Semi-Monolithic Crystals in Preclinical PET: A Leap Forward in TOF and DOI Technologies

Semi-monolithic crystals have the potential of combining the timing capabilities of pixelated crystals and the 3D positioning accuracy of monolithic crystals. We present here a preclinical PET scanner consisting of 2 rings with an inner diameter of 106 mm, and an axial length of 52 mm. Each ring consists of 14 semi-monolithic arrays of 22 slabs of $25.6 \times 12 \times 0.97 \mathrm{~mm}^{3}$ each, coupled to $8 \times 8$ SiPMs arrays. A timestamp averaging method, including time skew and time walk corrections, was applied, achieving a detector time resolution as good as 178 ps FWHM. A neural network technique based on a multilayer perceptron architecture has been implemented for the prediction of the annihilation photon impact position inside each detector module along the monolithic direction. The depth of interaction has been determined using the Energy/$I_{\max}$ estimator and calibrated using a Voronoi method. A full NEMA protocol evaluation has been carried out. A spatial resolution of $0.96 \pm 0.07 \mathrm{~mm}$ was achieved at ¼ of the axial Field of View. The sensitivity at the center of the scanner was 3.1% with an energy window of 30%. A NECR value of 10 kcps at a $280 \ \mu \mathrm{Ci}$ was achieved. The Recovery Coefficients (RCs) and Spill-Over-Ratio (SOR) values were obtained for the image quality phantom. RCs of 0.27, 0.71, 0.90, 0.98 and 0.99 were achieved for the 1, 2, 3, 4 and 5 mm rods, respectively. The SOR values were 0.18 for air and 0.27 for water.