A critical challenge in positron emission tomography (PET) is to develop affordable systems having high spatial resolution. We imagine a PET system, for imaging a region of interest (ROI) with ultra high performances and other regions with normal performances by use of inhomogeneous performances PET detectors. We propose such a prototype, consisting of a few ultra high intrinsic spatial resolution PET detectors and quite a few normal ones. In this paper, we investigate the effect of the location and the number of high resolution detectors. The high resolution detectors are grouped in a continuous partial ring. The initial results show that such a prototype is capable to bring the advantages of significantly improving the spatial resolution in a ROI. For a specified area, the improvement is influenced by the location and fraction of high resolution detectors in a ring. The encouraging results also indicate the possibilities to regulate detectors to achieve high spatial resolution according to the application, which is possibly formulated.