(622f) Genetically Encodable Acoustomagnetic Reporters for Background-Free Molecular and Cellular MRI

Technologies for non-invasive whole-organ imaging of molecular targets and cellular processes such as gene expression are essential to support biological studies, medical diagnostics, and the development of cellular therapeutics. While optical methods provide high-resolution images, they are fundamentally limited by the penetration depth of light in opaque tissues. In comparison, magnetic resonance imaging (MRI) and ultrasound are powerful and complementary non-invasive imaging modalities and have been widely used in clinics. Yet they have never been paired for molecular imaging. Such a pairing, especially in a way that one modality can control the image content from the other modality, could take advantage of the unique features of each modality to provide information not available via either technology alone. Here, we describe the first contrast agents for acoustomagnetic imaging. These agents are based on gas vesicles (GVs), a unique class of genetically encoded gas-filled protein-only nanostructures formed by photosynthetic microbes as a means to regulate buoyancy. These agents are genetically encodable, robustly detectable using susceptibility-dependent MRI sequences and â??erasableâ? at specific ultrasound pressures. Thus they provide, for the first time, a direct method to eliminate the endogenous background, a major drawback for susceptibility imaging including functional MRI and superparamagnetic iron oxide (SPIO) nanoparticle imaging. In addition, clustering of these agents based on the presence of a specific analyte produces changes in MRI contrast that could enable the use of these agents as functional sensors.

We reasoned that the gaseous interior of GVs would have a different magnetic susceptibility from the surrounding aqueous media, and would thus induce a local magnetic field gradient that would alter the precession of nearby aqueous ¹H nuclear spins. Indeed, we show that GVs can be imaged at nanomolar concentration using gradient echo MRI and quantitative susceptibility mapping. Furthermore, GVs are â??erasableâ? MRI contrast agents, since ultrasound pressure applied in a spatially selective manner can collapse these nanostructures, eliminating their gas compartment and thus the MRI contrast. This is very useful because, many endogenous tissue properties, such as the presence of blood vessels and the interface between different types of tissue, can produce hypointense contrast in gradient echo images, and the complication of endogenous contrast has been a major drawback to the design of susceptibility-dependent T2* MRI contrast agents. Due to their ability to be imaged before and after acoustic collapse, GVs enable, for the first time, the acquisition of background-free T2* contrast. We demonstrate the concept of acoustically erasable background-free imaging using GVs injected into mouse brains and collapsed using ultrasound.

Furthermore, acoustic collapse occurs at different specific pressures for GVs from different organisms, enabling multiplexed â??acoustomagneticâ? imaging using a pressure-scan paradigm. The paradigm could enable the tracking of multiple GV-labeled cell types using MRI. Finally, GVs can potentially be engineered as sensors of specific biological signals because the clustering of GVs produces a marked change in gradient echo images, as demonstrated in vitro by streptavidin-biotin cross-linking.

Overall, these results support the introduction of GVs as the first acoustomagentic reporters, which use the combination of ultrasound and MRI to enable background-free and multiplexed imaging. The ability of GVs to be genetically encoded and engineered opens the possibility of using this new form of contrast in a wide range of biological applications, especially in synthetic biology and cellular therapeutics.