SUMMARY

Ultrasound imaging is a technique that can be applied to a vast array of body systems thanks to its safety and versatility. However, there are still several drawbacks to ultrasound imaging, namely that the resolution pales in comparison to optical techniques (hundreds of micron resolution vs. hundreds of nanometer scale) and lacks any ability for long-term imaging with current commercially-available ultrasound contrast agents. As a result, there is a critical need for ultrasound contrast agents that can improve ultrasound resolution and enable long-term imaging for diagnostic applications, cell tracking, tissue graft monitoring, and more.
Two ultrasound contrast agents, perfluorocarbon nanodroplets and gas vesicles, have recently emerged and are becoming more commonplace in academic research. Perfluorocarbon nanodroplets are phase-change contrast agents that can extravasate into tumors via endothelial gaps in blood vessels and can remain in the body for days, enabling long-term imaging, but the use of these nanodroplets for single-cell imaging has yet to be explored. Gas vesicles are produced by certain cyanobacteria and archaea to help with buoyancy, and recent work has enabled gas vesicle expression in mammalian cells for prolonged periods of time using mammalian acoustic reporter genes (mARGs). However, these mARGs have only been integrated in certain cell lines, and the methods to isolate cells that successfully express gas vesicles are complex. These two nanoscale ultrasound contrast agents both have the potential to be used for deep tissue, high resolution, and long-term in vivo imaging, but require alterations to achieve this. This thesis proposal seeks to modify these ultrasound contrast agents and employ them for long-term, deep tissue imaging in vitro. For perfluorocarbon nanodroplets, I will optimize their outer shell parameters to improve nanodroplet phase-transitioning, utilize patch clamping to isolating the nanodroplets in individual cells, monitor the long-term cell health to ensure the nanodroplets do not hinder cell viability, and ultrasonically image the nanodroplets injected into the cell within a gel tissue phantom. For the genetically expressed gas vesicles in mammalian cells, I will expand gas vesicle integration into other mammalian cells, specifically induced pluripotent stem cells, and simplify the procedures for generating gas vesicle-expressing mammalian cells using improved plasmids and drug selection techniques.