Millimeter spatial resolution and sub-200 ps timing in 32 mm x 32 mm x 22 mm monolithic LYSO:Ce crystals with dual-sided digital photon counter readout

602 Objectives The need to the reduce PET scanning times while improving image quality and quantitative accuracy translates into a demand for detectors with better detection efficiency, spatial resolution, and time resolution. Digital photon counter (DPC) arrays are fully digital, solid-state photosensors with promising characteristics for use in TOF-PET detectors. DPC arrays are almost transparent to 511 keV gamma quanta, enabling new detector designs. Here we study the improvement in spatial and time resolution that can be obtained by reading out a monolithic crystal made from standard LYSO:Ce with DPC arrays coupled to its front- and back surface in so-called dual-sided readout (DSR) configuration, compared to conventional back-side readout (BSR). Methods DPC arrays consisting of 8 x 8 DPC pixels at a pitch of 4 mm were optically coupled to commercially available LYSO:Ce crystals with dimensions of 32 mm x 32 mm x 22 mm in BSR or DSR configuration. The spatial-, time- and energy-resolutions of both detectors were measured. Results An average spatial resolution close to 1 mm FWHM was achieved with the DSR detector, combined with a depth-of-interaction (DOI) resolution of about 2.5 mm FWHM. With the BSR detector, an average spatial resolution of ~1.5 mm FWHM and an average DOI resolution of ~4 mm FWHM were obtained. The coincidence resolving time (CRT) measured with the BSR and DSR detectors were ~225 ps FWHM and ~170 ps FWHM, respectively. This can be compared with the CRT of 185 ps FWHM previously obtained with 20 mm thick, Ca-codoped, monolithic LSO:Ce crystals readout in BSR configuration. Conclusions The CRT obtained with standard LYSO:Ce in DSR configuration is slightly better than that obtained with Ca-codoped LSO:Ce in BSR configuration. Moreover, DSR significantly improves the spatial- and DOI resolutions. Monolithic scintillator detectors with dual-sided DPC readout appear promising for PET applications in which high spatial resolution, high time resolution, and high detection efficiency all are crucial. Research Support This work was supported in part by EU FP7 project SUBLIMA, Grant Agreement No 241711 (www.sublima-pet-mr.eu).