Positron emission tomography (PET) is a non-invasive medical imaging technique which provides an image of a positron emitting radio pharmaceutical and its distribution in the body on a metabolic level. At the present time, the performance of this instrument is limited by the nuclear sensors used. The effort proposed here will examine an approach to improve the resolution in the imaging detector of PET systems by combining the signals of two types of sensors. A photomultiplier tube is the primary detector. In addition, solid state silicon photodetectors are used to obtain better spatial resolution. The goal of Phase I of this program was to demonstrate the feasibility of using silicon drift photodiodes as the semiconductor photodetector to make this approach work. We achieved this goal by demonstrating theoretically the advantages of using the drift photodiode structure over conventional silicon photodiodes. We also demonstrated in the laboratory that standard VLSI fabrication techniques, used by the semiconductor industry to construct microprocessors, can be used to construct drift photodiodes. The goal of the Phase II program will be to improve the drift photodiodes to the extent that they can be used in commercial PET machines. If successful this approach promises to be the breakthrough needed for the construction of a new generation of high resolution PET imaging systems.