We have developed two LSO block detectors for single-plane small animal PET instruments which have a spatial resolution of approximately 1 mm. They have served mainly as demonstration prototypes but have also been used for numerous rat and mouse imaging studies. By extending the axial length of the detector element from these systems and adding a second ring of photomultiplier tubes, a high-resolution 2-dimensional block using analog position sensing in both the radial and axial dimensions has been implemented. It consists of ten 1.2 mm wide×7 mm deep×20 mm high LSO crystals that are discrete in the radial direction and continuous in the axial direction. An instrument based on this design results in a substantial increase in sensitivity. Experiments with this detector element have demonstrated 1.2 mm radial resolution and approximately 1.6 mm axial coincidence resolution over the central 15 mm of the 20 mm high block element. A preliminary design for an instrument based on the 2-dimensional block has been evaluated by simulation studies.