Investigation of a scalable phoswich crystal design for a TOF-DOI PET detector

We are developing a phoswich PET detector capable of measuring both 511 keV photon time-of-fight (TOF) and depth-ofinteraction (DOI). In our previous work, we have demonstrated the effectiveness of using time-over-threshold (ToT) for pulse shape discrimination in a two-layer TOF phoswich detector. However, in that work, the LSO:Ce,Ca (calcium co-doped lutetium oxyorthosilicate) crystals used to construct the phoswich detectors and the polished surface treatment of the crystals may not be realistic for scaling to a full clinical PET system. Therefore, in this work, we investigated the DOI and timing performance of two-layer phoswich detectors made of commercially manufacturable, non-polished (as-cut and chemically etched) LSO-only crystals. Eight two-layer phoswich detector configurations comprising a mixture of combinations of 2 LSO crystal types (fast and slow decay time, both 3.2 × 3.2 × 10 mm
³
), 2 surface treatments (as-cut and etched) and 2 orientations (fast crystal coupled to SiPM and slow crystal coupled to SiPM) were tested. Two different types of pulse shape discrimination, ToT and pulse integral, to identify hits in the two phoswich layers were used. The relative layer separation accuracy for the ToT spectra was 5%-24% better than for the pulse integral spectra. To evaluate the effect of DOI measurement on coincidence timing performance, layer separation was performed with pulse integral except for the detector configurations that could not be separated by pulse integral, for which ToT was used. The coincidence timing resolution (CTR) improved by 2%-20% with DOI measurement, depending on the surface treatment. A CTR of ≈200 ps FWHM was achieved for all phoswich detector configurations. Phoswich detectors made of fast (as-cut) and slow (etched) crystals with the slow crystal coupled to SiPM showed the largest layer separation (with a figure of merit ranges from 0.8 to 1.1) among the phoswich detector configurations studied in this work.