specific drug delivery. To reach this goal, microbubbles capable of carrying a drug payload are being developed. To ensure that these microbubbles reach sufficient local concentration at disease sites, specific targeting for diseased tissues can be accomplished using several strategies. These strategies rely on either the intrinsic properties of microbubble shells or conjugation of monoclonal antibodies or other ligands to these shells that recognize antigens expressed in regions of disease. Site-specific delivery of antiinflammatory, antineoplastic, and thrombolytic drugs with microbubbles can be further enhanced by the ability to locally destroy microbubbles within an acoustic field, thereby releasing drugs and improving drug efficacy without systemic adverse effects. In the case of thrombi, ultrasound-mediated microbubble destruction also may facilitate the process of clot lysis. This review also will consider current limitations and technological advances required for the development of this field.