Impact of Laser-Processed X'tal Cube Detectors on PET Imaging in a One-Pair Prototype System

The X'tal cube is our original PET detector, which is being developed to achieve isotropic 3D positioning detectability. The X'tal cube is based on a 3D segmented crystal block for which all surfaces are covered with photo-detectors. Previously, instead of our initial approach of gluing segmented pieces of crystals, we successfully constructed a crystal block segmented by laser processing, and we developed the X'tal cube with the laser-processed 3D square grids of 2 mm length. In this paper, we extend the laser processing to 3D square grids of 1 mm length. The volume of a 1-mm crystal segment is 1/8 of that of a 2-mm crystal segment. We also evaluate imaging resolution performance with a newly developed one-pair prototype system to simulate a ring-type scanner, while our previous reports were limited to development and evaluation of a single detector. In particular, we compare 1-mm X'tal cube detectors to 2-mm X'tal cube detectors. The one-pair prototype system consisted of two X'tal cubes, two rotating stages, and a 192-channel data acquisition system. Each X'tal cube consisted of the LYSO cubic crystal block of 18 × 18 × 18 mm³ in which the 3D grids of 1 mm pitch were fabricated by internal laser processing. The 4 × 4 arrays of multi pixel photon counters were optically coupled to each surface of the crystal block. The detector positions were automatically controlled to simulate a ring-type PET with a 14.6 cm diameter. Data were collected for all assumed detector positions and then a sinogram was obtained. The data were reconstructed using filtered backprojection. The average spatial resolution of the 2-mm X'tal cube was 1.9 mm full width at half maximum (FWHM) over the field-of-view (FOV). On the other hand, the average spatial resolution of the 1-mm X'tal cube was 1.3 mm FWHM over the FOV. By applying deconvolution with the assumption that the point source was a Gaussian function of 1.0 mm FWHM, we estimated the average spatial resolution of the 2-mm X'tal cube and the 1-mm X'tal cube as 1.6 mm FWHM and 0.83 mm FWHM, respectively. The average spatial resolution of the 1-mm X'tal cube was improved approximately 48% compared to the 2-mm X'tal cube. In conclusion, we confirmed the potential of the X'tal cube for uniform and high resolution imaging.