LAUSR

Purpose Important advancements have been made in PET image reconstruction algorithms, among the newest modelling of the point spread function (PSF) and noise statistics modelling (NSM) can be accounted for. Quantitative results of the application of these reconstruction techniques on 3 state-of-the-art PET/CT scanners with and without time-of-flight (TOF) technique are presented. Methods and materials A 68Ge-NEMA phantom based on a standard IQ body phantom (Data Spectrum, Durham, NC) filled with 68Ge in an epoxy resin was used. The activity concentrations in the spheres (internal diameter-ID = 6, 10, 13, 17, 22, 28, 37 mm) and in the background were measured with a radionuclide calibrator tested against a NIST traceable source and were respectively 40.67kBq/ml and 10.58 kBq/ml at reference time (uncertainty ± 3%). Nominal sphere-to-background ratio was 3.84. Total activity in the phantom was 108.4 MBq at calibration. Acquisition time was adjusted according to the age of the phantom to meet 1.5 min per frame at the reference date on each PET scanner and reconstructed with variable reconstruction parameters (iterations: 48, 72; amplitude of Gaussian filter: 5, 6.4, 8 mm) with traditional (OSEM) and by combining advanced modalities, when present: PSF, TOF, TOF+PSF, NSM. For each PET scanner and each reconstruction modality maximum Recovery Coefficient (RCmax) values were evaluated for each visible sphere. RCmax were normalised to 37 mm ID sphere result. Results For each reconstructed modality, RCmax (mean ± sd) were averaged on different reconstruction parameters and PET scanners and for increasing sphere ID respectively 6, 10, 13, 17, 22, 28, 37 mm resulted: - OSEM: (0.31  ±  0.03), (0.51  ±  0.05), (0.63  ±  0.03), (0.82  ±  0.03), (0.89  ±  0.03), (0.95  ±  0.04), (1  ±  0) - OSEM+TOF: (0.32  ±  0.01), (0.56  ±  0.08), (0.64  ±  0.07), (0.84  ±  0.03), (0.91  ±  0.00), (0.97  ±  0.04), (1  ±  0) - OSEM+PSF: (0.30  ±  0.01), (0.50  ±  0.04), (0.65  ±  0.05), (0.88  ±  0.04),(0.93  ±  0.02), (0.95  ±  0.04), (1  ±  0) - OSEM+PSF+TOF: (0.30  ±  0.01), (0.56  ±  0.08), (0.70  ±  0.11), (0.91  ±  0.03), (0.94  ±  0.00), (0.97  ±  0.04), (1  ±  0) - NSM: (0.30  ±  0.02), (0.67  ±  0.19), (0.84  ±  0.14), (0.94  ±  0.05), (0.98  ±  0.03), (0.98  ±  0.06), (1  ±  0) Conclusion The RC obtained with the 3 state-of-the-art non digital PET/CT scanners demonstrate clearly that, independently of the reconstruction method used, modern PET/CT scanner outperforms respect to older one described by EARL curve. Higher differences are recorded for smaller spheres. Moreover, applying more advanced reconstruction techniques progressively enhances RC, NSM reconstruction being the most promising algorithm. Even if RCmax of different scanners converge at larger ( > 22 mm) and smaller (10 mm) spheres, the overestimations observed in the size between 1 and 2 cm are currently under investigation.