A methodology for optimizing the acquisition time of a clinical PET scan using GATE

The acquisition time of a PET scan is a critical parameter when designing imaging protocols for clinical nuclear medicine studies. The statistical quality of the projection data increases when longer acquisition times are selected. However, very large scanning periods can limit the number of PET studies performed and, moreover, increase the probability of motion artifacts. The competing objectives of good statistical quality and short acquisition time are both depending on the counting rate performance of the system. The noise equivalent count rate (NECR), which measures the rate in which statistically important coincidence events are counted by a PET system, is employed in this study to quantify the counting-rate performance. Thus, higher NECR values allow for acquisition of relatively larger number of true coincidence counts at the same scanning time. NECR is directly depending, for a particular patient-scanner system, on the amount of radioactive dose injected into the patient and acquires a peak value for a certain range of dose values. Thus, a minimal acquisition time can be achieved by estimating this optimal dose range prior to a scan. In this simulation study we propose an alternative optimization methodology. Initially, a regular low dose is selected and used as a constant. Then the NECR response is modeled, using Geant4 Application for Tomography Emission (GATE) simulation package, as a function of the parameters of the patient´s body size, the coincidence time window, the dead-time response and the energy window. Subsequently, the optimal scanning time is estimated, based on the simulated NECR, as the minimal scanning time necessary to acquire 20 million noise equivalent counts (NEC) per bed position. For this purpose, we employed a validated Biograph PET/CT scanner model, where six hypothetical dead-time responses were simulated as well as three coincidence time windows. Finally, we used three NCAT phantoms of different size, and four energy windows.