Optimization of tracer injection for 3D18F-FDG whole body (WB) PET studies using an acquisition-specific NEC (AS-NEC) curve 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 deadtime 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 20×70cm cylinder) phantom and validated with the NEMA-94 (20×20cm 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 ¹⁸F-FDG WB studies in a range of body mass index (BMI) between 16 and 31 kg/m² (6 under-weight (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 me 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: 336.7MBq (sd=22.2), 329.3MBq (sd=33.3), 344.1MBq (sd=48.1) for uw, nw and ow respectively. The average NEC-p activity for all patients was 336.7MBq (sd=40.7). The mean values of the radioactivity at 1%, 3% and 5% with respect to the NEC-p were: 284.9MBq (sd=40.7), 247.9MBq (sd=33.3) and 225.7MBq (sd=29.6) respectively. These results indicate the possibility to use, with the Discovery ST, a single injection protocol of 444MBq (for the range of BMI considered) to have an activity at the acquisition time (after 45 min of uptake) of 337MBq, 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 329MBq while preserving suitable NEC performance (-3%) with respect to the NEC-p.