Depth-of-interaction study of a dual-readout detector based on TOFPET2 application-specific integrated circuit

Depth-of-interaction (DOI) capability is important for achieving high spatial resolution and sensitivity in dedicated organ and small animal positron emission tomography (PET) scanners. The dual-ended readout is one of the common methods that can achieve good DOI resolution. The aim of this study is to evaluate a dual-ended readout detector based on silicon photomultiplier (SiPM) and TOFPET2 application-specific integrated circuit (ASIC). The detector is based on 4 x 4 lutetium-yttrium oxyorthosilicate (LYSO) units, each unit contained 6 x 6 LYSO crystals, and the crystal size was 1 x 1 x 20 min(3). The four lateral surfaces of LYSO crystals were mechanically ground to W14 (surface roughness 10-14 mu m), and the two ended surfaces were polished (surface roughness <0.5 mu m). The reflector was Toray Lumirror E60, and the packing fraction of the LYSO block was 86.5%. Each LYSO unit was read out from both ends with two Hamamatsu S13361-3050AE-08 SiPM arrays. The analog output signals of SiPM were digitized by PETsys TOFPET2 ASIC and acquired by PETsys SiPM Readout System. The ASIC and SiPM were cooled by a fan and a Peltier element. To investigate the crystal resolvability, different light guide thicknesses including 0.8,1,1.2 and 2 min were tested. The light guide was made of optical glass (H-K9L-Foctek Photoincs), and the size and refractive index were 6.45 x 6.45 mm(2) and 1.53 (at 420 nm), respectively. To characterize the detector performance at different depths, another 1 x 25.8 x 20 mm 3 single LYSO slab was used. Data were acquired at 10 depths (1, 3, ... ,19 mm), and each depth had a 10 min acquisition time and about 40 thousand coincidence events. During the experiment, the SiPM temperature was controlled as 27.6 +/- 0.4 degrees C. The results showed that the 1.2 mm light guide offered the best crystal resolvability. The energy, coincidence time, and DOI resolution full-width at half-maximum of the detector were characterized as 15.66% +/- 0.66%, 602.98 +/- 10.58 ps, and 2.33 +/- 0.07 mm, respectively. The good DOI resolution indicates the potential of utilizing the detector for high-resolution PET applications.