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

We report on the evaluation of a prototype PET detector with depth of interaction (DOI) encoding. The detector consisted of an 8×8 array of mixed lutetium oxyorthosilicate (MLS) crystals that were read out on the top and bottom by position sensitive avalanche photodiodes (PSAPD). Each crystal had dimensions of 1.65 mm × 1.65 mm × 22.00 mm, and the array had a pitch of 1.75 mm. The 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, while the summed signals from the two PSAPDs were 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 2.9 mm. Energy resolution and timing resolution (vs. 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 edge of the PSAPDs.