Iron oxide nanoparticle-containing microbubble composites as contrast agents for MR and ultrasound dual-modality imaging Zhe Liu a , Twan Lammers a , Josef Ehling a, b , Stanley Fokong a , Jörg Bornemann c , Fabian Kiessling a , Jessica Gätjens a, * a Department of Experimental Molecular Imaging (ExMI), Helmholtz Institute for Biomedical Engineering, Medical Faculty, RWTH Aachen University, Aachen 52074, Germany b Institute of Pathology, University Hospital Aachen (UKA), Aachen 52074, Germany c Electron Microscopic Facility, University Hospital Aachen (UKA), Aachen 52074, Germany article info Article history: Received 24 February 2011 Accepted 5 May 2011 Available online 31 May 2011 Keywords: Nanoparticle MRI (magnetic resonance imaging) Contrast agent Polymerization Molecular imaging abstract 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 MReUS 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 nano- particles 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 MReUS imaging, and their enhanced relaxivities upon US-induced destruction recommend them as potential vehicles for MR-guided US-mediated drug and gene delivery. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction In the field of disease-specific imaging, magnetic resonance (MR) and ultrasound (US) imaging are widely used modalities for various experimental and clinical applications [1,2]. Magnetic nanoparticles or metal complexes have been developed as power- ful contrast agents for MR imaging, which is non-invasive, and capable of providing morphological and functional information with a high spatial resolution and with excellent soft-tissue contrast. In comparison, ultrasound is highly cost-effective, and suitable for real-time imaging. Stabilized gas microbubbles are routinely used as US contrast agents, and they can be functionalized with antibodies or peptides to specifically bind receptors over- expressed on vascular endothelial cells, thereby generating sensi- tive intravascular molecular imaging probes. Moreover, they can also be loaded with drugs and genes, and release their payloads either upon magnetic actuation or upon exposure to destructive ultrasound pulses [3e7]. These features of MR and US contrast agents not only make them favorable tools for precisely visualizing biological and physiological processes at molecular and cellular level, but also suggest that they might render synergistic efficacy if one single imaging probe can be developed for both modalities. Recently, nanoparticle-containing composites have attracted unprecedented attention for functional and molecular imaging investigations [8e10]. It has been reported that some solid nano- particles, such as silica, polystyrene and superparamagnetic iron oxide (SPIO) nanoparticles, are able to boost the acoustic impedance, increase backscattered signals, and consequently contribute to contrast enhancement for US [11e 13]. On the other hand, gas-filled microbubbles proved to be detectable by MR since the shell oscil- lations of bubbles result in a proportionate contribution to magnetic susceptibility [14,15]. Therefore, constructing a hybrid contrast agent would be highly useful for both imaging modalities, and could facilitate MR-guided US-mediated transfection and gene delivery, MR-guided high-intensity focused ultrasound treatment, hyper- thermia therapy, and even broader biomedical applications [16e21]. As appealing carrier materials for nanoparticles, polymer- based hard-shell microbubbles show obvious advantages over * Corresponding author. Tel.: þ49 241 8080116; fax: þ49 241 8082006. E-mail address: jgaetjens@ukaachen.de (J. Gätjens). Contents lists available at ScienceDirect Biomaterials journal homepage: www.elsevier.com/locate/biomaterials 0142-9612/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biomaterials.2011.05.019 Biomaterials 32 (2011) 6155e6163