Enhancement of Gas-Filled Microbubble R 2 * by Iron Oxide Nanoparticles for MRI April M. Chow, 1,2 Kannie W. Y. Chan, 1,2 Jerry S. Cheung, 1,2 and Ed X. Wu 1,2 * Gas-filled microbubbles have the potential to become a unique intravascular MR contrast agent due to their magnetic suscep- tibility effect, biocompatibility, and localized manipulation via ultrasound cavitation. However, microbubble susceptibility effect is relatively weak when compared with other intravascu- lar MR susceptibility contrast agents. In this study, enhance- ment of microbubble susceptibility effect by entrapping monocrystalline iron oxide nanoparticles (MIONs) into poly- meric microbubbles was investigated at 7 T in vitro. Apparent T 2 enhancement (DR 2 *) induced by microbubbles was measured to be 79.2 6 17.5 sec 21 and 301.2 6 16.8 sec 21 for MION-free and MION-entrapped polymeric microbubbles at 5% volume fraction, respectively. DR 2 * and apparent transverse relaxivities (r 2 *) 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 micro- bubble susceptibility enhancement for MRI application. This study indicates that gas-filled polymeric microbubble suscepti- bility 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 63:224–229, 2010. V C 2009 Wiley-Liss, Inc. Key words: MRI; contrast agent; microbubbles; susceptibility; iron oxide nanoparticles Gas-filled microbubbles were originally developed as an intravascular contrast agent in ultrasound imaging to enhance acoustic backscattering. Recently, gas-filled microbubbles have been employed in therapeutic appli- cations due to their unique cavitation and sonoporation properties (1,2). Local microbubble cavitation by spa- tially focused ultrasound can be applied in achieving site-specific release of incorporated drugs or genes inside microbubbles. Microbubble-mediated sonoporation can dramatically increase cell permeability and intracellular uptake, with no apparent tissue damage and toxicity. Furthermore, the unique microbubble cavitation phe- nomenon has been exploited to achieve several therapeu- tic interventions, like sonothrombolysis, transient open- ing of the blood-brain barrier potentially for delivery of both low- and high-molecular-weight therapeutic com- pounds, and enhancing high-intensity focused ultra- sound therapy by increasing the local heating rate (3–5). Microbubbles can potentially be used as an intravascu- lar MR susceptibility contrast agent in vivo due to the induction of large local magnetic susceptibility differ- ence by the gas-liquid interface. Moreover, microbubbles can be locally cavitated and destroyed by focused ultra- sound (2); hence, the MR signals can be temporally and spatially manipulated because microbubble disappearan- ces will diminish the susceptibility effect. Early experi- ment with Albunex V R (Molecular Biosystems Inc., San Diego, CA), an ultrasound contrast agent consisting of air-filled microbubbles with human albumin shell, illus- trated the potential of air-filled microbubbles as a MR susceptibility contrast agent (6). Feasibility of microbub- bles as an MR pressure sensor, based on the susceptibil- ity change caused by pressure-induced microbubble size change, has been explored through theoretical and phan- tom studies (7,8). Linear relationship between apparent T 2 (R 2 * ¼ 1/T 2 *) and volume fraction for Optison V R (Amersham Health, Princeton, NJ) microbubbles of human albumin shells with perfluorocarbon as core gas, was first reported by our group at 7 T (9). Recently, we investigated the sus- ceptibility effect of lipid-based microbubble SonoVue V R (Bracco Diagnostics, Milan, Italy) and air-filled custom- made albumin-coated microbubbles (10). R 2 * dependency on microbubble volume fraction was also reported for Levovist V R (Schering AG, Berlin, Germany), air-filled microbubbles with palmitic acid shells, through an in vitro phantom study at 1.5 T (11). Magnetic suscepti- bility enhancement induced by a gas-liquid interface was demonstrated recently by simulations and MR experiments using air-filled cylinders in water (12), consolidating the feasibility of gas-filled microbubbles as an MR susceptibility contrast agent. However, microbubble susceptibility effect is relatively weak when compared with other intravascular MR suscepti- bility contrast agents. The dosage used in the in vivo experiments reported so far substantially exceeded the maximum ultrasound dosage recommended for a 10-min human myocardial study (9,10). Microbubbles are generally composed of a shell of bio- compatible materials, such as proteins, lipids, or poly- mers, with filling gas. The microbubble shell can be stiff (denatured proteins or polymers) or flexible (phospholi- pids). The shell thickness ranges from 10 nm to 200 nm, with thinner shells typically used for protein and lipid microbubbles, while thicker shells are used for polymeric microbubbles (PMBs) (13). The effective microbubble magnetic susceptibility can be manipulated by changing the shell thickness and the magnetic suscep- tibility of the shell or filling gas (14). As the type of 1 Laboratory of Biomedical Imaging and Signal Processing, The University of Hong Kong, Pokfulam, Hong Kong. 2 Laboratory of Biomedical Engineering, Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam, Hong Kong. Grant sponsor: Hong Kong Research Grant Council; Grant number: CERG HKU 7642/06M. *Correspondence to: Ed X. Wu, Ph.D., Laboratory of Biomedical Imaging and Signal Processing, Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam, Hong Kong. E-mail: ewu@eee.hku.hk Received 23 January 2009; revised 21 July 2009; accepted 27 July 2009. DOI 10.1002/mrm.22184 Published online 1 December 2009 in Wiley InterScience (www.interscience. wiley.com). Magnetic Resonance in Medicine 63:224–229 (2010) V C 2009 Wiley-Liss, Inc. 224