PET Scintillator Arrangement on digital SiPMs

Abstract

The common way to build a PET detector is to place an array of scintillator elements on top of a photo detector. In order to achieve high spatial resolution the scintillator footprints are often smaller than the pixel size of the photodetector. This requires light sharing and some kind of algorithm like Anger-Logic in order to identify the correct scintillator element in which the event took place. The digital Silicon Photomultiplier DPC3200-22-44 (Philips Digital Photon Counting) is a fully digital photo sensor device [1]. Each pixel consists of 3200 individual micro cells which are charged and read out under digital control. The device (Tile) is organized as an array of 8 by 8 pixels each of 3.9x3.9 mm2 size and is realized as a PCB equipped with 16 dice. One die provides four pixels together with the corresponding triggering, validation and readout electronics. Depending on the configuration the detection of an event on one die can cause the other dice to transmit their data as well (neighbor logic). The obvious solution of using neighbor logic and a scintillator matrix with light guide covering the whole tile shows some drawbacks. After each event all 16 dice will be busy and all pixels need to be read out. This results in increased dead time and a lot of data. Furthermore it turned out that sometimes pixels are missing because dice were already busy and could not transmit data when the event was detected. This will complicate the identification of the event position. A better performance can be obtained when the light is shared only within the four pixels of each die and the dice work independent from each other. We investigated the positioning capability of different scintillator matrices and light guides. These are arranged in such a way, that a single die can only receive the light from a 4 by 4 array of LYSO crystals which covers exactly the die dimensions. The results show that clear crystal identification can be achieved with such an arrangement. [1] Haemisch et al., Physics Procedia 37 (2012) 154