Project Summary This project aims to develop a microscopic nuclear imaging instrument that can provide the 3D distribution of clinical radiotracers within a thick, living multi-cellular organism. The proposed instrument will improve our understanding of how clinical radiotracers are distributed at a cellular level and thereby help us to better interpret clinical nuclear imaging. It will also allow us to test and select new radiopharmaceuticals using patient-derived organoids for safe and effective therapy. Microautoradiography (MAR) is the current gold-standard method for microscopic nuclear imaging. However, it can provide a high resolution only for thin (~ 5 µm) sliced specimens using low-energy radionuclides; the resolution is only moderate with the high-energy radionuclides used for clinical imaging, even for a thin sliced specimen, let alone single intact cells or cell spheroids. The resolution of MAR and other existing methods rapidly decreases with the sample thickness, because the positrons obliquely incident onto the scintillator spread more widely as the distance between the source and the scintillator increases. The scattering of positrons in the sample, which increases with the sample thickness, will further blur the image. We will develop a new instrument that can address these challenges by using radioluminescence microscopy (RLM) as a base platform. RLM is a state-of-the-art technique recording the scintillation tracks of individual positrons. The enabling idea is to record the 3D orientations of scintillation tracks at the crystal surface and trace them backward to the source (i.e., the radionuclide that emitted the positron) in the sample volume. To record the 3D orientations of scintillation tracks, which decay within tens of microseconds, we will use snapshot projection optical tomography (SPOT). SPOT was previously demonstrated with fluorescent or absorbing specimens, which is extended here to record the 3D orientations of scintillation tracks, for the first time to our knowledge. For a thin specimen, we can trace back the scintillation tracks using filtered backprojection. For a thick specimen, we will investigate statistical reconstruction, which can handle the scattering of positrons in the sample and the ill-posedness of the inverse problem. Using radiolabeled beads, [18F]FDG-filled microdroplets, and biological specimens incubated with [18F]FDG, we will demonstrate the feasibility of 3D microscopic nuclear imaging of a thick, living multi-cellular organism using the same radiotracers as used in clinical imaging and therapy. In particular, we aim to demonstrate subcellular resolution (< 1 µm) for confluent living cells and cellular resolution (10–20 µm) for thick cell spheroids.

Project Narrative This project aims to develop a microscopic nuclear imaging technique that can provide the 3D distribution of clinical radiotracers (instead of tritiated forms or analogs) in a thick, living biological specimen (instead of a fixed and sliced specimen). The proposed instrument will improve our understanding of how clinical radiotracers are distributed at cellular level in a living, multi-cellular organism, and thereby will help us to better interpret clinical nuclear imaging. It will also allow us to test and select new radiopharmaceuticals using patient-derived organoids for safe and effective therapy.