Elsevier

NeuroImage

Volume 46, Issue 3, 1 July 2009, Pages 658-664
NeuroImage

Microbubbles as a novel contrast agent for brain MRI

https://doi.org/10.1016/j.neuroimage.2009.02.037Get rights and content

Abstract

Gas-filled microbubbles have the potential to become a unique MR contrast agent due to their magnetic susceptibility effect, biocompatibility and localized manipulation via ultrasound cavitation. In this study, two types of microbubbles, custom-made albumin-coated microbubbles (A-MB) and a commercially available lipid-based clinical ultrasound contrast agent (SonoVue®), were investigated with in vivo dynamic brain MRI in Sprague–Dawley rats at 7 T. Microbubble suspensions (A-MB: 0.2 mL of ∼ 4% volume fraction; SonoVue®: 0.2 mL of ∼ 3.5% volume fraction) were injected intravenously. Transverse relaxation rate enhancements (ΔR2) of 2.49 ± 1.00 s 1 for A-MB and 2.41 ± 1.18 s 1 for SonoVue® were observed in the brain (N = 5). Brain ΔR2 maps were computed, yielding results similar to the cerebral blood volume maps obtained with a common MR blood pool contrast agent. Microbubble suspension ΔR2 was measured for different volume fractions. These results indicate that gas-filled microbubbles can serve as an intravascular contrast agent for brain MRI at high field. Such capability has the potential to lead to real-time MRI guidance in various microbubble-based drug delivery and therapeutic applications in the central nervous system.

Introduction

MRI provides superb soft tissue contrast with high spatial resolution when compared with other imaging modalities. While MR image contrast can be flexibly controlled by varying pulse sequences and parameters, it is determined by the intrinsic tissue properties such as proton density, longitudinal relaxation time (T1) and transverse relaxation time (T2). At present, the exogenous contrast agents available for brain MRI mainly fall into three categories, i.e. gadolinium chelates, manganese chelates, and superparamagnetic iron oxide particles. Their effects are usually described by longitudinal relaxation rate (R1) and transverse relaxation rate (R2/R2), where R1, R2 and R2 are defined as 1/T1, 1/T2 and 1/T2 respectively. Susceptibility contrast agents exhibit large R2/R1 ratios and predominantly induce signal loss through spin dephasing by strong magnetic susceptibility effects. Their T2 shortening effects are usually much stronger than the baseline T2 effects. Dynamic susceptibility contrast MRI is an effective tool to measure cerebral blood volume and perfusion (Belliveau et al., 1990). Gadolinum chelates have been widely used as susceptibility contrast agents to assess vascular characteristics through dynamic imaging by investigating first-pass effects of bolus injections in brain (Rempp et al., 1994, Rosen et al., 1991), while superparamagnetic iron oxide particles have also been used for the same purpose by utilizing both the first-pass (Simonsen et al., 1999) and steady-state effects (van Bruggen et al., 1998, Wu et al., 2003, Wu et al., 2004a).

Gas-filled microbubbles were originally developed as an intravascular contrast agent to enhance backscattering in ultrasound imaging. Microbubbles can potentially be used as a MR susceptibility contrast agent in vivo due to the induction of large local magnetic susceptibility differences by the gas–liquid interface. Moreover, microbubbles can be locally cavitated by spatially focused ultrasound (Bouakaz et al., 2005), and hence the MR signals can be temporally and spatially manipulated by external ultrasound irradiation because microbubble destruction will diminish the susceptibility effect. Due to their unique cavitation (Liu et al., 2006, Unger et al., 2001) and sonoporation (Mehier-Humbert et al., 2005, Wu et al., 2006) properties, gas-filled microbubbles play an expanding role in therapeutic applications. Site-specific release of incorporated drugs or genes inside microbubbles can be potentially achieved by local microbubble cavitation, while microbubble-mediated sonoporation dramatically increases cell permeability and intracellular uptake. Microbubble-mediated therapy has been used to deliver genes or drugs to specific tissues (Bekeredjian et al., 2003, Hauff et al., 2005, Shimamura et al., 2004, Taniyama et al., 2002) utilizing microbubble cavitation and sonoporation effects, including neural tissues, skeletal muscles, myocardium, kidneys, vessels, and tumors. Furthermore, microbubble cavitation phenomenon has been put into practical use in achieving several therapeutic interventions. Sonothrombolysis, which employs the local shock waves produced by microbubble cavitation to fragment clots on microscopic scale and restores blood flow, has been developed as a minimally invasive recanalization technique in treating vascular thrombosis (Culp et al., 2004, Daffertshofer and Hennerici, 2003). Besides, transient opening of blood–brain barrier through microbubble-enhanced sonoporation can be accomplished by applying transcranial ultrasound with intravenous injection of microbubbles without damaging the neurons (Sheikov et al., 2004). Delivery of both low and high molecular weight therapeutic compounds to the central nervous system can be potentially attained through this noninvasive procedure. Microbubbles are also used to enhance the noninvasive high intensity focused ultrasound therapy by increasing the local heating rate (Kaneko et al., 2005).

