Microbubbles as a novel contrast agent for brain MRI

Low-boiling Point Perfluorocarbon Nanodroplets as Dual-Phase Dual-Modality MR/US Contrast Agent

Detection of bare gas microbubbles by magnetic resonance (MR) at low concentrations typically used in clinical contrast-ultrasound studies was recently demonstrated using hyperCEST. Despite the enhanced sensitivity achieved with hyperCEST, in vivo translation is challenging as on-resonance saturation of the gas-phase core of microbubbles consequently results in saturation of the gas-phase hyperpolarized 129 Xe within the lungs. Alternatively, microbubbles can be condensed into the liquid phase to form perfluorocarbon nanodroplets, where 129 Xe resonates at a chemical shift that is separated from the gas-phase signal in the lungs. For ultrasound applications, nanodroplets can be acoustically reverted back into their microbubble form to act as a phase-change contrast agent. Here, we show that low-boiling point perfluorocarbons, both in their liquid and gas form, generate phase-dependent hyperCEST contrast. Magnetic resonance detection of ultrasound-mediated phase transition demonstrates that these perfluorocarbons could be used as a dual-phase dual-modality MR/US contrast agent.

Magnetic Resonance Detection of Gas Microbubbles via HyperCEST: A Path Toward Dual Modality Contrast Agent

Gas microbubbles are an established clinical ultrasound contrast agent. They could also become a powerful magnetic resonance (MR) intravascular contrast agent, but their low susceptibility-induced contrast requires high circulating concentrations or the addition of exogenous paramagnetic nanoparticles for MR detection. In order to detect clinical in vivo concentrations of raw microbubbles via MR, an alternative detection scheme must be used. HyperCEST is an NMR technique capable of indirectly detecting signals from very dilute molecules (concentrations well below the NMR detection threshold) that exchange hyperpolarized 129 Xe. Here, we use quantitative hyperCEST to show that microbubbles are very efficient hyperCEST agents. They can accommodate and saturate millions of 129 Xe atoms at a time, allowing for their indirect detection at concentrations as low as 10 femtomolar. The increased MR sensitivity to microbubbles achieved via hyperCEST can bridge the gap for microbubbles to become a dual modality contrast agent.

Molecular magnetic resonance imaging of Alpha-v-Beta-3 integrin expression in tumors with ultrasound microbubbles

Microbubbles (MB) are used as ultrasound (US) contrast agents and can be efficiently targeted against markers of angiogenesis and inflammation. Due to their gas core, MB locally alter susceptibilities in magnetic resonance imaging (MRI), but unfortunately, the resulting contrast is low and not sufficient to generate powerful molecular MRI probes. Therefore, we investigated whether a potent molecular MR agent can be generated by encapsulating superparamagnetic iron oxide nanoparticles (SPION) in the polymeric shell of poly (n-butylcyanoacrylate) (PBCA) MB and targeted them against α_vβ₃ integrins on the angiogenic vasculature of 4T1 murine breast carcinomas. SPION-MB consist of an air core and a multi-layered polymeric shell enabling efficient entrapment of SPION. The mean size of SPION-MB was 1.61 ± 0.32 μm. Biotin-streptavidin coupling was employed to functionalize the SPION-MB with cyclic RGDfK (Arg-Gly-Asp) and RADfK (Arg-Ala-Asp) peptides. Cells incubated with RGD-SPION-MB showed enhanced transverse relaxation rates compared with SPION-MB and blocking α_vβ₃ integrin receptors with excess free cRGDfK significantly reduced RGD-SPION-MB binding. Due to the fast binding of RGD-SPION-MB in vivo, dynamic susceptibility contrast MRI was employed to track their retention in tumors in real-time. Higher retention of RGD-SPION-MB was observed compared with SPION-MB and RAD-SPION-MB. To corroborate our MRI results, molecular US was performed the following day using the destruction-replenishment method. Both imaging modalities consistently indicated higher retention of RGD-SPION-MB in angiogenic vessels compared with SPION-MB and RAD-SPION-MB. Competitive blocking experiments in mice further confirmed that the binding of RGD-SPION-MB to α_vβ₃ integrin receptors is specific. Overall, this study demonstrates that RGD-SPION-MB can be employed as molecular MR/US contrast agents and are capable of assessing the α_vβ₃ integrin expression in the neovasculature of malignant tumors.

