Optimization of tracer injection for 3D 18F-FDG whole body (WB) PET studies using an acquisition-specific NEC (AS-NEC) cure generation

Aim of this work was the implementation and validation, for the Discovery-ST PET/CT (GE medical systems) system, of the acquisition specific noise equivalent counts (AS-NEC) method to establish the amount of the tracer to be injected in 3D ¹⁸F-FDG whole body (WB) studies to achieve the peak of the NEC (NEC-p) at the acquisition time. The AS-NEC model uses prompts, delayed events and system dead time of a reference acquisition, to calculate the full shape of the NEC curve. The method was implemented using a 3D decay series of the NEMA-01 (line source in a Teflon 20×70 cm cylinder) phantom and validated with the NEMA-94 (20×20 cm uniform cylinder) and NEMA-01 image quality phantoms. The AS-NEC curves generated by single frames of the phantom series well correlated with the experimental NEC curves proving the validity and the generality of the method. The AS-NEC model was then retrospectively applied on 40 3D F-FDG WB studies in a range of body mass index (BMI) between 16 and 31 kg/m² (6 underweight (uw), 18 normal-weight (nw), 16 over-weight (ow)). For each frame acquisition of each patient study, the activity at the acquisition time, corresponding to the NEC-p was identified on the AS-NEC curves. Furthermore, as the NEC curves show a large region around the NEC-p with small variations (nearly plateau), the values of radioactivity corresponding to reduction of 1%, 3% and 5% with respect to NEC-p were also considered as possible alternative dose to be injected in the patients. The results show that the average activity corresponding to the NEC-p was comparable for the three BMI classes: 336.7 MBq (sd=22.2), 329.3 MBq (sd=33.3), 344.1 MBq (sd=48.1) for uw, nw and ow respectively. The average NEC-p activity for all patients was 336.7 MBq (sd=40.7). The mean values of the radioactivity at 1%, 3% and 5% with respect to the NEC-p were: 284.9 MBq (sd=40.7), 247.9 MBq (sd=33.3) and 225.7 MBq (sd=29.6) respectively. These results indicate the possibility to use, with the Discovery ST, a single injection protocol of 444 MBq (for the range of BMI considered) to have an activity at the acquisition time (after 45 min of uptake) of 337 MBq, to reach the NEC-p. Nevertheless, the plateau near the NEC-p suggests the possibility to reduce the dose- to be injected in clinical studies at 329 MBq while preserving suitable NEC performance (-3%) with respect to the NEC-p.