Assessing the Spatial Resolution of the Modular J-PET Scanner using the Maximum-Likelihood Expectation-Maximization (MLEM) algorithm

Introduction: The presented study evaluates the spatial resolution of the Modular J-PET scanner using the National Electrical Manufacturers Association (NEMA) NU2-2018 standard. The Modular J-PET, constructed with BC-404 plastic scintillators in an axial arrangement and coupled with analogue Silicon Photomultipliers (SiPMs) at both ends, offers a 50 cm axial field of view and a bore diameter of 73.9 cm. The study compares results from GATE simulations with experimental data. Objective: The primary objective of this study is to assess the spatial resolution of the Modular J-PET scanner, using Time-of-Flight (TOF) and non-TOF image reconstruction, based on NEMA NU2-2018 guidelines. Methods: Spatial resolution was evaluated using a Na-22 point-like source as recommended by NEMA NU2-2018. Both TOF and non-TOF list mode acquisitions were performed, with a comparative analysis of the results from experimental and simulated data. Results: Radial spatial resolution, obtained based on the experimental data when taking into account TOF, is equal to 4.92 ± 0.56 mm, 7.38 ± 0.49 mm, and 6.94 ± 0.38 mm at positions 1 cm, 10 cm, and 20 cm from the detector centre, respectively. The tangential spatial resolution for TOF image reconstruction was determined as 7.38 ± 0.51 mm, 7.37 ± 0.10 mm, and 14.67 ± 0.31 mm at the same positions based on experimental data, while axial spatial resolution was calculated as 30.73 ± 0.52 mm, 30.73 ± 0.64 mm, and 31.96 ± 0.29 mm based on experimental data. Simulated radial spatial resolution for TOF image reconstruction methods was found to be 4.80 ± 0.59 mm, 7.26 ± 0.55 mm, and 6.67 ± 0.42 mm at positions 1 cm, 10 cm, and 20 cm from the detector centre, respectively. The simulated tangential spatial resolution for TOF image reconstruction methods was determined as 7.27 ± 0.47 mm, 7.27 ± 0.59 mm, and 15.1 ± 0.4 mm at the corresponding positions, while the simulated axial spatial resolution was determined as 29.97 ± 0.49 mm, 30.53 ± 0.74 mm, and 31.78 ± 0.11 mm. Conclusions: The Modular J-PET meets NEMA NU2-2018 standards, with TOF mode providing better spatial resolution than non-TOF, validating the system’s high-resolution imaging capabilities.