DoI detector design and characterization for open-field mouse brain PET

`Open-field' PET imaging of freely moving rodents is a powerful paradigm because it enables the correlation of functional and behavioral outputs in response to environmental or pharmacologic stimuli. Use of motion compensation techniques provides a feasible framework for imaging a moving animal. However, to overcome the geometric and performance limitations encountered in adapting motion compensation techniques to existing scanners, we are developing a purpose-built PET imaging system for freely moving mice. Two important innovations of this scanner are panel detectors with thick (20 mm) scintillators arranged in a tightly packed box geometry to provide high sensitivity (~15%) and a motion-controlled sliding gantry moving in synchrony with the animal. Here we describe the design and characterization of the depth-of-interaction (DoI) detector module forming the basis of a prototype system. The detector comprises a 23×23 LYSO crystal array, 0.85 mm pitch, 20 mm length, read out at both ends using 6×6 arrays of through-silicon-via (TSV) silicon photomultipliers. The crystal, photodiode arrays and light guide exactly matched to minimize gaps between neighboring modules. A cross-wire readout with resistive weighting was used to provide 4 position and one timing output per array, i.e. 10 outputs/module. Average energy resolution was 17.6 ± 5.3%. Average DoI resolution near the center of the crystal (10 mm) was 2.8 ± 0.35 mm and 3.5 ± 1.0 mm near the edges. This almost meets our target DoI of <;3 mm to provide near-isotropic spatial resolution of ~1 mm throughout the FoV. The timing resolution was 1.4 ± 0.14 ns. Manufacturing flaws in our scintillator array, particularly the integrity of the reflector material and its bonding to the individual crystals, likely compromised light collection and uniformity in our characterization of DoI and energy resolution. Therefore, although the results are promising, we expect improved performance in these parameters with a new crystal array.