Anger cameras (SPECT etc.) presently used in nuclear medicine employ as active detector NaI crystals, obtaining intrinsic spatial resolutions ≥ 3mm. Arrays made of optically isolated single crystal elements of YAP:Ce, having sub-millimeter aperture size, read out by position sensitive photomultipliers, allow to build active detectors to employ in SPECT systems, with intrinsic spatial resolution below the millimeter, and with time resolution of the order of tens of nanoseconds. In this paper preliminary results of measurements carried on different kinds of YAP:Ce arrays are reported. The measurements have been performed aiming to optimize the geometrical and physical parameters of the crystals in order to accomplish a SPEM (Single Photon Emission Mammography) camera detector.