Early experiment with Albunex®, an ultrasound contrast agent consisting of air-filled microbubbles coated with human albumin shells, suggested the potential of air-filled microbubbles as a MR susceptibility contrast agent for tumor imaging (Moseley et al., 1991). Feasibility of microbubbles as a MR pressure sensor, based on the susceptibility change caused by pressure-induced microbubble size change, has been explored through theoretical (Dharmakumar et al., 2002) and phantom (Alexander et al., 1996) studies. The first in vivo investigation of susceptibility contrast induced by microbubbles was reported in liver by our group previously using Optison®, microbubbles of human albumin shells with perfluorocarbon as core gas, at 7 T in rat liver (Wong et al., 2004). Dependency of R2 on microbubble volume fractions was also reported using Levovist®, air-filled microbubbles with palmitic acid shells, through a phantom study at 1.5 T (Ueguchi et al., 2006). Magnetic susceptibility enhancements induced by gas–liquid interface were demonstrated recently by simulations and MR experiments using air-filled cylinders in water (De Guio et al., 2008), consolidating the feasibility of gas-filled microbubbles as a MR susceptibility contrast agent. In this study, we aim to further investigate and demonstrate the in vivo MR susceptibility effect induced by both custom-made albumin-coated microbubbles and commercially available lipid-based microbubbles in rat brain at 7 T using dynamic susceptibility weighted MRI.

Section snippets

Methods

All MRI experiments were performed on a 7 T MRI scanner with a maximum gradient of 360 mT/m (70/16 PharmaScan, Bruker Biospin GmbH, Germany). All animal experiments were approved by the local institutional animal ethics committee.

Characterization of microbubble suspensions

The light micrographs of a representative batch of A-MB and SonoVue® microbubbles were depicted in Figs. 1a and b, respectively. The estimated size distribution was from 1 to 23 μm (with 9.21 μm mean diameter) for A-MB and 1 to 10 μm (with 2.95 μm mean diameter) for SonoVue®, as shown in Figs. 1c and d. The estimated mean diameter of SonoVue® was slightly higher than that provided by the manufacturer (2.5 μm). This was likely due to the aggregation of SonoVue® microbubbles in suspension (seen

Discussions

In this study, gas-filled microbubbles were successfully demonstrated as a novel intravascular contrast agent for brain MRI for the first time. Two different types of microbubbles were investigated for their susceptibility effects by dynamic brain imaging at 7 T. Microbubbles induced ΔR2 maps were observed to be similar to those caused by MION, a standard intravascular contrast agent, reflecting the blood distribution in tissue vasculature (Wu et al., 2003, Wu et al., 2004c, Wu et al., 2004d).

Conclusions

In this study, we investigated the feasibility of gas-filled microbubbles as an intravascular MR susceptibility contrast agent in brain at 7 T. Considerable susceptibility induced changes were observed and characterized in rat brain using the custom-made albumin-coated microbubbles and a commercially available clinical ultrasound microbubble contrast agent. The results indicate that microbubbles can serve as a unique intravascular MR contrast agent in vivo at high field. Such capability has the

Acknowledgments

We thank Dr. Joseph C.K. Leung of the Department of Medicine and Dr. Ke Xia Cai at the Laboratory of Biomedical Imaging and Signal Processing of the University of Hong Kong for assistance. This work was supported by the Hong Kong Research Grant Council (CERG HKU 7642/06M).

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