Air microbubbles as MR susceptibility contrast agent at 1.5 Tesla

Microbubble Agents: New Directions

Microbubble ultrasound contrast agents have now been in use for several decades and their safety and efficacy in a wide range of diagnostic applications have been well established. Recent progress in imaging technology is facilitating exciting developments in techniques such as molecular, 3-D and super resolution imaging and new agents are now being developed to meet their specific requirements. In parallel, there have been significant advances in the therapeutic applications of microbubbles, with recent clinical trials demonstrating drug delivery across the blood-brain barrier and into solid tumours. New agents are similarly being tailored toward these applications, including nanoscale microbubble precursors offering superior circulation times and tissue penetration. The development of novel agents does, however, present several challenges, particularly regarding the regulatory framework. This article reviews the developments in agents for diagnostic, therapeutic and "theranostic" applications; novel manufacturing techniques; and the opportunities and challenges for their commercial and clinical translation.

Evaluation of microbubbles as contrast agents for ultrasonography and magnetic resonance imaging

Background

Microbubbles (MBs) can serve as an ultrasound contrast agent, and has the potential for magnetic resonance imaging (MRI). Due to the relatively low effect of MBs on MRI, it is necessary to develop new MBs that are more suitable for MRI. In this study, we evaluate the properties of SonoVue® and custom-made Fe₃O₄-nanoparticle-embedded microbubbles (Fe₃O₄-MBs) in terms of contrast agents for ultrsonography (US) and MRI.

Methodology/Principal Findings

A total of 20 HepG2 subcutaneous-tumor-bearing nude mice were randomly assigned to 2 groups (i.e., n = 10 mice each group), one for US test and the other for MRI test. Within each group, two tests were performed for each mouse. The contrast agent for the first test is SonoVue®, and the second is Fe₃O₄-MBs. US was performed using a Technos^MPX US system (Esaote, Italy) with a contrast-tuned imaging (CnTI™) mode. MRI was performed using a 7.0T Micro-MRI (PharmaScan, Bruker Biospin GmbH, Germany) with an EPI-T₂* sequence. The data of signal-to-noise ratio (SNR) from the region-of-interest of each US and MR image was calculated by ImageJ (National Institute of Health, USA). In group 1, enhancement of SonoVue® was significantly higher than Fe₃O₄-MBs on US (P<0.001). In group 2, negative enhancement of Fe₃O₄-MBs was significantly higher than SonoVue® on MRI (P<0.001). The time to peak showed no significant differences between US and MRI, both of which used the same MBs (P>0.05). The SNR analysis of the enhancement process reveals a strong negative correlation in both cases (i.e., SonoVue® r = −0.733, Fe₃O₄-MBs r = −0.903, with P<0.05).

Conclusions

It might be important to change the Fe₃O₄-MBs' shell structure and/or the imagining strategy of US to improve the imaging quality of Fe₃O₄-MBs on US. As an intriguing prospect that can be detected by US and MRI, MBs are worthy of further study.

Microbubbles in Imaging: Applications Beyond Ultrasound

Since their introduction as ultrasound contrast agents, microbubbles have demonstrated the potential to revolutionise the use of ultrasound at the bedside. Aside from clinical application, where microbubbles are used to enhance ultrasonic assessment of myocardial perfusion, they have demonstrated potential in an exciting host of pre-clinical ultrasound imaging and therapeutic applications. These include the ability to target specific cellular markers of disease, provide dynamic blood flow estimation, deliver localised chemotherapy, potentiate the mechanisms of gene therapy, enhance lesion ablation through cavitation, and spatiotemporally permeabilise the blood-brain barrier. The unique and flexible construction of microbubbles not only enables a variety of ultrasound applications, but also opens the door to detection of microbubbles with modalities other than ultrasound. In this review, non-ultrasound imaging applications utilizing microbubbles are discussed, including MRI, PET, and DEI. These various imaging approaches illustrate novel applications of microbubbles, and may provide the groundwork for future multi-modality imaging or image-guided therapeutics.

Gas-filled microbubbles--a novel susceptibility contrast agent for brain and liver 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, in vivo demonstration of microbubble susceptibility effect is limited so far and microbubble susceptibility effect is relatively weak when compared with other intravascular MR susceptibility contrast agents. In this study, two types of microbubbles, custom-made albumin-coated microbubbles (AMBs) and a commercially available lipid-based clinical ultrasound contrast agent (SonoVue), were investigated with in vivo dynamic brain and liver MRI in Sprague-Dawley rats at 7 Tesla. Transverse relaxation rate enhancements (DeltaR2*) maps were computed for brain and liver, yielding results similar to those obtained with a common MR blood pool contrast agent. These results indicate that gas-filled microbubbles can serve as an intravascular MR contrast agent at high field. Enhancement of microbubble susceptibility effect by entrapping monocrystalline iron oxide nanoparticles (MIONs) into microbubbles was also investigated at 7 T in vitro. This is the first experimental demonstration of microbubble susceptibility enhancement for MRI application. This study indicates that gas-filled microbubble susceptibility effect can be substantially increased using iron oxides nanoparticles. With such approach, microbubbles can potentially be visualized with higher sensitivity and lower concentrations by MRI. Such capability has the potential to lead to real-time MRI guidance in various microbubble-based drug delivery and therapeutic applications.

Superparamagnetic iron oxide nanoparticle-embedded encapsulated microbubbles as dual contrast agents of magnetic resonance and ultrasound imaging

An encapsulated microbubble (EMB) of a novel construct is proposed to enhance the magnetic resonance imaging contrast by introducing superparamagnetic iron oxide (SPIO) nanoparticles (mean diameter is 12 nm) into the polymer shell of the microbubble. Such microbubble vesicle has nitrogen gas in the core and its mean diameter is 3.98 microm. An in vitro MR susceptibility experiment using a phantom consisting EMBs has shown that the relationship between the transverse relaxation rate R(2) and the Fe(3)O(4) nanoparticle concentration in the shell (the volume fraction of EMBs is kept constant) can be fitted to a linear function and an exponentially growth function is observed between R(2) and the SPIO-inclusion microbubble concentration. The in vivo MRI experiments also show that the SPIO-inclusion microbubbles have longer contrast-enhancement duration time in rat liver than non-SPIO-inclusion microbubbles. An in vitro ultrasound imaging experiment of SPIO-inclusion microbubbles also shows that they can enhance the ultrasound contrast significantly. Additionally, the interaction between the SPIO-inclusion microbubbles and cells indicates that such microbubble construct can retain the acoustic property under the ultrasound exposure by controlling the SPIO concentration in the shell. Therefore, the proposed SPIO nanoparticle-embedded EMBs potentially can become effective MR susceptibility contrast agents while also can be good US contrast agents.