Nuclear medicine techniques, such as SPECT, have a potential to become powerful new tools for imaging biological processes on a cellular or molecular level. With ever increasing number of human disease models, particularly in smaller animals such as mice and rats, the potential for high-resolution nuclear imaging technologies to contribute unique information on such a fine level is becoming apparent. Recently, the demand for small animal SPECT has also been driven by the pharmaceutical industry where in vivo quantification of biological processes to measure an agent's mechanism of action and its concentration at the site of action is necessary.The main barriers to using SPECT in studies of laboratory animals have traditionally been poor spatial resolution, low sensitivity, and high cost. To address these deficiencies, it is proposed to develop a novel detector, based on a unique design of a scintillator coupled to a position sensitive detector. Specifically, we propose to develop a SPECT detector that can achieve extremely fine spatial resolution and high sensitivity in a cost-effective manner. The effort will focus on the development of a complete small-animal SPECT system as a prototype product.