PET Detector based on a Semi-Monolithic Crystal with DOI and TOF Capabilities

A detector block based on semi-monolithic scintillators consists of a monolithic crystal segmented in one direction in different pieces called slabs. In this configuration, optical photons are preferred to only propagate in the monolithic direction, but not in the segmented direction. Semi-monolithic crystals are intended to overcome the limitations of monolithic and pixelated crystals, offering both accurate time of flight (TOF) and depth of interaction (DOI) information, as well as good spatial resolution. In the work presented here, the characterization of two single LYSO slabs of different dimensions coupled to two different photodetector arrays has been carried out. The dimensions of the slabs are 50×18×3 mm
³
and 25×18×3 mm
³
with all faces polished. The faces of the slabs have been laterally wrapped with ESR and a retroreflector film added to the entrance. The employed photodetectors consist of a linear array of 1×16 SiPMs and a matrix of 8×8 SiPMs, in both cases with elements of 3x3 mm
²
active area each, 50 μm cell size and 3.2 mm pitch. The readout electronics was based on the TOFPET2 ASIC in all cases. The interaction position has been calculated with analytical methods. The preliminary spatial resolution is around 2 mm FWHM both in the long monolithic axis and in the DOI directions. The coincidence time resolution (CTR) of the detectors under study with a reference detector of 3×3×10 mm
³
ranges from 368 to 295 ps FWHM (50 and 25 mm slabs, respectively). This value improves to 318–260 ps FWHM CTR if energy weighted averaging of the timestamps belonging to the same event is applied.