Assessment of ${\rm Lu}_{1.8}{\rm Gd}_{0.2}{\rm SiO}_{5}$ (LGSO) Scintillators With APD Readout for PET/SPECT/CT Detectors

Fusion of functional PET or SPECT images and anatomical X-ray CT images would ideally be performed using data recorded concurrently with the same detection system under all three modalities to avoid inaccurate co-registration due to patient movements and normal physiological processes like respiratory motion. One possible approach to achieve this goal is to use an avalanche photodiode (APD) based scintillation detection system. Requirements on scintillators for such an application are high density and high timing resolution for PET, good energy resolution for SPECT, large light output and fast scintillation decay time for counting mode X-ray CT. The purpose of this work is to investigate new Lu_1.8Gd_0.2SiO₅:Ce (LGSO) scintillators with improved performance developed purposely for PET applications, but having suitable characteristics for SPECT and CT detection. LGSO with 90% Lu content has high density (7.3 g/cc), large light output (90-120% of Nal), good energy resolution (~10 - 12% @ 511 keV), and fast decay time that can be adjusted from 34 to 45 ns by varying Ce concentration. In this work, LGSO with 0.025, 0.15 and 0.75 mol% Ce was assessed with APD readout for PET/SPECT/CT applications by measuring energy spectra at 511 keV (PET), 140 keV (SPECT) and 60 keV (CT), and timing resolution in reference to a fast plastic scintillator. LGSO-APD detectors can be considered as promising candidates for combined multimodality imaging scanners.