Performance of 256ch flat panel PS-PMT with small crystals for a DOI PET detector

A 256ch flat panel position sensitive photomultiplier tube (256ch FP-PMT) was evaluated for application to a depth of interaction (DOI) PET detector composed of small crystal elements. The 256ch FP-PMT has 52 mm×52 mm opening area with 89% useful area. Its 256 anodes are placed at a 3.04 mm interval. To estimate the FP-PMT performance, light spread functions of anodes at central and peripheral regions of a prototype 256ch FP-PMT were measured with a Gd₂SiO₅ (GSO) crystal sized 1.42 mm×1.42 mm×4.5 mm, one quarter of the area of the FP-PMT anode area. It was found that light spread functions of peripheral anodes were similar to the functions of central anodes and also the functions measured with a 2.9 mm×2.9 mm×7.5 mm GSO crystal, almost the same size as the anode area. Transit time fluctuation among the 256ch FP-PMT anodes was estimated by measuring time resolutions of a LuYSiO₅ (LYSO) crystal at some central and peripheral positions. The LYSO crystal has the same dimensions as the smaller GSO crystal BaF₂ was used for a reference detector. The obtained distributions show about the same time resolution between the central and peripheral positions and the full width at half maximum is (366±15) ps. The transit time fluctuation is ±106 ps. Two-dimensional position histograms of a 32×32 GSO crystal array and two layers of 9×9 GSO crystal arrays are obtained by uniform gamma-ray irradiation. The crystal dimensions are 1.42 mm×1.42 mm×4.5 mm. The 32×32 array covers all the useful area. The 9×9×2 array is coupled to peripheral region. The resultant histograms confirm that a 256ch FP-PMT has enough capability for identifying crystals of this size even on the periphery.