The impact of time-of-flight, resolution recovery, and noise modelling in reconstruction algorithms in non-solid-state detectors PET/CT scanners: - multi-centric comparison of activity recovery in a 68Ge phantom

The reconstruction algorithms implemented on PET/CT scanners offer gain in activity recovery of small lesions at an extent that is not full known yet. METHODS: A cylindrical phantom with warm background and hot spheres filled with a ⁶⁸Ge epoxy was acquired with four non-state-solid-detectors PET/CT scanners: mCT, Ingenuity TF, Discovery 710, and IQ. Images were reconstructed switching on and off time-of-flight (TOF), point spread function (PSF) modelling, and Bayesian penalised likelihood (BPL). Images were reconstructed with the default parameters recommended by the manufacturers. The recovery coefficient (RC_max), defined as the ratio of the measured maximum activity concentration in each sphere and the actual one, and the coefficient of variation (CoV_BAC) defined as the ratio of the standard deviation and the average of background activity concentration were measured. RESULTS: While with IR alone, complete recovery of the activity concentration is achieved down to the 22 mm diameter's sphere, with TOF, TOF + PSF and BPL it is achieved down to the 17 mm diameter one. At smaller dimensions, the difference among the various studied reconstruction algorithms is substantial for the 13- and 17-mm diameters' spheres for all scanners and for all reconstructions with a considerable gain in RCmax when PSF and BPL are used. At 10 mm diameter's sphere the difference among the algorithms is significantly reduced, except for BPL which still guarantees a gain in RCmax.