Timing performance of two PET detector designs capable of time-of-flight and depth-of-interaction measurement: Phoswich and offset crystal layers

Abstract

PET detectors capable of measuring 511 keV photon time-of-fight (TOF) and depth-of-interaction (DOI) can improve resulting image quality and accuracy. In this work, we studied the timing performance of two TOF-DOI PET detector designs: (1) phoswich and (2) offset crystal arrays. The phoswich detector comprises a 3 × 3 × 20 mm³ element made from a 3 × 3 × 10 mm³ LYSO:Ce crystal with 42 ns decay time optically epoxied to a 3 × 3 × 10 mm³ calcium co-doped LSO:Ce,Ca(0.4%) crystal with 33 ns decay time. The offset crystal block comprises a 4 × 4 array of 3 × 3 × 10 mm³ LYSO crystals in the bottom layer and a 4 × 3 array of LYSO crystals of the same size in the top layer. The top array is offset by half pixel pitch from the bottom array. The coincidence timing resolution for photopeak events acquired by two opposing phoswich detectors ranges from 164.6 ± 0.9 ps to 183.1 ± 4.2 ps FWHM, depending on the interaction layer combinations between two identical, dual layer elements. The coincidence timing resolution for photopeak events acquired by the offset crystal layers against a reference detector comprising a single 3 × 3 × 20 mm³ LYSO crystal ranges from 177.8 ps to 272.6 ps FWHM and averaged to 219.7 ± 35.5 ps FWHM. These results show that both designs are capable of TOF and DOI measurement.