Recent Advances on Ultrasound Contrast Agents for Blood-Brain Barrier Opening with Focused Ultrasound

The blood-brain barrier is the primary obstacle to efficient intracerebral drug delivery. Focused ultrasound, in conjunction with microbubbles, is a targeted and non-invasive way to disrupt the blood-brain barrier. Many commercially available ultrasound contrast agents and agents specifically designed for therapeutic purposes have been investigated in ultrasound-mediated blood-brain barrier opening studies. The new generation of sono-sensitive agents, such as liquid-core droplets, can also potentially disrupt the blood-brain barrier after their ultrasound-induced vaporization. In this review, we describe the different compositions of agents used for ultrasound-mediated blood-brain barrier opening in recent studies, and we discuss the challenges of the past five years related to the optimal formulation of agents.

Kinetics of albumin microbubble dissolution in aqueous media

The effectiveness of microbubbles as ultrasound contrast agents and targeted drug delivery vehicles depends on their persistence in blood. It is therefore necessary to understand the dissolution behavior of microbubbles in an aqueous medium. While there are several reports available in the literature on the dissolution of lipid microbubbles, there are no reports available on the dissolution kinetics of protein microbubbles. Moreover, shell parameters such as interfacial tension, shell resistance and shell elasticity/stiffness which characterize microbubble shells, have been reported for lipid shells but no such data are available for protein shells. Accordingly, this work was focused on capturing the dissolution behavior of protein microbubbles and estimation of shell parameters such as surface tension, shell resistance and shell elasticity. Bovine serum albumin (BSA) was used as a model protein and microbubbles were synthesized using sonication. During dissolution, a large portion of the protein shell was found to disengage from the gas-liquid interface after a stagnant dissolution phase, leading to a sudden disappearance of the microbubbles due to complete dissolution. In order to estimate shell parameters, microbubble dissolution kinetic data (radius vs. time) was fit numerically to a mass transfer model describing a microbubble dissolution process. Analysis of the results shows that the interfacial tension increases drastically and the shell resistance reduces significantly, as protein molecules leave the gas-liquid interface. Furthermore, the effect of processing conditions such as preheating temperature, microbubble size, and core gas and shell composition on the protein shell parameters was also evaluated.

Magnetic resonance imaging of oxygen microbubbles

Oxygen loaded microbubbles are being investigated as a means of reducing tumour hypoxia in order to improve response to cancer therapy. To optimise this approach, it is desirable to be able to measure changes in tissue oxygenation in real-time during treatment. In this study, the feasibility of using magnetic resonance imaging (MRI) for this purpose was investigated. Longitudinal relaxation time (T1) measurements were made in simple hydrogel phantoms containing two different concentrations of oxygen microbubbles. T1 was found to be unaffected by the presence of oxygen microbubbles at either concentration. Upon application of ultrasound to destroy the microbubbles, however, a statistically significant reduction in T1 was seen for the higher microbubble concentration. Further work is needed to assess the influence of physiological conditions upon the measurements, but these preliminary results suggest that MRI could provide a method for quantifying the changes in tissue oxygenation produced by microbubbles during therapy.

Design and testing of microbubble-based MRI contrast agents for gastric pressure measurement

PURPOSE

This work demonstrates specifically tailored microbubble-based preparations and their suitability as MRI contrast agents for ingestion and measuring temporal and spatial pressure variation in the human stomach.

METHODS

Enhanced alginate spheres were prepared by incorporating gas-filled microbubbles into sodium alginate solution followed by the polymerization of the mixture in an aqueous calcium lactate solution. The microbubbles were prepared with a phospholipid shell and perfluorocarbon gas filling, using a mechanical cavitational agitation regime. The NMR signal changes to externally applied pressure and coming from the enhanced alginate spheres were acquired and compared with that of alginate spheres without microbubbles. In vivo investigations were also carried out on healthy volunteers to measure the pressure variation in the stomach.

