Evaluation of a prototype small-animal PET detector with depth-of-interaction encoding

We report on the evaluation of a prototype small-animal positron emission tomography (PET) detector with depth-of-interaction (DOI) encoding. The detector consisted of an 8×8 array of scintillator crystals that were read out on the top and bottom by position sensitive avalanche photodiodes (PSAPD). Each scintillator crystal had dimensions of 1.65 mm×1.65 mm×22.00 mm, and the array had a pitch of 1.75 mm. The 14×4 mm PSAPDs were coupled to compact, discrete transimpedance preamplifiers. The DOI response was measured by illuminating the detector module from the side with an electronically collimated fan beam of 511 keV gamma rays. The signals from the corner contacts of the PSAPDs were used to identify the crystal of interaction, and the ratio of the total signals from the two PSAPDs was used to calculate the depth z at which the interaction took place. By stepping the crystal array in the z direction (perpendicular to the fan beam), we were able to determine the DOI resolution for each individual crystal. For measurements made at 10°C, the average DOI resolution was better than 3 mm. Energy resolution and timing resolution (versus a fast plastic scintillator coupled to a photomultiplier tube) were made under the same operating conditions. The average energy resolution across the array was 16.6% at 511 keV, and the average timing resolution was 3.9 ns. Importantly, the detector performance was maintained all the way out to the crystals at the edges of the PSAPDs.