Wavelength discrimination (WLD) detector optimization for time-of-flight positron emission tomography with depth of interaction information

Abstract

Parallax error in positron emission tomography (PET) scanners degrades image quality away from the center of the field-of-view (FOV). Depth of interaction (DOI) encoding is a common method for mitigating this parallax error in PET scanners. A DOI encoding method based on wavelength discrimination (WLD) has recently been proposed, showing promising results in terms of figure of merit (FOM) values. This method employs an optical filter and a phoswich detector, comprising of LYSO (bottom layer) and GAGG (top layer) scintillators. The main idea behind this method is to place an optical filter between the phoswich detector and one of the two adjacent photodetectors that are used to acquire light photons emitted from the phoswich scintillators, thereby blocking the emission from one of the scintillators. This enables identification of the interacting scintillator, based on the signals detected by the two photodetectors. However, the method has a drawback: light photon loss due to the optical filter degrades the detector's energy resolution. In this work, experiments were performed to optimize detector configuration of this WLD method. GFAG, a scintillator with a relatively short decay time, was chosen to replace GAGG to improve timing performance. The timing performances of the conventional setup (S50) were measured to be 276 ps and 216 ps for top (GFAG) and bottom (LYSO) layers, respectively. The coincidence resolving time (CRT) for each setup was acquired by using a 3 mm x 3 mm x 5 mm Ce:LYSO scintillator as reference detector. In terms of energy performance, GAGG provided better energy resolutions of 12.9% (top) and 12.6% (bottom) due to its higher light output, compared to 14.7% FWHM and 13.0% for GFAG. Further optimization was performed by reducing the area covered by the optical filter to 25% (S25) and 15% (S15) of the total detector area. The results for S25 were 249 ps (GFAG) and 213 ps (LYSO) for timing performance, and 11.8% and 11.9% for energy resolution of the top and bottom layers, respectively. For S15, the CRTs were 250 ps and 201 ps, while the energy resolutions were 11.2% and 10.8%. Thus, the combination of two fast scintillators (Ce:LYSO and GFAG) and better optical filter placement delivered improvements in timing and energy resolution while preserving WLD's excellent DOI decoding capability.