Dual APD array readout of LSO crystals: Optimization of crystal surface treatment

We are developing a compact PET detector module by coupling an LSO scintillator array with two APD arrays to achieve high sensitivity, and high and uniform spatial resolution. We report studies on improving the depth-of-interaction (DOI) resolution by optimizing the crystal surface treatment, and on the effect of crystal geometry on DOI resolution. Three 2×2×20 mm LSO crystals were treated with different surface finishes along their length: raw saw cut, polished with 12 mm grade AlO2 paper, and fine mirror polish. The 2×2 mm ends were fine mirror polished. The ratio of the signals from the APD arrays was used to measure DOI, and the sum of the signals to measure the total light output. Crystals finished with the 12 mm grade paper gave the best overall detector performance, with DOI resolutions ranging from 3.1 to 3.9 mm for all interactions with energy above -50 keV threshold, and uniform light output for different DOI positions. The energy resolution averaged -17%. A 1×1×20 mm and a 2×2×30 mm LSO crystals finished with saw-cut and 12 μm grade paper were also measured, and gave DOI resolutions in the range of 2.7 to 4.4 mm, and 4.7 to 6.6 mm, respectively.