DOI encoding on the PET detector using 2 × 2 PMT array

We have begun to develop the PET detector which is composed of a scintillation crystal array and a 2times 2 array of inexpensive photomultiplier tubes (PMTs). The detector is intended for use for a low cost commercial whole body PET scanner. Anger type calculation of the four PMT outputs gives the 2-dimensional (2D) position histogram on which response of each crystal element appears and the scintillated crystal is identified on the histogram. The array of inexpensive PMTs however does not provide small sampling intervals like a position sensitive PMT and some crystal responses may overlap in the histogram due to insufficient scintillation light spread to the other three PMT outputs indirectly connected to the light-emitted crystals. To get more light spread and avoid the overlap of the responses, a light guide is generally inserted between the crystal array and the PMT array. We propose a new way to identify crystals which uses no light guides and which removes reflectors between some crystals and inserts some reflectors between the crystal array and the PMT array. The inserted reflector not only promotes light spread but also controls the light path to improve the crystal identification performance. Since the reflector is sufficiently thin, the method can be applied to a depth of interaction (DOI) detector with a new optical structure. By inserting a reflector between DOI crystal layers, it is possible to get a different light distribution between crystals in any DOI layer, which results in discrimination of all crystal responses in the 2D position histogram. We demonstrated the validity of this method and showed that it improved crystal identification ability without degradation of light output, and in turn, energy resolution. We also applied this method to a 2-layer DOI crystal array and verified its effect.