This revised proposal is in response to PAR-15-090 BRAIN Initiative: Development, Optimization, and Validation of Novel Tools and Technologies for Neuroscience Research. Our overall goal is to enable positron emission tomography (PET) as a practical and effective quantitative tool in understanding brain function and health. In-vivo functional imaging of molecular tracers will play a key role in unraveling the physiology and functioning of the human brain in normal and abnormal states. A fundamental understanding of molecular pathways and distribution in the brain will also play a key role in finding cure for ailments that affect the brain. However, PET imaging systems currently used for whole-body oncology have insufficient resolution and are too costly to meet the needs for expanding neuroimaging research. There are no commercially available neuroimaging PET scanners, in part due to the prohibitive expense of dedicated brain-PET scanners using current state-of-art detectors and system design. For neuro-PET imaging technology to be impactful it cannot compete with hospital purchasing decisions for PET/CT, SPECT/CT, MRI or CT systems. It must be compact, easy to use, transportable, and affordable so that it can be operated in outpatient clinics, physicians? offices, and research facilities (e.g. neuroscience departments or pharma). We introduce two key concepts to reduce system costs while improving performance compared to existing PET scanners. The first concept is a dual-sided position-sensitive sparse-sensor (PS3) array, which is a high- resolution, depth-of-interaction capable scintillation detector with fewer sensors (by half) than traditional single- sided-readout scintillation detectors. The second concept is to strategically introduce gaps in parts of every other axial detector ring (a ?gap-bridge? design). In this manner, we reduce cost, while still satisfying Orlov completeness condition for accurate image reconstruction. Furthermore, openings in the scanner rings can then be used to insert visual monitoring and display equipment. In this phase I proof-of-concept application, we will validate the performance of the novel PET detector by building and testing a single detector ring prototype as preparation for a phase II proposal to develop a full commercial prototype. In addition, we will use Monte Carlo simulation to study the performance trade-offs between fully populated and our detector ring system with gaps. Our hypothesis is that by using both sparse sensor array detectors and a detector ring system with gaps, we will be able to achieve our design goals of better than 2.5 mm FWHM image resolution throughout the imaging field of view and greater than 5% absolute detection efficiency.