RESULTS

The MR signal changes in the contrast agent exhibits a linear sensitivity of approximately 40% per bar, as opposed to no measurable signal change seen in the control gas-free spheres. This novel contrast agent also demonstrates an excellent stability in simulated gastric conditions, including at body temperature. In vivo studies showed that the signal change exhibited in the meal within the antrum region is between 5% and 10%, but appears to come from both pressure changes and partial volume artifacts.

CONCLUSION

This study demonstrates that alginate spheres with microbubbles can be used as an MRI contrast agent to measure pressure changes. The peristaltic movement within the stomach is seen to substantially alter the overall signal intensity of the contrast agent meal. Future work must focus on improving the contrast agent's sensitivity to pressure changes.

Monitoring of acoustic cavitation in microbubble-presented focused ultrasound exposure using gradient-echo MRI

BACKGROUND

Gadolinium-based contrast agents can be used to identify the blood-brain barrier (BBB) opening after inducing a focused ultrasound (FUS) cavitation effect in the presence of microbubbles. However, the use of gadolinium may be limited for frequent routine monitoring of the BBB opening in clinical applications.

PURPOSE

To use a gradient-echo sequence without contrast agent administration for monitoring of acoustic cavitation.

STUDY TYPE

Animal and phantom prospective.

PHANTOM/ANIMAL MODEL

Static and flowing gel phantoms; six normal adult male Sprague-Dawley rats.

FIELD STRENGTH/SEQUENCE

3T, 7T; fast low-angle shot sequence.

ASSESSMENT

Burst FUS with acoustic pressures = 1.5, 2.2, 2.8 MPa; pulse repetition frequencies = 1, 10,100 Hz; and duty cycles = 2%, 5%, 10% were transmitted to the chamber of a static phantom with microbubble concentrations = 10%, 1%, 0.1%. MR slice thicknesses = 3, 6, 8 mm were acquired. In flowing phantom experiments, 0.1%, 0.25%, 0.5%, 0.75%, and 1% microbubbles were infused and transmitted by burst FUS with an acoustic pressure = 0.4 and 1 MPa. In in vivo experiments, 0.25% microbubbles was infused and 0.8 MPa burst FUS was transmitted to targeted brain tissue beneath the superior sagittal sinus. The mean signal intensity (SI) was normalized using the mean SI from pre-FUS.

STATISTICAL TESTS

Two-tailed Student's t-test. P < 0.05 was considered statistically significant.

RESULTS

In the static phantom, the time courses of normalized SI decreases to minimum SI levels of 70-80%. In the flowing phantom, substantial normalized SI of 160-230% was present with variant acoustic pressures and microbubble concentrations. Compared with in vivo control rats, the brain tissue of experimental rats with transmission of FUS pulses exhibited considerable decreases of normalized SI (P < 0.001) because of the cavitation-induced perturbation of flow.

DATA CONCLUSION

Observing gradient-echo SI changes can help monitor the targeted location of microbubble-enhanced FUS, which in turn assists the monitoring of the BBB opening.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2020;51:311-318.

Microbubble Formulations: Synthesis, Stability, Modeling and Biomedical Applications

Microbubbles are increasingly being used in biomedical applications such as ultrasonic imaging and targeted drug delivery. Microbubbles typically range from 0.1 to 10 µm in size and consist of a protective shell made of lipids or proteins. The shell encapsulates a gaseous core containing gases such as oxygen, sulfur hexafluoride or perfluorocarbons. This review is a consolidated account of information available in the literature on research related to microbubbles. Efforts have been made to present an overview of microbubble synthesis techniques; microbubble stability; microbubbles as contrast agents in ultrasonic imaging and drug delivery vehicles; and side effects related to microbubble administration in humans. Developments related to the modeling of microbubble dissolution and stability are also discussed.

