Characterization of a PET Detector based on Semi-Monolithic Crystals

A semi-monolithic scintillator crystal consists of a monolithic crystal segmented in one direction in different pieces called slabs. Semi-monolithic crystals are intended to overcome the limitations of monolithic and pixelated crystals usually employed in PET detectors, offering simultaneously accurate time of flight (TOF) and depth of interaction (DOI) information, as well as good spatial resolution. In this work, the characterization of four different semi-monolithic detectors has been carried out, comparing the performance of rectangular versus trapezoidal shape, as well as ESR versus air as separator between the slabs.The two rectangular blocks consist of an array of 24 LYSO slabs of 25.4×12 mm2and approximately 1 mm width. The two trapezoidal detectors consist of 16 LYSO slabs of 25.4-20.6×12 mm2and approximately 1.5 mm width. In all cases, all faces are polished. The semi-monolithic crystals are coupled to a matrix of 8×8 SiPMs of 3×3 mm2active area each with 50 μm cell size and 3.2 mm pitch. The 64 individual signals are read out by the TOFPET2 ASIC. The energy resolutions of the whole block range from approximately 16% up to 22% FWHM at 511 keV. Regarding timing, the coincidence time resolution (CTR) between the detector under characterization and a reference detector (single LYSO pixel of 3×3×10 mm3) ranges from 272 ps to 291 ps after time skew and time walk correction. The effect of applying energy weighted averaging of the timestamps belonging to the same event has also been assessed.