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Although gamma cameras have emerged in the sixties, their spatial resolution is still not sufficient to detect small tracer concentration abnormalities. Examinations like scintimammography requires high spatial resolution and then the possibility to position the detector as close to the explored organ as possible . The emergence of the new position sensitive photomultipliers tubes(PSPMT), from HAMAMATSU, permitted us to develop a compact gamma ray imaging probe which fulfils these requirements. The major interest of the new R8520-00-C12 PSPMT generation is their very low height (27mm) which allows to build a very compact and relatively light gamma ray detector. Their square shape (25.7x25.7mm2) and their very thin dead edges (1.85mm) authorize their juxtaposition in order to obtain a large detection area. In this study we investigate the characteristics of a prototype using a square 2x2 array of HAMAMATSU R8520 position sensitive photomultiplier tubes coupled to a pixelated NaI(Tl) crystal array containing 24x24 pixels each made of 2 x 2 x 5 mm3 crystals with 2.2 mm centre to centre spacing. We present the first results regarding intrinsic spatial resolution, energy resolution and homogeneity . Illuminating the detector, without scintillating crystal, with a light source simulating a scintillation at 140kev, we obtain an intrinsic spatial resolution better than 1mm on the whole field of view also including dead areas between PSPMTs. By coupling this detector to the crystal scintillator previously described, an energy resolution better than 10% FWHM at 140kev is obtained in PSPMT centers. These performances and the inherent scalability of detectors built using arrays of square tubes, make it an attractive choice for use in dedicated nuclear medicine instruments, including small animal imaging.
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