Intrinsic spatial resolution and parallax correction using depth-encoding PET detector modules based on position-sensitive APD readout

We are developing PET detectors with depth of interaction (DOI) capability based on lutetium oxyorthosilicate (LSO) scintillator arrays coupled at both ends to position-sensitive avalanche photodiodes (PSAPDs). The detector module consists of a 5×5 block of LSO crystals (1.5 × 1.5 × 20 mm³) coupled to 8×8 mm² PSAPDs at both ends. We present the intrinsic spatial resolution for two complete modules and the radial resolution component obtained with and without parallax correction using DOI information. DOI resolution was measured to be ∼3 mm. Intrinsic spatial resolution (FWHM) averaged 1.15 mm for opposing crystal pairs. The detectors were then rotated and moved closer together consistent with a 16 cm diameter cylindrical geometry scanner. Resolution was measured at positions corresponding to a radial offset of 4.7 cm and 7 cm, with the annihilation photons incident on the detector surface at 30° and 52° respectively. At an angle of 52° the intrinsic spatial resolution was degraded to 7.7 mm. By incorporating DOI information, the measured spatial resolution (FWHM) was improved to 1.8 mm. This demonstrates that approximately 90% of the resolution degradation due to the parallax error can be removed using these depth-encoding detectors for a field of view with a diameter that is 87.5% of the detector separation.