Towards Full-body Small Aquatic Animal PET: a Simulation Study of MERMAID-v2

The MERMAID (Multi-Emission Radioisotopes-Marine Animal Imaging Device) project aims to make positron emission tomography (PET) for small aquatic animals available, particularly for biomedical research with zebrafish. One limitation of the current prototype, MERMAID-v1, is the lack of depth-of-interaction (DOI) information and the need for module rotation. We are using Monte-Carlo simulation to implement a step-by-step system upgrade towards a cost-effective full-body small animal PET solution. For MERMAID-v2, the next upgrade, we are considering adding two more modules, different from the existing ones. The new modules should provide DOI, extend the axial field-of-view, and reduce the number of rotation steps. This hybrid configuration, unique in its kind, poses several challenges for simulation and reconstruction. We have addressed these challenges through a dedicated reconstruction based on the List-mode Maximum Likelihood Expectation Maximization (LM-MLEM) algorithm with multiray random sampling. The system model also takes into account the geometric efficiency of each detector type and was validated using Monte-Carlo simulated data. The reconstructed images of a simulated modified NEMA-NU4 2004 image quality phantom demonstrate significant improvements in spatial resolution, uniformity, and contrast. Despite only two of the six modules of MERMAID-v2 providing DOI information, the new configuration seems to be capable of achieving sub-millimeter resolution and resolving the shrinkage problem induced by parallax errors. The next steps include the simulation of a fish-like phantom, a more comprehensive image quality assessment, improving the reconstruction, and implementing axial detector motion.