Iron oxide nanoparticles: Diagnostic, therapeutic and theranostic applications

Many different iron oxide nanoparticles have been evaluated over the years, for a wide variety of biomedical applications. We here summarize the synthesis, surface functionalization and characterization of iron oxide nanoparticles, as well as their (pre-) clinical use in diagnostic, therapeutic and theranostic settings. Diagnostic applications include liver, lymph node, inflammation and vascular imaging, employing mostly magnetic resonance imaging but recently also magnetic particle imaging. Therapeutic applications encompass iron supplementation in anemia and advanced cancer treatments, such as modulation of macrophage polarization, magnetic fluid hyperthermia and magnetic drug targeting. Because of their properties, iron oxide nanoparticles are particularly useful for theranostic purposes. Examples of such setups, in which diagnosis and therapy are intimately combined and in which iron oxide nanoparticles are used, are image-guided drug delivery, image-guided and microbubble-mediated opening of the blood-brain barrier, and theranostic tissue engineering. Together, these directions highlight the versatility and the broad applicability of iron oxide nanoparticles, and indicate the integration in future medical practice of multiple iron oxide nanoparticle-based materials.

Drug-carrying microbubbles as a theranostic tool in convection-enhanced delivery for brain tumor therapy

Convection-enhanced delivery (CED) is a promising technique for infusing a therapeutic agent through a catheter with a pressure gradient to create bulk flow for improving drug spread into the brain. So far, gadopentetate dimeglumine (Gd-DTPA) is the most commonly applied surrogate agent for predicting drug distribution through magnetic resonance imaging (MRI). However, Gd-DTPA provides only a short observation duration, and concurrent infusion provides an indirect measure of the exact drug distribution. In this study, we propose using microbubbles as a contrast agent for MRI monitoring, and evaluate their use as a drug-carrying vehicle to directly monitor the infused drug. Results show that microbubbles can provide excellent detectability through MRI relaxometry and accurately represent drug distribution during CED infusion. Compared with the short half-life of Gd-DTPA (1-2 hours), microbubbles allow an extended observation period of up to 12 hours. Moreover, microbubbles provide a sufficiently high drug payload, and glioma mice that underwent a CED infusion of microbubbles carrying doxorubicin presented considerable tumor growth suppression and a significantly improved survival rate. This study recommends microbubbles as a new theranostic tool for CED procedures.

Ultrasound/Magnetic Targeting with SPIO-DOX-Microbubble Complex for Image-Guided Drug Delivery in Brain Tumors

One of the greatest challenges in the deployment of chemotherapeutic drugs against brain tumors is ensuring that sufficient drug concentrations reach the tumor, while minimizing drug accumulation at undesired sites. Recently, injection of therapeutic agents following blood-brain barrier (BBB) opening by focused ultrasound (FUS) with microbubbles (MBs) has been shown to enhance drug delivery in targeted brain regions. Nevertheless, the distribution and quantitative deposition of agents delivered to the brain are still hard to estimate. Based on our previous work on superparamagnetic iron oxide (SPIO)-loaded MBs, we present a novel theranostic complex of SPIO-Doxorubicin (DOX)-conjugated MB (SD-MB) for drug delivery to the brain. Magnetic labeling of the drug enables direct visualization via magnetic resonance imaging, and also facilitates magnetic targeting (MT) to actively enhance targeted deposition of the drug. In a rat glioma model, we demonstrated that FUS sonication can be used with SD-MBs to simultaneously facilitate BBB opening and allow dual ultrasound/magnetic targeting of chemotherapeutic agent (DOX) delivery. The accumulation of SD complex within brain tumors can be significantly enhanced by MT (25.7 fold of DOX, 7.6 fold of SPIO). The change in relaxation rate R2 (1/T2) within tumors was highly correlated with SD deposition as quantified by high performance liquid chromatography (R² = 0.93) and inductively coupled plasma-atomic emission spectroscopy (R² = 0.94), demonstrating real-time monitoring of DOX distribution. Our results suggest that SD-MBs can serve as multifunction agents to achieve advanced molecular theranostics.

