Magnetic Resonance Properties of Gd(III)-Bound Lipid-Coated Microbubbles and their Cavitation Fragments Jameel A. Feshitan, † Michael A. Boss, ‡ and Mark A. Borden* ,† † Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309, United States ‡ Electromagnetics Division, National Institute of Standards and Technology (NIST), Boulder, Colorado 80305, United States ABSTRACT: 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. ■ INTRODUCTION Magnetic resonance imaging-guided focused ultrasound surgery (MRIg-FUS) is a rapidly developing medical technique that uses high-intensity focused ultrasound to ablate tissue and magnetic resonance (MR) thermometry to monitor treat- ment. 1-3 The formation and cavitation activity of gas-filled microbubbles nucleated from dissolved gases in tissue and blood play an integral role in the efficacy of this therapy. 2 However, the nucleation of cavitation bubbles is unpredictable and can lead to deleterious effects outside the targeted region. Thus, it is preferable to use preformed, stabilized microbubbles (1-10 μm diameter) that can interact with ultrasound waves in a more predictable manner. 2 Lipid-coated microbubbles are currently approved by the U.S. Food and Drug Administration (FDA) for echocardiography and are being developed for expanded imaging capabilities 4,5 and therapeutic applications in drug, gene, and gas delivery. 6-10 One potential therapeutic application of microbubbles is the noninvasive, localized, transient opening of the blood-brain barrier (BBB) for drug delivery to the brain. Previously, lipid-coated microbubbles were shown to reduce the acoustic threshold needed for opening the BBB in vivo. 11-15 In addition to disrupting vasculature, microbubbles also have been designed to deliver a therapeutic payload. 6,16 However, methods are currently unavailable to use MRI for tracking microbubbles and their interactions with ultrasound. It is therefore desirable to develop MR-detectable microbubbles so that MRI can be used to monitor and control not only thermal ablation but also pharmaceutical delivery. In a previous study, MRI contrast agent Gd(III)-DOTA was conjugated to the lipid shell of size-selected, gas-filled microbubbles by the use of a postlabeling technique. 17 Gd(III)-DOTA was conjugated to the primary amine on the headgroup of phosphatidylethanolamine (PE). The effect of Gd(III)-bound microbubble cavitation on the MR signal was determined in vitro by comparing r 1 and r 2 * of 4 to 5 μm gas- core-containing Gd(III)-bound microbubbles to those of microbubbles that were fragmented by inertial cavitation and heating to lipid fragments. A 9.4 T vertical MRI system was used to acquire turbo spin echo (RARE-VTR) images with variable repetition times and multislice multiecho (MSME) images with variable echo times for longitudinal (T 1 ) and Received: August 13, 2012 Revised: October 1, 2012 Published: October 8, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 15336 dx.doi.org/10.1021/la303283y | Langmuir 2012, 28, 15336-15343