Small animal PET scanner is a very powerful tool to study biologic processes in vivo. The performance of the detector module is the central piece for a small animal PET scanner. Discrete crystal detectors have traditionally been used to achieve high spatial resolution for small animal PET scanners. But the cost of fabrication increases considerably as the cross-section of the crystal element gets smaller and smaller. The dead space between the elements also goes up when the number of crystal elements per unit area increases, which equivalently reduce the stopping power of the detector module....