Real-time monitoring of inertial cavitation effects of microbubbles by using MRI: In vitro experiments

PURPOSE

To investigate the feasibility of half-Fourier acquisition single-shot turbo spin-echo (HASTE) for real-time monitoring of signal changes because of water flow induced by inertial cavitation (IC) during microbubbles (MBs)-present focused ultrasound (FUS) exposure.

THEORY AND METHODS

Strong turbulence produced in MB solution at the onset of IC results in the difficulty to refocus signal echoes and thus the decrease in signal intensity (SI). Fundamental investigations were conducted using an agar phantom containing MB dilutions exposed to 1.85-MHz FUS. The effects of various experimental conditions including MB concentrations, imaging slice thicknesses, chamber diameters, acoustic pressures, duty cycles, and pulse repetition frequencies (PRFs) were discussed.

RESULTS

Continuous 2.8 MPa FUS exposure resulted in SI changed from 11% to 55% when MBs concentrations increased from 0.025% to 0.1%. When slice thickness increased from 3 mm to 6 or 8 mm, smaller SI changes were observed (84%, 59%, and 46%). Images acquired with chamber diameter of 6 and 3 mm showed SI changes of 84% and 35%, respectively. In burst modes, higher duty cycles exhibited higher SI changes, and lower PRFs exhibited smaller and longer SI decrease.

CONCLUSION

Under various conditions, substantial signal changes were observable, suggesting the feasibility of applying HASTE to real-time monitor IC effect under FUS exposure. Magn Reson Med 77:102-111, 2017. © 2015 Wiley Periodicals, Inc.

Using microbubbles as an MRI contrast agent for the measurement of cerebral blood volume

The susceptibility differences at the gas-liquid interface of microbubbles (MBs) allow their use as an intravascular susceptibility contrast agent for in vivo MRI. However, the characteristics of MBs are very different from those of the standard gadolinium-diethylenetriaminepentaacetic acid (Gd-DPTA) contrast agent, including the size distribution and hemodynamic properties, which could influence MRI outcomes. Here, we investigate quantitatively the correlation between the relative cerebral blood volume (rCBV) derived from Gd-DTPA (rCBV(Gd)) and the MB-induced susceptibility effect (ΔR(2*MB)) by conventional dynamic susceptibility contrast MRI (DSC-MRI). Custom-made MBs had a mean diameter of 0.92 µm and were capable of inducing 4.68 ± 3.02% of the maximum signal change (MSC). The MB-associated ΔR(2*MB) was compared with rCBV(Gd) in 16 rats on 4.7-T MRI. We observed a significant effect of the time to peak (TTP) on the correlation between ΔR(2*MB) and rCBV(Gd), and also found a noticeable dependence between TTP and MSC. Our findings suggest that MBs with longer TTPs can be used for the estimation of rCBV by DSC-MRI, and emphasize the critical effect of TTP on MB-based contrast MRI.

Magnetic resonance properties of Gd(III)-bound lipid-coated microbubbles and their cavitation fragments

