Detector characteristics of the micro crystal element scanner (MiCES)

This work reports on the performance characteristics of detector modules for the micro crystal element scanner (MiCES). MiCES is a small animal PET scanner that is designed for imaging mice. It will consist of four rings of 18 detector modules. The crystal array for each detector is a 22 × 22 grid of 0.8 × 0.8 × 10 mm mixed lutetium silicate (MLS) crystals set at 0.9 mm centers and framed for rigidity to compose a 19.94 mm square. A technique to mount multi-element crystal arrays to position sensitive photomultiplier tubes (PSPMTs) is described. The crystal arrays are slightly larger than the dimensions of the anode strips of the PSPMT. Therefore the detector module assembly requires precise alignment of the crystals with the PMT optics for optimum decoding. The crystal arrays are glued to the PSPMT using Histomount (National Diagnostics) glue. It has excellent transmittance at MLS´ peak emission wavelength. In addition, tapping down the crystals while the glue was curing was found to significantly improve the amount of light collected. We introduce a semi-automatic boundary segmentation technique to generate look-up-tables used for decoding the crystal of interaction from the collected PSPMT signals. The algorithm is based on valley detection followed by line segment tracking. The software tool provides a more convenient and precise way to create the crystal map for each detector module. Performance characteristics of each of the first 18 detector modules that were assembled and processed with the described techniques are presented. The overall results are an average energy resolution of 23.8% and an average peak-to-valley ratio of 7.6. These averages, however, include modules that had the crystal arrays removed and re-glued multiple times as part of our learning curve of how to optimize the gluing and positioning procedures.