Title: Engineering Lipid-stabilized Microbubbles for Magnetic Resonance Imaging guided Focused Ultrasound Surgery
Description: Lipid-stabilized microbubbles are gas-filled microspheres encapsulated with a phospholipid monolayer shell. Because of the high echogenicity provided by its highly compressible gas core, these microbubbles have been adapted as ultrasound contrast agents for a variety of applications such as contrast-enhanced ultrasonography (CEUS), targeted drug delivery and metabolic gas transport. Recently, these lipid-stabilized microbubbles have demonstrated increased potential as theranostic (therapy + diagnostics) agents for non-invasive surgery with focused ultrasound (FUS). For instance, their implementation has reduced the acoustic intensity threshold needed to open the blood-brain-barrier (BBB) with FUS, which potentially allows for the localized delivery of drugs to treat neurodegenerative diseases such as Alzheimer's, Parkinson's and Huntington's diseases. However, the effectiveness of microbubbles for this application is dependent on successful microbubble engineering. One necessary improvement is the development and utilization of monodisperse microbubbles of varying size classes.

Another design improvement is the development of a microbubble construct whose fragmentation state during or after FUS surgery can be tracked by magnetic resonance imaging (MRI). Thus, in this thesis, we describe a method to generate and select lipid-coated gas-filled microbubbles of specific size fractions based on their migration in a centrifugal field. We also detail the design and characterization of size-selected lipid-coated microbubbles with shells containing the magnetic resonance (MR) contrast media Gadolinium (Gd(III)), for utility in both MR and ultrasound imaging. Initial characterization of the lipid headgroup labeled Gd(III)-microbubbles by MRI revealed that the Gd(III) relaxivity increased after microbubble fragmentation into non-gas-containing lipid vesicles. This behavior was explained to stem from an increase in interaction between water protons and the Gd(III)-bound lipid fragments due to an increase in lipid headgroup area after microbubble fragmentation. To explore this hypothesis, an alternative construct consisting of Gd(III) preferentially bound to the protective poly(ethylene glycol) (PEG) brush of the lipid shell architecture was also designed and compared to the lipid headgroup-labeled Gd(III)-microbubbles.

Nuclear magnetic resonance (NMR) analysis revealed that, in contrast to the headgroup labeled Gd(III)-microbubbles, the relaxivity of the PEG-labeled Gd(III)-microbubbles decreased after microbubble fragmentation. NMR analysis also revealed an independent concentration-dependent enhancement of the transverse MR signal by virtue of the microbubble gas core. The results of this study illustrated the roles that Gd(III) placement on the lipid shell and the presence of the gas core may play on the MR signal when monitoring Gd(III)-microbubble cavitation during non-invasive surgery with FUS.