Gas-filled microbubbles are potentially useful theranostic agents for magnetic resonance imaging-guided focused ultrasound surgery (MRIgFUS). Previously, MRI at 9.4 T was used to measure the contrast properties of lipid-coated microbubbles with gadolinium (Gd(III)) bound to lipid headgroups, which revealed that the longitudinal molar relaxivity (r(1)) increased after microbubble fragmentation. This behavior was attributed to an increase in water proton exchange with the Gd(III)-bound lipid fragments caused by an increase in the lipid headgroup area that accompanied the lipid shell monolayer-to-bilayer transition. In this article, we explore this mechanism by comparing the changes in r(1) and its transverse counterpart, r(2)*, after the fragmentation of microbubbles consisting of Gd(III) bound to two different locations on the lipid monolayer shell: the phosphatidylethanolamine (PE) lipid headgroup region or the distal region of the poly(ethylene glycol) (PEG) brush. Nuclear magnetic resonance (NMR) at 1.5 T was used to measure the contrast properties of the various microbubble constructs because this is the most common field strength used in clinical MRI. Results for the lipid-headgroup-labeled Gd(III) microbubbles revealed that r(1) increased after microbubble fragmentation, whereas r(2)* was unchanged. An analysis of PEG-labeled Gd(III) microbubbles revealed that both r(1) and r(2)* decreased after microbubble fragmentation. Further analysis revealed that the microbubble gas core enhanced the transverse MR signal (T(2)*) in a concentration-dependent manner but minimally affected the longitudinal (T(1)) signal. These results illustrate a new method for the use of NMR to measure the biomembrane packing structure and suggest that two mechanisms, proton-exchange enhancement by lipid membrane relaxation and magnetic field inhomogeneity imposed by the gas/liquid interface, may be used to detect and differentiate Gd(III)-labeled microbubbles and their cavitation fragments with MRI.

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.

Theranostic Gd(III)-lipid microbubbles for MRI-guided focused ultrasound surgery

We have synthesized a biomaterial consisting of Gd(III) ions chelated to lipid-coated, size-selected microbubbles for utility in both magnetic resonance and ultrasound imaging. The macrocyclic ligand DOTA-NHS was bound to PE headgroups on the lipid shell of pre-synthesized microbubbles. Gd(III) was then chelated to DOTA on the microbubble shell. The reaction temperature was optimized to increase the rate of Gd(III) chelation while maintaining microbubble stability. ICP-OES analysis of the microbubbles determined a surface density of 7.5 × 10(5) ± 3.0 × 10(5) Gd(III)/μm(2) after chelation at 50 °C. The Gd(III)-bound microbubbles were found to be echogenic in vivo during high-frequency ultrasound imaging of the mouse kidney. The Gd(III)-bound microbubbles also were characterized by magnetic resonance imaging (MRI) at 9.4 T by a spin-echo technique and, surprisingly, both the longitudinal and transverse proton relaxation rates were found to be roughly equal to that of no-Gd(III) control microbubbles and saline. However, the relaxation rates increased significantly, and in a dose-dependent manner, after sonication was used to fragment the Gd(III)-bound microbubbles into non-gas-containing lipid bilayer remnants. The longitudinal (r(1)) and transverse (r(2)) molar relaxivities were 4.0 ± 0.4 and 120 ± 18 mM(-1)s(-1), respectively, based on Gd(III) content. The Gd(III)-bound microbubbles may find application in the measurement of cavitation events during MRI-guided focused ultrasound therapy and to track the biodistribution of shell remnants.

Iron oxide nanoparticle-containing microbubble composites as contrast agents for MR and ultrasound dual-modality imaging

Magnetic resonance (MR) and ultrasound (US) imaging are widely used diagnostic modalities for various experimental and clinical applications. In this study, iron oxide nanoparticle-embedded polymeric microbubbles were designed as multi-modal contrast agents for hybrid MR-US imaging. These magnetic nano-in-micro imaging probes were prepared via a one-pot emulsion polymerization to form poly(butyl cyanoacrylate) microbubbles, along with the oil-in-water (O/W) encapsulation of iron oxide nanoparticles in the bubble shell. The nano-in-micro embedding strategy was validated using NMR and electron microscopy. These hybrid imaging agents exhibited strong contrast in US and an increased transversal relaxation rate in MR. Moreover, a significant increase in longitudinal and transversal relaxivities was observed after US-induced bubble destruction, which demonstrated triggerable MR imaging properties. Proof-of-principle in vivo experiments confirmed that these nanoparticle-embedded microbubble composites are suitable contrast agents for both MR and US imaging. In summary, these magnetic nano-in-micro hybrid materials are highly interesting systems for bimodal MR-US imaging, and their enhanced relaxivities upon US-induced destruction recommend them as potential vehicles for MR-guided US-mediated drug and gene delivery.

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.