Parallax correction in PET using depth of interaction information

The authors have investigated the restoration of radial resolution for transaxially off-center sources in a PET ring detector configuration using directly measured depth of interaction (DOI) information. Lutetium oxyorthosilicate (LSO) crystals are coupled to a photodetector along the radial dimension of the PET ring configuration. Four millimeter wide crystals are segmented into 4 mm and 10 mm lengths in the radial dimension and optically isolated so that scintillation light is localized to the radial extent of the segment, giving 4 mm and 10 mm DOI resolution. The authors also segmented 2 mm wide crystals into 10 mm lengths. This DOI technique is unambiguous but not practical for a multi-ring PET scanner. The objective here is to quantify the intrinsic radial resolution as a function of position within the FOV and DOI resolution. The authors found the degradation in radial resolution across 75% of the field of view was reduced by 45% (50%) with DOI resolution equal to one-half (one-third) the crystal length. Improved resolution via narrower crystals and worse resolution due to non-colinearity were both overshadowed by DOI effects at 75% off-center positions in 30 mm deep crystals with 10 mm of DOI resolution.