The overall objective of this project is to develop a new detector module for multi-layer positron emission tomography that provides high sensitivity, high maximum counting rates, and high resolution. The advantages over existing block detector designs is a smaller block size, which results in higher maximum coincident event rates, and individual coupling of scintillator crystals to photodetectors, which improves the spatial resolution. Special features of this approach are: .Individual readout of narrow, heavy-atom scintillation crystals for high maximum event rates, maximum sensitivity, and 3.8 mm in-plane and 3.0 mm axial spatial resolution. .Solid-state photodetectors for identifying the crystal of interaction. .Evaluation of thallium bromide as a solid-state photodetector with high signal-to-noise ratio at room temperature. .Evaluation of newly discovered fast, heavy-atom scintillators and novel solid state photodetectors. This detector design is being developed for the quantitative measurement of tracer compounds with high spatial and temporal resolution to measure specific perfusion and metabolism. This project proposes only to do proof of principle studies culminating with test table images - no complete tomographs will be constructed. The design, when completed, will be offered to potential collaborators in private industry (CTI, Inc. of Knoxville, TN has already shown interest in this design) who may incorporate it into complete scanners.